National Academies Press: OpenBook

Materials and Man's Needs: Materials Science and Engineering (1974)

Chapter: Opportunities in Material Research

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Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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OPPORTUNITIES IN MATERIALS RESEARCH

Priority Analysis

In order to gather many viewpoints on opportunities in materials research, both basic and applied, COSMAT solicited the opinions of a broad cross section of the technical community both on specific materials topics that deserve attention and on relative priorities for research among various classes of materials, materials properties, and processes. These inquiries went to the presidents of materials and materials-related technical societies, to pertinent Gordon Research Conferences, and to individuals known for their work in the materials field. In all, information was received from nearly 1,000 persons, including the 555 usable responses to the COSMAT questionnaire on Priorities in the Field of Materials Science and Engineering. The information was handled by two task forces: one analyzed the quantitative responses to the priority questionnaire; the other developed brief descriptive summaries of some of the research opportunities that were identified most frequently. The main results of the quantitative analysis appear below, followed by the descriptive summaries.

The methodology used in analyzing the priority questionnaire is described in Appendix A.

To assess priorities in applied research, each respondent was asked to indicate on a scale of 1 (very high) to 5 (very low) the priority that should be given to applied research and engineering in a given materials specialty (out of a list of 46) to assure progress toward a national objective (nine Areas of Impact and 52 Subareas) in

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

which he claimed to be knowledgeable. Other information from the respondent allowed the degree of his familiarity in each instance to be taken into account in the analysis. The results of this study, integrated for each of the nine areas of impact, are summarized in Table 15. (See Appendix A for more information on priorities for applied research in various areas of impact and lists of specific research topics identified as having high priority.)

It is evident that some materials specialties are considered to be a high priority in certain areas of impact, but not in others. A few specialties, on the other hand, appear to have very broad relevance (Table 16). We would emphasize that the overall priority for a specialty cannot be established simply by totaling the stars across Table 15; this would presuppose that all areas of impact have equal levels of materials priority and correspond to sectors of comparable importance to the nation’s well-being. Rather, given the goal of advancing a selected area of impact, Table 15 indicates the relative priorities of the materials specialties in that context.

Respondents were asked also, in connection with each materials specialty, to assign priorities to basic research problems not necessarily identified with any particular area of impact. The results differed somewhat in emphasis from those for applied research, but the two overlapped considerably; problems described under the basic research heading were often the same as those described by others under the applied research heading.

The questionnaire results for priorities in basic research are summarized in Table 17. The various types of ratings designated in the right-hand columns are described in Appendix A. The materials specialties

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

TABLE 15

Priorities for Applied Research in Materials by Area of Impact

(x—indicates above-average priority; xxx—indicates highest priority.)

 

Communications, Computers, Control

Consumer Goods

Defense & Space

Energy

Environmental Quality

Health Services

Housing & Other Construction

Production Equip.

Transportation Equipment

PROPERTIES

 

Atomic Structure (Crystallography and Defects)

xx

 

x

xx

 

x

 

Microstructure (Electron Microscope Level)

xx

x

xx

xx

 

xx

 

x

xx

Microstructure (Optical Microscope Level)

x

x

x

x

 

x

 

x

Thermodynamic (Phase Equilibria; Change of State, etc.)

x

x

x

xx

 

x

 

Thermal (Thermal Cond., Phonons, Diffusion, etc.)

x

x

x

x

 

Mechanical and Acoustic (Strength, Creep, Fatigue, Damping, etc.)

 

x

xxx

xx

 

xx

xx

x

xxx

Optical (Emission, Absorption, Luminescence, Excitation, etc.)

xx

x

 

Electrical (Cond., Electron Trans., Ionic Cond., Thermoelec., Injection, Carrier Phen.)

xxx

x

x

x

 

x

x

x

x

Magnetic (Ferromagnetic, Resonance, Paramagnetic)

x

 

Dielectric (Ferroelectric, Breakdown, Loss, Piezoelectric, etc.)

xx

x

 

Nuclear* (Radiation Damage. Absorption, Surface States, Catalysis, etc.)

x

 

x

 

Chemical & Electrochemical* (Corrosion, Battery Phen., Oxidation, Flammability, etc.)

x

x

x

xxx

xx

xxx

xx

x

xx

Biological (Toxicity, Biodegradibility, etc.)

 

x

 

xx

xxx

x

x

x

MATERIALS

 

Ceramics

xx

 

x

xx

x

x

x

x

x

Glasses and Amorphous Materials

xx

x

x

 

x

x

x

 

Elemental and Compound Semiconductors

xxx

 

x

 

Inorganic, Non-Metallic Elements and Compounds

xx

 

x

x

x

 

Ferrous Metals and Alloys

 

x

x

x

 

x

xx

xx

Non-Ferrous Structural Metals and Alloys

 

xx

x

x

x

x

xx

xx

Non-Ferrous Conducting Metals and Alloys

x

 

x

 

Plastics

x

xxx

xx

x

xx

xxx

xx

 

xx

Fibers and Textiles

 

x

 

x

xx

x

 

x

Rubbers

 

x

 

xx

 

xx

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

 

Communications, Computers, Control

Consumer Goods

Defense & Space

Energy

Environmental Quality

Health Services

Housing & Other Construction

Production Equip.

Transportation Equipment

Composites

 

x

xx

x

x

xx

xx

 

xx

Organic and Organo-Metallic Compounds

x

x

 

x

x

xx

x

x

x

Thin Films

xxx

 

Adhesives, Coatings, Finishes, Seals

x

xx

xx

 

x

xx

x

xxx

Lubricants, Oils, Solvents, Cleansers

 

x

 

xx

xxx

Prosthetic and Medical Materials

 

xxx

 

Plain and Reinforced Concrete

 

x

 

Asphaltic and Bituminous Materials

 

x

 

x

 

Wood and Paper

 

xx

 

x

xx

x

 

PROCESSES

 

Extraction, Purification, Refining

xx

 

x

xx

 

Synthesis and Polymerization

xx

xx

x

x

x

xx

x

 

x

Solidification and Crystal Growth

xx

 

x

x

 

Metal Deformation and Processing

 

x

x

x

 

x

xx

Plastics Extrusion and Molding

x

xxx

x

x

 

xx

xx

x

x

Heat Treatment

 

x

x

 

x

xx

Material Removal (Machining, Electrochemical, Grinding, etc.)

x

x

x

 

xx

Joining (Welding, Soldering, Brazing, Adhesive Bonding, etc.)

x

x

xx

x

 

x

xx

x

xx

Powder Processing

 

x

x

 

x

x

Vapor and Electro-Deposition, Epitaxy

xxx

 

Radiation Treatment (Ion Implantation, Electron Beam, UV, etc.)

xx

x

x

 

x

 

Plating and Coating

xx

x

x

x

 

x

x

 

x

Chemical (Doping, Photoprocessing, Etching, etc.)

xxx

x

x

 

x

 

Testing and Non-Destructive Testing

x

x

xx

xx

x

xx

x

xx

x

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

 

Communications, Computers, Control

Consuner Goods

Defense & Space

Energy

Environmental Quality

Health Services

Housing & Other Construction

Production Equip.

Transportation Equipment

DISCIPLINES

 

Earth Sciences

 

x

 

x

 

Analytical Chemistry

x

x

 

x

x

 

Physical Chemistry

x

x

x

x

x

x

 

Organic and Polymer Chemistry

x

xx

x

 

x

xxx

xx

 

x

Inorganic Chemistry

x

x

 

x

x

 

Solid State Chemistry

xxx

x

x

xx

 

x

x

 

Solid State Physics

xxx

x

x

xx

 

Ceramics and Glass

xx

x

xx

x

x

x

xx

x

x

Polymer Processing

x

xxx

xx

 

x

x

 

x

Extractive Metallurgy

 

x

 

xx

 

Metals and Inorganic Materials Processing

x

 

xx

x

x

x

 

x

x

Physical Metallurgy

x

 

xx

xx

x

 

x

x

Chemical Engineering

x

 

x

 

xx

 

Mechanical Engineering

 

x

xx

x

x

x

x

x

x

Electronic Engineering

xxx

x

xx

x

 

x

 

x

Aerospace Engineering

 

xx

 

Nuclear Engineering

 

x

xx

 

Bioengineering

 

x

 

x

xxx

 

Civil and Environmental Engineering

 

x

 

xxx

 

x

 

*Due to a typographical error in the original questionnaire, Nuclear and Surface Properties were entered as one item. However, respondents generally read it as Nuclear and included Surface Properties under Chemical and Electrochemical.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

TABLE 16

Applied Materials Research Problems of Broad Implication

The combined opinions of a number of materials scientists and engineers suggest upon analysis that high priority be assigned to the generic problems in applied materials research, listed below. These problems are characterized by their broad implications and, for that reason, might well be considered by academic investigators. The problems were selected from among several thousand proposed,

Properties

Chemical: corrosion; stress corrosion; flammability; catalysis.

Biological: biocompatibility; toxicity; allergenicity; biodegradability.

Mechanical: fracture; fatigue; creep; friction; wear; lubrication.

Defects and Microstructure: effects of impurities and crystallographic imperfections on properties.

Electrical: superconductivity.

Materials

Composites: fracture toughness; interfacial phenomena; reliability.

Thin Films: reliability; plating and coating.

High Performance: superalloys; ceramics and glass.

Plastics: property-structure relations; high performance.

Processes

Testing: nondestructive testing; characterization; analysis; interaction with optical, acoustical, and other forms of radiation.

Joining: adhesives; welding.

Polymer Processing: synthesis; extrusion; molding; recycling.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

TABLE 17

Priorities for Basic Research in Materials

(x—indicates above-average priority; xxx—indicates highest priority. Appendix A for explanation of analysis.)

Rank, Allowing for Familiarity

 

 

Chemists

Physicists

Matallurgists

Engineers

 

Uncorrected for Familiarity

Corrected for Familiarity

Experts

Overall Rating

OUT OF 13

 

PROPERTIES

 

6

7

7

1

Atomic Structure (Crystallography and Defects)

xxx

xx

x

xx

4

4

3

3

Microstructure (Electron Microscope Level)

xxx

xx

x

xx

13

13

13

12

Microstructure (Optical Microscope Level)

 

12

8

9

5

Thermodynamic (Phase Equilibria, Change of State, etc.)

xx

x

 

x

10

12

12

8

Thermal (Thermal Conductivity, Phonons, Diffusion, etc.)

x

 

5

9

2

6

Mechanical & Acoustic (Strength, Creep, Fatigue, Damping, etc.)

xxx

xx

xxx

xxx

9

4

6

9

Optical (Emission, Absorption, Luminescence, Excitation, etc.)

x

x

xx

xx

3

3

8

7

Electrical (Conduction, Electron Trans., Ionic Cond., Thermoelectric, Injection, Carrier Phen.)

xx

xx

xx

xx

8

11

10

13

Magnetic (Ferromagnetic Resonance, Paramagnetic, etc.)

 

11

10

11

11

Dielectric (Ferroelectric, Breakdown, Loss, Piezoelectric, etc.)

 

7

6

5

10

Nuclear* (Radiation Damage. Absorption, Surface States, Catalysis)

x

x

xx

xx

2

2

1

2

Chemical & Electrochemical* (Corrosion, Battery Phen., Oxidation, Flammability, etc.)

xxx

xxx

xxx

xxx

1

1

3

4

Biological (Toxicity, Biodegradability, etc.)

x

xxx

xxx

xxx

OUT OF 19

 

MATERIALS

 

3

5

1

5

Ceramics

xxx

xxx

xxx

xxx

6

1

6

4

Glasses and Amorphous

xxx

xxx

xxx

xxx

7

8

7

8

Elemental and Compound Semiconductors

xx

xx

xxx

xx

12

11

13

11

Inorganic, Non-Metallic Elements and Compounds

x

x

xx

x

10

16

18

16

Ferrous Metals and Alloys

xx

 

x

x

5

10

14

13

Non-Ferrous Structural Metals and Alloys

xx

x

x

x

13

12

19

15

Non-Ferrous Conducting Metals and Alloys

x

 

4

7

3

3

Plastics

xx

xxx

xxx

xxx

11

14

11

14

Fibers and Textiles

 

14

15

12

12

Rubbers

 

1

2

2

1

Composites

xxx

xxx

xxx

xxx

16

6

10

9

Organic and Organo-Metallic Compounds

 

x

x

x

9

4

8

6

Thin Films

x

xx

xx

xx

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

Rank, Allowing for Familiarity

 

 

Chemists

Physicists

Metallurgists

Engineers

 

Uncorrected for Familiarity

Corrected for Familiarity

Experts

Overall Rating

8

9

4

2

Adhesives, Coatings, Finishes, Seals

xx

xx

x

xx

15

13

9

10

Lubricants, Oils, Solvents, Cleansers

 

x

 

2

3

5

7

Prosthetic and Medical Materials

x

xxx

xxx

xxx

17

17

15

17

Plain and Reinforced Concrete

 

19

18

17

19

Asphaltic and Bituminous Materials

 

18

19

16

18

Wood and Paper

 

OUT OF 14

 

PROCESSES

 

2

4

5

8

Extraction, Purification, Refining

x

xx

xxx

xx

4

1

3

2

Synthesis and Polymerization

xx

xxx

xx

xx

8

5

9

3

Solidification and Crystal Growth

xxx

x

xx

xx

6

11

12

12

Metal Deformation and Processing

x

 

13

12

7

10

Plastics Extrusion and Molding

 

11

14

14

14

Heat Treatment

 

10

13

13

13

Material Removal (Machining, Electrochemical, Grinding, etc.)

 

5

9

2

5

Joining (Welding, Soldering, Brazing, Adhesive Bonding, etc.)

xx

xx

xxx

xx

3

10

4

7

Powder Processing

x

x

xx

x

9

3

10

4

Vapor and Electro-Deposition, Epitaxy

x

x

xx

x

7

2

8

9

Radiation Treatment (Ion Implantation, Electron Beam, UV, etc,)

 

x

xxx

x

12

8

6

11

Plating and Coating

x

 

14

6

11

6

Chemical (Doping, Photo-Processing, Etching, etc.)

 

xx

 

1

7

1

1

Testing and Non-Destructive Testing

xxx

xxx

xxx

xxx

*Due to a typographical error in the original questionnaire, Nuclear and Surface Properties were entered as one item. However, respondees generally read it as Nuclear and included Surface Properties under Chemical and Electrochemical.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

given highest priority (three stars in Overall Rating) for basic research are:

  • Properties: biological; chemical, particularly surfaces; mechanical.

  • Materials: ceramics; composites; glass and amorphous; plastics; prosthetic.

  • Processes: testing and nondestructive testing.

As with applied research, lists of basic research topics in the various specialties appear in Appendix A.

Information from the priorities questionnaire also allowed comparisons to be made among respondents grouped according to their fields of highest degree. The left side of Table 17 shows the rankings arrived at in this way by four groups—chemists, physicists, metallurgists (including ceramists), and engineers—taking into account average familiarity within each group for each specialty. The rankings display both good correspondence and intriguing differences. It appears, for example, that those who would be expected to know most about a given specialty sometimes rate it lower than do materials professionals in the other disciplinary groups. Thus metallurgists rate the priority of basic research on ferrous metals lower than do any of the other disciplinary groups; physicists, who have much to contribute to nondestructive-testing methods and instrumentation, rate it seventh among processes, while the other three groups rate it first.

A possible interpretation of the rank orderings on the left side of Table 17 is that they are arranged roughly in accordance with the degree of opportunity as perceived by the four groups of professionals. That is, the highest-ranked items are those of greatest scope and need for generating new knowledge.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

These rankings, however, clearly must be interpreted with care. In particular, they should not be taken necessarily to indicate relative increments of needed research support; rather they might be taken to suggest the relative sizes of programs within overall materials research budgets. Nor do the rank orderings show the existing importance of various specialties to the related applied research and engineering. Ferrous metals and alloys, for example, are essential to the economy, but evidently the respondents (even the metallurgical group) felt that basic research in this field might be expected to yield diminishing returns today, perhaps because of extensive research in the past. Materials like concrete, asphalt, and wood, in contrast, have not been subjected to comparable basic research, so that the corresponding fundamental understanding may not yet be advanced to the point where research opportunities are recognizable, even by experts in the field. Yet, in view of the enormous role of the latter materials in the nation’s economy and way of life, a modest investment in research could ultimately yield a relatively large return compared with that from many other research areas.

Selected Priority Problems in Materials Research Based on Questionnaire Responses

Corrosion. Although much progress has been made in understanding the thermodynamics and kinetics of the corrosion process, the mechanisms of localized corrosion are not well understood, nor are those for imparting resistance or protection against aqueous or gaseous corrosion.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

For localized corrosion like 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. Corrosion can be initiated also at surface inhomogeneities, but the types have not been characterized clearly.

The propagation of stress-corrosion, hydrogen-embrittlement, and corrosion-fatigue cracks demands further investigation. As the use of high-strength materials increases, these problems become more important. Susceptibility to hydrogen embrittlement, for example, increases with the strength of the steel. The mechanism of stress corrosion probably differs in detail from system to system. Problems pertinent to many systems include the role of mechanical fracture; the effect of stress on the rate of anodic dissolution; continuous versus discontinuous cracking; the relevancy of continuum mechanics as opposed to atomistic analyses of crack propagation; the effect of defect structure and of chemical composition and distribution at the macro and micro levels in the metal; and the role of hydrogen generated at the crack tip.

High-technology industries often must cope with unexplored conditions. Thus, research is required for corrosion in aqueous media at high temperature, high pressure, or both; 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, whose properties may differ radically from those of the bulk material.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

The corrosion of alloys in gaseous environments can cause surface roughening, most likely because of preferential attack on the less noble constituent, and can result in poor surface finish and poor adhesion of films. The theory of surface instability and the mechanisms of surface roughening require further attention.

Small changes in chemical composition can radically change the corrosion resistance of an alloy because of subtle alterations in the characteristics of surface films. Research must determine the effects of alloy composition and structure on surface films and relate these effects to the problems of internal and external oxidation, the adhesion and the spalling of corrosion films, their resistance to breakdown, and the mechanism of self-healing. More specifically, the following must be examined: the crystallography of surface films and the factors that determine crystal size and transitions between the crystalline and amorphous state; the defect structures; the conductivity of, and diffusivities within, the films and their effects on film-growth kinetics; the mechanical properties of corrosion films; and the thermodynamics and kinetics of the transformation from one corrosion product to another during the high-temperature gaseous corrosion of complex alloys. Such studies could lead to new alloys with better corrosion properties, or to less costly compositions.

Protective coatings fall into two classes: inhibitors, which are of monomolecular dimensions and reduce the anodic and cathodic reaction rate; and thicker films, which provide a physical barrier. The interaction of inorganic inhibitors like chromates with a metal surface requires elucidation. Are they adsorbed? Are electrons

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

transferred (i.e., is the metal oxidized)? Are all surface sites affected equally? The application of metallic coatings can result in the formation of intermetallic phases at the interface between the metals. Their role in adhesion and in corrosion protection is not sufficiently clear. The possibility of using metals like chromium or aluminum, which form corrosion-resistant oxides, as coatings on the refractory metals for service at high temperature (above 1,200ºC) should be studied more systematically, along with the resulting chemical and metallurgical problems. For organic films, basic research is needed on the mechanism of adhesion.

Flammability of Polymers. Flammability, an especially fast form of surface chemical reaction, is particularly important in the use of polymers. The controlling variables of burning must be determined more quantitatively. The oxygen index test, for example, rates the ease of burning of individual materials quite well; counterparts must be developed for entire materials systems or products in terms of end-use environments.

The high-temperature, free-radical reactions of polymer combustion encompass oxidation and pyrolysis in both the flame and the degrading polymer. These reactions can be described at present only in qualitative terms. Learning how the important physical and chemical processes may be slowed or altered by adding various fire retardants is a challenge to research very similar to that posed by catalysis.

The success of empirical efforts to devise better fire retardants for flammable materials has apparently peaked. Recent progress has mainly involved optimizing the forms and amounts of antimony, halogen,

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

and/or phosphorus in particular materials or products. Such treatments, however, are now known to increase greatly the formation of smoke and toxic gases. Inherently nonflammable polymers like the polyimides are available, but they are not economical except in limited, small-volume applications.

The greatest improvements in fire protection will come probably from careful design and engineering to give system-wide, rather than individual material, protection. The age-old, reliable sprinkler system is a simple instance. An excellent example of innovation in this area is the recent experiment in which intumescent insulation alone protected the inside of an entire aircraft fuselage for more than 10 minutes in an inferno of burning fuel.

Biomaterials. The development of materials and devices for use in medicine and surgery is an exciting growth area for materials research. At this interface between the animate and inanimate worlds, many questions must be answered at a most basic level.

Among typical research topics in the field is the surface architecture of biomaterials, including surface energy and changes that can result from contact with body fluids. Examples of such changes would be the development of monolayers of lipids or proteins on the material in question.

Further research is in order on the mechanisms of the degradation of polymers by water, lipids, proteins, and enzymes. Needed, too, are studies on the corrosion of metals and the degradation of ceramics, frequently by hydrolysis. Also important is the converse area, the passivation of metals to make them less susceptible to corrosion and

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

the development of coatings to protect them. Corrosion protection for ceramics and polymers demands further work as well.

Another target for research in biomaterials is the mechanism of bonding by adhesives between metals or polymers and, for example, hydroxy apatite and collagen. The topic is important in both dental and orthopedic applications. In dentistry particularly there is pressing need for an adhesive to seal the margins between the tooth enamel and a restorative material. Such problems in bonding have much in common with a general challenge in materials science and engineering —the striving for better understanding of phenomena at the interface between two constituents and of the possible degradation at the interface.

Glass ceramics based on the calcium phosphate glasses are potentially useful in areas that include degradable orthopedic devices. Only a small research effort is under way on these materials, however, and more seems warranted.

Substantial development has been carried out on pyrolytic carbon for heart valves, but more work is needed on graphite and carbons, which are compatible with the human body. Major consideration should be given to fabrication.

New techniques, such as freeze drying, should be studied for fabricating materials with potential as implants. Particularly intriguing also is the replamineform process, which replicates life forms or structures in the appropriate material. Much remains to be done in the development of membranes suitable for diffusion of gases. These are needed for devices that measure oxygen, carbon dioxide, and other

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

gases, as well as for long-term artificial-lung devices. Measurement of gases is important for in vivo physiological studies on the body. Another significant field is membranes for kidney dialysis. The big problem here is cost, since membranes currently exist that maintain life. As the emphasis shifts toward full rehabilitation of patients, however, there will be a need for a totally implantable artificial kidney of superior diffusion and surface properties as well as acceptable cost.

Reversible physico- or chemi-adsorption requires further investigation. The topic may become central in the area of drug release or its reverse, adsorption of toxic materials. Both polymeric and ceramic or glass systems have potential for such uses. One objective is to be able to “dial in” the release rate, so that the depot material releases the drug into the blood-stream at a preset rate.

Another research area is the application of analytical techniques in monitoring variations in tissue components and fluids. Such an instance would be changes in the conformation of polymers or proteins in the body, such as hyaluronic acid, a polysaccharide which is important in arthritis and is part of the synovial fluid in joints; another would be changes in the proteins collagen and elastin in blood-vessel walls, which is important in atherosclerosis. One would also like to know how conformational and other changes may alter the calcium binding.

The effects of stress on the development of biopotentials in natural tissue is still an open field. Stress is known, for example, to cause changes in the rate of dissolution or deposition of hard tissue.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Blood clotting at implant sites remains an enormous question. A major part of the problem concerns the surface-chemical architecture of the implant material and its effect on protein adsorption. To date, several materials, such as pyrolytic carbon, block copolyetherurethanes, and heparin-treated polymers, have shown encouraging results as anticlotting materials. Each has limitations in use, however. Also demanding attention is the fluid mechanics of blood flow and surface adhesion together with the effect of the implant material in terms of fibrous ingrowth and calcification.

Fracture Mechanisms, Defects. Fundamental understanding of fracture mechanisms is important to the design of safer, more reliable engineering structures. Relatively sophisticated theories of fracture, which take into account polycrystallinity or microstructure, have led lately to better testing procedures and tougher materials. Such improvements in knowledge can conserve materials by minimizing the need for over-design, but much remains to be done in this connection. A relatively new aspect of materials science and engineering in this context is the application of fracture mechanics to rocks and geological structures. Involved here are structure/property relationships on a vast scale, having many implications for tunnel excavating, underground blasting, and seismic damage. Ultimately we have to know more about the actual breaking strengths of atomic or chemical bonds and the role of lattice vibrations in a distorted or defect-riddled structure. Particularly pertinent to the severe technological problem of stress-corrosion cracking is a deeper knowledge of the chemical reactivity of strained interatomic bonds resulting from strained lattice structures.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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The first direct observation of dislocations (line defects), in the mid-1950’s by transmission electron microscopy, gave great impetus to experimental and theoretical study of dislocations in both single crystals and polycrystalline materials of commercial interest. This work greatly facilitated the analysis of plastic deformation in terms of dislocation motion and interactions. Modern research and development of structural materials is now aided by the concepts and methods of dislocation theory. A current thrust in research on dislocations includes the combination of dislocation theory with continuum mechanics to give a continuum theory of dislocations. Work is needed particularly on the transition from dislocation behavior to continuum behavior under dynamic conditions. Another active area is the use of computers to average individual dislocation reactions into macroscopic plastic deformation.

Dislocation theory is now being applied through analytical models relating the dynamic behavior of dislocations and point defects to mechanical behavior, including such practical applications as creep and hot pressing of crystalline solids. This work is done by computer “mapping,” in which 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 on mechanical behavior are considerable at high temperature, where redistributions of point defects and dislocations may occur. Effects on high-temperature strength are related to vacancy transport (diffusion) and the formation of stable dislocation substructures and networks. Particularly interesting and important is the combined action of high temperature and neutron flux,

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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like that prevailing in a fast breeder nuclear reactor. Here, the dynamics of vacancy agglomeration into voids poses a stiff challenge to research, as does the broader area of gas (hydrogen and helium) generation and agglomeration. The attendant swelling limits severely the practical lifetime of nuclear-fuel elements.

Superconductivity. One of the most tantalizing challenges to materials scientists is to find practical superconductors with higher transition temperatures than those now known. High-temperature superconductors have great potential value in electric power generation and transmission, novel forms of high-speed ground transportation, magnetic ore separation, and many other applications. Hundreds of elemental and compound superconductors have been discovered or synthesized, but the highest transition temperature achieved yet is only 23.2 K (–250ºC) in a compound of niobium-germanium. Means of fabricating the compound into useful shapes without depressing the transition temperature have yet to be devised. Indeed, difficulty in large-scale processing is a major obstacle to widespread use of other superconductors having relatively high transition temperatures, such as niobium-tin. Stoichiometry and atomic order are critical material parameters, so that basic research in phase equilibria and kinetics of phase transformations is a necessary prelude to new or improved processing techniques.

On the theoretical side, development of the Bardeen-Cooper-Schrieffer theory in 1957 did much to explain the mechanism of superconductivity and to rationalize various experimental observations. Questions remain, however, on the fundamental limitations imposed on

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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the transition temperature by the lattice and electronic structures of real solids. Much of the search for higher-temperature superconductors has been devoted to finding the appropriate electronic structure, but it has become more apparent lately that the dynamic properties of the lattice (phonons) are at least as important. In particular, lattices that undergo structural transformations accompanied by or triggered by soft modes of the lattice vibrations exhibit some tendency to be high-temperature superconductors. To pursue these clues rigorously and quantitatively is an urgent challenge to materials science. Other recent research results, meanwhile, have stirred excitement in the possibilities of discovering new superconductors that might be lurking in organic materials.

Composites, Concrete. A composite material generally combines two or more mutually discrete macroconstituents—glass, plastic, ceramic, or metal—differing in composition or form. Fiber-resin composites usually consist of glass fibers in an epoxy or polyester resin. Many metal-matrix composites have been studied, including steel or boron fibers in aluminum, aluminum oxide fibers in iron, and tungsten fibers in copper or stainless steel. Perhaps the most prevalent composite is concrete, the single most-used man-made construction material.

Fiber-resin composites can be tailored to many needs and are used widely in such applications as filament-wound tanks, automobile bodies, boat hulls, and translucent glazing. Glass fiber is the most frequently used strengthening agent in these materials because of its low cost. But where performance requirements outweigh cost, fibers like graphite and boron, as well as special glass fibers, are finding

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

increased usage, as in space vehicles and aircraft. Fiber-resin composites, because of their temperature limitations, cannot replace metals entirely. It is clear, however, that they can do so to a significant extent, and this may be important as metal resources dwindle.

Research and development is required on fiber-resin composites in four primary areas. One need is improved production technology for fibers and resins to reduce overall cost. This is particularly true for advanced fibers and the higher-temperature resins. The second area is the development of more advanced production-processing methods. Many of the fabrication methods used today for fiber-resin composites are relatively expensive and time-consuming. Further work is required also on new fibers and resins to improve overall properties at reasonable cost. The fourth primary need is advanced design methods and analytical techniques to help the designer exploit the great flexibility of fiber-resin composites more fully. Structural analysis of these complex materials is still in its infancy. Other advances to be sought include better joining methods, higher resistance to erosion by rain and dust, greater toughness, and the development of structural-property data banks. In some uses, problems with flammability may call for work on resins with flame-retardant properties and nontoxic combustion products.

Metal-matrix composites are still in the development stage. Their primary advantages over conventional materials are their high strength and stiffness relative to weight. These materials, in addition, have a good service-temperature range. A number of manufacturers have made and tested metal-matrix composites, often as prototypes.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Prospective applications include aerospace engines, fan and compressor blades in advanced gas turbines, thermal protection surfaces for space vehicles, and a variety of commercial functions.

Concentrated effort is needed to improve the fibers available for metal-matrix composites and to lower their cost. Fibers now under development, including glass, have potential for easy, low-cost manufacturing. Glass-coated aluminum oxide fiber, for example, could have four times the specific modulus of conventional glass fibers and be made by a relatively inexpensive process. The promising high-performance fibers should be combined with metal-matrix materials and studied for mechanical, thermal, and chemical (or metallurgical) compatibility. Equipment should be designed especially for producing fiber-metal matrix composites. And parts or prototypes of components will have to be fabricated and tested under service or simulated conditions to build confidence in these composite materials.

Basic research on concrete is a necessity to help fill the growing demands of construction. The material is a rather complex composite, and the introduction of any single constituent will affect the properties of the others. Thus the problem lies not only in working out the properties of each constituent—cements, aggregates (sand, stone), reinforcement—but also in clarifying the inter-relationships among them.

Research on physicochemical properties as a function of composition will aid the 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

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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with improved resistance to weathering and physical, chemical, and thermal attack. Challenging problems include studies of the properties of admixtures and their chemical reactions with each other and with reinforcing materials.

The replacement of stone with light-weight aggregates will improve the strength-to-weight ratio of concrete, allowing it to be applied more effectively in tall buildings and long spans. These light-weight aggregates include waste products like fly ash as well as inexpensive natural aggregates indigenous to local construction sites. The physicochemical properties of these materials in relation to the other constituents of concrete require thorough investigation.

Reinforcing materials such as steel rods and fibers increase the tensile strength and toughness of concrete. Studies of adhesion and the reactivity of steel with cements and admixtures of controlled composition are essential to learn to retard the degradation of properties with time. Alternatives to steel, such as organic-coated glass fibers, may broaden the utility of reinforced concrete.

Superalloys. Alloys based on nickel, cobalt, or iron and intended for service above 500ºC are frequently termed superalloys. More than 50 superalloy compositions are commercially available in this country, but the nickel-base system is the most widely adopted. Such superalloys account for some 50 percent of the weight of aircraft gas-turbine engines, where they are used for turbine and compressor disks, turbine vanes and blades, and other hot components. It appears that, to an appreciable extent at least, the ability of engine designers to

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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achieve their plans for advanced engines will depend on progress in superalloy metallurgy.

Alloys developed recently in the United States and Britain have maximum practical use limits varying from 1,000º to 1,050ºC. Further improvement in superalloys seems likely to come from two directions: overcoming the temperature limitations based on environmental attack, and increasing the strength by advances in processing methods. There is also an urgent need to improve the correlation between laboratory tests and service conditions so that the service life of components can be predicted more accurately.

The next generation of superalloys will operate at temperatures too high for traditional chromium-oxide protective scales. One potential solution is to develop a family of superalloys protected by aluminum-oxide films. The latter tend to spall during thermal cycles, but this may be inhibited by introducing dispersed oxides into the alloy. It has been found only recently that dispersed oxides are quite beneficial in combating high-temperature corrosion in addition to their favorable effects on high-temperature creep. Major questions to be answered in the development of protective coatings include the influence of alloying additions on the diffusivity of the constituents; on the thermodynamics and kinetics of formation of competing oxide films; on competition between internal and external oxidation; and on vacancy behavior and its possible role in spalling.

Processing offers many possibilities for enhancing the properties and performance of superalloys. Promising approaches include the use of directionally solidified eutectic alloys, electroslag remelting,

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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composite structures joined by diffusion bonding, and improved powder-metallurgy processing. Mechanical alloying in attritor mills can effectively disperse oxide phases. Control of recrystallized structures to yield interlocked elongated grains aligned in the loading direction can be achieved by adjustment of dispersed phases or through zone-controlled recrystallization. The high-temperature benefits resulting from elongated, interlocked grains in nonsag tungsten can likely be extended more broadly to superalloys and other high-temperature materials.

Ceramics, Glass. A major aim of ceramics technology is to improve the physical and mechanical properties of polycrystalline ceramics, which compete with metals and glasses as engineering materials. The general approach is to develop new compositions and processing techniques that permit superior properties to be achieved through close control of composition, density, and grain structure. Thus, increasing emphasis will be placed on the relationships among composition, microstructure, and material properties.

The traditional uses of ceramics rely heavily on the materials’ durability and resistance to thermal and chemical attack. There is much opportunity for improvement here, in new facing materials and glazing systems for buildings, in prefabrication materials such as blocks and foams, in brick and pipe with improved weatherability and resistance to frost. The upgrading of the thermal-shock characteristics of ceramics for furnaces and thermal reactors depends on deeper insight into the chemistry of oxide formation and the variables of composition and particle size and shape. Such ceramics are used increasingly in applications such as incinerators and kilns.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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For new compositions, basic study in solid-state physics and chemistry should clarify electrical, optical, magnetic and mechanical phenomena that may be peculiar to ceramics. In the recently discovered lead-lanthanum zirconate titanate ceramic, for example, only a small addition of lanthanum improves optical transparency and electro-optic memory characteristics. This suggests that effects of impurities on physical properties of ceramics should be examined more systematically.

High-temperature structural ceramics are receiving new emphasis. One example is the current work on silicon carbide and silicon nitride for service in gas turbines, where they would permit higher operating temperature and thus greater efficiency. Ceramics are not tough enough for this application at present. One approach is to learn to design around the shortcoming through deeper understanding of fracture mechanics. A second direction is to find means of improving the toughness of materials like silicon carbide, which should be a superior high-temperature material, but which cannot yet be fabricated economically with sufficiently close control of its structure.

Recent studies have indicated that improved high-temperature properties may be possible in complex systems such as solid solutions of nitrides and oxides of silicon or aluminum; here, high density can be achieved by sintering at relatively low temperatures. Accordingly, ceramics based on silicon-aluminum-oxygen-nitrogen and related systems are promising candidates for structural materials.

Composite materials containing ceramic fibers represent a way to combine a high-strength fiber with a ductile matrix, but the interface-bonding problem inhibits full realization of their potential.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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The limited ductility of ceramics also restricts their use as lightweight armor. The wear resistance of ceramics makes them good cutting tools but, again, their limited fracture-toughness restricts their applicability.

Ceramics with good thermal-shock resistance have been developed for stove tops and have many other potential uses. Lucalox, the transparent lamp-envelope ceramic, was developed through an understanding and control of sintering behavior; it points the way to other transparent ceramics. Important here is knowledge of basic processes: mixing of ceramics; preparing and processing highly reactive starting powders (such as aluminum oxide); control of sintering behavior; and stabilization of properties at high temperatures.

Glass formation is basically a kinetic phenomenon, and much remains to be learned of the associated dynamic problems, including diffusion, conductivity, and polarization. The separation of homogeneous glass into two amorphous phases, or into amorphous and crystalline phases, may be either troublesome or useful, depending on the application. Neither the thermodynamics nor the kinetics of these processes are sufficiently well in hand to allow the occurrence or absence of phase separations to be predicted in the more complex glasses.

A common limitation in the utilization of bulk glass is its brittleness. Investigations of brittle fracture, ultimate strength, notch sensitivity, and static fatigue have led to more efficient use of the intrinsic strength of glass products. The fundamental limits on the mechanical properties of the material, however, remain unknown. The structure of glass is difficult to determine and even to define.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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The application of a combination of modern tools like nuclear magnetic resonance, Raman spectroscopy, and electron microscopy should produce further insights in this area.

A new opportunity in glass technology is the development of optical wave-guides for long-distance communications. A commonly envisioned configuration is a high-refractive-index optical fiber with a low-refractive-index cladding. It has been realized recently that, in some inorganic glasses, loss of light in the red and near-infrared spectral regions should be very small. This discovery has produced intense activity in preparing glass fibers of precise dimensions that are extremely pure and free of light-scattering and light-absorbing defects. The work has underscored the need for improved ultrapurification processes for glasses and chemical compounds.

Polymers. A variety of useful plastics, elastomers, and other polymers have become commercial products in recent decades. Launching a radically new polymer, however, is expensive. Semiempirical routes to practical materials are likely to be followed most often, and one of the most attractive is to explore the effects of blending polymeric materials already available. The properties of blends depend on many factors, the most important, perhaps, being the intimacy of the mixing. The degree of dispersion can vary widely. Indeed, some polymer pairs cannot be blended properly at all. Where this is so, various modifications may improve compatibility. One example is acrylonitrile-butadiene-styrene, in which the rubber particles added to increase impact strength are surface-modified by grafting to provide a good bond across the boundary with the base polymer.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

A more fundamental understanding of the characteristics of blends is needed to obtain materials with tailor-made properties, properties that are too specialized sometimes to warrant bulk production of a new polymer. Such research on polymer blends can be compared in many ways to the history of research and development with metal alloys.

Major achievement and continuing effort mark the field of rubberlike polymers. A recent product of basic studies is the family of ethylene-propylene copolymers. All rubberlike substances lose their viscoelastic properties and become far more rigid when the temperature is low enough. They assume a glassy character and behave as almost perfect elastic solids. A host of polymers are glassy at ordinary temperatures and many of them, like the inorganic glasses, have valuable optical properties. Fundamental work with monomers or combinations of monomers to yield polymers with desired properties, such as a particular optical absorption or refractive index, has been highly productive and can be expected to be so in the future.

Ability to control the structural regularity of polymer chains has been a striking achievement in polymer science. This chemistry of molecular shape (stereochemistry) is making it possible to synthesize highly ordered molecules that cluster into crystalline order. The individual crystalline regions are extremely small but highly organized. They form a superstructure or morphology that gives strength and dimensional stability to the polymer, somewhat like the way in which precipitates can strengthen metal alloys. The morphology is extremely complex, but is governed nevertheless by identifiable factors, such as

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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the rate of the crystallization and the distribution of molecular sizes. Further research in this area will lead undoubtedly to new and serviceable materials.

The relations between molecular structure and physical properties are 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 relationships are becoming well understood at the molecular level. Continued research in this area is likely to yield both nonbiological and biological uses for plastics.

The durability of polymers may become one of the most active areas for polymer research in the immediate future. Various stabilizers are added to protect commercial polymers against ultra-violet light, heat, and other kinds of degradation. Current studies on polymer durability center on stabilizer interactions, retention, and lifetimes under various conditions. Minor structural modifications have been found recently to improve stability markedly in polyvinyl chloride and polyoxymethylene without significant changes in physical properties. Further increases in stability probably can be expected from additional changes in the molecular structure of polymers.

Processing: Metals. Two main goals of innovation in metal processing are to improve mechanical and physical properties and to make finished parts more economically. Most new processing techniques are developed for specific materials and applications, but basic research in this connection should spawn new approaches.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Melt spinning, a relatively new means 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. Pertinent research problems include the hydrodynamics of the liquid jet and the chemistry of surface films developed to stabilize the jet. Another new technique, rheocasting, involves casting metal that is partially solidified; high fluidity is maintained by vigorous mechanical stirring. The lower pouring temperature reduces mold erosion, centerline shrinkage, and freezing time. If the stirring is stopped momentarily, the slurry stiffens and can be handled like a solid for die casting (thixocasting). Research problems here include fluid flow and rheology of partly solidified alloys and the microstructure and properties obtained in this type of casting.

The properties of practically all commercial alloys, as well as those of new alloys, can be improved by controlling the thermal and mechanical cycles of processing. Progress in thermo-mechanical processing will come from restudy of the complex interaction of deformation, recrystallization, texture development, and solid-state reactions in the important commercial alloys. In addition, the principles of property improvement by thermomechanical processing are rather well understood in many cases, but economical forming methods or systems have not been developed to reduce the principles to practice. Instead, the processes have been adapted to existing facilities, usually with little success. The stiffness of steel, for example, could be increased perhaps one third if the appropriate texture could be produced in polycrystalline iron under production conditions.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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One of the greastest opportunities for materials development lies in the use of powder metallurgy and powder consolidation to shape materials and components that cannot be formed by conventional techniques. The possibilities here need to be clarified and reduced to practice. The results should lead to more economical utilization of materials.

In joining methods like diffusion bonding, which is used to fabricate metal-matrix composites, progress requires further knowledge of adhesion as influenced by solid-state reactions under conditions that include heat, pressure, and surface films. For the newer welding techniques—plasma arcs, electron and laser beams—structural changes and the resulting properties in the region of the weld require study.

Work is needed also on the changes in structure and composition produced by finishing operations such as electric discharge machining, electrochemical machining, and laser machining. New approaches to coating—flame spraying, for instance—would benefit from research on the resulting microstructures and properties. Particularly important is the development of alloys with a built-in ability to generate protective coatings during service. One example is the incorporation of aluminum, chromium, and yttrium in nickel alloys to provide oxidation resistance without an external coating.

“Splat cooling,” in which molten alloys are shot onto a cold surface, has created vast possibilities for new materials. The technique quenches the melt so fast that it solidifies with a minimum of atomic diffusion. This leads to the formation of metastable (marginally

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

stable) phases, characterized by a range of unusual properties. The equilibrium form of Nb3Ge, a compound of niobium and germanium, for example, becomes superconducting at 7 K; splat cooling yields a metastable form of the compound whose transition temperature is 17 K. Splat cooling also produces metallic glass alloys that, mechanically, are among the strongest of the nonferrous materials.

Research on metastable states in the past has concentrated on structure, as in demonstrating the amorphous nature of the metallic glasses. Emphasis now is changing to the use of splat cooling to enhance specific properties of materials or to create new properties. Among many targets for study are the mechanical, corrosion, and transport properties of materials like metallic glasses, which have no grain boundaries or similar imperfections. The unusual properties of splat-cooled materials cannot be fully exploited, however, unless the difficult problem of fabricating them into practical forms can be solved.

Processing: Rubber, Plastics. Many methods have been examined to reduce the relatively high cost of processing conventional rubbers. Some rubbers are now sold as powders that can be mixed initially by blending and then fed directly to an extruder or injection-molding press. An attractive possibility is to mix a low-molecular-weight rubber as a liquid, thus avoiding the power-consuming shearing action required with solid rubbers. After mixing, the rubber could be chain-extended and crosslinked to give a product equivalent to that made by present methods. Advances required to achieve this end embrace a range of elastomers of proper reactivity for the chain extension and crosslinking, as well as the corresponding linking agents.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Recent developments in block copolymers have created elastomers in which the crosslinks are physical in nature rather than chemical and may be formed and broken reversibly by heat. These new rubbers, potentailly, could be processed as plastics. The high creep rates of this class of materials currently exclude them from many applications, but commercial rubbers could result from proper choice of monomers and the development of the necessary polymerization techniques.

Cold forming of both amorphous and crystalline polymers is an active field at present. The method avoids the energy consumption and time required to heat a polymer above its softening or melting point and then recool it to room temperature. Injection molding of thermosetting resins also looks promising. Cycle times for heavy-section moldings are now faster than for injection molding.

A further need in polymer processing is for more efficient recycling. (Biodegradation techniques for plastics are poorly developed thus far.) Reclamation of rubbers by mastication and of the monomer from polymethyl methacrylate by pyrolysis are well established processes, and a method has been developed for recovering polyethylene terephthalate (polyester) from textile mill tailings and photographic film. Means must be found of using polymer scrap as a raw material for new processes. Thus, chemicals can be obtained from automobile tires by destructive distillation, and carbon black by controlled combustion. Likewise inviting attention are recycling processes for polymers that emit unpleasant or poisonous fumes when burned. Polyvinyl chloride is perhaps the most important of these because so much of it is made. When burned, the plastic yields hydrochloric acid, which must be removed

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

from the combustion gas before it is discharged. The problem is to find a process that avoids the need for burning.

Most polymer-recycling processes require a substantial supply of clean scrap, and factory scrap is the first choice. Really efficient operation will depend on adequate separation of plastics from rubbish and garbage along with economical transportation to recycling plants.

Testing, Characterization, Evaluation. 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.”

*  

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

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

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 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.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

Analysis of ultrapure materials is challenging analysts seriously. 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 valuable current techniques, or techniques 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).

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Routine use of these methods in nondestructive testing, however, requires more understanding of the physics of the phenomenon 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.

Materials Research on Fundamental Properties

Historically, most new materials or properties have been worked out or discovered by empirical methods. Rarely indeed is a new material or property predicted from basic principles. An outstanding exception lies in single-crystal materials, especially those used for solid-state electronics. Scientists have achieved a degree of understanding, of the simpler crystals at least, that often allows them to prescribe in advance the compositions that will have the generally desired properties.

This progress has come largely because the single-crystal state of matter lends itself to theoretical analysis, particularly of electronic properties. Only recently have basic scientists begun to turn to the more complex forms of matter, the glassy, polycrystalline, and polymeric states found in most practical materials, particularly those employed structurally.

The urgency of fundamental knowledge will vary in different parts of materials science, and priorities will have to be set. Nevertheless, it would be unwise to conclude that even the most esoteric work will not prove useful in the future. The engineer often wishes

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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to produce practical results in a relatively short time, say one to five years. But usually he will not know beforehand what areas of materials science he will have to draw on. If the knowledge is to be ready when the engineer wants it, scientists may have to be working five to 20 years ahead.

To illustrate, in the early 1950’s, efforts to calculate the electron-band structures or energy distributions in crystalline semiconductors would have seemed remote from the everyday task of trying to make practical diodes and transistors. But such calculations, improved by the relatively large computers that were appearing then, have led to strikingly detailed insights into the electronic and optical properties of semiconductors. The calculations were steadily refined, particularly for semiconductors, and extended to other crystalline materials. Today, phenomena like the bulk negative-resistance effect in gallium arsenide (the Gunn effect), laser action in gallium arsenide, infrared photodetection in semiconductors, and light emission from junctions in gallium phosphide are understandable in terms of the detailed band structures of these materials. And the esoteric results of research of nearly two decades ago in theoretical solid-state physics are nowadays the starting point from which the electronic engineer can embark on specific short-term development projects. The status of similarly fundamental research, described selectively below, will be of special interest to scientists and engineers working in the field of materials.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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Selected Research Frontiers

Interatomic Forces, Chemical Bonding, Lattice Stability. There is no more basic property of a material than its very existence. Yet, despite enormous advances in solid-state theory, we are unable to predict from first principles and the appropriate atomic wave functions the configuration and dimensions of any crystal lattice except for a few very simple materials. Band-structure calculations have reached a point where the electronic properties of many crystals can be calculated with remarkable precision, given the crystal structure and atom spacings. But the fundamental challenge remains—to relate the properties of individual atoms to those of a crystalline solid composed of such atoms, particularly the imperfect solid. 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, chemical, and mechanical properties. But stating the problem this way makes us realize just how primitive is our quantitative knowledge of such basic matters as interatomic forces, chemical bonding, and configurational interactions.

Besides the need for theoretical progress, we shall continue to depend on 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 required to provide parameter inputs for these calculations, such as measurements of intermolecular potentials and charge distributions, and computer simulations of molecular dynamics. Meanwhile, to fill the immediate needs of the

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

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 modeling 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 basic properties of various materials can be expected to emerge steadily in the coming years.

Microscopic Understanding of Phase Transitions. Although the equilibrium crystal structure can be calculated for a few simple materials, we still lack the fundamental knowledge to predict from first principles the changes in crystal structure that occur with variations in temperature, pressure, or composition. And melting, perhaps the most dramatic phase transition of all, is still largely a mystery from a basic point of view. If we understood melting properly, we would have much more insight into the roles of interatomic forces, cooperative interactions, and related phenomena in determining the structure and stability of solids. The microscopic mechanisms that bring about phase transitions are an object of intense research at present, and part of the deep atraction of this study lies in the remarkable universality of certain general phenomena in the vicinity of higher-order phase transitions, regardless of the type of material or even of the type of phase transition—the liquid-gas (at the critical point), ferromagnetic, ferroelectric, local order, and superconducting phase transitions are very similar in certain rather profound ways. In each case the thermally

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

driven fluctuations in a particular variable become correlated over increasingly longer ranges as the transition is approached, and the time scale of these fluctuations increases markedly. Some consequences of this are the increase 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 has not been achieved. This is a very important challenge for materials research and for solid-state physics in particular.

In martensitic transformations, for example, there is a large body of evidence that special nucleation sites are necessary for initiating the reaction. The nature of these sites (possibly defect arrays) and the mechanisms of interface propagation during transformation have not been clearly determined. It is equally challenging to develop deeper insights into the mechanisms of phase-transitions in interacting systems, such as the coupling of electron spins and phonons near the magnetic transition.

Amorphous, Disordered State. Less is known about the glassy or amorphous state of matter than about crystalline matter. But the possibility that the completely disordered state offers the next most tractable model after the perfectly ordered or crystalline state is attracting widespread theoretical and experimental attention. A fully developed conceptual framework for amorphous materials is lacking, and close collaboration between experimentalists and theorists, as recommended

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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recently by a panel of the National Materials Advisory Board*, is a prerequisite for major progress to occur. On the theoretical side there is need for calculation of electron-energy diagrams, the counterparts of the energy-band diagram of the crystalline state, that will further understanding of the electrical and optical properties of physically realizable glass structures. In addition to continuing debate over the detailed changes occurring in bond structures on passing from the crystalline to the glassy state, there are questions concerning possible electronic phase transitions and high electric-field transport effects in semiconducting glasses. The correct interpretation of optical-absorption and photoconductivity spectra in terms of electron-energy states is by no means clear; nor are the mechanisms of the converse radiative recombination transitions that give rise to luminescence.

The experimental approach calls for better understanding of material-preparation variables and the glassy-to-crystalline transition; the effects of illumination on the kinetics of this transition seem particularly intriguing as well as possibly lending themselves to various optical writing and memory applications. In the same vein, more needs to be known about the photochemistry of glasses, that is, changes in the electronic states of impurities or imperfections as a result of irradiation with light. The characterization of the effects of radiation damage in an already disordered system have to be unraveled. What determines the mechanical strength and other physical

*  

Fundamentals of Amorphous Semiconductors, National Academy of Sciences, Washington, D.C., 1972.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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properties of glasses of various compositions and bond coordinations remains an important unanswered question. Indeed, it appears that we still do not comprehend properly the structural states that glasses may assume—recent work on ultrasonic attenuation at ultralow temperatures may help answer some of these questions.

Impurity Effects in Solids. When impurities are introduced into an otherwise perfect host crystal, all of the properties of the resulting system, in principle, are modified. The nature and extent of these effects depend on the impurity concentration, location, and interaction with the host material. In dilute amounts some 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 in impurity-enhanced optical damage in nonlinear optical materials. Despite the enormous amount of work that has been done on impurity effects in semiconductors, for example, we lack a general theory of the effects of impurities on material properties at the microscopic level. The dilute limit, while theoretically simplest, is experimentally difficult, while the converse is true for high concentrations of impurities.

These problems present several points of attack. 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 cooperative impurity modes (such as phonons, excitons, and magnons) at intermediate

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

concentrations. Theories are needed to explain the emergence of these phenomena from the single-impurity behavior at low concentrations and to distinguish them from the solid-solution behavior at high concentrations. Experiments are in order on systems in which the impurity-host interactions are sufficiently weaker than impurity-impurity interactions to compensate in a controlled way for the numerical abundance of the former. More consideration should be given to systems with simply 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, but not in the host, is particularly intriguing. Progress along these lines has been made already in thin-film studies of magnetic impurities in nonmagnetic, metallic hosts. Similar experiments on optical and electrical properties appear very promising. A lingering puzzle is the role of impurity excitons (electron-hole pairs) in semiconductor laser action. Optical studies have suggested the presence of excitonic molecules and have stimulated speculation on the possibility of creating, within a crystal, a fluid or perhaps even a solid phase composed entirely of electron-hole pairs.

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 school argues that the lattice-friction stress of the pure metal is inherently large at low temperature, while another contends that interstitials introduce lattice distortions, which are especially

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

effective in impeding dislocation motion at low temperatures. Although sophisticated experiments are required here, theoretical calculations based on interatomic forces could help decide the issue.

Surfaces. Surfaces and interfaces are possibly one of the most fruitful research topics in materials science. Knowledge at the most fundamental level in this area can be expected to be relevant to almost all uses of materials, from the processing and performance of integrated circuits to the corrosion of structural components, from frictional wear and energy loss 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 understanding of surfaces is, in contrast, in its infancy.

The aim is to develop more sophisticated insight into the electronic and chemical properties of surfaces. These properties are very sensitive to the detailed ways in which atoms are positioned at the surface, however, and in general these positions are not known. Surface properties are related also to the properties of the underlying bulk material, but in ways that are not often clear. And though bulk properties, by-and-large, are understood 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, and models for nucleation and growth.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

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

The electronic properties of surfaces in simple systems warrant considerable attention. There is some controversy about the extent to which surfaces can be treated as an extension of the bulk—that is, whether the discontinuity in properties at the surface is great enough to require new concepts and analytical procedures. Our theoretical models for surface electronic properties, surface relaxation, and surface structure are rudimentary. The extent to which surface states on semiconductors are intrinsic to 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 been shown recently 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 in detail for a variety of systems, with emphasis on the simpler systems. The kinetic 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. Classical surface nucleation theory is inadequate to

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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account for the results of such measurements, and major modifications of the theory appear to be necessary. Adsorbed atoms can be identified by 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 stage at which the position of surface atoms can be determined quantitatively with some accuracy. These methods are also being used extensively to monitor the cleanliness and structure of surfaces and 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 ultraviolet photoemission spectroscopy are used to obtain band-structure data. Knowledge of electronic and chemical bonding can be derived from ion neutralization spectroscopy. Infrared reflection spectroscopy gives information about chemical bonding, and insights concerning deep electronic levels can be 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 for investigating materials problems associated with thin films, grain boundary segregation, interdiffusion phenomena, and trace analysis. The trend toward miniaturization in electronics, resulting from economic, reliability, and high-frequency considerations, points toward growing importance of surfaces. The concepts of miniaturization are best embodied in the technology of large-scale integrated

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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circuits, where surface and grain-boundary diffusion often dominate bulk diffusion processes. This trend is expected to continue, particularly as optical microcircuitry is developed.

The elucidation 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 of catalysts have not been clarified in detail.

Notable advances have been made in the area of adhesion, where knowledge 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, but 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 metal 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

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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own vapor or in a vacuum, and it presents extra problems because the interface is not readily examined in situ. Some metals, such as silver, undergo surface rearrangement in aqueous solution at room temperature. Alloys generally undergo a change in their equilibrium or steady-state surface composition. The atomistics of these phenomena are poorly defined. There is much ignorance regarding the effects of surface stress, defect structure, and nonequilibrium conditions on the reactivity of metal surfaces, and these effects are of major importance in the performance of materials.

One- and Two-Dimensional Systems. Until recently, calculations of physical phenomena in one- or two-dimensional systems were considered to be mainly of academic interest. Onsager’s famous exact solution for a simple two-dimensional lattice inspired solid-state physicists and engendered hope for eventual similar success in three-dimensional systems. Within the past four or five years, however, a variety of magnetic, superconducting, and resistive materials have been prepared that exhibit exceedingly large anisotropies in their thermodynamic, transport, and collective properties. (See subsequent section on collective behavior.) The anisotropies are so pronounced that microscopic interactions along a line or within a plane may be several orders of magnitude greater than in the transverse directions. Tetragonal crystals of the K2NiF4 family, for example, exhibit inplane magnetic exchange forces several thousand times larger than the out-of-plane exchanges, with the result that below about –170ºC truly two-dimensional long-range magnetic order occurs. Neutron diffraction and optical experiments have confirmed the two-dimensional

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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nature of the electron-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. They have now been found to be essentially two-dimensional superconductors, whose properties can be altered markedly by chemically changing the spacing between layers. Certain organometallic complexes have exhibited one-dimensional manifestations of antiferromagnetism and the metal-insulator transition.

The recent evidence of unusually high electrical conductivity in some crystals made up of organic molecules (abbreviated to TTF/TCNQ) has excited considerable interest in the possibility of high-temperature superconductivity in such materials. Whether the high conductivity in fact is related to superconducting phenomena has yet to be demonstrated. But whatever the origin of the effect, if it is real it is a major breakthrough in the properties of organic materials.

These discoveries have kindled lively theoretical and experimental interest in the physics of less than three dimensions. The consequences of extreme anisotropy of microscopic interactions must be explored more fully. The effects of lower dimensionality on collective modes, e.g., electron and heat transport, must be clarified. Particularly intriguing is the effect of a microscopic upper limit to the interaction distance in certain directions on the critical properties near phase transitions in lower dimensional systems. While some magnetic transitions have been studied in this context, virtually nothing has been done on structural, order-disorder, or

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

ferroelectric transitions in less than three dimensions. Further advance in the physics and chemistry of two-dimensional systems is also essential to the eventual understanding of catalysis. Because of the extreme anisotropy in bonding strength in the layered-structure materials, study of their mechanical behavior could lead to superior lubricants or high-strength components, as demonstrated already in graphite. In the usual powder form, graphite is a widely used lubricant. Precursor polymer filaments can be processed to yield dense, highly oriented graphite fibers that exhibit axial strengths that are a significant fraction of the theoretical strength.

Because in some ways they are fundamentally different from bulk materials, thin films and filaments are of renewed interest to solid-state physicists. The fabrication of structures that extend only a few tens of angstroms in one or two directions has made clear the opportunity for more careful experiments and sophisticated interpretations in the physics of such structures. Two indicative examples are the observation of a nearly fivefold increase in the superconducting transition temperature in thin films of aluminum and the increased sound-attenuation coefficient of small-diameter glass fibers.

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 to the long-chain molecules of which they are composed and which set them apart from a host of other materials. Recognition of the key role of long-chain molecules was one of the singular discoveries of this century. It led to intense research to

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

find how variations in the structure of these giant molecules, through new approaches in chemical synthesis, could be invoked to cause valuable changes in the physical properties of plastics. Yet, to put the structure/ property relationship of polymeric materials on a firm, fundamental, and quantitative basis remains a prime challenge to materials research, even greater in complexity than the parallel challenge posed by amorphous inorganic materials.

In polymeric materials the molecules may be arranged in an orderly chain-folded fashion; in this form plastics bear some correspondence to the familiar inorganic crystalline materials. But more often the molecules are arranged in a haphazard fashion, resembling perhaps a bowl of spaghetti; this is the counterpart of the disordered, glassy state of inorganic matter. And as with inorganic glasses there can be partial devitrification in plastics. In view of the primitive state of theoretical concepts and analytical procedures for dealing with ordinary glasses, it is not surprising that we are a very long way from being able to go the whole distance of determining from first principles the fundamental properties of the polymeric molecules themselves and then the physical properties of the macroscopic plastic materials.

Collective Behavior. One of the most useful concepts in solid-state physics is that of the collective mode, that is, a simple excitation of a system of interacting electrons and/or atoms. This concept has permitted the handling of complicated many-body (1023) 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

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
×

responses to various types of disturbance. The complete set of these responses forms the so-called “normal modes” or “elementary excitations” of the system, which provide the basis for many of its static and dynamic properties. Since a single elementary excitation involves the participation of all the atoms in the system, the concept is quite powerful in elucidating the cooperative behavior among large numbers of particles that results in a particular phenomenon or property. As was indicated briefly in the discussion of phase transitions, the collective-mode concept has been fruitful in describing even anomalous material properties. Although the elementary excitation concept has become very familiar to physicists (the words phonon, plasmon, magnon, etc., are well incorporated into the solid-state vocabulary), it still has great potential for significant growth. Extensions of the concept should prove valuable in at least two directions: (a) nonlinearities and interactions among elementary excitations; and (b) elementary excitations in systems lacking long-range order.

  1. Recent experimental advances have permitted fairly direct and precise study of the more familiar excitations on the one hand, and the generation, detection, and study of some new excitations on the other. In the former category are inelastic scattering (both light and neutron) and acoustic, magneto-optic, and certain solid-state plasma experiments. The latter include super high-frequency phonon and second-sound generation by electron-pair deexcitation in superconductors; the launching of stable-amplitude pulses of both mechanical (e.g., solitons) and electromagnetic (e.g., self-induced transparency) nature; and propagating electroacoustic domains in semiconductors. For the future, better understanding can be expected

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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of the interactions among these excitations, leading to optimized manipulation of such interactions for energy or information transfer.

  1. Less straightforward, perhaps, but certainly no less important is the second direction: studies of elementary excitations in systems lacking long-range order. In amorphous solids and liquids, effort of this kind has been under way for some time. Already, for example, some microscopic understanding of electronic, optical, and acoustical 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 viscoelastic 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 investigation and should illuminate that important intermediate regime between well-developed long-range order (crystal) and the more transient short-range order (liquid).

Another attractive possibility lies in extending the collective-mode concept to large but finite structures, particularly to macro-molecules. 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 as unusual challenges to both theorist and experimentalist in

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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solid-state physics. A true science base for “molecular engineering” rests largely on progress in this direction.

Nonequilibrium Systems. 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 limitations of thermodynamics and statistical mechanics), the increasing importance of nonequilibrium phenomena requires that substantial effort be directed to alleviating these deficiencies. Lasers and negative-resistance semiconductor devices are familiar examples of nonequilibrium physics in action. Recent progress in clarifying the transient and threshold behavior has illuminated analogies with equilibrium higher-order phase transitions. It is intriguing to consider more general instabilities such as hydrodynamic, magnetohydrodynamic, and plasma phenomena from this point of view. The problem of turbulence may be the most challenging and important of these. Autocatalytic chemical reaction systems give rise to large spatial and temporal variations in composition. The familiar convective instability can cause extreme problems in crystal growth from the melt. Indeed, the behavior of the atmosphere, the oceans, and even of the earth’s crust is strongly influenced by such hydrodynamic instabilities.

With new laser techniques, materials under extreme transient conditions (shock waves and high electric, magnetic, or optical fields) can be studied in real time with a resolution of ~10–12 second. Scattering, absorption, and fluorescence experiments, which have proved so valuable in guiding theories of materials at equilibrium, should begin soon to do the same for nonequilibrium systems. A

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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foretaste of what might be in store is the use of these fast laser pulses to study short-lived excited states of radicals and molecules, with consequent insights into the detailed sequence of atomic or molecular events taking place in chemical reactions.

Suggested Citation:"Opportunities in Material Research." National Research Council. 1974. Materials and Man's Needs: Materials Science and Engineering. Washington, DC: The National Academies Press. doi: 10.17226/10435.
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