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Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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CHAPTER 6

OPPORTUNITIES IN MATERIALS RESEARCH*

*  

This chapter was prepared by a Task Force that included E.A.Chandross, G.Y.Chin, A.G.Chynoweth (chairman), T.D.Dudderar, P.A.Fleury, R.Frankenthal, F.T.Geyling, K.A.Jackson, P.L.Key, R.A.Laudise, L.D.Loan, S.Mahajan, D.K.Rider, M.D.Rigterink, T.D.Schlabach, W.P.Slichter, J.H.Wernick, and F.H.Winslow. As part of its work, this Task Force analyzed and summarized the large number of letters that were received in reply to invitations sent out by COSMAT to scientists and engineers in the materials field seeking their opinions about materials research opportunities. Most of these scientists and engineers were reached through the professional societies. Others were reached through appropriate Gordon Research Conferences. A few were identified individually.

The Professional Societies whose help was solicited in this part of the COSMAT Study included:

American Ceramic Society

American Chemical Society

American Concrete Institute

American Foundrymen’s Society

American Institute of Aeronautics and Astronautics

American Institute of Chemical Engineers

American Institute of Mining, Metallurgical, and Petroleum Engineers

American Iron and Steel Institute

American Nuclear Society

American Physical Society

American Society of Civil Engineers

American Society of Mechanical Engineers

American Society for Metals

American Society for Nondestructive Testing

American Society for Quality Control

American Society for Testing and Materials

American Welding Society

Association of Iron and Steel Engineers

Electrochemical Society

Electron Microscopy Society of America

Federation of Societies for Paint Technology

Forest Products Research Society

Institute of Electrical and Electronic Engineers

Instrument Society of America

National Association of Corrosion Engineers

National Association of Power Engineers

Optical Society of America

Society of Aerospace Material and Process Engineers

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

   

   

Society of Automotive Engineers

Society for Experimental Stress Analysis

Society of Manufacturing Engineers

Society of Plastics Engineers

The Gordon Research Conferences (1972) through which solicitations were made included:

Analytical Chemistry

Atomic and Molecular Interactions

Chemistry and Metallurgy of Semiconductors

Chemistry of Molten Salts

Chemistry and Physics of Cellular Materials

Chemistry and Physics of Coatings and Films

Chemistry and Physics of Inorganic Phosphors

Chemistry and Physics of Liquids

Chemistry and Physics of Paper

Chemistry and Physics of Solids

Corrosion

Crystal Growth

Elastomers

Environmental Sciences: Air

Geochemistry

Inorganic Chemistry

Ion Exchange

Laser Interactions with Matter

Natural Products

Organic Photochemistry

Polymers

Science of Adhesion

Separation and Purification

Technology of Biomaterials

Thin Films

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

CHAPTER 6
OPPORTUNITIES IN MATERIALS RESEARCH

INTRODUCTION

The broad aim in materials research is to develop fundamental and general understanding of the properties and performance of materials and how these relate to their composition, structure and processing. Increasingly, this knowledge is expressed in terms of the fundamental, individual and collective properties of atoms and electrons. Increasingly, phenomenological or analytical models of materials at this basic atom-electron level are becoming a common language that spans many scientific disciplines, science and engineering, and catalyses effective communication and knowledge-transfer back and forth among the materials community. Whether the initial stimulus for seeking this knowledge is largely curiosity or the prospect of an application, the approach and objective is much the same—to acquire fundamental knowledge which can, in turn, be used in a rigorous, direct, predictive way to advance with confidence the frontiers of materials capability. The alternative to understanding at the microstructural and atom-electron level is to rely primarily on empirical methods which, though often expedient and dramatically successful, provide few guides to whether further improvement is possible.

Despite the many impressive achievements of materials research there is the awareness that only the surface of scientific capability has been scratched. The majority of advances have historically been made via the empirical approach. Most new materials or properties are arrived at or discovered by cut-and-try methods—new chemical or alloy compositions are prepared and characterized and their various properties are determined. There are usually underlying rationales or phenomenological models to this empirical approach but it is rare indeed for a new material or property to be predicted from basic principles. The principal exception to this situation is in the area of single crystal materials, particularly those used in solid state electronics. On the other hand, techniques and concepts of physical science are often essential for characterizing and reproducing the properties of even empirically-invented materials. With electronic materials, due to the combined talents of chemists, metallurgists, physicists and electrical engineers a degree of understanding has been achieved, at least for the simpler crystals, so that material compositions having the desired physical properties can often be prescribed beforehand.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

An important key to this progress in electronic materials is the single crystal state of the material—a state which lends itself to theoretical analysis. Only relatively recently has the attention of basic research scientists been turning towards other, more complex forms of matter, such as the glassy, polycrystalline and polymeric states characteristic of the majority of practical materials, particularly those used in structural applications. It is reasonable to hope that the levels of sophistication that materials scientists and engineers have achieved with single crystal materials will, in due course, be paralleled by achievements with these more complex states of matter.

In the following paragraphs some illustrative examples will be given of opportunities for research in materials. Whether these opportunities are curiosity- or applications-stimulated they have in common a primary aim of arriving at fundamental knowledge at the atom-electron level. Some of this knowledge, when obtained, may have immediate applicability, some may be many years ahead of its application, and some may never find practical use although it may still contribute to general conceptualisation of understanding about materials. While the need and urgency for acquiring knowledge will vary in different parts of materials science, leading to the setting of priorities, it is impossible to conclude that even the most esoteric and apparently irrelevant research topics in materials science, judged by today’s standards, will not prove of value at some time in the future.

This brings up the question of the time lag between today’s basic research and tomorrow’s technology. The engineer is usually concerned with achieving practical results on a relatively short timescale, say one to five years. But often he will not know precisely, beforehand, what areas of materials science he will have to draw on so that a shrewdly developed stockpile of scientific knowledge and the techniques for rapidly acquiring new knowledge are vital for current and future engineering projects. If the knowledge is to be ready when the engineer wants it scientists may have to be working five to twenty years ahead. Thus today’s basic research may be the engineer’s handbook fifteen years hence. Much of the basic research of fifteen years ago is, in a sense, in the engineer’s handbook today.

The occurrence of such timescale effects may be better understood with a specific example, such as provided beautifully by research into the band structures of semiconductors. In the early fifties efforts to find ways to calculate the electron band structures, or energy distributions, in crystalline semiconductors probably seemed rather remote from the tasks of trying to make practical junction devices. But with the aid of the relatively large computers that were beginning to make their appearance such calculations led to marvelously detailed insight into the electronic and optical properties of semiconductors. Calculations were steadily refined and extended to other crystalline materials, but particularly semiconductors. As a result, such phenomena as the Gunn effect and laser action in gallium arsenide and light emission from junctions in gallium phosphide are all understandable in terms of the detailed band structures of these materials. In fact, so well accepted have band structure calculations become that the modern, sophisticated solid state electrical engineer, trying to develop a more efficient Gunn effect oscillator, or a laser, or a light emitting diode,

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

or many other semiconductor devices, would first consider the band structures of available materials. So the esoteric, seemingly remote theoretical solid state physics research of nearly a couple of decades ago is nowadays the base from which an electronic engineer embarks on specific short term development projects.

In what follows, illustrative examples will be given of current opportunities in materials research, both curiosity- and application-motivated.

These examples were extracted from a considerable number of written inputs from those knowledgeable in the field of materials science and engineering.

These opportunities in materials research can be conveniently arranged into four groups:

  1. particular classes of materials,

  2. materials processing,

  3. basic properties of materials that are clearly relevant to eventual applications,

  4. basic properties of materials where the specific relevance is not yet apparent.

CLASSES OF MATERIALS

Ceramics

Polycrystalline ceramics compete with metals and glasses as engineering materials. In the future, as in the past, the major aim of ceramics technology will be the development of new compositions and processing techniques to achieve superior physical and mechanical properties. These properties will be gained through close control of composition, density, and size, shape and orientation of the grain structure. Thus increasing emphasis will be placed on the relationships among composition, microstructure and material properties. The following areas of research are vital to the realization of that aim.

For new compositions, basic study in solid state physics and chemistry is necessary in order to gain new insights concerning electric, optical, magnetic and mechanical phenomena which may be peculiar to ceramics. In the recently discovered lead-lanthanum zirconate titanate ceramic, for example, it was found that the addition of lanthanum to PbZrO3-PbTiO3 resulted in compositions with improved optical transparency, electro-optic memory characteristics and linear and slim-loop quadratic electro-optic characteristics. If the role of lanthanum in relation to these properties is better understood, new ceramics with still better physical properties may be developed. Other examples of opportunities in devising new compositions are found under the section on electronic materials.

In the area of mechanical properties, an improved understanding of the relation of crystal structure and bonding behavior to crystal plasticity, as well as a refinement in the treatment of fracture mechanics coupled with microstructures, could lead to ceramics which are strong and tough. High

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

temperature structural ceramics are receiving increased emphasis as is evident by work on silicon nitride and silicon carbide for gas turbines. These materials exhibit high temperature strength coupled with good thermal shock resistance on account of low value of thermal expansion and high thermal conductivity. Recent studies have indicated that improved properties may be possible in complex systems such as solid solutions of β-Si3O4 and Al2O3. In addition, high density could be achieved by sintering at relatively low temperatures. Hence ceramics based on Si-Al-O-N and related systems appear promising as high temperature structural materials.

Other areas of research interest in ceramics include behavior in severe mechanical environments (abrasion, cutting, and ballistic loading), physiological environment (bioceramics) high pressure environment (deep-submergence components) and radiation environments (laser damage, swelling of nuclear fuels).

In the area of processing techniques: (a) More needs to be learned about the chemistry of oxide formation with the exception that this research would lead to novel techniques of preparing high-purity oxide particles of controlled composition and particle size and shape. Such improved starting materials would enable better attainment and control of microstructure in subsequent processing operations. (b) Continued study of the basic mechanisms of sintering, particularly in complex systems of several ceramic components, is a must if the achievements of dense, transparent ceramics made in simple systems are to be extended to new systems. Thermodynamics of phase relations, kinetics of reactions, nature and behavior of surfaces and interfaces, and plastic deformation behavior are major topics in this area. (c) Basic studies of novel processing techniques are of paramount importance. These include extension of hot pressing to forging and other types of hot deformation, as well as computer analysis and control of complex processing variables involving temperature, time, pressure and gaseous environment. Since many physical and mechanical properties of ceramics are anisotropic, attention might be paid to texture development in processing.

Glass

Many “discoveries” have been made in glass science in the last 25 years. In general, these have not been instances of isolated discoveries as such, but rather a continual building on an accumulated body of experimental facts, until a “discovery” or rather an “understanding” was achieved. In this vein continued investigation is needed in such areas as: Kinetics—glass formation is basically a kinetic problem and much more needs to be known about the general mechanisms of glass formation for various types of composition as well as associated dynamic problems including “network” and ionic relaxation (diffusion, conductivity, polarization, etc.). Phase separation—separation of a single phase, homogeneous glass, into two or more amorphous phases, or amorphous and crystalline phases may be either troublesome or useful. The understanding of both the thermodynamics and kinetics of these processes have been advanced substantially in recent years but is not yet at the stage where the occurrence or absence of phase separation in the more complex glasses can be predicted.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

Brittle fracture—one of the major drawbacks in the use of glass products is usually rapid deterioration of strength. An understanding of the process of brittle fracture, ultimate strength, “notch” sensitivity and static fatigue, have led to some improvements in useful strength, as well as more efficient use of the inherent strength of glass products, but here again a knowledge of the fundamental limits to mechanical properties has not yet been reached.

Structure—the achievement of an “understanding” of glass structure is difficult as well as difficult to define. The use of new tools (NMR, EPR etc.) should produce much additional knowledge in this area—clarification of the “borate anomaly,” recognition of the occurrence and role of pairing and clustering and elucidation of the character of site distortions, for example.

Electronic Properties—studies of “electronic” behavior have led to such discoveries as the switching and memory effects in amorphous semiconductors. Studies of the behavior of 3d and 4f ions in glassy hosts has produced structural information as well as an understanding of the physics of optical absorption and fluorescence in glassy solids. But in view of the increasing role expected for glass in electronic applications such as optical communications, there is a need for greatly improved understanding of the spectroscopic properties of various ions in various glassy hosts.

Optical waveguides—A particularly demanding and exciting challenge to glass technology is the development of practical long-distance optical waveguides for communications. A commonly envisaged configuration is a clad optical fiber with a high refractive index core and a low refractive index cladding. The realization that the fundamental limits to loss mechanisms in some inorganic glasses in the red and near infra-red spectral regions should be only a few dB/km has led to intense activity aimed at the preparation of glass fibers of extreme purity, extreme freedom from light scattering and absorbing defects, and extreme dimensional control. Such needs have emphasized, in particular, how crude the present state of technology is in the area of ultra-purification of chemical compounds—for all but a handful of materials there is no counterpart to the elegant zone refining process.

Metals

A continuing challenge to basic research in metals is to discover alloys capable of meeting exacting performance criteria under ever more hostile environmental conditions, such as those used in jet engines and in nuclear reactors. Much exploratory effort remains in the search for high-field superconductors with high transition temperatures. In the complex world of microelectronics, thin film metallization plays a central role, along with attendant problems in short circuit diffusion, electromigration, corrosion resistance, etc. associated with thin surfaces. A large class of metals and alloys acts as “contacts”, as in switches, relays and commutator brushes. Wear, tarnish and electrical erosion are some of the perrenial problems. These topics are discussed below as illustrative examples of metals research.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

Many of these are related to high performance technologies and thus call for much effort in broad “alloy mapping” programs. It is recognized that steady improvements are also expected in conventional ferrous and nonferrous alloys. The most promising lines for progress for these materials, however, seem not to lie so much in broad “alloy mapping” programs as in devising more efficient and economical processing techniques, as discussed in a later section. These processing techniques are to be based on knowledge of material properties and behavior gathered over the last several decades.

Superalloys

Alloys based on nickel, cobalt or iron which are intended for service above 1000ºF are frequently termed superalloys. The nickel-based system is the most advanced and most widely used of these alloys in current aircraft gas turbine engine applications. A major contribution to high temperature creep strength is derived from a high volume fraction of very fine coherent, ordered γ′ precipitates, which are stabilized by alloying additions based on considerations of low diffusivity, low interfacial energy or low solubility. In addition, the grain structure may be stabilized with insoluble phases, carbides or oxides. Further improvements in superalloys appear probable from two directions: One based on overcoming temperature limitations resulting from environmental attack; the other on increasing strength from processing improvements. There is also a need for improving the correlation between simple laboratory tests and service conditions so that life of components can be predicted with greater accuracy.

The next generation of superalloys will operate at temperatures that are too high for the traditional Cr2O3 protective scales, because evaporation of chromium via CrO3 takes place to an increasing extent above 1800ºF, requiring the use of relatively brittle coatings. One promising direction is to develop a new family of superalloys protected by Al2O3 scales, which are not subject to evaporation, and grow very slowly, because of the low cationic diffusion of A13+ through the scale. Al2O3 scales tend to spall during thermal cycles, but this may be overcome by dispersed oxides, which may be added intentionally as in the TD alloys (thoria dispersed) or formed by internal oxidation of reactive elements like yttrium. It has been discovered only recently that oxide dispersions have significant beneficial effects on high temperature corrosion in addition to their well-known beneficial effects on high temperature creep. The rate of scale formation is much lower, and the adherence of the scale is greatly improved. The dispersed oxides reduce the reactive alloying content (like chromium) needed to produce external scales rather than internal oxides. There is also evidence that the improvement in adherence is due to the internal oxides acting as vacancy sinks, preventing vacancy agglomeration into voids at the scale-coating interface. Thus, major questions to be answered in the development of coatings include: effect of alloying additions (1) on the diffusivity of the alloy constituents; (2) on the thermodynamics and kinetics of formation of competing oxide films; (3) on competition between internal and external oxidation; and (4) on vacancy behavior and its possible role in spalling.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

Processing is a major arena for improved properties and performance of superalloys, particularly through the use of directionally-solidified eutectic alloys, electroslag remelting, composite structures joined by diffusion bonding, and improved powder metallurgy processing. Mechanical alloying in attritor mills can effectively disperse oxide phases. Control of recrystallized structure to produce interlocked, elongated grains aligned in the stress direction can be provided by control of dispersed phases or through zoned recrystallization. The high temperature benefits resulting from elongated, interlocked grains in nonsag tungsten can be extended more broadly to superalloys and other high-temperature materials.

In alloy development for still higher temperature service, there is an attractive possibility for new class of alloys based on the refractory metals. Major problems are similar to those for superalloys (protective coatings, optimum alloying and processing), but only more so at the present time.

Radiation Resistant Reactor Materials

The operating conditions within fast breeder reactors, i.e., high temperatures (~575ºC) and high neutron fluxes, impose very stringent materials requirements; the materials requirements for a fusion reactor system are still more severe. Under these conditions swelling of the reactor components, resulting from the formation of voids, leads to dimensional instability of the structural components. In order for reactors to compete favorably with conventional energy sources, the reactor system should have as long a life as possible. To find out how to control swelling phenomena is of high importance.

During neutron irradiation, self-interstitials and vacancies are produced in equal numbers in structural materials. However, interstitials are preferentially removed from solution leading to the super saturation of vacancies which, under the right conditions, precipitate as voids. It is, however, clear that each of the following metallurgical variables have an effect on the formation of voids: (i) impurities, (ii) irradiation dose and temperature, (iii) dislocation density and distribution, (iv) fine precipitates and (v) grain boundaries. Metals and alloys containing second phase particles may be more resistant to swelling. Since the operating temperatures are fairly high, superalloys and refractory metals and alloys are the best prospects for satisfying the stringent material requirements. In fusion reactors the walls must be compatible with lithium, sputtering, fast neutron damage, and higher temperatures. The ductility of refractory materials is often insufficient for fabrication into complex shapes. Either the ductility of these materials has to be improved or new fabrication techniques developed. Another factor which needs investigation concerns the use in fast breeder reactors of liquid sodium and liquid sodium containing dissolved gases.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×
Superconductors

One area posing a great challenge to metals research is the development of practical superconductors of possible use in thermo-nuclear power generation, high-speed transportation, propulsion, magnetic-ore separation, secondary water treatment, and desulfurization of fuel oil and coal. There is a need for research aimed at translating the broad theoretical guidelines of solid state physics, such as the need for high density of states and large electron-phonon interactions, into available material parameters such as chemical composition, crystal structure, elastic constants, lattice parameter, and melting points. Thus far transition-metal compounds with the beta-tungsten and rock-salt structures (at room temperature) have yielded the highest superconducting transition temperatures (Tc). It has been found, however, that certain phases of materials like Mo-Re, and more recently, Nb3Ge, which are stable at elevated temperatures but metastable at room temperature, also exhibit high Tc. If new high Tc materials turn out to be metastable phases, there will then be a major challenge of processing these materials economically. Indeed, uneconomical processing is a major problem preventing the wide-spread use of known high Tc materials such as Nb3Sn. The compound Nb3Ge has the highest known Tc of 23.2ºK, but a method of fabricating it into useful shape and retaining the high transition temperature has yet to be devised. Since stoichiometry and atomic order are two critical material parameters, basic research in phase equilibria and kinetics of phase transformations is a necessary prelude to new or improved processing techniques. Concerning the goal of raising the critical current density, there is still much need for detailed fundamental understanding of the role of various imperfections such as inclusions, dislocations and grain boundaries in pinning the flux lines. Such understanding should lead to substantial improvement in critical current density through structural control by processing.

Contact Materials

The life and reliability of an electrical contact is ultimately limited by one or more of the following phenomena; arc erosion, tarnish film formation, polymer formation, wear or particulate contamination. Which one or combination of these eventually determines contact life depends on the circuit parameters, the contact material(s), and the chemical and mechanical environment of the contact. Contact failure is generally characterized by the development of excessive contact noise and/or resistance.

In spite of the complexity of this subject, opportunities for fairly well-defined materials research on contact materials do exist. The first such area is that of tarnish film formation. Tarnish film formation is the limiting failure mode in many low-energy or dry circuits involving switching or semipermanent contacts. Gold- or platinum-group-based alloy systems have been the traditionally-preferred contact materials in these instances because of their limited film forming tendencies. Research directed towards

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

limiting film formation on less noble or base metal alloys exposed to nominal contact environments would appear worthwhile. Oxide and sulfide films are those of principal interest.

A second area is that of the arc erosion behavior of contact materials, particularly as used in low- to medium-energy circuits. Here, the objectives are to understand those properties of a contact that determine arc duration and energy and which lead to lateral as opposed to localized erosion. Studies over a range of well-defined circuit conditions are required here.

Additional research opportunities exist with regard to frictional polymer formation, mechanical wear of contacts and the reaction of mercury with contact support and contact materials in mercury-wetted contact systems. These areas, however, are judged to be of lesser overall importance than those of arc erosion and tarnish film formation.

Plastics

Substances, such as cotton, wool, and silk have been known to mankind longer than recorded history. Others such as rubber, rayon, and celluloid were developed for practical use through empirical processes long before their basic molecular character was correctly known. But the single important feature that sets these seemingly unique substances with remarkable physical and chemical properties apart from a host of well recognized materials is that they are made of very big molecules which obey all the recognized chemical and physical laws but which have added properties that stem from their giant size. Intense research has yielded knowledge of how variation in the structures of these giant molecules, through new approaches in chemical synthesis, can be invoked to cause valuable changes in physical properties. Fundamentally, these marcomolecules, both natural and man-made, consist of chemical combinations of small molecular entities, monomers, that are part of the huge family of chemical compounds. It is the combined effect of the monomeric structures that determines the physical and chemical character of the polymer. While research along these lines began 40 years ago, it remains a vital, increasingly sophisticated part of chemical science.

During the past decades a wide variety of new and useful polymers have been brought into commercial production. More new polymers with interesting mechanical, electronic and chemical properties will undoubtedly be produced in the future but launching a radically-new polymer is expensive. Semi-empirical routes to useful materials are likely to be followed most often, one of the most attractive of these being to explore the effects of blending of polymeric materials available at present. The properties of polymer blends are not merely the means of those of the two individual components. The resultant properties depend on a variety of factors including, perhaps most importantly, the intimacy or heterogeneity of the mixing. The degree of dispersion can however vary greatly and indeed some polymer pairs cannot be properly blended at all. Where blending of pure polymers is impossible various modifications may be made to improve compatability. As a known

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

example the rubber particles used to increase impact strength in ABS (derived from Acrylonitrite-Butadiene-Styrene) are surface modified by grafting to provide a good bond across the boundary.

Increased fundamental understanding of the properties of blends is needed to enable us to readily obtain materials with a wide range of properties, properties which are sometimes too specialized to warrant bulk production of a new polymer. Furthermore, combinations of properties not possible with single polymers will be achievable using blends, not only of two polymeric components but several. This need for research and exploration in the field of polymer blends can be compared in many ways to the history of metal alloy research and development.

Several important research activities will illustrate this continuing appear for increased understanding of molecular structures and physical properties. Major achievement and continuing effort occur in the field of rubber-like compounds. Nature has given us, in the product of the rubber tree, a molecule that possesses a high degree of flexibility. This property is partly owing to the nature of the chemical bonds in the molecular chain, which are relatively free in their movement. The flexibility also stems from the structural asymmetry of the monomers which constitute the chain. This asymmetry keeps the chains from clustering closely and thus allows the freedom of motion that is essential to rubber-like behavior. Research into the synthesis of polymers possessing the attributes of natural rubber was a critical activity in the logistical efforts of World War II. With the growth of new knowledge of the control of synthesis, fundamental studies can develop new, superior forms of rubber-like compounds. The new family of polymers made from ethylene and propylene is a recent example of achievement of rubber-like properties through exploration.

All rubber-like substances lose their visco-elastic properties and become far more rigid when the temperature is sufficiently reduced. These materials are then glassy in character and behave as almost perfect elastic solids. There is a host of polymers that are glassy at ordinary temperatures. As with the inorganic glasses, many of these materials are valuable for their optical properties. Fundamental studies with monomers or combinations of monomers to yield polymers with desired properties, such as a chosen optical absorption or refractive index, have been and can be expected to be highly productive.

A striking achievement in polymer science has been the discovery of controlling the structural regularity of polymer chains. This chemistry of molecular shape, sterochemistry, is making possible the synthesis of highly-ordered molecules which, by virtue of their symmetrical character, are able to cluster into crystalline order. It has been found that the individual crystalline regions are extremely small but highly organized, and that they form a superstructure or morphology which confers strength and dimensional stability to the polymer somewhat analogous to ways in which precipitates can lead to strengthening of metal alloys. Recent research has shown that the morphology, although extremely complex, is nevertheless governed by identifiable factors, such as the rate of the crystallization and the distribution of molecular sizes. Further research in this area will undoubtedly lead to new practical materials.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

The relations between molecular structure and physical properties is central to the behavior of polymers of biological interest. The proteins responsible for form and strength in much of living matter, notably collagen and Keratin, are examples of substances in which these relations are now becoming well understood at the molecular level. Continued research in this area is likely to benefit both inanimate and biological or medical applications for plastics.

Although similar in many ways the plastics and rubber industries differ in one major respect—processing. Conventional rubbers must be compounded with fillers and curatives, molded and cured, whereas thermoplastics in general require less compounding and no curing. Recent developments in block copolymers have led to the synthesis of elastomers in which the crosslinks are formed through association of the more rigid sections of the polymer chain. They are thus physical rather than chemical in nature and may be formed and broken reversibly by heat. These new rubbers have the potential of being processed as plastics. The early members of the class show creep rates which excluded them from many applications but an appropriate choice of the component monomers, together with the development of appropriate polymerization techniques, should lead to commercially-useful rubbers.

What is likely to be one of the most important and active areas for polymer research in the immediate future concerns their durability. Fluoropolymers and poly (methyl methacrylate) have excellent resistance to weathering, but other polymers destined for outdoor exposure require protection against ultraviolet degradation. Polyolefins, natural rubber, poly (vinyl chloride), polystyrene and cellulose derivatives are especially vulnerable to deterioration. Protectants function as light screens, ultraviolet absorbers or deactivators. At elevated temperatures the polyolefins, natural rubber and cellulose materials are stabilized by antioxidants that destroy hydroperoxides and inhibit radical chain reactions. Hydrocarbons in contact with transition metals are also protected by sequestering or chelating agents. Various basic metal salts are added to poly (vinyl chloride) to suppress discoloration and neutralize hydrogen chloride. Current studies center on stabilizer interactions, retention, and life-times under various conditions. Minor structure modifications have recently produced marked improvement in the stability of poly (vinyl chloride) and polyoxymethylene without significant changes in physical properties. Further increases in stability probably can be expected from additional changes in molecular structure of polymers.

Composites and Concrete

A composite material generally is defined as a combination of two or more mutually-insoluble macroconstituents differing in composition or form. The microconstituents may be glass, plastic, ceramic or metal. One normally thinks of composites, however, in a more restrictive sense, as high-modulus reinforcing fibers (most often glass) supported in a thermoset resin matrix (generally epoxy or styrenated polyester). Glass fiber reinforced thermoplastic resins (RTP) are also finding increased usage in structural

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

applications, particularly in the automotive field. New high-performance fibers—boron, carbon (graphite) etc.—with still greator modulus are available, but only at prices that preclude widespread commercial use at this time.

The development of sheet molding compounds (SMC) and bulk molding compounds (BMC) shows promise of enlarging appreciably the scope of application of these composites, particularly in the transportation field. The pultrusion process has already had an impact on the economical fabrication of structural shapes for use especially in the chemical process and electrical industries. The former is an important major field for adoption of composites, but the chemical resistance of the resin and of the resin-glass interface are possible constraining factors.

A large potential market also exists in construction, but here certain material shortcomings become apparent. For example, flammability needs to be reduced by the use of halogen-containing raw materials or other fire-retardant additives, but these add to the cost and generally degrade weatherability. Weatherability, in turn, can be enhanced by laminated coverings of poly (vinyl fluoride) film or suitable paints, but these too add to the cost and offset the advantages of integral color and surface texture inherent in composites. Unitized bathrooms are now molded from these materials but durability becomes a question because of their rather poor scratch and mar resistance.

True structural applications for composites become feasible, for example, for high-performance aircraft, only with the use of the newer but more costly boron and graphite fibers. Entire aircraft structures based on epoxy-glass have been made for small private planes, but this remains a minor application at present. In any case, the lack of adequate design data (including the incorporation of anisotropy of properties in aligned composite), understanding of failure modes, and the high cost of suitable materials hamper the more rapid expansion of aerospace uses into commercial aircraft.

Major needs in the exploitation of composite materials are for attention to such factors as: raw materials cost; low-cost; high-speed manufacturing techniques; modulus and ductility; flammability; weatherability; and suitable design data. Expanded use of the newer, high-performance materials should develop ultimately as increased demand leads to lower materials costs.

In addition to realizing their application in aerospace due to an improved strength-to-weight ratio, composites can be expected to see increased use for improvement of other physical properties not attainable with individual components. An active area for research concerns all-metal composite strip and wire. Two- and three-layer composites are available and widely used but there is need for five-layer composites (e.g. outer, corrosion-resistant, joinable layers, inner strength layers and a high-conductivity core). These are not currently exploited because of their high cost derived from low yields, but improved methods of fabrication and yield could stimulate their use. Other examples of potential uses for composites are in copper-clad superconducting filaments for increased critical current carrying capacity, fiber reinforced or dispersion strengthened copper for combination of high strength and electrical conductivity, fiber reinforced magnets for creep-resistant high temperature rotors, and the possibilities of using directionally-solidified eutectics for magnetoresistance, electromagnetic and radiation detector applications.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Three major areas of research are needed to broaden the future use of composites. First, study of reactions between fiber or dispersant and matrix (e.g. phase equilibria and kinetics of phase transformations, surface reactivity) and wetting. This study should lead to improved adhesion between fiber or dispersant and matrix and to retention of individual characteristics of both components, thus permitting the attainment of property improvements expected from theory. Secondly, research on fiber and whisker growth, particularly fibers oriented for subsequent processing. Also important are innovations in handling of fibers to maintain orientation and prevent breakage, as well as new techniques of joining. This area of research is necessary for improvement of processing of composites starting with separate components. Finally, since there is economical advantage in fabricating a composite using a single operation, further research should be conducted in directional solidification of eutectic systems and in directional alignment of precipitates and other dispersants in the solid state. Rheocasting may offer a new method of producing matrix-dispersoid systems.

Perhaps the most widely-used composite is concrete, the single most-used, man-made construction material. Because it is a rather complex composite, basic research into the properties of the constituents and their interrelationships is an absolute necessity to fill the ever-expanding demands of construction. The following areas for materials research are noted.

Cements

Basic research on the physico-chemical properties as a function of composition will enable the custom development of cements designed for specific functions. These include expansive cements for shrinkage control or for self-stressing, cements with controlled setting time, and cements with improved resistance to weathering and to physical, chemical and thermal attacks. Particularly challenging areas of research include studies of basic properties of admixtures and chemical reactions among them as well as with aggregates and reinforcement materials.

Aggregates

Basic studies conducted on the replacement of stone with light-weight material will improve the strength-to-weight ratio of concrete and permit increased use in tall buildings and long spans. These aggregates include waste products such as fly ash and other recycled products. They also include other natural aggregates indigenous to local construction sites for obvious economic reasons. Again, the physico-chemical properties of these materials in relation to the other constituents in concrete need to be studied thoroughly.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Reinforcement

Reinforcement materials such as steel are used to increase the tensile strength of concrete. Studies of adhesion and reactivity of steel with cements and admixtures of controlled composition are important to prevent strength degradation. In addition, alternatives to the conventional steel bars, such as chopped steel fibers and organics-coated glass fibers, need be pursued to broaden the scope of use offered by the reinforcement constituent of concrete.

In short, concrete is a complex composite of several constituents. The introduction of any single constituent will affect the properties of the other constituents. Hence the challenge in basic research lies in terms of the broad interrelationships among all constituents.

Electronic Materials

The term electronic materials embraces a wide variety of active materials which are usually and conveniently labelled according to their electronic property of practical importance. These include semiconductor, optical, magnetic, piezoelectric, photochromic, etc. materials. The central position in electronic materials is occupied by:

Semiconductors

Semiconductors are the heart of solid state electronics. The fundamental properties and other materials aspects of the elemental semiconductors, silicon and germanium, are now very well established but much more work needs yet to be done to achieve even greater control over the impurity and defect content of crystals of these materials. The influence of defects and impurities becomes proportionately greater as the dimensions of devices, integrated circuits, and so on, become smaller and smaller. Crystal perfection is also important for high voltage power applications.

Explorative work on group III-group V semiconductors (such as GaAs and GaP) as structural and electronic analogs to the group IV semiconductors (Ge and Si) started in the 1950’s. During the 1960’s several important new devices based on these materials made their appearance, such as microwave oscillators and varactor amplifiers, electroluminescent diodes and injection lasers. Fundamental understanding of these materials is well along but much more needs to be done on the solid state chemical aspects and on defect, impurity and stoichiometry control. Recently, complex ternary III–V compound structures for injection lasers based on Ga1-xAlxAs, have been developed to a high point of precision using liquid-phase and molecular-beam epitaxy.

The explorative front in semiconductor materials is probably now centered on ternary analogs of the group IV and III–V semiconductors. The II–IV–V2 compounds e.g., CdSnP2, have received considerable attention. Their band structure has been shown to be very similar to that of the III–V

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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compounds and their electrical properties have been investigated. However, defect, phase and thermodynamic investigations are very scarce. The I–III– VI2 compounds (e.g., CuGaS2) are also receiving attention. It has been shown that their band structure is similar to that of the II–VI compounds. The copper compounds investigated have shown p-type conductivity in contrast to the II–VI sulfides and selenides. All I–III–VI2 compounds investigated so far have been found to have direct band gaps. In several materials, visible stimulated emission has been observed at low temperature. These various properties might allow the design of heterojunctions involving n-type II–VI-sulfides and selenides. Such structures might be expected to emit visible light and exhibit laser action; a particularly pressing need is for an efficient blue electroluminescent diode. Much work remains to be done on these materials, both on their physical and chemical properties. Furthermore, the techniques for the controlled preparation of single crystals is still rather primitive. Phase diagrams are generally not known, especially for the more promising compounds like CuGaS2 and CuAlS2. An even bigger problem is the defect chemistry, which is more complicated but may provide more possibilities than for binary compounds. Thermodynamic investigations are virtually nonexistent. While some experimental band structure investigations have become available recently, the more detailed nature of valence and conduction bands are unknown and it is conceivable that some of these materials might be well suited for certain devices like Gunn oscillators.

Recently there has been a marked increase in theoretical and experimental studies of amorphous semiconductors. This interest is due, in part, to the electrical switching effects that have been observed in these materials but more generally to curiosity about the effects of a lack of long range structural order on physical properties. Theory has predicted that, upon going from the regular three-dimensional structure of a crystal to the disordered structure of an amorphous solid, the electronic structure changes from that of a normal band model to that of band edge tailing (smearing of band edges) and the localization of states in the gap. Because of coulombic repulsion, a fraction of the highly localized states will be singly occupied (unpaired). However, theory is not yet able to predict quantitatively the concentration, degree of localization, and the energy spectrum of these states; nor the fraction singly occupied. There is considerable question whether experiments will confirm the theoretical models. Experimental studies have included transport (conductivity, field effects, thermostimulated currents, drift mobility), optical, magnetic susceptibility, and spin resonance, but results thus far are often ambiguous or conflicting.

Some properties can be markedly affected by introducing structural disorder. For example, there are glass compositions which are 10–11 orders of magnitude more resistive than the corresponding crystal. Because of the ability of glass-forming materials to transform fairly reversibly between the glassy and crystalline states and the associated changes in electrical conductivity and optical properties, amorphous semiconductors have been

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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suggested for use as electrical and optical memories. At low temperatures, the electrical conduction properties appear consistent with a hopping transport process between localized states. At fields exceeding about 104 V/cm, amorphous semiconductors suffer an electrical instability which results in a switching effect. It is not yet fully resolved whether this instability is triggered by an electronic mechanism, a thermal mechanism (thermal avalanching) or a combination of the two.

The interpretation of optical effects is hindered by the lack of a detailed theory of absorption in amorphous materials. However, localized states in the gap are expected to produce absorption effects at energies below the absorption edge. Experimentally, absorption tails are observed but they are both structure and impurity sensitive and distinguishing fundamental from such secondary causes of absorption tails is proving to be a very difficult challenge. Experimental results can often be interpreted as due to the presence of inhomogeneities or impurities. As an example: Is the observed spin resonance in Ge, Si and SiC due to unpaired localized spins in the amorphous matrix, or due to surface states at the surfaces of internal voids? Likewise, the observed frequency dependence of the a.c. conductivity, and the field dependence (super ohmic) of the d.c. conductivity, can be interpreted in terms of a carrier hopping process in the matrix, or interpreted as due to structural and chemical inhomogeneities. Clearly, in future research, materials preparation and characterization, including chemistry (impurities), homogeneity, and structure, must achieve the level of sophistication of that currently done on crystals. In this area there is much to be done.

Magnetic Materials

Magnetic materials embrace metals and alloys on the one hand and single crystal or polycrystalline insulators on the other. There are two major opportunities in the future of practical magnetic metals and alloys. First, in the search for materials with improved magnetic properties there is much need for basic understanding of various magnetic phenomena such as saturation magnetization, magnetocrystalline anisotropy and magnetostriction and the development of guidelines relating the phenomena to available material parameters such as composition and crystal structure. Secondly, there is need for understanding the relationship between microstructure and the technical magnetic properties (e.g. permeability, remanence, coercivity) to be exploited, so that economical processing techniques can be developed for fabricating the materials into useful devices.

Permanent magnets—Permanent magnets of alloys of cobalt with rare earths have yielded energy products and coercive forces superior to those available in the past. There is need for understanding the structure and phase relations in these permanent magnet compounds so that more economical processing techniques can be devised. In addition, research on the origin

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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of high magnetocrystalline anisotropy energy, which is chiefly responsible for the large coercivity of these magnets, in relation to composition and structure may yield other and superior permanent magnet alloys.

High temperature magnets—Magnetic materials may be used as rotors in electric generators that operate at elevated temperatures. Hence there is a need for materials with high magnetic flux density together with high mechanical strength at the operating temperatures. Studies on the role of dispersoids and composites in magnetic and mechanical hardening would be useful in optimizing both properties.

Giant magnetostriction materials—Metals and alloys with large magnetostriction can compete with piezoelectric materials as transducers often with the advantage of being ductile and tough. Giant magnetostrictions (~10–3) have been recently reported in rare-earth transition metal compounds, but these values are gained only at large applied magnetic fields. Some are also brittle and pyrophoric. A better understanding of the origin of magnetostriction may lead to more practical alloys.

Magnetic recording materials—Metallic particles such as Co-Fe and Co-P exhibit greater magnetic moment and high Curie point compared with iron oxide. There is need here for a more economical mass production of these fine particles if they are to compete with existing oxide technology.

Magnetooptical materials for information storage—Compounds such as MnBi and MnAlGe are being evaluated for information storage using Curie point writing techniques employing laser beams. There is much need for studying the microstructure of these compounds, particularly in thin film form. Also needed is a search for other materials exhibiting large Kerr and Faraday effects. As yet the magnitude of these effects cannot be predicted from a knowledge of composition and crystal structure.

Amorphous magnetic materials—alloys such as Fe-P-C have been prepared in the amorphous state and shown to be ferromagnetic. There is recent indication that amorphous Co-Gd films exhibit uniaxial anisotrophy and “bubble” domain behavior. Amorphous chalcogenide films might prove useful in optical storage systems. Study of the structural origin of intrinsic parameters such as saturation magnetization, magnetic anisotropy and magnetooptical coefficients could thus lead to the development of a new class of magnetic alloys.

Conventional magnetic materials—Because of the large volume in use for conventional magnetic materials such as silicon iron, even small improvements could lead to large savings. One recent example is the commercial development of highly-oriented, Goss-textured silicon iron using aluminum nitride needles to restrain normal grain growth. The sharper texture leads to higher initial permeability. By applying a surface coating which produces tensile stresses on the strip, magnetostriction and total loss are decreased.

Dissipative losses, such as those caused by eddy currents in magnetic metals, are generally too large to permit their use as inductors at mega-hertz frequencies. As a result an interest in the insulating magnetic oxides arose in the late 30’s, culminating in the development of inductor and permanent magnet ferrites in Japan and Holland. Work subsequent to World

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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War II has seen marked advances in permeability and quality and extension of the use of inductors to frequencies approaching a gigahertz. The ferrites have also found applications as memory elements, as microwave components (to >50 gigahertz) and as low cost permanent magnet materials for closing refrigerator doors and (hopefully) magnetic levitation of high-speed trains. More recently single crystal films of magnetic garnet deposited on a nonmagnetic matching substrate have become of interest for miniature memory and logic (bubble domain) devices. These developments rest firmly on a basic understanding of the physics of magnetism and on magneto-chemistry.

Clearly, there now exists a need for even better inductors and higher coercivity permanent magnets. To achieve these ends more subtle control of the properties, subdivision and orientation of particles in dense compacts must be realized. Controlled inhomogeneity can be of particular importance in inductors. In contrast, inhomogeneity is to be avoided in the thin garnet film technology. Here defects must be avoided, film uniformity in composition and thickness must be preserved and allied skills in crystal preparation, processing, masking, etc., must be optimized. Research, in chemical additives and preparative techniques, in new flux systems for liquid phase epitaxy film growth and control of chemical vapor deposition processes are expected to provide some of the needed technological advances.

Improvement in our detailed understanding of dynamic processes such as the physics of domain wall motion can be expected to increase markedly the technological properties of magnetic materials. Improved understanding of less than three-dimensional solids—metal organic cluster complexes, layer and chain compounds will provide new insights into magnetic interaction with improved materials as an output.

Optical Crystals

The demonstrations of maser and laser action in solids count as great events in materials science. The laser in particular has been developed into an important energy source for micromachining, for drilling dies, adjusting microcircuits, trimming resistors, etc. It has also found application in telemetry, range-finding and ordnance. However, its full potential for low cost, broad-band communication has yet to be realized.

The present high level efficiency in optically-pumped lasers rests on two scientific cornerstones—a detailed understanding of the spectroscopy of the rare earths and an understanding of the influence of the host crystal on spectroscopic linewidth, in particular the role of mass action interactions in charge compensating Nd3+ when it is present in sites of different valence. Trivalent Nd is thus the most favorable host environment for low threshold lasers at present, further research might lead to more economical performance in other host materials which admit to stronger Nd3+ optical absorption bands more favorably with respect to 0.93µ, the pump wavelength available from gallium arsenide laser diodes.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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The potential use of lasers in communications systems has stimulated intense searches for efficient nonlinear optical materials useful for modulators, amplifiers, frequency mixers, and so on. Interest has centered particularly on ferroelectric classes of large, band-gap crystals for use in and near visible wavelengths and on ternary semiconductors for use in the infra-red. These materials were discovered mainly using structure and polarizability as search criteria. Although present theories now readily account for the optical susceptibility of materials, detailed a priori predictions of nonlinear optical properties are still not possible. The need for quantitative prediction of susceptibilities should provide stimulus and motivation for a fundamental approach based on chemical bonding and structural considerations. Perhaps materials science has more closely approached the ideal of permitting “molecular engineering” based on fundamental knowledge in this sphere than anywhere else to date.

The systematic study of nonlinear effects may also provide important new structural information and details concerning polarization and bonding that can give insight into interactions between amino acids and other materials important to life. Indeed the measurement of susceptibilities provide one of the few direct measurements of the polarizability of electrons in bonds available to the chemist.

From the fabrication viewpoint there is a need to develop ways of miniaturizing optical integrated circuits, through controlled epitaxial growth and other thin film processes. In addition, improvement in crystal preparation techniques to minimize nonuniformities, defects, etc. are needed. These improvements can be expected to flow from an understanding of the hydrodynamics of crystal growth, phase equilibria and the connection between the physical chemistry of growth and the physical and chemical perfection of grown materials.

Dielectrics

Piezoelectrics, especially quartz, were among the first electronic materials utilized and one of the triumphs of materials synthesis is the large scale preparation of quartz. Detailed understanding of the physical chemistry of the growth process and the partition of impurities has allowed the growth of quartz superior to the natural material under economic conditions. Lithium tantalate has recently been shown to be a practically useful piezoelectric and a wide variety of new materials are being investigated for stress-optic and surface-wave applications. An improved quantitative atomic understanding of piezoelectric and acoustic effects could be expected to have important device impact by pointing the way to superior materials.

A related topic receiving increasing attention both because of its usefulness in understanding the behavior of polar crystals and because of its applicability to infrared detection and imaging devices is pyroelectricity. Measurement of the pyroelectric coefficient yields valuable

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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information on spontaneous polarization, phase transitions, piezoelectric switching and domain kinetics. The polar materials which are also useful for nonlinear optics, ferroelectric ceramics, and sheets of polyvinyl fluoride polymers are all of interest. High frequency current measurements in pyroelectrics enable a direct study of nonradiative electronic and vibrational relaxation times. At very short times a second effect due to the change in dipole moment of the excited center has recently been demonstrated. These excited state dipoles contribute to a macroscopic polarization change only in pyroelectric crystals, and intense electrical pulses as short as 6 picoseconds have been generated with the mechanism using picosecond optical pulse evaluation. At the present time measurements of excited state dipole moments and nonradiative relaxation times have been made only for a few transition metal ions in LiNbO3 and LiTaO3 but these experiments may be extended to any absorbing center (ionic or molecular, electronic or vibrational) in any pyroelectric host. Using suitable input frequencies or short pulses, intense radiation from d.c. to submillimeter wavelengths can be generated and should enable the study of fundamental, short-life processes which cannot be reached by other techniques.

Photochromic and Electrochromic Materials

Color changes induced in materials by the action of light or electric fields can be useful or a nuisance. They can be used for light-responsive sunglasses and for holographic optical memories. Contrariwise, light-induced optical absorption changes may seriously impair the performance of both passive and optical devices or cause undesirable changes in appearance. Optical “damage” in nonlinear optical crystals due to laser radiation has been ascribed to optically-induced valance changes in impurities in the crystals, analagous to the role of F-centers in alkali halide crystals. Coloration changes in barium-sodium-niobate-crystals have been related to the degree of off-stoichiometry in the crystal. But in most cases the specific mechanisms of photochromic effects have not been established. For many applications it is desirable to have reversible photochromicity, using two different wavelengths of light. Such photochromics are usually slow, however, because for each color center created one photon is required. An area for future research is for a reversible photochromic with gain, i.e., many color centers per photon. This is possible in some irreversible systems at present—(some polymers and the photographic plate being notable) and would lead to greater sensitivity.

Electrochromic materials are receiving increasing attention. These are generally long chain, highly polarizable molecules which can be oriented by an electric field. The electrochromic nature of the molecules leads to an electrically-controllable color change, potentially useful for visual display devices. Electrochromic effects have been observed in dyes—but the effects are fairly small though potentially of interest for electrically-tunable dye lasers. Large effects (~1000Å) are observed in long molecules.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Monolayers of these molecules can have electrochromic electron ratios of 500:1 over a large spectral bandwidth. Applications of such materials to devices calls for fast electrochromic response with a large extinction ratio. The present problem is that generally the longer the molecule the slower the response but the larger the extinction ratio.

Miscellaneous Materials

Solid Electrolytes

Conventional lead-acid storage batteries pose an intriguing challenge to the materials scientist and materials engineer to try to develop more compact, rugged, all solid state batteries using solid electrolytes. Solid electrolytes can be broadly grouped into two mechanistic classes: 1) Defect conductors, such as βAgI, CaO·AO2 (A=Zr, Hf, Th, Ce) and M2O3·ZrO2 (M=La, Sm, Y, Yb, Sc) in which interstitials and/or vacancies are the ion-conducting species; 2) Disordered cation structures, exemplified by the modified AgI compounds such as RbAg4I5 and αAg3SI, and the various β-Al2O3 phases. This second class can be subdivided into two groups: a) the modified AgI phases which are “tunnel” conductors, in which the sparsely-occupied silver sublattice sites are arranged into essentially one-dimensional tunnels which are located parallel to each crystal axis in the various unit cells. b) the β-Al2O3 phases which have the cation (Na, K, Ag, etc.) sublattice sites arranged in two-dimensional layers parallel to the a and b crystallographic planes.

The defect conduction phases are of little interest for solid state battery development because of their low conductivity at ordinary temperatures. In the second group no really optical ionic conductor has been found. The layer structures are suitably conductive but the two-dimensional materials are inherently fragile in the wrong (a and b) direction in the single crystal form, and have drastically lowered ionic conductivity as polycrystalline ceramics.

On an a priori basis, a tunnel conductor would be ideal, but the only known high conductivity tunnel structures are based on AgI—an inherently high cost, low energy system. The pertinent questions are: 1) What is special about AgI? 2) Are there any other compounds with the same special properties of AgI and can they be used to synthesize new electrolytes?

Silver iodide (stable above 146ºC) has the highest ionic conductivity of any solid not close to its melting point. Its crystal structure is curious in that it has both tetrahedrally- and octahedrally-coordinated Ag sites, and is of significantly lower density than the tetrahedral AgI structure. It is this anomalous low density, combined with tetrahedral cation vacancies with shared faces that is typical of all AgI-type conductors. Moreover, if a small mol fraction of a 6-coordinate binary compound (e.g. RbI, etc.) is combined with AgI, a highly ion-conductive ternary

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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solid is formed which is stable at room temperature—i.e., RbAg4I5. The tetrahedral sites in these ternary structures are somewhat distorted in order to accommodate both the mixed coordination and also the wide differences in density between the two binary parents. It is this ability to withstand considerable distortion of the tetrahedral coordination sphere (“softness”) without a large energy increase (which would destabilize the structure) that permits the freakish, low-density AgI-type conductors. Measurements of this property of softness for AgI may lead to other potential binary parents. Other soft binaries appear to be MgSe and MgS; and BaxMg6–xSe6 has been shown to be a member of a class of solid state Mg++ conductors. This phase has been prepared by sintering methods and shows usable amounts of ion conduction at room temperature in cells of the type Mg|BaxMg6–xSe6|NbSe2–I2.

Liquid Crystals

The investigation of the properties and uses of liquid crystalline materials has encountered an explosive revival during the last decade and particularly during the last five years because of their intriguing device applications and the ready availability of room temperature nematic liquid crystals (Schiff bases). Although cholesteric liquid crystals were first used in temperature-sensitive display devices, it is now apparent that the major device application of liquid crystals lies in the area of electrooptic display devices. The major advantage of these devices is their extremely small power requirements. Commercial numeric display devices are now appearing on the market in electronic watches and calculators and it is apparent that developments of new materials and device applications will continue at a rapid pace in the future.

One recurring problem is the need for more chemically-stable, room-temperature, liquid-crystal materials. Related to this problem is a better theoretical understanding of the forces contributing to liquid crystallinity, and the development of nonempirical approaches to the design of liquid crystalline materials having specific phase ranges. These problems are further related to the measurement and better understanding of the properties (thermodynamic and physical) of liquid crystals and the role of impurities in inducing changes in these properties (i.e. the mechanism of storage and the role of surface properties in inducing liquid crystal orientations). From a better understanding of liquid crystal properties, extensions might then be made toward the development of better models for the more disordered liquid state.

In recent years it has become apparent that lytropic liquid crystals may be important in biological processes. Certain viruses are known to possess the rod-like shape of liquid crystal molecules. In the future it will be of much interest to assess the influence of such materials on biological phenomena.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Biomaterials

A most promising growing area for materials research concerns the development of materials and devices for surgical and health use. At this interface between the inanimate and animate worlds answers to many intriguing questions will have to be found at a most basic level. Typical research topics in this area include:

  1. Surface Architecture—This also includes surface energy and changes which can occur as the result of contact with body fluids; for example, development of monolayers of lipids, proteins, etc.

  2. Degradation—There is a need for further research on the mechanisms of degradation of polymers by water, lipids, proteins, and enzymes. This area also includes further work on the corrosion of metals and also the degradation of ceramic materials, frequently by hydrolysis. The converse area is also important; namely, passivation of metals to make them less susceptible to corrosion or the development of coatings to protect them. This can also apply to ceramics and polymers.

  3. Bonding—The mechanism for the bonding by adhesives between metals or polymers and, for example, hydroxy apatite and collagen. This is important to both dental and orthopaedic needs; it is particularly a pertinent topic in dentistry where there is a great need for an adhesive between a restorative material and enamel to seal the margins.

  4. Composite Materials—Especially development of a better understanding of the interface between the two components and the possible degradation of the interface. There are many common denominators between this problem and that already mentioned under (iii).

  5. Glass Ceramics—There is only a small amount of work being carried out on potential glass-ceramic materials based on the calcium phosphate glasses. These have potential for degradable devices in the orthopaedic field, among others.

  6. Graphite and Carbons—Although there has already been much development of pyrolytic carbon for heart valves, more work needs to be done in the development of these materials which are well established as being compatible with the human body. Probably the major emphasis here, however, should be in the fabrication rather than the fundamental scientific aspects,

  7. New Fabrication Techniques—Such as the freeze-drying technique for the fabrication of materials which have potential use for implants. Also, particularly intriguing is the replamineform process (replicated life forms or structures).

  8. Membranes—A great deal still needs to be done on the development of membranes suitable for diffusion of gases (needed for devices for the measurement of oxygen, carbon dioxide, etc. as well as for long-term artificial lung devices). This is important in the area of in vivo physiological measurements on the body. There is still a great deal to be done in membranes for kidney dialysis. The big problem here, however, is cost since membranes currently exist which allow maintenance of life. However, as emphasis shifts towards full rehabilitation of patients there is a need for a totally implantable artificial kidney at acceptable cost and with superior performance.

  9. Adsorption Kinetics—Reversible physico- or chemi-adsorption needs further development. This is particularly important in the coming area of drug release or its reverse, adsorption of toxic materials. Both polymer

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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and ceramic or glass systems have potential in this area. The ultimate goal, for example, is to be able to “dial in” the release rate, particularly to develop a constant rate.

  1. Analytical Techniques—The application of current or new analytical techniques for studying change in tissue fluids or components; for example, changes in the conformation of polymers or proteins in the body such as hyaluronic acid, which is important in arthritis and is part of the synovial fluid in joints. Also, how these changes in conformation, etc., may change the calcium binding.

  2. Biological Potentials—The effect of strain (or stress) on the development of biopotentials in natural tissue is still poorly understood. They are known to effect changes in the rate of dissolution or laying down of hard tissue, for example.

  3. Reactions at Implant Sites—Blood clotting at implant sites is still an enormous problem. A major part of this problem revolves around the surface chemical architecture and the effect on protein adsorption. To date heparin-treated polymers with high surface energy have proved very successful as anticlotting material. In addition, there is a fluid mechanics problem associated with blood flow and surface adhesion and the interaction of the material in terms of fibrous ingrowth and calcification.

MATERIALS PROCESSING

Processing and Manufacturing Techniques for Metals

There are two main directions for innovation in processing: (1) attainment of microstructure not possible with conventional techniques, and (2) attainment of finished part more economically than otherwise. The latter includes finding ways to consolidate processing operations into fewer ones. In particular, all processing operations need to be viewed within the broad perspective of energy requirements. Although most innovative processing techniques are primarily developed for specific materials and applications, basic research in this area should spawn new approaches. Examples from four major categories of processing are illustrated below. In addition, two research topics relevant to processing (metastable phases and computer simulation) are discussed.

Extractive and Process Metallurgy

The need for new and improved beneficiation and extractive metallurgy processes is widely recognized and has acquired a certain sense of urgency in light of mineral demand trends, decreasing ore quality, increasing energy costs and environmental constraints. Many of these needed advances will come from process improvements that retain the basic chemistry and basic design of an existing process and, as such, fall within the realm of engineering development. Additional advances will come from new processes which

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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retain the same basic chemistry but invoke new or novel concepts of momentum, heat, or mass transfer or of fluid flow. The final category, and that of most interest here, relates to new processes involving novel chemistry.

In the area of ore or mineral beneficiation research is required to more fully understand the properties of interphase and grain boundaries so that mineral aggregates can be efficiently and preferentially separated at these boundaries. Further research is also required on the fracture and flow characteristics of mineral phases as a function of temperature, and perhaps in the presence of surface active agents, in support of improved, preferential separation techniques. A second, broad research area in ore beneficiation, involves the handling and efficient separation of fine particles (<20–30µm) produced from prior comminution. Ultraflotation techniques, selective agglomeration, electro-phoretic methods, and the utilization of ultrahigh magnetic or electrostatic fields are only some of the techniques which may lead to new and useful technologies for the efficient recovery and separation of fine particles. A corollary here is that of tailor-making chemical reagents for flotation and solvent extraction use. Improved knowledge of the properties of mineral-solution interfaces and the relationship between the structure of flotation reagents, mineral crystal structure and flotation and extraction behavior is essential for making significant advances in these areas. Research on wholly dry separation techniques based on the gravitational, electrostatic and magnetic properties of particles should not be ignored since these will be particularly relevant to beneficiation in arid regions.

New processes which retain the same basic chemistry but invoke new or novel process concepts, i.e. the interaction of this basic chemistry with the physical phenomena of momentum, heat and mass transfer, offer challenging research opportunities. There is, for example, the continuing need for modeling flow and transport behavior in heterogeneous systems which are those most often encountered in real processing. Some of the newer computer simulation methods for use here are described elsewhere. The coupling and refining of such modeling with innovative techniques of experimental process analysis is also a legitimate and much-needed research activity. The success of an analytical method or model depends much less on the amount of theory which it incorporates than on its reliability as a mirror of reality—and that reality is provided by experimental process analysis. It must be noted that much of our current materials processing, particularly process metallurgy, suffers from a lack of real-time data acquisition and feedback control and that such control will be a necessary feature of new processes to optimally exploit the research results derived from the modeling and experimental process analysis indicated.

The discovery of new processes involving novel chemistry provides a wide-ranging field of research. Much of this “novel chemistry” may already exist in the immense chemical and metallurgical literature although additional research will be needed to amplify and extend our understanding of promising systems. However, significant areas of novel chemistry remain to be explored. These include aqueous chemistry at elevated temperatures and pressure, vapor— phase chemistry, fused salt and nonaqueous solvent chemistry, electrochemical methods in aqueous and nonaqueous systems, ion exchange and solvent extraction. The use of novel reductants such as solvated electrons and atomic hydrogen may offer new and interesting possibilities.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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The possibilities of novel chemistry in pyrometallurgical processes should not be ignored either, even though our knowledge here is fairly extensive. Careful control of thermodynamic and kinetic conditions may offer new extractive possibilities not previously thought feasible. Novel chemistry involving metastable phases should be considered. The pyrometallurgy of oxide, sulfide and metal phases will remain a major process for the production of large-tonnage metals and, as such, warrants our continuing close attention.

Casting

Melt spinning—This relatively new technique of casting metal filaments by extruding a liquid jet through a fine orifice may lead to high-speed production of fine wires in a single step. The challenges in this area include hydrodynamics of the liquid jet, chemistry of surface films developed to stabilize the jet, and casting defects, solid state reactions and resultant properties associated with high cooling rates.

Rheocasting—Another new technique, involving material cast in the partially-solidified state. High fluidity is maintained by vigorous mechanical stirring. Because of lower pouring temperature, there is less mold erosion, centerline shrinkage and freezing time. If the stirring is momentarily stopped, the slurry stiffens and can be handled like a solid for die casting (thixocasting). Or, particulate materials can be added and uniformly blended in composite fabrication (compocasting). Two notable areas calling for study are: (1) fluid flow and rheology of partially-solidified alloys, in particular as to the size, shape and distribution of the solidified portions, and (2) microstructure (e.g. microporosity, dendrite structure) and properties resulting from this type of casting. New alloys may have to be designed to realize the full potential of rheocasting.

Melt Saturation—A technique for preparing dispersion hardened alloys by mixing two or more molten alloys, e.g. Cu-Th and Cu-B to obtain ThB dispersion strengthened Cu, taking advantage of a single step of dispersoid formation and of the faster reaction kinetics at elevated temperatures. Studies of reaction kinetics and fluid flow of mixed molten alloy systems and resultant structure and properties are needed, perhaps in combination with rheocasting.

Other areas to be exploited include continuous casting, ferrous die casting, electroslag remelting, and directional solidification.

Working

Thermomechanical Processing—Properties of practically all commercial alloys as well as new alloys can be optimized by controlling the thermal and mechanical cycles of processing. Severe cold working of several nominally single-phase Cu alloys followed by moderate temperature annealing, for example, results in enhanced strength for given ductility. The enhancement comes from a superposition of texture sharpening, increased dislocation density and deformation-enhanced precipitation not achieved with small degrees of prior deformation. As another example, the development of stable

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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elongated grain structure in dispersion strengthened Ni and Al alloys by TMP, has resulted in a considerable improvement in high temperature creep strength. Future advances will come from a reexamination of the complex interaction of deformation, recrystallization, texture development and solid-state reactions in the important commercial alloys. For selected applications where deformation is not possible, shock by explosives, pulsed lasers, gas guns, etc. may be used for textured metal formation.

Powder Metallurgy—Two areas for research are: (1) Novel means of preparing powders of controlled size, shape, composition and structure with a view toward optimizing the properties of the finished parts. Examples of such processes include splat cooling, electric spark discharge, cold substrate deposition, freeze drying and various redox reactions. New directions for such materials include acicular Co-Fe magnet powders for enhanced shape anisotropy, and amorphous powders for enhanced chemical reactivity. (2) Thermal and mechanical behavior of powder preforms with a view to optimizing properties via thermomechanical processing (e.g. improved density and texture development via hot working of preform). Advances in this area could lead to economical production of specially-designed alloys or composite structures.

Hydrostatic Pressure Forming—In addition to continuing development of the engineering aspects such as those providing for continuous operation, there is need for extensive study of the effects of temperature and reactivity of the pressurizing fluid with the alloy (e.g. protective coatings). In addition, studies are required of the structure and properties of parts formed under hydrostatic pressure to insure optimum alloy design from the fabrication standpoint and to extend this technique to the forming of wider range of materials. Volume changes in plastic deformation, such as may be manifested in S-D (strength-differential) effects, should be examined critically both microstructurally and in terms of plasticity theory.

Superplastic Forming—Structural requirements such as fine grain size are fairly well characterized. Future efforts will likely be on novel techniques for achieving the required microstructure (e.g. powder metallurgy for fine grain size), particularly in normally hard-to-form materials (e.g. ceramics and refractory alloys) where superplastic forming may be of added advantage. Problems of die material for high temperature operation need to be met, and ways may need to be developed to alter the microstructure to inhibit superplastic flow during service.

Joining and Finishing

In joining techniques such as diffusion bonding used in fabricating metal matrix composite materials, basic studies of adhesion as influenced by solid state reactions in the presence of heat and pressure, surface films, etc. are important. For the newer welding techniques such as those using plasma arcs and electron and laser beams, structural changes and resultant properties in the weld region need to be studied. Similarly, structural and compositional changes resulting from finishing operations, such as electric discharge machining, electrochemical machining, and laser machining need to

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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be determined more thoroughly. New approaches to coating, such as flame spraying, again require studies of resultant microstructures and properties. Particularly important is research aimed at developing metals with a built-in ability to generate protective coatings during service, such as by the incorporation of Al and Cr to TD nickel to provide outstanding oxidation resistance without external coating. The role of dispersed oxides and rare earth additions (e.g. Y) in improving adherence of scale in coatings also needs clarification.

Metastable Phases

Nucleation and growth transformations require time for the process to start, and to continue. Thus, if insufficient time is allowed for atomic diffusion, then transformations can be suppressed and those which do occur are kinetically fastest; generally producing metastable phases. This kinetic effect has been exploited by the “splat cooling” technique in which molten alloys are shot on to a cold substrate. Splat cooling is the fastest quench obtainable-for quenching materials directly from the liquid state, and quenches of from 107 to 109ºC/sec have been achieved. Splat cooling has produced many new materials; metastable solid solutions (in which precipitation has been suppressed), metastable crystal structures (even simple dubic), and the first metallic glasses, in which crystallization itself is by-passed. Many of these metastable phases have novel and useful properties—e.g. 1) the superconducting compound Nb3Ge forms a metastable material whose transition temperature is 17ºK, an increase of 10ºK over that of the equilibrium, not splat-cooled, form; 2) new semiconducting materials such as crystalline metastable solid solutions in the system Ge-Ga-Sb, and glassy tellurium-based alloys; 3) magnetic alloys with increased coercive force; 4) splat-cooled metallic glass alloys that are among the mechanically strongest of the nonferrous materials; and 5) ferromagnetic metallic glasses in the systems Pd-Co-Si, Pd-Fe-Si, and Fe-P-C. The alloy Pd.68Co.12Si.20 has an unexpectedly high coercive force of 160 Oe, whereas Fe-P-C alloys have the low coercive force expected for disordered, amorphous structures.

Clearly, the ability to produce metastable states has opened up vast possibilities for the preparation of new materials. Past research has concentrated on structure; demonstrating the amorphous nature of the metallic glasses, for example. There is presently, however, a change in emphasis to that of using splat-cooling to enhance, or originate, specific properties. Many questions need to be answered—for example, what are the mechanical, corrosion, and transport properties of a material, such as a metallic glass, which has no grain boundaries or other imperfections. To utilize the technology important properties of splat-cooled materials, methods of fabrication into usable forms must also be devised.

Computer Techniques in Processing

The ability to predict macro-phenomena of a material based on microtheories under the complex conditions of processing is essential to the

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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understanding and development of new and efficient processing techniques. In this regard modern computer simulation of material behavior in continuum physics is indispensible. Two complementary computer methods have been developed.

The finite element methods are most suitable for small displacements. Applications have included three-dimensional stress analysis, viscoplasticity, heat transfer, fluid dynamics, electrodynamics and electromagnetics. It should be possible to extend these techniques to analyses of deformation of composites, thixocasting, injection molding, nucleation mechanisms in crystal growth, and the terminal phases of processes such as dynamic super-plasticity and splat cooling.

In contrast to the finite-element methods, the particle-in-cell techniques show their forte in modeling large and complex material displacements and various mixing or transport mechanisms, including turbulent flow fields. These methods could thus simulate not extrusion, the initial high-velocity phases of dynamic, superplastic forming or splat cooling, and the convection and diffusion processes surrounding crystal growth.

As part of a systematic exploitation of these computer methods, a significant effort needs to be directed toward having the two methodologies complement each other for the most effective and computationally-efficient approach to specific problems. The stakes in terms of programming effort and machine time can be high.

Rubber and Plastics

The processing of conventional rubbers is quite a costly step in manufacture. Many possible ways of reducing or eliminating certain steps have been investigated. As an example of current interest some rubbers are now marketed in powder form so that the initial mixing may be carried out by blending. This blending to form a mixed powder could then be followed by direct feeding to extruder or injection molding press.

Another more attractive possibility involves mixing of a low molecular weight rubber as a liquid. This would avoid the power-consuming shearing action necessary with solid rubbers. The low molecular weight rubber would, after mixing, be chain extended and then crosslinked, ideally in a single step, to give a product equal in properties to that obtained by present day processes. To achieve this end a number of advances are necessary. The action of reinforcing fillers is often found to be dependent upon high shear mixing. Understanding of this process would hopefully lead to the development of methods making this unnecessary. A range of elastomers with chemical reactivity appropriate for the chain extension and crosslinking together with the corresponding linking agents are required. Some of these difficulties have already been overcome in polyurethane systems giving hope of further extension into other rubbers.

An area that is exciting interest concerns the cold forming of both amorphous and crystalline polymers, a method that avoids the energy and time required to heat a polymer above its softening point or melting point and then cool it down again to room temperature. Injection molding of thermoset resins also looks promising—cycle times for heavy section moldings are currently faster than with injection molding.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

Perhaps one of the most urgent technical problems in the plastics processing field is to find efficient methods of recycling. Recycled polymers continue to expand in both variety and volume of production. Reclamation of rubbers by mastication and of monomer from poly (methyl methacrylate) by pyrolysis are now well established processes. Likewise, much newsprint is now recycled, and a method has also been developed for recovering poly (ethylene terephthalate) from textile mill tailings and photographic film. Perhaps effort has tended to be more concentrated on recovery rather than recycling, and there is need for developing ways to use scrap as a raw material for new processes. Thus, used automobile tires may be used as a source of chemicals (by destructive distillation) or carbon black (by controlled combustion). The established reclaiming process to reform a rubber is more complex and yields a material which might be more accurately classified as a compounding ingredient rather than a raw rubber. A process which competes with recycling polymers is to use them as fuels—for example, tires yield 50 percent more heat than coal—but a most pressing need is to find effective recycling processes for polymers which give unpleasant or poisonous fumes on burning. Polyvinyl chloride is perhaps the most important such polymer because of its large production, but there are many others. It yields hydrochloric acid on burning which must be removed and prefarably used before discharge of the combustion gases.

Techniques involving biodegradation are poorly developed. So far in many cases they may lead to useful products and as a minimum lead to non-dangerous and easily disposable materials. In all considerations of recycling it must be remembered, however, that we have a people problem and an economic problem. Most of the processes require a substantial supply of clean material (scrap). Factory scrap is the first choice; but to really operate efficiently, provision must be made for adequate separation of refuse with economic transportation to recycling plants. The creation of such a system is perhaps not the province of the scientist but is essential to the application of science to recycling.

Electronic Materials

Single crystals are the cornerstones of solid state electronics. As in the past, future innovation in electronic materials will depend heavily on furthering the art and science of crystal growth. The search for new crystals with novel properties often occurs beyond the reaches of the predictive power of physical theory and has to rely heavily on intuitive interpolation and extrapolation of trends in the periodic table, atom sizes, crystal structures, bond polarizabilities, etc. The electronic materials of the future will be more difficult to prepare and the quality and perfection requirements will be more stringent. Higher melting points, higher volatilities, ternary and quaternary compounds, pose greater problems than did the elemental semiconductors, germanium and silicon. Yet even with silicon the demands of modern integrated circuit technology make it necessary to find ways to achieve even greater control over the presence of impurities and imperfections than is presently typical.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Effective search, strategies for new electronic materials call for close liaison, often on a day-to-day basis, between those engaged in the chemical and metallurgical aspects of crystal growth, and those primarily concerned with their physical properties. More complete knowledge of each other’s area of expertise can only help the synergistic process.

A wide range of crystal growth and preparation techniques is now available: pulling from the melt, hydrothermal growth from hot aqueous solutions, flux growth from other solutions, growth directly from the melt, epitaxial methods for thin films, chemical vapor deposition, ion implantation, etc. Each method offers advantages and disadvantages for a given material, but in all cases the trend is toward greater control over the resulting crystal composition, purity, and homogeneity. More complete understanding of crystal growth processes and the factors affecting them, such as convection instabilities in the liquid phase, and more detailed knowledge of the factors determining phase diagrams and which impurities, if any, can be used to dope crystals p- or n-type are needed. Theoretical crystal growth studies at the atomic level, particularly by computer modeling, together with careful experimental studies of growth and initial nucleation on clean surfaces using, for example, molecular beams, are advancing to the point where theory and practice should begin to mesh.

Control of the nature, density and distribution of point, line and planar defects in single crystals is essential in the interpretation and control of structure sensitive properties. For example, isolated or clustered vacancies, interstitials and impurity atoms, second phase particles, dislocations and dislocation arrays, stacking faults, antiphase boundaries and magnetic domains can be present in the as-grown condition or form after growth as a consequence of imposed conditions. The control of these defects requires, firstly, their detection and characterization and, secondly, knowledge of how the defect state varies with imposed conditions (e.g. heat treatment, impurity doping, thermal-mechanical treatment, irradiation, oxidation, electrical or magnetic fields, etc.). Given these factors it is, in principle, possible to manipulate crystal growth variables so as to produce a particular as-grown defect structure (e.g. dislocation “free” silicon) and to utilize as well as operate within the boundaries of imposed conditions so as to both design a particular defect structure and maintain or minimize changes in desirable defect structures.

To a large extent the detection and detailed characterization of lattice defects is now possible because of continual development of sophisticated X-ray and electron optical instrumentation over the past twelve years and simultaneously because of major advancements in the theories of diffraction and image formation. Using X-ray diffraction and topography techniques defects present at the micron scale can be studied whereas defects present on the near atomic scale can now be studied with electron optical equipment which combines high, resolution electron diffraction and microscopy with energy analysis and selection techniques. Most studies, however, have been concerned with, surveys of defects in as-grown, quenched, irradiated or mechanically-damaged metals, metal alloys and, to a lesser extent, oxides (i.e. structural materials). The knowledge derived from these studies has been used primarily to better our understanding of the fundamentals of

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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deformation and fracture processes. Similar attention has not been given materials whose electrical, superconducting, magnetic and optical properties are of principal importance. This neglect has produced a significant knowledge gap in design, fabrication and quality control areas where these materials are utilized.

Nowhere in electronic materials are such exacting demands made on crystal quality and defect control as in integrated circuit technology. At the same time, the trend has been to even greater extremes of miniaturization. But in one respect, electronics has remained unaltered over the years. Components both active and passive have been interconnected according to the precepts of classical circuit theory. What has changed, and changed remarkably, is our ability to fabricate smaller components and make the interconnections at progressively lower cost and higher reliability. The cost of a transistor in complex circuits has decreased some ten thousand fold since 1960, and yet the reliability of large scale integrated arrays containing thousands of components approaches that of single discrete components. Concurrently with the increasing scale of integration on monolithic silicon devices, the need in analog circuits for precision resistors and capacitors has led to the development of these components in precision thin film form. When interconnected on a common substrate with silicon devices, a powerful and adaptive hybrid integrated circuit technology results.

The development of large scale silicon monolithic integrated circuits for both the bipolar and metal-oxide-semiconductor functions has been made possible mainly by evolutionary improvements in materials and processes which have steadily reduced the numbers of defects introduced at each stage in the complex processing sequence, by defects which can cause a device malfunction. Defects can enter a device in three ways: intrinsic defects in the starting materials; processing defects where a process step fails locally to generate the required structure; and defects due to the photolithography process or the photomasks themselves.

At present, the last two categories are the most critical. Many of the processing steps used in fabricating integrated circuits are less than perfectly understood, and would benefit substantially from basic study. Examples are the structure sensitivity of electrical properties in SiO2; factors affecting the surface state density and charge mobility at Si/SiO2; interfaces; the thermodynamics and kinetics of the formation of ohmic contracts between metals and silicon.

The photolithographic process and particularly our ability to make defect-free masks is now one of the most crucial obstacles to further increase in the scale of integration. Radical new approaches are needed here since detail on current silicon integrated circuits is delineated to limits too close to the optical diffraction limit, and further substantial improvement in the process is impossible. In view of this, many of the major semiconductor concerns are now developing electron beam lithographic processes for direct lithography on silicon. Crucial to this effort is the synthesis of new polymeric electron resists and improvements in our understanding of the physical processes occuring during the interaction between electron beams and these polymer films. The substantial impetus behind electron lithography is the factor of 102–103 increase in packing density over the

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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present state of the art which the technique ultimately offers, with its attendant large reduction in cost. But once this is attained, the fundamental size limitation on both bipolar and metal oxide semiconductor (MOS) devices will have been reached.

Further progress at this point will be possible only by reducing the total number of components in a system. To do this, our old approach of reproducing physically the elegance of classical circuit mathematics will have to be abandoned in favor of functional electronics—where basic properties of matter are used directly to perform circuit functions.

One of the oldest and best examples of a functional device is the piezoelectric quartz crystal resonator. Nowhere within the quartz can the inductive or capacitive components of its classical circuit analog be isolated. Other examples are quartz and glass delay lines, Gunn effect and Impatt oscillators and amplifiers, acoustic wave transducers, elastic wave amplifiers made from piezoelectric semiconductors, magnetic bubbles, acoustooptic and electro-optic modulators. The latter two are particularly interesting in view of the prospects for early use of optical communications systems. The hybrid approach to integrated electronics offers an easy vehicle for the introducing and interfacing of optical devices with other more conventional electronic elements. Major opportunities for long term materials research lie in this field of functional electronics.

Instrumentation, Analysis and Testing

The practice of testing and delineating the characteristics of materials, especially those related to performance, runs through all technology. Testing is required for quality control; for establishing standards to ensure in-service durability, reliability, and safety; for sensing in production processes and automation; and to avoid environmental degradation.

In a 1967 report, Characterization of Materials,* the Materials Advisory Board stated, “Attempts to provide the superior materials that are critically needed in defense and industry are usually empirical and often wasteful of efforts and funds. That is so, chiefly because we do not yet have a fully developed science of materials that affords predictable and reliable results in devising and engineering new materials for specific tasks.” A definition was proposed—“Characterization describes those features of the composition and structure (including defects) of a material that are significant for a particular preparation, study of properties, or use, and suffice for the reproduction of the material.”

Destructive or nondestructive tests are required to determine the many characteristics of materials; mechanical, electrical, optical, and other physical properties; composition and structure; defects and impurities. Understanding of the relationships between properties and performance, of the mechanisms of degradation and failure, and of the interaction of matter with

*  

Publication MAB-229-M, National Academy of Sciences-National Academy of Engineering, Washington, D.C., 1967.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

various forms of radiation is essential to the development of testing methods and equipment. The latter, in addition, must be designed to function in the pertinent service environment.

Technology and basic research interact strongly in the development of instrumentation. The initial models of many sophisticated instruments are built, as a rule, for specific research projects. Often this instrumentation eventually becomes standard for production or quality control. One example is the thermocouple, which resulted from basic research in the 19th century on the thermoelectric effect. The thermoelectric properties of many materials were determined, and this led to the adaptation of the phenomenon to measure temperature. Other well-known examples are X-ray diffraction, the optical and electron microscopes, and spectrochemical analysis.

The realization that the composition of the surface of a solid usually cannot be inferred from measurements of the bulk material has stimulated the development of new spectrometric instruments for surface analysis. Much of the current effort is aimed at establishing the full potential of these tools, which include the ion probe, the X-ray photoelectron spectrometer, the Auger spectrometer, and the ion-scattering spectrometer.

Analysis of ultrapure materials is seriously challenging analysts. Improvements are required in the mass spectroscopy of solids and in activation analysis. Ecological concerns are largely responsible for an upsurge of interest in the detection of organic compounds, such as those present in trace amounts in biological materials. Advances will be sought, as a result, in mass spectroscopy, infrared techniques, gas chromatography, electrophoresis, and other analytical methods.

Nondestructive testing is among the areas of materials technology requiring urgent attention. In the past, nondestructive testing generally meant testing only for geometric size, defects, and some mechanical properties, but it should be interpreted much more broadly—testing for composition, microstructure, and the full range of physical properties. Basic research in solid-state physics and chemistry, aimed at detecting and understanding certain properties of materials, has spawned many of the modern techniques for nondestructive testing. The methods depend heavily on the interaction of matter with optical, electromagnetic, acoustical, and other forms of radiation. A few examples of techniques of current value or being developed for nondestructive testing are: electron paramagnetic resonance (fracture of polymeric solids, stress analysis); nuclear magnetic resonance (chemical analysis; Mössbauer spectroscopy (surface-chemical and phase analysis, stress analysis); optical correlation (surface distortion); infrared spectroscopy (thermal analysis, flaw detection); microwave attenuation (moisture content); optical and acoustical holography (stress analysis, flaw detection); acoustic emission (flaw detection).

Routine use of these methods in nondestructive testing, however, requires more understanding of the physics of the phenomena involved, its quantitative relationship to the physical property to be monitored, and the limits of applicability. Required also is instrumentation that offers improved signal detection and reliability as well as greater physical ruggedness and ease of testing, especially in portability and automatic readout of easily interpretable data.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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BASIC RELEVANT PROPERTIES OF MATERIALS

Superconductivity

One of the most tantalizing challenges to materials scientists is that of finding practical superconductors with, higher transition temperatures than those currently known. There is great potential technological value of high temperature superconductors for electric power generation, transmission, and novel modes of high-speed ground transportation. Hundreds of elemental and compound superconductors have been discovered or synthesized and transition temperatures as high as about 23.2ºK. have been achieved but the advance is slow—only about 5ºK over the last decade despite much effort and many materials preparations. On the theoretical side developments in 1957 (the BCS theory) did much to clarify the mechanism of superconductivity and rationalize various experimental observations. There remain unanswered questions regarding the fundamental limitations which the lattice and electronic structure of real solids impose on the transition temperature. Until recently much of the search, for higher temperature superconductors concentrated on trying to find the “magic” electronic structure. Recently it has become more apparent that the dynamic properties of the lattice (phonons) are at least as important; in particular, lattices which undergo structural transformations accompanied by, or triggered by, certain of the lattice vibrations going to very low frequency (soft modes) have been found to be particularly prone to be high temperature superconductors. To put these results and ensuing phenomenological models on a rigorous and quantitative basis is a particularly exciting and urgent challenge to materials science. In the meantime, intriguing progress is being made by focussing the search on materials with relatively unstable or metastable lattices, either as a basic property or as an artefact attained by some preparative technique such as rapid quenching.

Extensions of Laser Action

Though lasers are by now well known and understood and new applications for them are steadily being found there are three areas where orders-of-magnitude improvement are needed—higher power, shorter pulses, and shorter wavelength.

The power limitations for solid lasers are not yet known nor is much understood about the limiting mechanisms of optical breakdown. Recent development of ultraviolet-assisted large volume CO2 gas lasers appears to have opened a new era in the direction of rugged, inexpensive, ultrahigh power sources. There is hope that the same principles can be applied to shorter wavelength lasers such as the 3371Å N2 laser or even the recent H2 laser which has operated down to 1161Å. With such actual and anticipated developments comes the need for solid materials for optical elements: windows, mirrors, lenses and nonlinear optical elements. The anticipated use of high power lasers for nuclear fusion initiation will likewise provide tough challenges to materials science.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
×

The production and control of short ( sec) optical pulses calls for a more thorough understanding of the very short time dynamics of materials. The recently-developed optically activated Kerr-type shutter whose response is limited by the molecular reorientation time of a liquid molecule is an example. As times become shorter and laser pulses more intense, much of our physical understanding of materials which is largely based on thermal equilibrium states will prove inadequate.

The achievement of shorter wavelength laser sources—ultimately into the X-ray region—is an extremely important goal. Attempts to use solids with larger bandgaps would suggest the rare gas solids (and possibly liquids) as fertile fields of research. The possibility of pyramiding nonlinear optical processes up to the 15th or higher harmonics of the fundamental is already under serious considerations, using phase matchable metal vapors as the nonlinear material. Much more effort is called for on direct soft X-ray lasers, probably using low-lying nuclear energy levels. This is an area where a novel approach is most likely required.

The potential benefit to materials science and engineering from such research is obvious in several ways: X-ray holography, direct measurement of bond charge distributions, and inelastic scattering from excitations of even the smallest wavelength would revolutionize our methods of studying and characterizing materials.

Continuously tunable laser sources are now a reality in several embodiments: dye lasers, magnetic spin-flip Raman lasers, pressure-tuned direct gap semiconductor lasers and parametric oscillators. Without exception these devices are based on basic research of a decade or more previous. The tuning, modulation and control of the higher power, short pulse, short wavelength lasers of the future will undoubtedly make use of the interaction of the laser energy levels with external perturbations, in markedly nonequilibrium conditions.

Present research in materials science is developing knowledge concerning the interaction of lasers with materials. Areas such as laser-induced shock strengthening of metal surfaces, laser welding, and machining of ceramics and metals with lasers will benefit from such materials research.

Fracture Toughness

The importance of a fundamental understanding of fracture toughness to the design of safer and more reliable engineering structures is obvious. But as in so many other areas of materials science and engineering our present understanding at the atomic level is not sufficient to provide a reliable predictive capability. Instead, the most fruitful approach has been in the area of fracture mechanics. Most research to date has revolved around continuum models of crack propagation, particularly concerning energy release criteria and stress intensity factors for crack propagation. More recently somewhat more sophisticated theories of fracture which take into account polycrystallinity, or microstructure, have resulted in improved understanding, better testing procedures, and the development of tough materials. As the ignorance factor is diminished so there can be significant materials

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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savings by minimizing the need for over-design. But much, more needs to be done. Improved models are needed to close the gap between continuum and microstructural theories of fracture by taking into account the nature of the polycrystallinity, anisotropy, and time-dependent material behavior. More understanding is needed of the dynamics of crack propagation and the effects of the statistical nature of the microstructural elements. Here, innovative use of computer modelling techniques can be expected to play an important role. But ultimately we need to know more about fracture initiation with regard to actual breaking strengths of metallic or chemical bonds, the role of lattice vibrations in a distorted or defect-riddled lattice and, potentially pertinent to the severe technological problem of stress-corrosion cracking, the dynamics of the chemical reactivity of stretched or otherwise distorted lattice structures and individual interatomic bonds.

Dynamic Behavior of Defects

When dislocations first were directly observed by transmission electron microscopy in the mid 1950’s, great impetus was given to both experimental and theoretical study of dislocations in both single crystals and polycrystalline materials of commercial interest. As a result, the treatment of plastic deformation of crystalline materials by dislocation motion and interactions has been greatly facilitated. Modern research and development of structural materials is now related to the concepts and methodology of the dislocation treatment. There is still a great deal of basic work left to do in advancing the dislocation theory and its application to structural materials development. A current thrust in research on dislocations includes the combination of dislocation theory with continuum mechanics, resulting in a continuum theory of dislocations. Work is needed particularly on the transition from dislocation behavior to continuum behavior under dynamic conditions. Another area of research is the use of computers to average individual dislocation reactions into a net plastic deformation. Still a third area involves dislocation dynamics under high strain rate conditions, such as those prevailing in the vicinity of an advancing crack and those inherent in shock deformation and laser pulsing.

The field is at the stage where applications of the theory are being made through analytical models relating the dynamic behavior of dislocations and point defects to mechanical behavior; e.g., such practical applications as creep and hot pressing of crystalline solids. This is accomplished by computer “mapping”, whereby various theoretical constitutive equations are used to predict in stress-temperature space, regions where specific mechanisms of high temperature mechanical behavior are operative.

The effects of point defects in the mechanical behavior of materials are of particular importance at high temperatures, where redistribution of point defects and dislocations may occur. Effects on high temperature strength performance are related to vacancy transport (diffusion) and the formation of stable dislocation substracture and dislocation networks. Of particular importance is the combination of high temperature and neutron flax, such as

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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that prevailing in a fast breeder nuclear reactor. Here the dynamics of vacancy agglomeration into voids poses a severe challenge, as well as the broader area of gas (hydrogen and helium) generation and agglomeration.

Our ability to treat the dynamics of surfaces or interfaces at elevated temperature rests on our ability to treat dynamics of point defects (interstitials and vacancies) and line defects (dislocations). These are inextricably related to the structure of area defects (surfaces and interfaces). There is little doubt but what high temperature structural strength is directly related to the stability of the three-dimensional network of area type defects, which can be facilitated by solute and particle pinning.

Flammability of Polymers

Flammability, an especially fast form of surface chemical reaction, is of particular concern in the use of polymers. There is need for better quantitative determination of the important variables of burning. For example, counterparts to the oxygen index test, used for rating precisely the ease of burning of individual materials, have to be developed for entire materials systems or products under conditions reflecting their performance in end-use environments.

The high-temperature, free radical reactions of polymer combustion, encompassing oxidation and pyrolysis in both the flame and degrading polymer, are now understood in only qualitative terms. The interaction and sequence of the important physical and chemical processes, and how they may be slowed or altered by the addition of various fire retardants, represent a challenge to modern research very similar to that posed by catalysis.

In a second category, the results of practical efforts to devise significantly better fire retardants for flammable materials have apparently peaked. Little real progress has been made other than to optimize the form and amounts of antimony, halogen, and/or phosphorus in a particular material or application. Furthermore such treatments are now known to increase greatly smoke and toxic gas formation. Although synthesis of inherently nonflammable polymers has given us materials such as the polyimides, they have not so far been economically feasible except in limited use applications.

The most promising improvements will probably come from careful design and engineering to give system-wide rather than mere individual material protection. The age-old, reliable sprinkler system is a trivial example. The recently-reported experiment of protecting the inside of an entire aircraft fuselage for over ten minutes in a raging inferno of burning fuel solely with intumescent insulation is an excellent instance of innovation in this area.

Photochemistry

Chemical changes resulting from exposure of materials to light can be deleterious or useful. Sunlight can cause plastics (such as automobile

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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upholstery) to discolor, decompose and crack. On the other hand, photochemical reactions are the basis of photography. Related photochromic effects offer promise for information storage and holography.

The renaissance of organic photochemistry began only a decade ago. Many basic principles and reactions were discovered but a deeper understanding of reaction mechanisms is still necessary not only if photo-induced material degradation is to be avoided, but if the full synthetic utility of photochemical processes is to be realized. There is considerable promise for those reactions, which frequently occur under mild conditions, in the synthesis of chemically-sensitive species of pharmaceutical and biological interests.

There is still much improvement possible in silver halide photography and the development of grainless nonsilver imaging systems is still in its infancy. The related areas of high density information storage with photochromic or holographic techniques requires more fundamental investigation of photochromic compounds and photopolymerization. Photo-induced changes in the electronic configurations of certain dye molecules have led to tunable dye lasers. Further research on the electronic configurations and properties of organic molecules may be expected to yield both important scientific information and practical benefits.

Corrosion Resistance

Although much progress has been made in understanding the thermodynamics and kinetics of the corrosion process, the mechanisms of localized corrosion and the mechanisms for imparting corrosion resistance or protection against aqueous or gaseous corrosion are not well understood.

For localized corrosion, e.g., pitting and stress corrosion, initiation is distinct from propagation. Initiation may involve the breakdown of a surface film; important factors to be studied are variations in film composition and microstructure down to the atomic level and their interaction with the environment. Initiation can also occur at surface inhomogeneities; the types have not been clearly characterized. The propagation of stress-corrosion, hydrogen-embrittlement, and corrosion-fatigue cracks is not understood. As the use of high strength materials increases, these problems become more important; for example, susceptibility to hydrogen embrittlement increases with the strength of the steel. The mechanism of stress corrosion is probably different for each system. Problems pertinent to numerous systems include (1) the role of mechanical fracture, (2) the effect of stress on the rate of anodic dissolution, (3) continuous vs. discontinuous cracking, (4) the relevancy of continuum mechanics, as opposed to atomistic analyses of crack propagation, (5) the effect of defect structure and of chemical composition and distribution at the macro and micro levels in the metal, (6) the role of hydrogen generated at the crack tip. A high hydrogen fugacity in the vicinity of the crack tip is not a prerequisite for hydrogen embrittlement. However, it is not clear which of numerous other proposed mechanisms, if any, explains this phenomenon satisfactorily.

The corrosion of alloys by gaseous environments can cause surface roughening, most likely due to the preferential attack of the less noble

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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constituent, and can result in a poor surface finish and in poor adhesion of surface films. The theory of surface instability and the mechanism of surface roughening are poorly understood.

High technology industries operate under unexplored conditions. Research is required for corrosion in aqueous media at high temperatures and/or pressures and in the ocean near the surface and at great depth, in highly corrosive body fluids for prostheses, and in gaseous media for thin metal films, the properties of which may differ radically from the bulk ones.

Research in the protection of metals falls into two classes: alloy composition and protective coatings. Although the specifics may vary, the basic questions to be answered are similar for most corrosion systems, including most alloys, aqueous and gaseous environments, and temperatures from below 273 K to above those at which the superalloys can be used economically at the present.

Small changes in chemical composition can radically change an alloy’s corrosion resistance, due to an intrinsic change in the alloy or due to composition changes in surface films. Research must determine the properties imparted by and the role of the alloying addition as affected by its presence in solid solution, in microsegregates, and in second phase particles. Research must also determine the effect of alloy composition on film composition and microstructure and then relate these to the problems of the transition from internal to external oxidation, the adhesion and the spalling or corrosion films, the resistance to breakdown of these films, and the mechanism of self healing. More specifically, the following must be studied: the crystallography of the films and the factors that determine crystal size and transitions between the crystalline and amorphous states; the defect structure, the conductivity of and diffusivities within the films and their effect on film growth kinetics; the mechanical properties of corrosion films; the thermodynamics and kinetics of the transformation from one corrosion product to another during high temperature gaseous corrosion of complex alloys. The above studies should lead to new alloys with better corrosion properties or to cheaper alloys.

Protective coatings fall into two classes: inhibitors are of monomolecular dimension and reduce the anodic or cathodic reaction rate, whereas thicker films provide a physical barrier. The interaction of inorganic inhibitors, e.g., chromates, with a metal surface is not understood. Are they adsorbed? Are electrons transferred, i.e., is the metal oxidized? Are all surface sites equally affected? The application of metallic coatings can result in the formation of intermetallics at the interface between the metals. Their role in adhesion and in corrosion protection is not always understood. The possibility of using metals that form corrosion-resistant oxides, e.g., chromium or aluminum, as coatings on the refractory metals for high temperature (> 1400 K) applications should be studied along with the resulting chemical and metallurgical problems. For organic films basic research is needed on the mechanism of adhesion.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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BASIC GENERAL PROPERTIES OF MATERIALS

Interatomic Forces, Chemical Bonding, and Lattice Stability

There is no more Basic property of a material than that it exists. Yet, despite the enormous advances in solid state theory we are not able to predict from first principles and the appropriate atomic wavefunctions the configuration and dimensions of any crystal lattice except for a handful of very simple materials. Band structure calculations have reached a point of sophistication where the electronic properties of many crystals can be calculated with remarkable precision given the crystal structure and atom spacings. The fundamental challenge of relating the properties of individual atoms to those of a crystalline solid composed of such atoms, particularly the imperfect solid, remains. Ideally the goal of research in this area should be to predict the conditions under which the material forms, its structure, its stability, and its electronic and mechanical properties. But stated this way makes us realize just how primitive is our understanding of such basic matters as interatomic forces, chemical bonding, configurational interactions, and so on. Besides the need for further theoretical developments there is likely to be a need for a long time to come for sensitive experimental determinations of such basic descriptions of the solid as the band structure, the phonon spectra and the Fermi surface with which to test the soundness of theoretical calculations. Other experiments are needed to provide parameter inputs to these calculations such as measurements of intermolecular potentials, charge distributions, and computer experiments on molecular dynamics. In the meantime, to fill the immediate needs of the materials scientist, efforts should be exerted to provide the best available theoretical descriptions of the imperfect solid (e.g. stacking fault energy, stress fields around vacancies, impurity atoms and dislocations). A fruitful approach has been the computer modelling of the defect lattice using interatomic potentials. Much of the groundwork for cooperative experimental-theoretical progress in these areas appears to have been laid and advances in the understanding of the inherent, basic properties of various materials can be expected to emerge steadily in the coming years.

Microscopic Understanding of Phase Transitions

Though the equilibrium crystal structure can be calculated for only a very few simple materials the fundamental ability to predict the abrupt changes in crystal structure that occur as the temperature, pressure or composition is varied is in even worse shape. And the most dramatic phase transition of all, namely melting, is still very much a mystery from a fundamental point of view. If we properly understood melting, we would have much more insight into the roles of interatomic forces and cooperative interactions, etc, in determining the structure and stability of solids. The microscopic mechanisms that bring about phase transition are an object of

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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intense research at present and part of the deep attraction of this study of phase transitions lies in the remarkable quantitative universality of certain general phenomena in the vicinity of second-order phase transitions regardless of the type of material or even of the type of phase transition—the liquid-gas (at critical point), ferromagnetic, ferroelectric, order-disorder (some) and superconducting phase transitions are in certain rather profound ways all the same. In all cases the thermally-driven fluctuations in a particular variable become correlated over increasingly longer ranges as the transition is approached and the time scale of these fluctuations increase markedly. Some consequences of this are the well-known increases in magnetic susceptibility and dielectric constant at the ferromagnetic and ferroelectric transitions, respectively, and critical opalescence at the liquid-gas critical point. While quantitative correspondences between various transitions have been established (the scaling laws) a true microscopic understanding of phase transition mechanisms is lacking—a truly important challenge for materials research and solid state physics in particular. In martensitic transformations, for example, there is a large body of evidence for pre-existing embryos providing sites for nucleation. The nature of these embryos (presumably defect clusters) and the mechanism of interface propagation during transformation are by no means understood. Perhaps equally important is to develop an understanding of phase transition mechanisms in interacting systems, of which the coupling of spins and phonons near the magnetic transition is but one example.

Amorphous, Disordered State

Though fundamental understanding is still in a very primitive state regarding the crystal and electronic structures of crystalline solids it is much worse off as regards the glassy or amorphous state of matter. As was recommended by a recent NMAB panel,* fully-developed conceptual framework for amorphous materials is lacking and close collaboration between experimentalists and theorists is needed for progress to occur. On the theoretical side there is a need for calculations of electronic potentials, energy levels, and transport properties directly applicable to physically realized glass structures. The experimental side calls for better understanding of material preparation and the glassy-to-crystalline transition. The question of what determines the mechanical strength and other physical properties of glasses of various compositions and bond coordinations is pretty well wide open and there persists much uncertainty about the electronic band structure of semiconducting glasses.

Impurity Effects in Solids

When impurities are introduced into an otherwise perfect host crystal in principle all of the properties of the resulting system are modified. The nature and extent of the effect of the impurities depends on their concentration, location and interactions with the host material. In dilute amounts

*  

Fundamentals of Amorphous Semiconductors, Report of ad hoc Committee on The Fundamentals of Amorphous Semiconductors, National Materials Advisory Board, NAS/NRC (1972).

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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the impurities can be viewed as a nonperturbing probe of the microscopic properties of the host (as in spin resonance experiments) but in high concentrations they can lead to new phases (alloys) and phenomena (e.g. order-disorder transitions). Impurities may be desirable, as in most semiconductor phenomena, or undesirable as, for example, in impurity-enhanced optical damage in nonlinear optical materials. Yet, despite the enormous amount of work that has been done on, for example, impurity effects in semiconductors, a general microscopic understanding and theory of the effects of impurities on material properties is lacking. The dilute limit, while theoretically simplest, is experimentally the most difficult, while the converse is true for high-impurity concentrations. The directions for improvement on these problems are obvious. The intermediate domain, in which impurity-impurity interactions are no longer negligible, constitutes a prime challenge to both theory and experiment. Recent experiments have shown the existence of collective impurity modes such as phonons, excitons, and magnons at intermediate concentration levels. Theories are needed to explain the emergence of this behavior from the single impurity behavior at low concentrations and to distinguish it from the mixed crystal or alloy behavior at high concentrations. Experiments are needed on systems where impurity-host interaction strengths are sufficiently weaker than impurity-impurity interactions to compensate in a controlled way for the numerical superiority of the former. More consideration should be given to systems with simple structured and/or inert hosts, such as helium and the other rare gas solids, so as to provide theoretically tractable, experimentally accessible model systems for impurity effects. The possibility of long-range order (e.g. magnetic) in the impurities which is absent from the host is particularly intriguing. Properly controlled impurity experiments in the nondilute range could open new opportunities for direct observation of microscopic interactions. Progress along these lines has already been made in thin-film studies on magnetic impurities in nonmagnetic, metallic hosts. Similar experiments on optical, elastic, and dielectric properties promise exciting results. A particular lingering puzzle is the role of impurity excitons in semiconductor laser action. Optical studies have suggested the presence of excitonic molecules and have stimulated speculation on the possibility of an excitonic liquid or even a solid phase. Another continuing controversy concerns the role of interstitial impurities in increasing the low-temperature yield strength (thereby enhancing brittleness) of body-centered cubic metals. One camp argues that lattice friction stress of the pure metal is inherently large at low temperature, while another argues that interstitials introduce lattice distortion which is especially effective at low temperature. Although sophisticated experiments are needed here, theoretical calculations based on interatomic forces should help decide the issue.

One- and Two-Dimensional Systems

Until fairly recently calculations of physical phenomena in one- or two-dimensional systems were considered to be of mainly academic interest. Onsager’s famous exact solution to the two-dimensional Ising model provided

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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inspiration to solid state physicists and engendered the hope for eventual similar success in three-dimensional, or “real” systems. Within the past four or five years, however, a variety of magnetic, superconducting and resistive materials have been prepared which exhibit exceedingly large anisotropies in their thermodynamic, transport, and collective properties. The anisotropies are sufficiently large that the microscopic interactions along a line or within a plane may be several orders of magnitude larger than in the transverse directions. For example, tetragonal crystals of the K2NiF4 family exhibit in-plane magnetic exchange forces several thousand times larger than the out-of-plane exchanges, with the results that below about 100ºK truly two-dimensional long range magnetic order occurs. Neutron susceptibility and optical experiments have confirmed the two-dimensional nature of the spin dynamics (magnons) and the critical behavior as well. Similar striking behavior in one-dimensional antiferromagnetism has been observed. Layered structure transition metal dichalcogenides (MoS2, etc.) have long been recognized as effective lubricants. More recently they have been found to be essentially two-dimensional superconductors, whose properties can be altered markedly by chemically changing the spacing between layers. Certain organo-metallic complexes have exhibited one-dimensional manifestations of antiferromagnetism and the metal-insulator transition. These discoveries have enkindled lively theoretical and experimental interest in the physics of less-than-three dimensions. Consequences of extreme anisotropy of microscopic interactions must be explored more fully. The effects of lower dimensionality on collective modes, electron and heat transport must be understood. Particularly intriguing is the effect of a microscopic upper limit to the correlation length in certain directions on the critical properties near phase transitions in lower dimensional systems. While some magnetic transitions have been studied, virtually nothing has been done on structural, order-disorder or ferroelectric transitions in less than three dimensions. Improved understanding of the physics and chemistry of two-dimensional systems is essential to the eventual understanding of catalysis. Because of the extreme anisotropy in bonding strength, study of the mechanical behavior of the layered structure materials could lead to superior lubricants or high strength components, as already demonstrated in graphite. In usual powder form graphite is a widely-used lubricant. Through processing of precursor polymer filaments, dense and highly oriented graphite fibers have been prepared which exhibit axial strengths that are a significant fraction of the theoretical strength.

Although in some ways fundamentally different, thin films and filaments of otherwise three-dimensional materials are a subject of renewed interest to solid state physics. The fabrication of structures which extend only few tens of angstroms in one or two directions has made clear the need for more careful experiments and sophisticated interpretations to understand the physics of such structures. Two indicative examples are the observation of a nearly five-fold increase in the superconducting transition temperature in thin Al films and the increased sound attenuation coefficient in small diameter glass fibers.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Physics and Chemistry of Surfaces

Surfaces are possibly one of the most fruitful research, areas in materials science. Knowledge at the most fundamental level in this area can be expected to have relevance to almost all uses of materials, from the processing and reliable performance of integrated circuits to the corrosion of structural components, from frictional wear and tear to catalysis and flammability, from crystal growth to adhesion. The variety and complexity of surfaces and surface layers are at least comparable to the variety and complexity of bulk properties but our level of understanding of surfaces is, in contrast, in its infancy.

While the aim is to develop a more sophisticated understanding of the electronic and chemical properties of surfaces, our present level of ignorance is illustrated by the fact that these properties are very sensitive to the detailed ways in which atoms are positioned at the surface, and in general these positions are not known. Surface properties are related to the properties of the underlying bulk but in ways which are not often clear. And though it can be said that bulk properties are understood by-and-large in principle, if not always in detail, this is not true of many of the surface properties where the broad outlines of the phenomenology are only now being drawn. This phenomenology concerns, for example, the details and statistical mechanics of surface topology, local bond and electronic structures, the energy states of electrons at surfaces, models for nucleation and growth, and so on.

Surfaces offer an extra degree of freedom for arrangement of atoms statistically on the lattice sites. The statistical mechanics of this situation, extending with three dimensions over several atomic layers, needs considerable development. While the roughness of a surface on the atomic scale has a major impact on adsorption, surface diffusion and crystal growth processes, very little is yet known about the details of the role of surface roughness in these processes.

The electronic properties of surfaces in simple systems need considerable attention. There is some controversy about the extent to which surfaces can be treated as an extension of the bulk or whether the discontinuity in properties at the surface is sufficiently great to require new concepts and analytical procedures. Our theoretical models for surface electronic properties, surface relaxation and surface structure are in a rudimentary state at present. The extent to which surface states on semiconductors are an intrinsic property of the surface or associated with surface impurities is under debate. Surface states occur both at free surfaces and at interfaces, such as the silicon-silicon oxide interface. It has recently been shown that various surface states on semiconductors correlate with various surface structures as revealed by low energy electron diffraction.

Surface nucleation, vapor deposition, adsorption, and surface contamination, topics with clear practical significance, are currently being investigated experimentally in detail for a variety of systems, with emphasis on the simpler systems. Much more work in this area needs to be done. The kinetics and thermodynamic properties of vapor deposits can be obtained by mass spectrometric methods and the distribution of clusters on the surface can be determined by diffraction methods. Simple classical surface nucleation theory is inadequate

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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to account for the measurements and major modifications of the theory appear to be necessary. Adsorbed atoms can be identified using Auger spectroscopy even at a small fraction of a monolayer coverage. Auger spectroscopy coupled with ion bombardment can be used for profiling, to get at bulk composition profiles below the surface. Low energy electron diffraction is just entering the quantitative stage where the position of surface atoms can be determined with some accuracy. These methods are also being used extensively to monitor the cleanliness and structure of surfaces as well as to investigate production problems involving contamination at surfaces. The electronic and chemical properties of surfaces and adsorbed species are being investigated by a variety of methods. Photoelectron spectroscopy and UV photoemission spectroscopy are used to obtain band structure information. Electronic and chemical bonding information can be obtained from ion neutralization spectroscopy. IR reflection spectroscopy gives information about chemical bonding, and information about deep electronic levels can be also obtained from the analysis of Auger spectra.

The techniques developed for surface research, such as ion mass analysis and Auger spectroscopy, are providing the best, and often the only, methods available for investigating materials problems associated with thin films, grain boundary segragation, interdiffusion phenomena, and trace analysis. The trend towards miniaturization in electronics resulting from economic, reliability and high frequency considerations points towards growing importance of surfaces. The concepts of miniaturization are best embodied in the technology of large scale integrated circuits where surface and grain boundary diffusion often dominate over bulk diffusion processes. This trend is expected to continue, particularly as optical microcircuitry is developed.

The understanding of catalytic processes is not detailed in most cases. Considerable qualitative insight is available, but the roles of surface structure, surface defects, surface geometry, surface electronic properties and even the bulk properties are not understood in detail.

Significant advances have been made in the area of adhesion, where the understanding of the role of adlayers and their interaction has contributed significantly. Friction is understood in some detail, especially the role and interaction of the asperities in sliding contact, but the process is difficult to treat from a fundamental standpoint, let alone circumvent in practice. From a practical point of view, the lubrication of sliding contacts is fairly well understood. Cold welding can be a serious problem in electrical contacts. Erosion, corrosion and contamination of electrical contacts as a result of arcing remain serious problems.

Deeper knowledge of the behavior of surfaces can also be expected to improve our control over the important practical problem of corrosion—the interaction of a material with its environment. The presence of water or an electrolyte solution changes the physics and chemistry of metal surfaces significantly. The surface energy is altered and becomes a strong function of the charge in the electrical double layer at the metal/solution interface. The equilibrium surface structure may be different from that in the presence of the metal’s own vapor or a vacuum and it presents extra problems in that the interface is not readily examined in situ. Some metals, e.g., silver, undergo surface rearrangement in aqueous solution at room temperature. Alloys

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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generally undergo a change in their equilibrium or steady-state surface composition. The atomistics of the above are poorly understood. There is much ignorance regarding the effects of surface stress, defect structure, and nonequilibrium conditions on the reactivity of metal surfaces.

Physical Properties of Polymeric Materials

Polymeric substances, whether natural such as cotton, wool, and silk, or synthetic, including rubber, rayon and celluloid, owe their remarkable physical and chemical properties which set them apart from a host of well recognized materials to the very long chain molecules of which they are made. Recognition of the key role of long chain molecules was one of the singular discoveries of this century. It led to intense research to discover how variation in the structure of these giant molecules, through new approaches in chemical synthesis, could be invoked to cause valuable changes in physical properties. Yet to put the structure-property relationship of polymeric materials on a firm, fundamental, quantitative base remains a prime challenge to materials research, akin in complexity to the parallel challenge posed by amorphous inorganic materials, perhaps more so.

Collective Behavior

Perhaps the single most fruitful concept in solid state physics has been that of the collective mode or elementary excitation; this concept has permitted the handling of complex many (1023) body systems in terms of a very few degrees of freedom. The basic idea is to regard the structure and composition of the system as given and to seek its responses to various types of disturbance. The complete set of these responses form the so-called “normal modes” or “elementary excitations” of the system in terms of which many of its static and dynamic properties can be expressed. Since a single elementary excitation involves the participation of all the atoms in the system, the concept is quite fruitful in elucidating the cooperative behavior among large numbers of particles which result in a particular phenomenon or property. As was briefly indicated in the discussion of phase transitions, the collective mode concept is quite fruitful in describing even anomalous material properties. The elementary excitation concept has become so familiar to physicists (the words phonon, plasmon, magnon, etc. are well incorporated into the field’s vocabulary) that it may not often be recognized as still having potential for significant growth. However there are at least two directions in which extensions of the concept should prove of significant value: (1) nonlinearities and interactions among elementary excitations and (2) elementary excitations in systems lacking long range order. Recent experimental advances have permitted more precise and complete direct study of the more familiar excitations on the one hand, and generation, detection and study of some new excitations on the other. In the former catagory are included inelastic scattering (both light and neutrons) acoustic, magneto-optic and certain solid state plasma experiments. The latter includes super high-frequency phonon and second sound generation by quasiparticle recombination

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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in superconductors; the launching of stable finite amplitude pulses of both mechanical (solitons) and electromagnetic nature (self-induced transparency); propagating electroacoustic domains in semiconductors, etc. For the future better understanding can be expected of the interactions among these excitations leading to optimized manipulation of such interactions for energy or information transfer.

Perhaps less straightforward but certainly no less important is the second direction: studies of elementary excitations in systems lacking long range order. This includes obviously, amorphous solids and liquids, where effort of this kind has been underway for some time. Already, for example, some microscopic understanding of electronic, optical and acoustic properties of such materials has emerged. Recent generalizations of the hydrodynamic equations to shorter length and higher frequency domains have revealed the smooth transition from collective, phonon-like behavior to diffusive and even single particle behavior in liquids. Some of these trends should also be evident in visco-elastic solids, but the picture is not yet clear. Similar mathematical techniques have been employed to describe elementary excitations in the paramagnetic (disordered) phase of a spin system. The collective modes of the liquid crystal state are under present investigation and should illuminate that important intermediate regime between well-developed long range order (crystal) and the more transient short range order (liquid).

A most exciting possibility lies in the extension of the collective mode concept to large but finite structures, particularly to macromolecules. From the point of view that a large molecule approximates a small solid, the existence of collective motions within the molecule is clear. However, the detailed nature of such excitations and their role in transport of charge, strain, spin, etc. within the molecule remain challenges to both theorist and experimenter in solid state physics. The hope for a true science base for “molecular engineering” largely rests on progress in this direction.

Nonequilibrium Systems

Despite the several unresolved problems indicated above, our basic understanding of the physics of materials under equilibrium conditions is far ahead of that for nonequilibrium systems. While the reasons are not hard to find (such as the inapplicability of thermodynamics and statistical mechanicś), the increasing importance of nonequilibrium phenomena requires that substantial effort be expended to alleviate these deficiencies. Lasers and negative resistance semiconductor devices are familiar examples of nonequilibrium physics in action. Recent progress in understanding the transient and threshold behavior has illuminated analogies with equilibrium second-order phase transitions. It is intriguing to investigate more general instabilities such as hydrodynamic, magneto-hydrodynamic, and plasma phenomena from this point of view. The problem of turbulence is perhaps the most challenging and important of these. Auto-catalytic chemical reaction systems give rise to large spatial and temporal variations in composition.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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The familiar convective instability can cause extreme problems in crystal growth from the melt. Indeed the behavior of the atmosphere, the oceans and even the earth’s crust are strongly influenced by such hydrodynamic instabilities. Instabilities in both gaseous and solid state plasmas have received much attention but are not fully understood.

On a slower time scale understanding metastable states and structures in solids and solid solutions (including spinodal decomposition) should benefit from attacks on these problems.

With new laser techniques, detailed investigation of materials under extreme transient conditions (shock waves, high electric, magnetic or optical fields) can be studied in real time with resolution of ~10–12 seconds and longer. Scattering, absorption and fluorescence experiments which have proved so valuable in guiding theories of materials at equilibrium, should soon begin to do the same for nonequilibrium systems. A foretaste of what might be in store is the use of these fast laser pulses for studying short-lived excited states of radicals and molecules with consequent insights into the mechanisms of chemical reactions.

Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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Suggested Citation:"6. Opportunities in Materials Research." National Research Council. 1975. Materials and Man's Needs: Materials Science and Engineering -- Volume II, The Needs, Priorities, and Opportunities for Materials Research. Washington, DC: The National Academies Press. doi: 10.17226/10437.
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