Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 305
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 306
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 307
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 308
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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,
OCR for page 309
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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: particular classes of materials, materials processing, basic properties of materials that are clearly relevant to eventual applications, 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
OCR for page 310
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 311
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 312
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 313
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 314
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 315
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 346
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 347
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 348
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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).
OCR for page 349
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 350
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 351
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 352
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 353
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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
OCR for page 354
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 355
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research 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.
OCR for page 356
Materials and Man’s Needs Materials Science and Engineering: Volume II The Needs, Priorities, and Opportunities for Materials Research This page in the original is blank.
Representative terms from entire chapter: