Research opportunities in new energy-related materials

149

Materials Science and Engineering, 50 (1981) 149 - 198

Review Paper Research Opportunities in New Energy-related Materials JOHN L. WARREN

Los Alamos National Laboratory, NM 87545 (U.S.A.) T H E O D O R E H. GEBALLE

Center for Materials Research, Stanford University, CA (U.S.A.) (Received April 2, 1980; in revised form March 28, 1981)

SUMMARY

This review is a s u m m a r y o f a study carried o u t for the Council on Materials Science, which was established by the Division o f Materials Sciences, U.S. Department o f Energy, to provide in-depth information needed to formulate research policy. It covers research on new materials including polymers, intermetallic compounds, amorphous solids, thin films, solid state ionics, catalysts and semiconductors. For each o f these materials an evaluation o f the state o f the art is given followed by a summary o f the research opportunities and needs and a discussion o f technological opportunities. The study concluded (1) that a large n u m b e r o f new materials can be prepared w i t h o u t a massive effort by imaginative application o f established techniques, (2) that a close coupling o f synthesis, characterization and measurement o f properties is highly desirable, (3) that amorphous and thin film materials offer a wide range o f systems that can increase basic understanding and can offer solutions to technical problems and (4) that the major share o f innovative inorganic synthesis has been undertaken recently in Europe with the U.S.A. playing a minor role.

1. I N T R O D U C T I O N

This review is the product o f a diverse panel of physicists, chemists and materials scientists. The Panel was organized at the request of the Council of Materials Sciences, Divisionof Materials Sciences, U.S. Department o f Energy, to generate a report [ 1 ] on the needs and opportunities in basic research on new materials with the intention to provide advice which could be used in planning future research pro0025-5416/81]0000-0000/$02.50

grams. Further, it was made clear that there should be a non-tenuous link between the basic research opportunities and the energyrelated problems facing technology. The Panel was given the license to take an uninhibited look at the situation, but to bear in mind the mission of the U.S. Department o f Energy. Any report by a committee put together in one working session has to present the consensus which prevails at that m o m e n t in time by those present. The report should be taken in the perspective in which it was prepared: a "best e f f o r t " of those present at Pajaro Dunes, CA, on August 19 - 24, 1979. A list of those present is given in Table 1. The topics to be included in the report were agreed on by the members of the Panel and a coordinator was chosen for each topic as indicated in Table 1. The list of topics is given in Table 2. At least one topic, ceramics, was omitted because of the comprehensive report on this subject submitted by the Bowen Panel last year [2, 3]. An a t t e m p t was made to structure each topic to give an informative discussion o f the present state of the science, specific areas where research opportunities lie, the needs of the scientific c o m m u n i t y which, if met, would help in exploiting research opportunities, and the needs and opportunities of technology that can be met if research opportunities are realized. Each topic evolved in a different way, so that in the final report a c o m m o n f o r m a t is scarcely recognizable. In preparing this paper for publication it was t h o u g h t advisable to recast the information in the original report into a more uniform structure. This turned out to be a non-trivial task. The original selection of topics was n o t based on any organizationai principle such as chemical composition, © Elsevier Sequoia/Printed in The Netherlands

150 TABLE 1 Contributors to the Report of the Panel on New Materials [ 1] Person a

Affiliation

Professor Nell Bartlett Professor John D. Corbett, 1

Chemistry Department, University of California, Berkeley, CA Ames Laboratory, Department of Chemistry, Iowa State University, Ames, IA IBM Research Center, San Jose, CA Center for Materials Research, Stanford University, Stanford, CA Argonne National Laboratories, Argonne, IL Exxon Research and Engineering, Linden, NJ Department of Applied Physics and Center for Materials Research, Stanford University, Stanford, CA Westinghouse Research and Development, Pittsburgh, PA Materials Science Department, California Institute of Technology, Pasadena, CA Physics Department, University of California, San Diego, CA Chemistry Department, Pennsylvania State University, PA Bell Laboratories, Murray Hill, NJ Dupont Central Research, Wilmington, DE IBM Research Center, San Jose, CA Division of Engineering and Applied Physics, Harvard University, Cambridge, MA Solar Energy Research Institute, Golden, CO Bell Laboratories, Murray Hill, NJ Exxon Research and Engineering, Linden, NJ

Dr. James Economy Dr. R. S. Feigelsonb Dr. Frank Fradin Dr. Fred R. Gamble Professor T. H. Geballe, Chairman Dr. John K. Hulm Professor William Johnson, 4 Professor M. Brian Maple, 3 Professor Gerd M. Rosenblatt Dr. John Rowell, 5 Dr. Arthur Sleight, 7 Dr. G. Brian Street, 2 Professor David Turnbull Dr. Sigurd Wagner, 8 Dr. Jack H. Wernick Dr. M. Stanley Whittingham, 6

a l - 8, the numbers identify coordinators of the corresponding topics in the original report; see Table 2. bThis speaker was invited to address the Panel on the subject of electrocrystallization.

TABLE 2 Organization o f the original report of the Panel CHARACTERIZATION I

Topic

1 2 3 4 5 6 7 8

Inorganic synthesis Basic research on new polymeric materials Intermetallic compounds Amorphous solids Thin films Solid state ionics Catalysis Semiconductors

morphology or area of application. Furthermore, there were overlaps. Many catalytic materials are ionic conductors and many new intermetallics are made as thin films. The topics were chosen because of their high interest from the point of view of basic research and energy technologies. In the end it was decided to leave the selection of topics intact b u t to reorganize each section on the basis of the synthesis loop illustrated in Fig. 1. The synthesis of new c o m p o u n d s and new forms of existing materials is driven by needs and opportunities. The characterization and eval-

i

/

i

APPLICATIONS

\

I

KNOWLEDGE

Fig. 1. The synthesis loop used to reorganize each section.

uation of new materials frequently lead to basic understanding and applications b u t more often motivate further synthesis, thus closing the loop. Somewhere hidden in the cycle is serendipity: the true hallmark of basic research. The discovery of cathodes for high energy density batteries resulting from the research on the superconducting properties of intercalated layered metal dichalcogenides is an example of h o w new technological opportunities are sometimes created by chance. To confound matters further, synthesis, characterization and evaluation have their own needs and opportunities. Synthesis is so important to the development of new materials

151 that a separate section, Section 2, is included so that its needs can be discussed. The final section, Section 10, contains some general recommendations and some specific suggestions to foster the development of new materials.

2. MATERIALS SYNTHESIS

2.1. Evaluation

Materials synthesis increases our knowledge of the Universe and provides the means to control it. Although it is impossible using digital computers to start with the equations of motion for 1022 atoms and to predict simple properties such as melting points as a function of composition, this information is economically and efficiently obtained by making measurements on a series of carefully prepared well-characterized samples. The synthesis-measurement process is like an analogue computer. On the practical side, all energy technologies are limited by material performance. It is an obvious but perhaps overlooked truism that, to find better materials, first the materials must be produced. Some needs will be met by purer or better forms of known materials or by alloy additions and control of morphology, but others will require the synthesis of totally new chemical structures. The number of new materials that can be synthesized appears to be limited more by critical synthesis conditions than by the intrinsic limitations of matter. Among the undiscovered compounds, some will be synthesized easily but have n o t yet been sought. A greater number require the discovery of critical synthesis conditions, not only as regards the usual variables but also with respect to the container purity, transporting agents, "mineralizers" etc. The number of remarkable new phases which are being discovered even in supposedly simple systems attests to the frequency with which these conditions have n o t been met, even in prior investigations which might have been judged as thorough. A presumably large group of materials remains undiscovered [4] either because the rate o f formation under any set of conditions is extremely low or because an upper temperature limit for synthesis fixed by some decomposition reaction is still too low

for a reasonable rate of formation. A good example is M06S8, a conceptual predecessor of m a n y Chevrel phases which can be made only by indirect routes; the analogous compounds W6S8 and W6Ses probably exist but have never been made, possibly because of low decomposition temperatures. Some untried solvent, transporting agent or vapor deposition route may make these accessible. It is useful to recognize that the intellectual input of the synthetic c o m p o n e n t to research can vary greatly. Four categories can be recognized. The first is the synthesis of known materials in a specified quality, quantity, crystallinity, shape, order etc. This effort almost always has a service component; however, it may also require creative efforts in the development of techniques and apparatus. The second is synthesis in order to study a specific property. The effort may range from basic research to development. The phenomenological orientation may often dominate, with synthesis p e r se making a minor contribution. The production of new structures from such synthesis efforts is rare. The third is synthesis within a structural or related class for the purposes of exploration, extension or extrapolation of properties. Systematic measurements of related physical properties are frequently the basis for extending predictive capabilities and for testing models. Investigations of magnetic and electronic properties are examples. This work overlaps with the second category, but the role of synthesis is more creative. The fourth is the synthesis of entirely new types of compounds. Research of this character frequently precedes those of the other three types. Success provides new solid state structures and the promise of novel and new properties, new p h e n o m e n a and new solutions for materials problems. These results also broaden our understanding of the solid state. The predictability of results in this category may be very small, but intuition is guided by a broad background in chemistry and theoretical ideas. Some degree of synthesis by design can be achieved. New phases can be created which have the combined attributes of van der Waals bonding to create open structures for easy chemical substitution and rapid ionic diffusion, metallic bonding for electrical and thermal properties and ductility, and ionic or covalent bonding for strength and hardness.

152 TABLE 3 Examples of inorganic compounds where an unusual structure or an unusual property signals a research opportunity

Compound

Unusual feature

Further discussion

NbsSe 4

Infinite chains of M6Xs-type clusters sharing faces Non-metal structure in a metal array; photoemission Strongly bonded columns of metal polyhedra Infinite chains of metal octahedra, high stability Infinite strongly bound double-metal sheet structures Mixed-valency (5d, 4f) compounds Superconducting cluster compound, high critical field High critical current, high Tc superconductors Coexistence of superconductivity and magnetism Layered chalcogenide charge density waves High ionic conductivity Unusual metal chains Infinite linear strings of Hg Heavy metal clusters in intermetallic phases

Ref. 5 Ref. 6 Ref. 7 Refs. 8 and 9 Ref. 10 Section 4 Section 4 Section 4 Section 4 Sections 4 and 7 Ref. 11 Refs. 12 and 13 Ref. 14 Ref. 15

Superconducting ionic clathrate Soft magnetic materials, corrosion resistance

Ref. 16 Ref. 17

Piezoelectric polymer

Section 3

CSllO3, Cs70 Ta2S NaMo406, Sc5C18 ZrCl, ZrBr StuB6, SInS (RE)Mo6Se s Nb3X (X -= AI,Ge,Si) (RE)Rh4B 4 M(Ch)2 Li3N Te3Cl2,BiI Hg3_xAsF 6 CallBilo, Na3Hg2, K4Sn 4 (Ag608)AgNO 3 PdSi (amorphous) and derivatives PVF 2

2.2. Opportunities The choice of specific systems in which to seek new materials is the most difficult aspect and may often depend on slim leads, chemical intuition or just exploration. It is useful to see what c o m p o u n d s have been made already. A collection of compounds with unusual structures or properties that offer opportunities for further research are given in Table 3. The large number of highly reduced materials more or less along the interface between metals and conventional salts are noteworthy. Many of these have been found in supposedly simple binary systems, supporting the earlier assertion that the number of truly unusual compounds to be discovered is large indeed. In synthesis, opportunities exist by virtue of newly recognized principles, advancement in techniques and equipment, the discovery of compounds with unusual features, or solely from the existence of unexplored areas of solid state chemistry. It was recently recognized that a number of novel materials can be prepared by high temperature routes where advantage is taken of the unusual {often complex) molecular species which form in high temperature gaseous systems as well as in high temperature liquids where kinetic barriers

are also minimized. Quenching experiments in liquid systems have also produced a number of interesting products, some of which are discussed in Section 5. New and useful transport reactions utilizing complex equilibria may be limited by the investigator's understanding of these systems and imagination in devising transporting agents. A proper choice of precursor species for synthesis may be invaluable. Finally, it is anticipated that new complex strongly bonded materials will be fabricated using a proper combination of transition metals and non-metals when size, electronegativity, electron count, admixture of covalent and metallic bonding, and qualitative band features are taken into account. With regard to new techniques, perhaps the ultimate approach in synthetic efforts is the use of epitaxial or molecular beam techniques to lay down reactants a few layers at a time. Application of this technique to thin films is discussed in Section 6. Molten-salt solvent systems as a synthetic technique have received relatively little attention, except for unusual compounds which frequently may be separated from binary systems. Electrochemical techniques for synthesis, discussed in Section 4, have been explored only roughly.

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2. 3. Needs A broad and pervasive need for new and better materials requires that increased efforts be devoted to their creation whether it is better examples of known materials in important applications, broad ranges of materials to aid and test our understanding of solid state structure and properties or totally new compounds for which applications and phenomena have not y e t been conceived. The long-range future depends particularly on the last factor. The history of major advances in solid state science and of significant applications and devices therefrom indicates that in nearly all cases the original p r o t o t y p e c o m p o u n d was synthesized without concern for application or property. An admittedly incomplete but perhaps representative list of such landmark advances in synthesis made during this century is given in Table 4. A novel structure is often the first indication of potentially interesting

properties and, conversely, the measurement of an unusual property signals an unusual structure. The synthetic effort is enhanced when there is a closed loop between the synthesis, characterization and property evaluation. Therefore, joint efforts between academic departments in universities should be encouraged in order to benefit from interdisciplinary inputs as is achieved in the larger industrial and national laboratories. Within the U.S.A., traditional methods of support for university-based research have n a t provided a climate in which this close coupling would thrive. A continuing national problem for the U.S.A. regarding the origin of landmark materials in Table 4 is apparent; in all fields, the amount and breadth of solid state inorganic synthesis in the U.S.A. is relatively small in comparison with the general situation in Europe. In Germany there are 25 universities

TABLE 4 Some inorganic syntheses leading to landmark development in solid state science

Prototype compound

The researcher who first reported the compound

Count~

Year

Subsequent development

Ko.5MoS 2 LiMoS 2 • 0.6NH 3 KC 8

Rudorff

Germany

1959

Fredenhagen

Germany

1926

~-A12O 3 (Mg,Ca) (Na) ZrO2(Y203) ZrO2(CaO) V3Si

Rankin Stillwell Nernst Ruff Wallbaum

U.S.A. U.S.A. Germany Germany Germany

1916 1926 1900 1929 1939

Battery cathodes (LixTiS2) Metallic graphite, high energy batteries Solid electrolyte

SnMo6S 8 (Mo6Se 8 )Mx Mo6Ch 8 K 2 [Pt(CN)4 ] Xo. 3

Espelund Chevrel Krogmann

Norway France Germany

1967 1971 1960

Bi2(MoO4) 3 HxMoO 3

Zambonini Glemser

Italy Germany

1915 1951

BaTiO 3 LiNbO 3

Tamman Sue

Germany France

1925 1937

BaFel2 O19

Adelskold, Schrewelius Thiel Klemm Barrer Konig

Sweden

1938

Oxide conductor 02 sensor A15 high temperature superconductors Superconductors, very high fields One-dimensional metals by oxidation Oxidation catalyst Electrochromic displays, proton mobilities in solids Ferroelectrics Piezoelectric, non-linear optics Ferrites, memories

Germany Germany England Germany

1910 1943 1948 1944

I I I - V semiconductors Strong magnets Many catalysts Solar cells

InP (RE)Ni5 a Synthetic zeolite Amorphous Si aRE, rare earth.

154

where at least one.professor directs research in solid state synthesis and characterization, and in addition there are three well-integrated synthesis programs at the Max-Planck-Institut, Stuttgart. The U.S. o u t p u t of novel synthesis, at least as described in the scientific literature, reflects the fact that students in chemistry are only rarely exposed to the challenges of solid state synthesis. Government and industrial laboratories have difficulty finding U.S.trained personnel in solid state synthesis and characterization. A recent survey of publications in the Journal o f Solid State Chemistry shows that contributions from the U.S.A. have fallen from about 40% at the beginning of this decade to less than 20% in 1979 [18]. There are substantial opportunities which can be taken advantage of if access can be obtained to state-of-the-art equipment. Although these new capabilities are expensive, means should be found to increase their availability in laboratories. Apparatus and tools that have become available or have been applied to synthesis problems in recent years include superior containers and means for their fabrication, excellent inert atmosphere facilities, high resolution X-ray powder techniques, automatic diffractometers and improved analytical methods, such as microanalysis, electron spectroscopy for chemical analysis, electron microscopy etc. The potential of theoretical guidance in the synthesis loop cannot be overrated. For example, both ionic diffusion and phase stability are of prime importance to synthesis. Many solid state synthesis reactions require good ionic diffusion at a given temperature in order to succeed; the problem of phase stability pervades this review both implicitly and explicitly. Although there exist a number of empirically based predictive theories of phase stability of simple intermetallic phases, e.g. those discussed in refs. 19 and 20, these have had only limited success and no applicability to a wide range of other materials which are n o t intermetallic. The search for new superconducting, hydrogen storage and other energy-related materials needs advanced theoretical insight for guidance. The search for new compounds with unusual properties also depends on ready accessibility to previously obtained data on the thermodynamic and physical properties of potential candidates and their analogues. Ready accessi-

bility requires a continuous effort to provide updated compilations of critically evaluated data of various types. The critical evaluation of binary phase diagrams published by Hansen and Anderko in 1958 [21] and supplemented by Elliott [22] and Shunk [23] has been particularly useful. For many purposes the phase diagram information generated in the 10 - 15 years since Shunk's review lies buried and unused by those who need it. The lack of critically evaluated data is felt so severely that it has led one industrial library to distribute current phase diagrams in a looseleaf format [ 2 4 ] , which is useful b u t limited by lack of completeness and criticality. The same is true of many other data needed by materials scientists, such as the alloy thermodynamic data compiled for an extended period by Hultgren et al. [25] and other thermodynamic data formerly c o m p u t e d by Kelly and King [ 26 ]. An example of a useful ongoing project is Roberts' continuing non-critical b u t complete compilation of superconducting data [27]. The need for data organization and evaluation exists over a broad range of physical sciences, and this problem has been studied by a recent U.S. National Academy of Sciences committee (the Committee on Data Needs) and has been c o m m e n t e d on by other Academy committees {the Committee on Chemical Sciences and the Committee on High Temperature Science and Technology). These Committees proposed that 0.1% - 0.2% of the U.S. federal research and development budget be specifically earmarked for critical evaluation and compilation of data in all fields of science. Federal agencies with the power to carry out this recommendation have n o t as y e t made any firm commitment, b u t discussions have been initiated. Since the problem is really international, the commitment to data evaluation should extend to all national governments and not just to that of the U.S.A.

3. P O L Y M E R S

3.1. Evaluation The importance of polymer materials to the major energy-generating and energy-using technologies is clearly illustrated by Table 5 which lists some present applications taken from a U.S. Department of Energy workshop report [ 2 8 ] . In general, polymers possess many

155 TABLE 5 Application of polymers in energy technologies

Technology

Application

End-use conservation

Lubricants; foams for thermal insulation in buildings; substitute structural materials for energy-intensive metals such as Al; ion exchange resins and membranes for water purification; piezoelectric and pyroelectric energy transducers in microphones, earphones and loudspeakers; substrates for catalysts; coating to prevent corrosion of metals and degradation of polymer substrates Polymer flooding for enhanced oil recovery from oil wells Electrical and thermal insulation in superconducting.magnets for plasma confinement in fusion reactor prototypes Flywheels for kinetic energy storage; casings and ion exchange membranes for electrochemical storage batteries; solid electrolytes in electrolysis cells for H2 energy storage High voltage insulation for electrical power cables Structural materials; adhesives; substrates and coatings for mirrors; glazings; heat transfer and storage fluids Packer seals between drill pipes and well walls; O ring seals in the bearings of drill bit cutter cones; cable coverings and sealants for well-logging equipment; corrosion-resistant concrete for well casings and mounting pads; fiber composites for fluid lines; cooling tower structures and vent chim-

Fossil energy Nuclear energy Energy storage Power transmission Solar energy Geothermal energy

neys

advantages over metals and ceramics, such as elasticity, plasticity, low density and low temperature process ability. Of course, n o t all polymers possess all these qualities and m a n y polymers suffer degradation with time in severe environments, just as m a n y metals c o r r o d e or ceramics suffer brittle fracture. The mo s t i m p o r t a n t attribute o f polymers is that through chemistry and processing their properties can be changed in what seems to be an endless n u m b e r of ways. Most polymers at present available are made from ethylene, benzene, butadiene, pro~ pylene or xylene, all o f which t o d a y com e from petroleum. This is n o t o f major concern because less than 2% of oil usage is for plastics. F u r t h e r m o r e there are ways o f obtaining these raw materials from coal, natural polymers such as cellulose, and even carbon m o n o x i d e and hydrogen if the need arises. This review is n o t intended to be a comprehensive s um m a r y on p o l y m e r properties and hence the reader is referred to r ecen t t e x t b o o k s [29, 30] in which the various classes o f p o l y m e r are listed and the p o l y m e r i z a t i o n processes are described. Only a few basic concepts needed later will be described here. In addition to molecular structure, we must also be c onc e r ned with supermolecular structure or m or phol ogy. Polymers

o f low molecular weights are liquids at r o o m temperature. P o l y m e r properties become evident at a molecular weight of around 1500. So-called high polymers have molecular weights greater than 10 000. Even in solution or at temperatures 100 °C above their melting points, some strongly linear polymers behave like liquid crystals and show anisotropic properties. The m o r p h o l o g y of m a n y solid polymers is described as semicrystalline. There are crystalline regions separated by a m o r p h o u s regions. A given p o l y m e r chain can be part of t w o or more crystallites. Other com pl ex configurations arise because o f isomerism, steric configurations, helical structures and p o l y m e r chains, e.g. p o l y e t h y l e n e , t hat fold back on themselves several times. It was quickly learned t hat the properties o f h y d r o c a r b o n pol ym ers could be improved by substituting fluorine, chlorine, amino groups and ot her radicals for hydrogen. Phosphorus and oxides o f sulfur can also be inserted in the chains. This has generally given a bet t er resistance to oxidation and also improved the thermal stability. The natural extension o f this is the synthesis of inorganic polymers [31, 32] such as silicones, borazanes and carborane - s i l o x e n e polymers. Inorganic polymers have been somewhat technologically disappointing to date

156

b u t still offer considerable potential for high temperature application. Another important concept in the synthesis of polymers is crosslinking. By the use of catalysts, heat treatment or radiation treatment the side chains on one polymer molecule can be chemically crosslinked to another polymer chain, making the whole structure more rigid. This emphasizes the importance of process parameters in affecting the final properties of polymers. There are basically three processes for the solidification of polymers. Polymers which solidify by the evaporation of solvents are called "lysotropic". Those which solidify from a melt are "thermotropic". If heat treatment causes cross-linking, the polymer is called "thermosetting". Thermotropic polymers are highly desirable because they permit ease of rapid melt processing into a myriad of forms such as injection-molded parts, extruded films and fibers, coatings etc. Lysotropic polymers are clearly less desirable, not only because of the energy required to evaporate the solvent b u t also because of the need to eliminate solvent wastes from effluents. Conditions during solidification and mechanical treatment can strongly influence polymer behavior. In long-chain linear polymers, for example, a spherulitic morphology (a collection of non-oriented spherical crystaUites) develops when the material cools in an unstressed state. Solidification under pressure can produce oriented extended-chain crystals. The values of Young's modulus for these t w o forms of the same material can be different by orders of magnitude. Placing polyvinylidene fluoride (PVF2) films in large electric fields, a process called "poling", induces pyroelectric and piezoelectric properties. Through processing, a polymer can be combined with other polymers, ceramics, metals and gases to produce materials with new properties. Some examples are fiber-reinforced composites, multilayer laminated sheets and films, t w o interpenetrating continuous phases, foams etc. Miscibility and interface phenomena are very important. The large range of possibilities means that we are on the verge of a new era in which materials can be engineered into intricate structures for particular functions. This ability to tailor materials is in its infancy only.

Two of the major problems with polymers that prevent an even more rapid development than is at present taking place are degradation and aging. In the presence of moisture, certain chemicals, light and other radiations, many polymer properties deteriorate. Even in the absence of environmental attack, many polymers age. The agin.g process is analogous to the crystallization of glass. It is a relaxation to a thermodynamically more stable state. Both degradation and aging can be inhibited by adding materials called "stabilizers". As can be seen from the above discussion, the field of polymer research is very broad. From this field, seven topics of special concern to energy technology have been selected for particular consideration. These include high temperature stability, mechanical strength, radiation sensitivity, permeability, solubility, electrical conductivity and piezoelectricitypyroelectricity. With respect to basic studies directed at high temperature polymers, there is considerable information of an empirical nature which has built up over the past 20 - 25 years on this subject [33]. Unfortunately, there is considerable confusion as to the limitations of present materials and, in particular, as to their mechanisms o f degradation. Thus elastomers such as silicones, polyphosphazenes and carborane polymers are thought to degrade at around 200 °C. A research program aimed at defining the exact limitations is needed. A second approach to high temperature elastomers involves the synthesis of new perfluoro polymer structures which can readily be crosslinked to yield useful elastomeric properties. Both the optimum design for such structures and their mechanisms of degradation are of considerable scientific interest. High strength high modulus materials encompass both rigid rod-like polymers such as Kevlar [34] and reinforcing agents such as carbon fibers. Work on rigid rod-like polymers has focused almost completely on the development of lysotropic systems that are spun from solutions into filaments. As such, a considerable body of knowledge has developed on such materials with respect to their rheological and morphological characteristics. In contrast, thermotropic polymers are only n o w being described in the literature [35] and there is essentially no basic understanding of their

157 morphology and theology. Thus there is a great need to develop a far better knowledge base of the fundamental behavior of these melts consisting of rigid rod-like structures, since they offer a far greater degree of versatility in terms of fabricating a wide variety of forms with outstanding mechanical properties. To date, only certain aromatic polyesters have shown these unique features and it is critical that the key structural features associated with thermotropic behavior be identified, permitting optimization of the aromatic polyesters and development of new systems. The topic of new or improved reinforcing agents is considered here because of its importance to high performance composites. It is especially critical in light of the almost total lack of research in th~s area in the U.S.A. since the late 1960s, at which time the U.S. Air Force and NASA shifted the focus of research on new reinforcing agents to p r o t o t y p e studies [36]. The strength-limiting features in carbon filaments have not been determined. They may be connected with the internal stresses in carbon fibers which arise from differences in crystalline orientation across the fiber diameter. A similar problem may exist in high strength modulus BN filaments; hence basic studies on this material would be highly desirable also. The development of ceramic composites reinforced with high strength modulus filaments and designed to withstand temperatures far in excess of 1000 °C represents a major challenge for the materials scientist. Candidate systems that might fulfill these requirements include B4C or SiC filaments embedded in an SiC or Si3N4 matrix. The next generation of reinforcing materials will be based on single-crystal flakes with aspect ratios of 100:1. Exploratory studies indicate that light-weight composites with planar modulus values of 200 - 275 GPa (30 40 Mlbf in - 2) can be fabricated; this represents a twofold to threefold increase over values obtainable with fiber-reinforced structures [37]. In some applications it is desirable to have polymers that are sensitive to radiation, e.g. photoresists such as p o l y m e t h y l m e t h a c r y l a t e used in photolithographic production of microelectronics, but in most applications radiation sensitivity is undesirable. Systematic studies to understand the nature of the interactions between polymer structures and

different types of radiation sources have y e t to be carried out. A large body of empirical knowledge has been generated on the photodegradation of polymers, resulting in partial solutions through the use of stabilizers [38]. Another area of particular interest is those polymers which display reversible responses on exposure to specific wavelengths in the UV. Thus cross-linked polymeric tribenzopyran or cis-biphenylazo structures in the form of films can display an expansion and contraction of 1% - 4% in the plane of the film [39]. This unusual phenomenon appears to depend on reversible ring opening-closing reactions or on cis-trans isomerization which are induced photochemically. Basic research to understand this phenomenon better and to synthesize new systems which can display larger volume changes with a high level of reversibility would seem highly desirable. Polymers exhibit a wide range of permeabilities to small molecules such as hydrogen, oxygen and water. For example, low density polyethylene is more than a 1000 times more permeable to oxygen than Saran is. Relatively impermeable coatings such as Teflon applied to metals or even to other polymers can prevent corrosion and degradation; this is important to energy conservation. Selectively permeable polymer membranes are used in water purification, ultrafiltration, electrodialysis cells for generating acids and bases, electrolytic production of hydrogen and other electrochemical processes. Permeation process mechanisms can be broadly classified as porous flow, electrostatic interactions and solutiondiffusion. In porous flow the flux and selectivity are determined by the molecular dimensions of the penetrant relative to the pore size. Electrostatic interactions may be d o m i n a n t when the membrane or the penetrant mixture or both have ionic groups in the presence or absence of an imposed electric field. New fluorocarbon ionomers have been developed from copolymers of tetrafluoroethylene with ether monomers containing perfluorinated sulfonyl fluoride functional groups, which have been converted to an acid salt by treatment with a strong base such as sodium hydroxide [ 40]. Membranes of these materials show good ion selective transport properties. There is even some theoretical work which yields quantitative results in fair agreement with experiment [41]. The

158 solution-diffusion mechanism depends on the size, shape, composition, concentration and perhaps other properties of the penetrant as well as on the composition, structure and morphology of the polymer. The division between these mechanisms is not always clearcut in practice. In the work on characterizing the permeability and diffusivity of polymers to moisture and corrosive gases, much progress has been made in defining the role of free volume, degree of crystallinity and nature of polymer functionality. More recent studies have begun to elucidate the effect of stresses on permeability and the tendencies for certain mixed gases to facilitate diffusion. When polymers dissolve in organic solvents or in water, they change the viscosity and flow properties of the fluid. With respect to water-soluble polymers for use in enhanced oil recovery, the urgency for pushing research in this area derives from the potential of recovering 280 billion {2.8 × 1011 ) barrels of oil that might otherwise be unrecoverable. The major problem encountered with simple water flooding of oil wells is fingering of the injected water through the oil-bearing rock, resulting in premature breakthrough from the water injection hole to the production well hole. High molecular weight water-soluble polymers reduce the fingering. Although there is an extensive amount of literature on the theological behavior of polymers in organic media, extrapolation to aqueous systems is complicated by strong hydrogen-bonding and ionic effects, especially with polyelectrolyte systems. The mechanisms of mobility control in porous media has not been elucidated fully. The process of shear thinning needs explanation. The nature of other degradation mechanisms in geological media is speculative at this time. Some work has been done on modeling the flow in porous media b u t it has little predictive value yet. There is a large amount of engineering literature dealing with polymers for enhanced oil recovery b u t very few molecular data, especially dealing with ways of synthesizing polymers with improved properties. The idea of having electrically conducting polymers is very attractive. Since the discovery of metallic and superconducting properties in the inorganic polymer (SN)x [42 - 4 5 ] , it has been found that high conductivity can be achieved in several organic polymers based on

polyacetylene [46, 4 7 ] , polypyrrole [48], polyparaphenylene [49] and polyparaphenylene sulfide [50]. Unlike (SN)x or brominated (SN)x these organic polymers are n o t intrinsically metallic nor do they exhibit superconductivity; they are in fact closed-shell insulators which become conducting to the degree they can be oxidized or reduced. Polyacetylene, for instance, is an insulator whose electrical conductivity can be varied from about 10 -9 to 10 a ~ - 1 cm-1 by treatment with oxidizing agents such as AsFs, I2, Br2, silver etc. or reducing agents such as the alkali metals. Similar properties characterize the polyparaphenylene-based polymers. The electrochemically prepared polypyrrole tetrafluoroborate is somewhat different. In this case, oxidation is achieved electrochemically during synthesis rather than by "doping" existing material as in polyacetylene powder. The conductivity of the polypyrrole films can at present be varied between 10 -a and 1 0 2 ~2-1 cm-1 by the incorporation of controlled amounts o f N-methyl pyrrole. The progress during the last 4 years reflects the fact that the area has attracted and sustained the cooperative interdisciplinary efforts of chemists and physicists, both crucial to meeting the challenges of this complex field. Piezoelectricity and pyroelectricity are related phenomena because both involve volume and dipole orientation changes and they are both ways of changing one form of energy, stress or thermal, into another form, electricity. One very important application, the non-destructive testing of materials and structures by acoustic imaging using conformable PVF 2 transducers, has been identified. This application would benefit greatly from films of increased piezoelectric modulus and reduced electrical loss tangent. PVF2 is a semicrystalline polymer which is currently of great interest in a variety of research areas because of its high piezoelectric and pyroelectric coefficients and because of its large non-linear optical coefficient and high dielectric breakdown strength [ 51 - 57]. The polymer chain composed of CH2CF 2 m o n o m e r units exhibits a strong transverse dipole m o m e n t which is responsible for most of its interesting properties. However, charge injection and trapping also occur, and there is currently some controversy as to which effect is the dominant mechanism. Fundamental

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questions such as the role of the amorphous region in charge trapping, charge conduction and dielectric loss have not been addressed. The opportunity to answer these questions exists by variations in synthesis and processing. Several avenues are currently being studied, including copolymerization with non-polar species such as tetrafluoroethylene (C2 F4 ), and low temperature homopolymerization which reduces chain defects such as head-to-head linkages. The crystalline-to-amorphous ratio in C2F4 is considerably greater than that in commercial material because of its lower defect density, and some variation in properties has been achieved. Although most work has been concentrated on PVF2, there are other systems which have been looked at. Of these the odd nylons are a prime candidate. Nylon 11 has been poled and surprisingly was found to be a poor piezoelectric material while having a relatively large pyroelectric response [58, 59]. All theories predict a direct correspondence between these two responses. This may be connected in some way with the semicrystalline structure of n y l o n polymers. Three recent developments in polymer processing deserve note because they offer the possibility of producing polymers with new properties. The first, electrochemical deposition, has been mentioned in connection with conducting polymers as a way of controlling the incorporation of N-methyl pyrroles. The second is plasma polymerization with subsequent vapor deposition or epitaxial growth. This could produce single-crystal polymer monolayers. The third is spin coating onto substrates of polymers that have liquid crystal properties. The orientation of the chains can be maintained during spinning, resulting in highly ordered arrays. The spinning process is also used in the production of exceptionally strong fibers of materials such as Kevlar. 3.2. Research needs and opportunities Research needs and opportunities can be divided into categories associated with synthesis, characterization and evaluation. One very important concern, which in part may explain the slow rate of progress in developing new polymers, is the almost complete absence of good facilities for synthesizing polymers in universities. Thus, although most polymer groups in universities have brought together scientists with outstanding skills in polymer

characterization for study of structure-property relationships, the complementary effort in synthesis is lacking. Historically, this balance was maintained primarily through the very strong synthetic efforts in industry; however, during the past decade the chemical industry has shifted its focus from the development of new polymers to the improvement of existing systems and processes. The potential still exists for developing new and unusual polymers tailored to specific goals through synthesis. This versatility is an attractive feature of polymer materials and the need for highly trained polymer synthetic scientists will increase sharply in the coming years. In the synthesis of high temperature elastomers, for example, another approach which may be of value is to take advantage of the high temperature rigid polymer technology developed over the past 25 years. Thus the modification of these various structures to reduce their glass transition temperatures Tg to below 150 °C could yield a material which behaves as an elastomer at temperatures above 175 °C. In the area of new polymers, significant technological benefits would be derived from polymers which, as well as displaying conducting properties, also possess the attractive physical properties of plastics, e.g. processability and stability. Further attempts should be made to prepare new conducting polymers from monomers, where the polymerization is initiated by the Lewis acid or the Lewis base required to induce conductivity. The role of electrochemistry in the field needs more emphasis, not only in producing new polymers byt also in helping to define the oxidation and reduction potentials of existing polymers but also in helping to define the importance to find new polymers capable of being subjected to n-type doping if the junction properties of polymeric semiconductors are to be applied. Although current successes have led to the stressing of the electrical properties of polymers, efforts should also be directed toward developing polymers which show associated properties such as high thermal conductivity and magnetic behavior. Polymers in general, and conducting polymers in particular, present significant barriers to characterization. It is important to learn how to adapt current techniques to these materials and also to develop new techniques. A second approach is to develop new polymers

160 which are either soluble or very crystalline and hence would be more amenable to standard characterization techniques. The outstanding opportunity to develop inorganicpolymer-fiber-reinforced ceramics also presents a characterization problem. Research studies to characterize the high temperature mechanical properties of the candidate filaments and their interface with the matrix materials appear basic to progress in this area. The most critical need in polymer science is the evaluation of existing data and the development of theoretical models which can relate structure to properties and one property to another property. With high temperature polymers, not only are the structural features deemed necessary for thermal stability still n o t clearly understood, but also the complex mechanisms of degradation observed u n d e r different environments have n o t been determined. For high strength modulus polymers and composites there is virtually no understanding of the rheological and morphological characteristics of thermotropic (rigid rod-like) polymers which would provide a basis for understanding their unusual behavior. Work on developing new higher strength modulus reinforcing agents for use in composites is at a standstill because of the lack of insight into the strength-limiting features in filaments with glassy, polycrystalline or ribbon-like morphologies. The area of radiation-stable or radiation-sensitive polymers remains more an art than a science. Fundamental understanding of the mechanisms of polymer reactions on exposure to different types of radiation including neutron, electron beam, X-ray, deep UV and near UV is desperately needed, since polymer scientists working on these materials are forced to operate on an empirical basis. With respect to permeability of gases through films, some understanding as to the mechanisms has been developed b u t as y e t little progress has been made on devising new more impervious films or semipermeable membranes based on these concepts. Basic studies to characterize in more detail the mechanisms o f hydrogen ion transport through films of sulfonated perfluoropolymers appear highly desirable, particularly in the search for improved systems. A similar argument could be made for developing new high molecular weight water-soluble polymers with theological and functionality features tailored to

minimize interreactions with different ionic environments; however, the basic research aspects are not so clear since these studies are far more meaningfully carried out in smallscale field tests. In the area of conducting polymers, structural requirements for conductivity are not y e t understood. Conceptual approaches to combine the unique electrical conductivity features with the desired rheological properties have as yet not been defined. Although considerable progress has been made in interpreting the semiconductor ideas, it is necessary to learn more about the physics and chemistry of defects and traps in these materials, and their influence on the conductivity needs to be evaluated. In particular, the search for experimental evidence for solitons in these materials should be extended. It is also critical that we learn more about the nature and distribution of the dopant species, particularly in the semiconducting region. Before piezoelectric polymers can achieve their full potential, much work must be done. Even the mechanism of generation of pyroelectric and piezoelectric activity in PVF2 is unclear. The relationship of morphology, orientation and crystallinity to activity has n o t been elucidated. There are at least three crystal forms and hints that there may be two more.

3. 3. Technological opportunities The existence of many technological opportunities either on the horizon or almost within our grasp is implicit in the above analysis. The automobile industry is already evaluating graphite-reinforced composites to replace drive shafts, leaf springs and many other steel components. Flywheels fabricated from new higher strength graphite fibers could provide a threefold increase in the figure of merit for flywheels. High temperature composites reinforced with filaments stable to 1300 °C could provide a meaningful alternative to the inherently brittle ceramics which are currently being developed for use as high temperature turbine blades. .The photoexpansion and photocontraction of some polymers is an effect waiting for a technological use. The development of polymers resistant to neutron fluxes would have major impact in the development of cryogenic insulation for fusion reactors built with superconducting magnets. The impact of submicroelectronics produced with the help of radiation-

161 sensitive lithographic polymers m a y be larger than the impact of the industrial revolution. The implication for enhanced oil recovery of using p o l y m e r flooding has already been mentioned. It remains to be seen whether truly useful polymers can be developed. Of a far more speculative nature is basic research directed at the development of new polymers which mimic the features of semiconductors for use in photovoltaic systems. It is t o o early to assess realistically the technological implication of conducting polymers. For many applications the technological aspects would be strongly emphasized by the demonstration that these conducting properties could be obtained in polymers possessing the attractive properties of plastics, e.g. plasticity, elasticity, low density and chemical and thermal stability. However, in view of the progress in this area to date, it is n o t unreasonable to predict a commercial future for organic conductors commensurate with the way organic p h o t o c o n d u c t o r s have begun to c o m p e t e with their inorganic analogues. There are t w o other aspects of p o l y m e r behavior having energy applications which have not been mentioned yet. The first is the latent heat of solidification and the second is the thermodynamic properties of elastomers. The heat of fusion of some polymers, like high density polyethylene, is large enough to be of interest for storing solar energy. Elastomeric material behaves thermodynamically like a one-dimensional gas. It has long been known that elastomeric solids could be used as t h e working material in engines designed to convert heat into mechanical work or in other reversible thermodynamic cycles. This has never been considered more than a novelty but today, with our re- examination of energy alternatives and our need for higher efficiencies, elastomers may deserve a reappraisal in connection with the extraction of energy from the oceans and other energy-pumping applications [60].

4. INTERMETALLIC COMPOUNDS 4.1. Evaluation An exact definition of the phrase "intermetallic c o m p o u n d " is elusive and somewhat arbitrary. There is n o t even universal agreem e n t on which elements are metals. The so-

called Zintl line dividing metals from other elements runs down through the periodic table between boron and carbon. This excludes tin and lead, which are ordinarily thought of as metals. For the purposes of this review the 71 elements to the left and below the diagonal line running from boron through astatine will be considered to be metals. The metalloid elements are defined to be boron, silicon, arsenic, tellurium, astatine, carbon, phosphorus, selenium and sulfur. The term "intermetallic" has come by general usage to include compounds between metals and metalloids as well as between metals only, which a literal meaning of the term implies. We shall follow the broader interpretation in this review. The more c o m m o n intermetallic c o m p o u n d s already play a vital role in technology. Examples of their uses includes dispersion-hardening agents (NisAI in nickel-based superalloys and CuA12 in high strength aluminum alloys), refractory carbides in grinding and machine tool applications, magnetic compounds (NiA1 in Alnico and Sm2Co17 in ram-earth-Co permanent magnets), and A15 crystal structure c o m p o u n d s (NbsSn is n o w being used in superconducting magnets [61]). Many of these well-known intermetallics were first discovered as intermediate phases in the process of exploring alloy phase diagrams during the nineteenth century. A great deal has been learned a b o u t binary phase diagrams since then using the techniques of thermal analysis, powder metallurgy, optical microscopy and X-ray diffraction. However, there remains an even richer field of intermediate phases with three or more types of atoms per unit cell, which are sometimes called "ternary phases", yet to be explored. Under favorable conditions these phases can have the combined attributes of partial van der Waals bonding or open channels for easy chemical substitution, metallic bonding for electrical properties and ductility, and ionic and covalent bonding for strength or hardness. There remains a potentially large b u t uncharted field of intermediate phases which have n o t been reached by traditional methods of preparation because of kinetic limitations at the relatively low temperatures at which they become stable. Because of their metallurgical applications, m e t a l - m e t a l c o m p o u n d s have been explored more fully than metal-metalloids. It is mainly the metal-metalloid c o m p o u n d s that will be

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of interest since they offer a greater opportunity for scientific and technological innovatiorL Five classes of compounds will be discussed: (1) A15 superconductors; (2) layered quasione-dimensional and quasi-two-dimensional compounds; (3) ternary molybdenum chalcogenides (Chevrel phases); (4) ternary rare earth rhodium borides; (5) intermediate valence rare earth compounds. Let us begin with the A15 structure, which is cubic and has eight atoms per unit cell. In NbsSn, for example, the tin atoms are located at the corner and the center of the cube, and the niobium atoms are at the (¼ 0, ~ ), (-~ 1 1 ( ~I S, z , 0 ) and (o,~,z) 1 3 0), (0,½ , (~, O, -~), positions [62]. The highest transition temperature superconductors form in the A15 phase. Although Nb3Sn, which is stable and easily formed from the elements by diffusion, is likely to serve as the workhorse of future superconducting large-scale technology, more remains to be learned about its superconductivity. The nature of defects and their role in flux pinning, the nature of surface barriers to the entry and exit of flux, and the way in which Nb 3 Sn performs when incorporated in composite structures to produce high magnetic fields are not well understood. The opportunity exists for studying the effect of welldefined physical and chemical changes on its macroscopic superconducting properties such as critical current and critical fields. The compound Nb 3 Si, which may turn out to be the A15 phase superconductor with the highest T¢, has been prepared as a metastable A15 phase only by vapor phase quenching, and then only in a niobium-rich form. Its synthesis is a greater challenge than the synthesis of the three highest temperature superconductors (Nb3A1 , NbsGe and NbsGa) known at present, none of which has been prepared properly as a pure ordered phase. Understanding the conditions which favor the stabilization of the A15 phase with its strong electron-phonon coupling may lead to the synthesis of even less stable pseudobinary ordered A15 phases. The marked susceptibility of A15 compounds to radiation damage not only needs to be well understood for technological reasons but affords an opportunity for generalizing our understanding to different classes of intermetallic compounds. The

¼),

severe degradation of the superconducting properties is easily removed by simple annealing. Some analyses show that there is a large redistribution of charge and atomic motion within the unit cell on being irradiated that may be understood qualitatively in terms of the formation of covalent bonds. An equally important and probably related problem is the strong dependence of the physical properties on stoichiometry. Tunnel junctions have recently been prepared with adequate characteristics to permit the application of electron tunneling spectroscopy to the problem of unraveling the microscopic properties of the electron-phonon interaction [63, 64]. There is a class of layered transition metalchalcogenide compounds (exemplified by the TaS2 and NbSe 3 structures), which behave as quasi-one-dimensional or quasi-two-dimensional materials. Not only are they superconducting, but also many of them exhibit a transition to a charge density wave ground state which lies below a transition temperature To, which for TaSe2 is 600 K [65]. The charge density wave state is one which displays long-period modulations in the electronic charge density which are driven by the topology of the Fermi surface. The period of these modulations need not be commensurate with the lattice periodicity and hence the material is no longer truly periodic. In essence the conduction electrons can form a separate lattice within the lattice of ions. Their mutual interaction destroys the periodicity. Much progress has been made in the past few years in understanding the relation between the electronic and lattice degrees of freedom. The van der Waals forces hold sheets or chains together giving the structures quasione-dimensional or quasi-two-dimensional character that is reflected in the physical properties. The ability to synthesize compounds where composition can be varied in a welldefined way permits properties to be related back to band structure. For instance, the lattice instability temperature To in TaS2 varies with the substitution of titanium for tantalum in a way which clearly establishes the driving force as a charge density wave derived from a particular wave vector spanning the Fermi surface [66]. The interaction and coexistence of charge density waves with superconductivity provide valuable experi-

163 mental data for understanding both p h e n o m e n a [ 6 7 ] . When two-dimensional superconducting crystals based o n TaS 2 are intercalated with organic molecules, the resulting superconducting transition temperatures are above 3 K and upper critical fields with a magnetic field parallel to the crystal layers have slopes dHc2/dT > 20 T K -1 , which is several times higher than that for any other superconductor. These intercalation c o m p o u n d s are extreme examples of ternary compounds and have some features in c o m m o n with the Chevrel and rare earth rhodium boride phases discussed below. The conduction band is well localized within the metallic layers of the unit cell in that substitutions at different sites affect different physical properties. Metallocene radicals incorporating 3d elements with uncompensated spins, such as Co(C5H5), can be introduced with little apparent effect on the superconducting properties indicating an example of the coexistence of magnetism and superconductivity as y e t uninvestigated. The ternary m o l y b d e n u m chalcogenides, MxMosXs or Chevrel phase compounds, also are important for studying the interplay of superconductivity and magnetism. The c o m p o n e n t M ranges throughout the periodic table of the elements {alkali metals, alkaline earths, transition metals, s - p metals, rare earths and actinides), X -- S, Se or Te or, in part, X = Br or I, and the value of the index x generally lies within the range 1 ~< x ~< 4. The c o m p o u n d s have rhombohedral s y m m e t r y and consist of MosX s clusters and open channels into which the M atoms can be inserted. The Mo6X s cluster is a distorted cube with X atoms on the comers and molybdenum atoms on the faces [ 6 8 ] . Some of the Chevrel phase c o m p o u n d s exhibit superior superconducting properties with critical temperatures Tc as high as 15 K [69] and upper critical fields H¢2 approaching 70 T [ 70 - 7 2 ] . Although it is generally believed that the extremely high values of H¢2, which are the highest known for any material, can be traced to the cluster character of these compounds, a fundamental understanding of the origin of the high H~2 values is lacking and systematic studies are clearly called for. In addition, experiments yielding knowledge of the electronic band structure and p h o n o n characteristics would be very useful. The Chevrel phases may represent a means of avoiding the elec-

t r o n - p h o n o n instability limitation. Very anharmonic behavior of the M element is allowed, while these degrees of freedom are contained by rather rigid Mo6X 8 cages. Information a b o u t the electronic structure of these materials may accrue from the exploration of mixed ternary systems and could possibly lead to higher values of T¢ and He2. The preparation of more dense polycrystalline and, in some instances, single-crystal specimens has recently been accomplished. This should enable better comparisons to be made with theoretical studies of the electronic band structure and lattice dynamics. In particular, it should be possible to address long-standing questions concerning the coexistence of superconductivity and various kinds of magnetic order or, more generally, critical phenomena involving coupled order parameters. The reason that the ternary rare earth m o l y b d e n u m chalcogenide cluster compounds, and other ternary rare earth cluster compounds, are so well suited to this type of investigation is twofold. The periodic disposition of rare earth ions with partially filled 4f electron shells and corresponding magnetic moments leads to long-range magnetic ordering via the R u d e r m a n - Kittel- Kasuya- Yosida (RKKY) interaction. As a result, the magnetic ordering temperature TM is well defined and the features in the physical properties at TM are sharp, e.g. ~,-type anomaly in the specific heat at TM • The clusters, which are apparently responsible for some of the remarkable superconducting properties, are relatively isolated from the rare earth ions. Consequently, the overlap between the conduction electrons and localized 4f electrons is rather small, leading to a value of the conduction electron spin-rare earth magnetic m o m e n t exchange interaction parameter that is nearly an order of magnitude smaller than typical in binary rare earth metallic compounds. Recent experiments on various (RE)xMosS s and (RE)xMosSe a compounds have yielded several examples in which superconductivity and antiferromagnetic order coexist [73 - 7 5 ] . In one example, H o t 2 M o 6 $8 [ 7 6 ] , superconductivity is destroyed at a second lower critical temperature by the onset of ferromagnetic order [ 7 7 ] . Further systematic studies of superconductivity and magnetism in these c o m p o u n d s will also shed light on the electronic structure of a class of complex intermetallic c o m p o u n d s that may be proto-

164 types for catalysts in important chemical reactions. The ternary rare earth rhodium borides constitute another class of cluster compounds which exhibit a rich interplay between superconductivity and magnetism. The structure can be viewed as a body-centered tetragonal arrangement of Rh4 B4 clusters with rare earth ions located at the center of the top and b o t t o m faces and midway along the vertical edges of the unit cell. The clusters are cubes with alternate corners being rhodium or boron, resembling two interpenetrating tetrahedra. In these materials the free energies of the superconducting and magnetically ordered states achieve such a delicate balance, as reflected in the original paper by Matthias et al. [ 7 8 ] , that some of them were superconducting, e.g. those with RE - Nd, Sm, Er, Tm and Lu, while the others were ferromagnetic, e.g. those with RE - Gd, Tb, Dy and Ho. In ErRh4B4, superconductivity is re-entrant [79] because of the onset of ferromagnetic order [80] while, in NdRh4B 4 [81] and SmRh4B 4 [ 8 2 ] , superconductivity and magnetic order coexist. In these compounds the role of crystalline electric fields appears to be central and needs to be elucidated. Since these materials have tetragonal symmetry, single-crystal specimens would be particularly valuable in achieving a detailed understanding of their remarkable behavior. Some pseudoternary rare earth rhodium boride systems have been investigated. Systematic studies may lead to other systems of ternary rare earth cluster compounds. During the past few years, there has been a great deal of interest in a certain class of metallic rare earth c o m p o u n d s which exhibit an intermediate or non-integral valence of the rare earth ion [83, 8 4 ] . In such c o m p o u n d s it is generally believed that the 4f electron shell fluctuates between t w o configurations 4f ~ and 4f n-1 , accompanied by the emission and absorption of an electron by the 4f shell. The rare earth ions that have been found to exhibit this t y p e of behavior include cerium, samarium, europium, thulium and ytterbium. Examples of materials of this t y p e include ~-Ce, CePda, SmBe, SmS in its collapsed metallic phase, EuCuuSi2, TmSe and YbCuAI. They have striking and unusual properties such as the behavior of the magnetic susceptibility below a characteristic temperature To of approxi-

mately 100 K. The magnetic susceptibility approaches a finite value as T -~ 0 K rather than diverging according to a Curie law (i.e. proportional to T - 1 ) or exhibiting some t y p e of magnetic order. The characteristic temperature To is related to the value of the interconfiguration fluctuation lifetime which is approximately 10-13 s; other characteristics include a large electronic specific heat coefficient, a peak in the thermoelectric power near To, and various types of anomalies in the electrical resistivity near To. This class of materials has also been studied by means of MSssbauer and X-ray photoelectron spectroscopy techniques, which support the basic picture of temporal fluctuations between two 4f configurations. A completely adequate theoretical description, particularly one that incorporates the role of the phonons, has yet to emerge. These materials may also have other interesting properties that are applicable to catalysis. Further charting of the intermetallics depends on the preparation of new materials. Traditional methods of preparation impose kinetic limitations (low reaction rates, and long diffusion times at the relatively low formation temperatures of these compounds). There are several innovative techniques being developed at the present time. These include vapor phase condensation, molten salt electrodeposition, growth from liquid metals and direct formation of composite materials in technologically useful configurations. Although vapor phase condensation is n o t new, the hardware and techniques that have been developed in the past few years make possible a major increase in the number of known compounds. Solid state reactions, which at low temperatures require months or longer, can occur in seconds. Thus portions of phase diagrams involving a refractory metal as one c o m p o n e n t can be explored at temperatures below 1000 °C. Similarly materials with low or very incongruent melting points can be prepared easily as can phases that become highly disordered or defect laden at temperatures necessary for classical synthesis. Non-equilibrium crystal structures can be produced, e.g. NbsSi. Normally the elements niobium and silicon react at elevated temperatures to form a stable c o m p o u n d with the E1-Fea(P , B) or TiaP structure. It has been found, however, that films deposited below 850 °C form in the A15 structure [85]. Classical metallurgical techniques alone would

165 never be able to isolate the A15 phase of NbsSi but, once the necessary conditions for obtaining it have been established, other methods of fabrication m a y become possible. Since vapor phase techniques are used in the preparation of thin films, more details will be given in Section 6. Techniques have been developed for the growth of single crystals of LaB 6 [86] and PrB 6 by the electrolysis of suitable molten salt solutions in the 700 - 800 °C temperature range. Large centimeter-sized crystals were produced at low current densities (approximately 20 mA cm -2). Examples of other c o m p o u n d s synthesized by these electrolysis techniques include Zr3B4, TiB2, MnB, Ni2Ge, M n f G e 3 , Cr3Ge, VS 4 and a range of Cr-B c o m p o u n d s including Cr3B4, CrB4, CrfBa, Cr2B , Cr3B 2 and Cr4B. The range of possibilities for preparing new and unusual compositions by this method is extremely broad. Although the flux growth o f high melting oxide crystals, e.g. garnets, and the growth of semiconductor single-crystal alloy layers for semiconductor devices from liquid metal solutions are well-established laboratory and technological processes, little has been done on the synthesis and growth of intermetallic c o m p o u n d s from low melting point metallic or salt solutions. Recently, the rare earth borides ((RE)B 4 and (RE)B 6 ) were synthesized from liquid aluminum [ 8 7 ] . There are, of course, other low melting point elements, eutectic alloys and mixtures [ 8 8 ] . This opens an o p p o r t u n i t y for the synthesis of many new three- and four-component intermetallics, particularly those that might contain stoichiometric amounts of the flux components. However, an etch is needed to remove the solidified flux from grown crystals, and this may limit the choice of fluxes. Synthesis from low melting solutions offers several advantages over conventional methods including the possibility of single-crystal growth. Synthesis and crystal growth from a novel solvent system offers the possibility of obtaining a purer phase if the impurities in the solutes segregate into the host liquid during growth. Finally, the melting points of the constituents in the c o m p o u n d may be such that they can only be grown from solution. For example, many of the intermetallic c o m p o u n d s of scientific and technological interest generally

contain transition elements and exhibit such high melting points that inert crucibles are not available. Some intermetallics may contain one c o m p o n e n t that is volatile and thus require a low temperature synthesis technique, or the c o m p o u n d s of interest may form incongruently from the melt. Fundamental research on materials systems is usually carried o u t on samples which are, as far as possible, homogeneous and defect free across the bulk of the material. Materials technology, however, frequently necessitates the deliberate introduction of a controlled variation in either composition or imperfections on various scales at the surface or in the interior of materials. A few outstanding examples of this include the semiconductor p - n junction, precipitation-hardened alloys for great mechanical strength, fiber-reinforced plastics for light-weight structures and the filamentary superconductors for high field magnets and electrical machines. Electrical machines represent an area in which technology for intermetallic c o m p o u n d s is rapidly developing. This will be discussed in more detail in order to illustrate the great potential of composite intermetallic compounds. High field high current density superconductors were discovered a b o u t 20 years ago. It was immediately apparent that these materials could be used to construct zero-loss electromagnets of great value to an important group of energy technologies including electric p o w e r magnetohydrodynamics and fusion reactors. The actual development of superconducting magnets then revealed that the superconducting wire could n o t be used in bulk form. This was due to magnetic instabilities or flux jumps which occurred during any change in current or field within the magnet. These instabilities caused an uncontrolled normalization of the magnets during excitation. To overcome the magnetic instability problem, composite conductors were prepared in which the superconductor consisted of a large n u m b e r of fine filaments embedded in a good normal c o n d u c t o r such as copper. This procedure reduced the large flux jumps to many small jumps, which released less energy per unit volume. Further advances included further segmentation of the copper with alloy zones to reduce electromagnetic losses. At present, fine-filament composite superconductors are produced by a multistep repetitive

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process of bundling copper and superconductor bars together and drawing down to small sizes. This is an expensive process. It lends itself very well to certain superconducting alloys, such as N b - T i . These alloys have rather low critical temperatures (about 10 K) and maximum upper critical fields around 10 T. Magnet designers would prefer to use other materials, in particular the A15 intermetallic compounds, for which Tc and Hc2 are roughly double the values for Nb-Ti. Unfortunately, the A15 c o m p o u n d s are brittle and cannot be drawn down to a fine-filament form in their native state. One solution to this problem is to draw d o w n one of the components of the A15 compound, e.g. niobium, within a matrix of bronze and then to form the A15 intermetallic at the end of the drawing process by a high temperature diffusion step. This has been quite successful, b u t it has certain disadvantages. The most serious one is the high residual tin impurity level in the copper stabilizer. Furthermore, for some applications the wire must be wound into its final configuration before the high temperature diffusion step produces the brittle A15 reaction product. To avoid this problem a number of approaches have been suggested. For example, high speed quenching techniques, such as are used to produce amorphous metals, can also produce finely divided crystalline systems. Alternatively, small grain composites can be produced by chemical vapor decomposition carried o u t in a plasma arc reactor. Precipitation of dendrites or filaments from a solvent c o m p o u n d or eutectic alloy is a third approach. Furthermore there is a possibility that these unconventional approaches may also be used to produce finely divided forms of metastable intermetallic c o m p o u n d s which cannot be attained by a diffusion reaction process.

4.2. Research needs and opportunities From the above discussion on preparation it can be seen that there is a need to seek o u t radically new methods of preparing intermetallic (metal-alloy) composite systems of controlled geometry with structures extending d o w n to the 10 nm scale. Innovation in this area will also give the materials designer new opportunities to attain novel performance in the electrical, magnetic, mechanical and physical performance of materials. The rapid development of new sputtering sources, and

of evaporation and vacuum techniques, makes the synthesis of new compounds from the vapor phase a promising and open-ended approach. The use of three or even more independently controlled sources permits the deposition to be carried out over a range of compositions. It is then possible to investigate compositional dependence and to synthesize new phases in a systematic way. The use of organometallic c o m p o u n d s for growth from the vapor phase offers another degree of choice and is an area that has n o t been investigated for the vapor phase growth of intermetallics. Synthesis from molten salt systems (other than the parent binary melts) has been explored little and represents an opportunity to grow large single crystals needed for fundamental studies of intrinsic properties. In view of the striking superconductive properties exhibited by the ternary m o l y b d e n u m chalcogenides and rare earth rhodium borides, some of which arise from the interplay between superconductivity and long-range magnetic order, it seems worthwhile to look for other systems of cluster compounds. It can reasonably be anticipated that, in addition to superconductivity and magnetism, new cluster compounds will be discovered that have other unusual and useful properties such as mechanical strength, resistance to corrosion catalytic activity, phase transitions, superionic conduction etc. and may conceivably exhibit unthought-of new phenomena. Although there has been a rapid advance in the development of new characterization tools for establishing deviation from ideal crystal structures in complex intermetallic compounds, the application of these advanced techniques needs to become more widespread. This is particularly crucial in the more complex intermetallic c o m p o u n d s because the physical properties are dependent on small deviations from stoichiometry or long-range order. Notew o r t h y among the new techniques are high resolution electron microscopy (including electron energy loss analysis or lattice imaging), ion beam analysis techniques (such as Rutherford backscattering, channeling and nuclear resonance scattering), extended X-ray absorption fine structure and diffuse scattering, and neutron techniques (including diffuse scattering). Advances in the above techniques for studying defect structures are expected as soon as atomic resolution imaging in scanning

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transmission electron microscopy becomes practical and high intensity synchrotron X-ray sources and intense pulsed neutron sources become available. In the fabrication of layered composites from the vapor phase, a technique such as Rutherford backscattering can be used to detect the diffusion of layer A into layer B and compound formation can be detected at much lower temperatures and in shorter times than conventional techniques permit. It would be very desirable to develop higher resolution spectroscopies for characterizing electron states in narrow band systems. Materials of current interest include A15 compounds, ternary cluster compounds and intermediate valence rare earth compounds. An energy resolution approaching 10 meV would be particularly useful. There is a need for a systematic study of the binary and pseudobinary A15 compounds as well as of the ternary and mixed ternary systems of Chevrel phases and rare earth rhodium borides, in order to unravel the electronic and ionic contributions to bonding and other physical properties such as superconductivity, magnetism and charge density waves. The ternary compounds represent a challenge to solid state theory since a description of the ground state of the coupled superconducting and magnetic system goes well beyond the Bardeen, Cooper and Schrieffe r theory of superconductivity. The theory of intermediate valence compounds also needs considerable development.

4.3. Technological needs and opportunities As we look to future applications of intermetallic compounds, it appears that they will enter into almost every phase of advanced energy technology, including production, conversion, storage, distribution and utilization of energy. There will be an increasing demand for hard or refractory coatings resistant to erosion, corrosion and ablation at a variety of temperatures. Such protective coatings will be needed in coal liquefaction and gasification plants, breeder reactors, magnetohydrodynamic power generators and in deep geothermal systems. They will be put in place by plasma spraying, chemical vapor deposition and probably ion implantation. Research on new methods of surface coating is vital.

There will be a need for new composite or multicomponent high strength refractory material systems for turbine blades in gas and steam turbines. These materials will enable higher inlet operating temperatures and thus greater efficiency in power plants. High temperature materials will be needed to withstand extreme nuclear radiation environments in the fuel system of breeder reactors and in the first wall of fusion reactors. These materials must retain strength and be resistant to swelling caused by intense neutron bombardment. Intermetallic compounds seem to offer the best candidates for these applications. New magnetic intermetallic compounds are vitally needed for application to electrical machinery. In particular, ways must be found of replacing cobalt in the rare earth-Co systems because of the scarcity and high cost of cobalt. If these systems can be reduced in' cost or a higher B .H product can be attained, they will find wide application in reducing losses in electrical motors and generators. Possibly, high saturation rare earth compounds can also be applied to cooling and heating systems (magnetic heat pumps). Superconducting technology is in the process of moving out of the laboratory and into prototype electrical power generators, ship propulsion systems and fusion power magnets. In all cases, the attraction is greater efficiency or energy saving. To accelerate progress in this new technology, we need better superconducting properties {To, He2, J¢) combined with high mechanical strength and electromagnetic stability. Intermetallic compounds of the A15 and related structures offer the best avenue of progress to these goals and intensified research on these superconductors is essential. The forthcoming decline in crude oil production and the substitution of alternative hydrocarbon resources to supply the fuel and petrochemical needs of the world will require new chemical reactions which will, in turn, require the availability of suitable catalysts. The compounds of metals with unfilled d and f bands are of particular interest as potential catalysts. Many compounds exhibit solid state phase transitions in which major changes in physical properties may occur over a narrow range of temperature. Steep variations in electrical

168 conductivity, dielectric constant or magnetic susceptibility will often occur at these phase changes. Such property changes provide an excellent basis for new sensor development. Sensors are required to provide critical temperature rise signals in power equipment or, in some cases, to provide direct feedback to limit electric current or voltage to the device. Alternatively, there is a need for new solid state sensors to detect the concentration of various gases in combustion systems, e.g. 0 2 , CO2, CO, SO2. This t y p e of sensor is vital to improving the efficiency of combustion and for control o f undesirable emissions. For the future development of hydrogen as a fuel, possibly coupled with its direct generation in solar photoelectrochemical cells, there is a need for better means of storing hydrogen gas to replace compressed gas cylinders. Many solids, e.g. palladium, absorb hydrogen very well b u t cannot be considered as bulk storage materials because of cost. There is a need for low cost intermetallic c o m p o u n d s to perform this function. Recent research on intercalated graphite c o m p o u n d s has suggested that it may be possible to synthesize conductors which at room temperature have an electrical conductivity higher than that of metallic copper. There are some difficulties connected with extreme anisotropy in such materials, b u t the importance of a breakthrough of this type to the electrical power industry is great enough to warrant further study.

5. AMORPHOUS SOLIDS 5.1. Evaluation There are certain universal characteristics associated with the amorphous state that can be discussed for all classes of amorphous material. These include formation of the amorphous state, its stability, its structure, the influence of impurities, atomic transport phenomena and various transformation processes. Special behavior such as superconductivity, easy magnetization, corrosion resistance, ductility, hardness, semiconductivity, ionic conductivity and resistance to leaching depend on the particular atomic composition. This evaluation will begin with a discussion of universal characteristics and end with a consideration of three classes of amorphous solids:

metallic alloy glasses, amorphous semiconductors and amorphous oxides. The areas where our knowledge is inadequate will be apparent. 5.1.1. F o r m a t i o n and stability It has been conjectured that every liquid would solidify into an amorphous form if crystallization did not intervene [ 8 9 ] . A liquid just below its liquidus temperature T1 may solidify slowly because the latent heat of crystallization must be removed from the bulk material or partial remelting will occur. Nucleation of crystallite embryos can occur homogeneously, i.e. at random in the material, or heterogeneously, i.e. at the location of impurities or at internal and external interfaces. If the liquid is cooled to a temperature well below T1, crystallization may be inhibited because diffusive atomic transport to sites of nucleation is slowed. This is particularly true in more complicated alloys near eutectic compositions where a considerable amount of atomic rearrangement is required in going from the fairly homogeneous liquid to the separate crystalline phases that exist below the liquidus. This is believed to be the reason that it is possible to make alloy glass from such solutions as Pds0Si20, Cu60Zr40, Fes0B20, La76Au24 and U70Cra0 [90, 9 1 ] . In other materials such as silicon, silicates, borates and mixed oxide materials, strong covalent bonding is responsible for inhibited atomic transport and hence crystallization. In addition to kinetic factors, there are thermodynamic factors of importance. The most important of these is the difference in the free energy of the amorphous state and that of the crystalline state. This in turn depends mainly on bonding energies and h o w these energies are influenced by the symmetry of electronic wavefunctions. Another important factor in determining the atomic configuration is the kinetic path taken. There are many ways to achieve an amorphous state including quenching from the melt, electrodeposition, sputtering, vapor phase condensation on cold substrates and even metamictization by heavy radiation damage. Discussion of melt quenching requires the definition of several characteristic temperatures [ 92]. The glass transition temperature Tg is generally defined as the point of inflection of the rising curve of specific heat as a function of temperature. It is also defined as the maximum in the second derivative of

169 the volume versus temperature curve. In either case there is some ambiguity because the location of the inflection point and the maxim u m depend somewhat on the rate of cooling. This is of minor concern because the range of Tg relative to the average value is usually less than 5%. Experience indicates that with no exception the reduced glass temperature Trg = Tg/Tl is less than unity, implying that all glasses are less stable than some crystallized state of the material. This crystallized state is seldom a single phase when the liquid is a solution. In fact, even in the amorphous state, some mixed oxides such as (BaO)~ (SiO2)100-x exhibit a mixture of amorphous phases as a reflection of the miscibility gap in the phase diagram [93] or inflection points in the free energy versus composition plot. The nonmetallic liquids, e.g. silica and o-terphenyl, which can be slowly cooled in large continuous masses to glass form, all exhibit 2 ~< Trg ~< 1.0. Further, the occurrence of homogeneous crystal nucleation in these materials appears to be negligible; they devitrify only if seeded. In contrast, for the metallic alloy glass formers so far identified it has been observed that 0.45 < T~g ~< 0.70 [90, 94, 9 5 ] . Another temperature of importance is Tm, the average per atom of the thermodynamic melting temperatures of the individual pure constituents. Generally the liquidus temperature of allo_y glass formers is greatly depressed relative to Tm [94 - 97] ; it is observed that 0.40 < TI/Tm < 0.79. If the glass is reheated at a specific rate from below Tg, there is a fairly definite onset of rapid crystallization of the glass at a temperature Tkc, which is called the kinetic crystallization temperature. The d i s p l a c e m e n t (Tkc -- Tg )/Tg is usually small (not more than 0.15) [90, 94] ;it is sometimes negative so that the glass transition is obscured. There apparently is no simple inequality involving T~ and Tg for alloy glasses that would distinguish them from non-metallic glasses. For many such alloys Tg/Tm is a b o u t 0.39 + 0.03, b u t there are some marked exceptions. The last temperature to be introduced is TN, the temperature at which the homogeneous crystal nucleation rate reaches measurable values when a liquid is undercooled. There are technological reasons w h y it is desirable to distinguish between the onset o f homogeneous and heterogeneous nucleation. If TN/T1 is less

than a b o u t 0.8 and the melt can be freed of heterogeneous nucleation sites, then homogeneous nucleation in those alloy glass formers for which Trg ~< 0.6 ought to be negligibly small at all reasonable cooling rates [ 9 8 ] . The technological implication is that we may be able to make some alloy glasses in bulk form instead of the thin layers n o w required for rapid cooling. Alternatively, if crystal nuclei are formed copiously at relatively slow quench rates (either by homogeneous nucleation or by secondary nucleation processes starting from impurities), it may be possible to obtain t w o types of microcrystalline structures by varying quench rates. The first would have a high density of microcrystallites dispersed in an amorphous matrix. This structure would have a high mechanical strength. If the amorphous matrix were superconducting, the microcrystallites would serve as flux-pinning centers. The second type is a completely microcrystalline structure which, depending on the composition, might exhibit outstanding mechanical strength or high critical currents in superconduction [ 9 9 ] . We note that the U.S. Government is supporting some research in these areas b u t the magnitude of the effort is small relative to that in other fields of amorphous metal research. There seems to be no p r o o f of the occurrence or non-occurrence of homogeneous nucleation in undercooled alloy-glass-forming melts. However, there are scattered observations which suggest that TN/Tl is less than 0.80. By way of reference TN/TI is less than 0.75 for most pure metals. As mentioned above, alloy glasses form most easily near eutectic compositions. As we move away from these compositions in the phase diagram, crystallization occurs more rapidly and hence higher quench rates are required to reach the amorphous state. The highest quench rate to date is nearly 1 TK S-1 attained in very thin (less than 0.1 pm) molten metal layers by picosecond laser pulsing [100]. This capability would seem to offer the o p p o r t u n i t y to extend markedly the composition range of glass formation by melt quenching and so to obtain glasses with Trg well below 0.45. There are some empirical correlations of Tg with various parameters but none appears to be wholly successful. There is no satisfactory microscopic theory for Tx. If there existed either a successful phenomenological

170 or a microscopic theory, it might be possible to identify those alloys that have Trg > 0.7 and so have a greater potential for bulk glass formation. Furthermore, there seems to be no clear answer from either theory or experience to the question of h o w low the impurity concentration must be before impurities no longer influence glass formation from the melt or the vapor phase.

5.1.2. Structure and defects The physical and electronic properties of amorphous materials depend to a great extent on the atomic scale structure of these materials. The nature and extent of chemical and topological short-range order are of fundamental importance. X-ray, electron and neutron diffraction methods give useful information in the form of atomic pair correlation functions [ 1 0 1 ] . Local structure near impurity sites in laser glasses can be investigated using laserinduced fluorescence line narrowing and optical site selection spectroscopy [ 1 0 2 ] . More detailed information concerning structure and dynamics of molecular clusters within the liquid and amorphous state is available from less commonly used probes such as nuclear magnetic resonance and MSssbauer spectroscopy. Little use has been made of recently developed X-ray absorption fine structure techniques [ 1 0 3 ] , which promise to yield considerable information on the atomicscale chemistry of amorphous materials. Smallangle X-ray scattering offers a rather unexploited means of studying defect structures in these materials. Internal friction measurements and possibly Raman spectroscopy may also be a useful means of detecting defect structures. The concept of a defect in amorphous materials needs further clarification. In amorphous semiconductors, defects can be identified by their influence on electron transport and optical properties. In amorphous metals, defects appear to govern the plastic deformation behavior of the materials. Other material properties are probably influenced by defect structures and this area of research should be explored. The same material prepared by different methods is frequently found to exhibit rather different properties. Could it be that materials prepared by different methods relax to different structures on thermal aging? Can the various materials be described by their deviation from an ideal amorphous structure?

Radiation damage provides a controlled means of altering the atomic-scale structure of amorphous materials. Little has been done y e t to determine the nature and effect of neutron, heavy ion and electron damage on the structure and properties. This is particularly important since existing data [104, 105] suggest that many properties of amorphous metals are highly insensitive to radiation damage. There are many promising applications o f amorphous materials to fusion technology. They include inner wall materials, structural materials and superconducting magnets. Finally, little or no attention has been paid to the structure, composition and short-range order of the surface and interface regions in amorphous materials. Since this is of importance for such areas as corrosion, wear and catalysis, advantage should be taken of the opportunities that exist to utilize recently developed surface techniques on amorphous materials.

5.1.3. Atomic transport and transformations An amorphous solid, metallic or nonmetallic, represents a frozen atomic configurational state less stable than some crystalline state and possibly less stable than another amorphous atomic configuration. If annealed for a short time, it may relax toward this other amorphous configuration. If annealed for a longer time or at a higher temperature, phase separations may occur and eventually complete crystallization may take place. The relaxation process depends on atomic transport, which in turn may be considered to be the product of a kinetic factor and a thermodynamic term determined by the difference in the free energies of the configurational states of the atoms. The kinetic factor will be proportional to some frequency or an average of the frequencies representing hopping rates between full and vacant sites or rates of attack against atomic potential energy barriers. There are several kinetic frequencies of interest. The frequency k , of flow or shear relaxation is proportional to the reciprocal of viscosity ~. The diffusive transport frequency k D is proportional to the atomic mobility. Crystallization is controlled by the frequency ku of interfaciai rearrangement. Because rapid cooling leaves vacancies in the structure, there is a frequency for volume relaxation processes called kv. In non-metallic-glass-forming liquids

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these constants often, b u t n o t always, scale together. Such scaling is very useful and often permits fairly reliable estimates of kD, ku and kv (which are fairly difficult to measure) from shear viscosities (which can be measured fairly easily to very high accuracies). When these scaling relations are fulfilled, the rates of crystallization, phase separation and volume relaxation, as well as o f shear relaxation, become negligible at temperatures only a few degrees Celsius below the glass temperature. There is evidence that, in some alloy glass formers, appreciable crystal growth and impurity transport occur at temperatures as low as 100 - 150 °C below Tg. Also there seem to be no reported measurements of volume relaxation rates in metal glasses. The atomic diffusivities in alloy glasses are in the region of 10 -18 cm 2 s -1 and lower [106] and hence are difficult to measure with present techniques. Because there has not been a complete set of transport frequency measurements on any alloy glass, there is no test of scaling. Another interesting p h e n o m e n o n that has been observed in alloy glasses is the occurrence of liquid-liquid phase separation on reheating to the glass transition range after melt quenching [ 107, 1 0 8 ] . However, the exploration for such a separation as well as its characterization has been quite limited. We might expect that its occurrence in ternary glassforming alloys might be as widespread as in silicate systems provided that crystallization can be avoided. The onset of crystallization can be minimized by careful selection of annealing temperatures and times. Also it is evident that, the larger (Tkc -- Tg)/Tg is, the easier it will be for liquid-liquid phase separation to be seen. If liquid-liquid phase separation can be carried o u t in such a way that t w o phases are interdispersed on a spatial scale as small as 3.0 - 5.0 nm, such structures might be exploited. The formation of porous metal membranes by selective dissolution of one of the phases could be useful as cathode materials in some electrochemical cell. By crystallization of such a mixed-phase material, we could obtain a high density of flux-pinning centers in a superconducting alloy [ 1 0 9 ] .

5.1.4. Properties of alloy glasses There are several review articles and books in which the properties of alloy glasses are discussed [91, 95, 110, 1 1 1 ] . Only a few

highlights and areas n o t well covered in the literature will be mentioned. Superconducting amorphous alloys have been prepared from both the vapor phase (NbsGe) and the melt (LasoAu20, (Moo.sRUo.2)soP20) [112]. Some of these materials exhibit high magnetic critical fields He2 [ 1 1 3 ] , high mechanical strength and good ductility; others do not. Superconducting transition temperatures Tc are generally between 3 and 8 K. The systematics of Tc in amorphous transition metals have been shown to differ substantially from those of the corresponding crystalline materials [ 112 ]. These differences reflect the fundamental influence of disorder on the electronic structure of d band metals and alloys. Atomic-like properties appear to have a more dominant influence on superconductivity in the amorphous state than they do in the crystalline state. Superconductivity has become a probe for investigating the electronic structure and the electron-phonon interaction in the amorphous state. On the practical side, the critical current density J~ is found to decrease rapidly with increasing magnetic field [ 1 1 4 , 1 1 5 ] , which implies the absence of fluxpinning interactions. As mentioned above, researchers are seeking metastable composites which contain both an amorphous and a crystalline superconducting phase interspersed in a highly refined morphology to increase the flux pinning. The understanding of the electronic properties of amorphous metals poses a unique challenge to solid state theory. As yet, no comprehensive theory of electronic transport in amorphous metals has emerged [ 1 1 6 ] . Among the variety of p h e n o m e n a observed are the anomalous temperature dependence of electrical resistivity [ 117 - 1 1 9 ] , Kondo-type anomalies [ 120, 1 2 1 ] , a positive Hall coefficient in some amorphous alloys [ 122] and a negative magnetoresistance. As yet, little experimental information is available on other transport properties such as the thermoelectric power. Much effort has been devoted to the study of the 3d transition metal alloys (chromium-, manganese-, iron-, cobalt- and nickel-base materials). These alloys have been fairly well characterized and are n o w being developed for technical applications. Several important problems remain in spite of considerable research. Magnetostrictive effects in these

172 materials are still poorly understood and pose a serious obstacle to practical use in transformer cores. The effect of thermal relaxation on magnetic properties is another technically important problem which deserves further study. Magnetic alloys containing rare earth metals have only recently been studied extensively. The nature of magnetic interactions among rare earth ions in amorphous alloys is not well understood. The I%KKY interaction which mediates this interaction is presumably attenuated by the strong electron scattering [123] in the amorphous state, as suggested by deGennes [ 1 2 4 ] . The effect of magnetic ordering on superconductivity in the amorphous state has n o t been studied at all. Let us turn now from electrical properties to chemical and mechanical properties. Several investigations have now indicated that certain F e - C r - m e t a l l o i d alloys are much more resistant to corrosion when in an amorphous form [ 1 2 5 ] . It appears that this exceptional corrosion resistance of amorphous alloys derives from their extraordinary compositional and structural homogeneity on a spatial scale greater than a few ~ingstrSms, and thus the composition and structural singularities that provide the preferred points of corrosive attack on passivating films, e.g. C r 2 O3, are largely lacking. In general it appears that the dissolution of the passive film is an interface-limited process and that the problem is to characterize the dissolution kinetics and the role of structural irregularities thereon. The microscopic theory for the exceptional strength and considerable ductility of a large number of glassy metal alloys is still far from clear [ 1 2 6 ] . However, the problem has attracted widespread attention, and there is now a large effort to characterize and explain the mechanical properties of alloy glasses. A number of amorphous metallic alloys have been found to exhibit Vickers hardnesses in the range 1000 - 2000 kgf mm -2 [127, 1 2 8 ] . The homogeneous nature of the amorphous phase, together with the high hardness strength and resistance to corrosion, suggests possible use of these materials as protective wearresistant coatings.

5.1.5. Properties of amorphous semiconductors The discovery that hydrogenated amorphous silicon (a-Sill x ) can be d o p e d n and p type

[ 129] and the subsequent demonstration of a photovoltaic device based on a-SiHx [130] has stimulated substantial research activity. This material is a challenge t o theorists and experimenters alike. It should be mentioned that, although a-SiHx is considered primarily as a candidate for an inexpensive thin film solar cell, applications as a selective photothermal absorber or as a thermoelectric material have also been discussed. a-SiHx can be made by d.c. or a.c. glow discharge in silane, reactive sputtering of silicon in the presence of hydrogen, chemical vapor deposition from silane and a number of less used techniques such as electrodeposition, ion plating, ion implantation and evaporation. The properties of the product film are affected by a large number of growth parameters including the deposition geometry. The effect of the respective techniques on the film properties is n o t understood. In response to this situation, several laboratories are augmenting their diagnostic capabilities. As a result of this, it has recently been recognized that the inadvertent doping of the material with oxygen, nitrogen and carbon {about 1%) during plasma deposition causes rather striking modifications of the dark current and photoconductivity even at room temperature [ 1 3 1 ] . The role of intentional dopant impurities (boron, ph osph orus) and of so-called modifiers {fluorine) is understood only in a qualitative way. The creation of new defects on the introduction of the boron p-type dopant is n o t y e t understood. Although there is a fairly good framework for understanding the existence of an energy band gap and a pinned Fermi level in amorphous semiconductors [ 132, 1 3 3 ] , there still are theoretical difficulties in interpreting charge transport measurements. Moreover, measurement techniques for carrier lifetime and diffusion length are difficult and controversial. The determination o f the residual density of states within the band gap also is difficult. It is known that hydrogen has t w o roles in a-SiH~. It passivates electrically active defects such as dangling bonds and it enters into the chemical bonding between silicon atoms making a new electronic material. Hydrogen can exist as either a positive or a negative ion; it may be behaving as a negative ion (analogous to oxygen in silicates) in the lattice and as an interstitial positive ion in analogy to sodium in NaSiO 3

173 glasses [ 1 3 4 ] . The hydrogen concentration can be up to 20 at.%. Estimates of the concentration are usually made on the basis of IR absorption, b u t there is no non-destructive routine technique for the precise determination of hydrogen content.

5.1.6. A m o r p h o u s oxides It has recently been shown that the crystalline octahedrally coordinated ferroelectric materials, L i N b O s , LiTaO 3 and BaTiO z [135 1 3 7 ] , can be quenched from the melt to form amorphous dielectrics. A unique unexplained feature of these materials is the fact that their dielectric constants are much larger than those of their crystalline counterparts. Very large dielectric anomalies also appear on heating prior to the onset of crystallization, and the microscopic nature of this effect is also n o t understood. Other properties, such as the pyroelectric effect, suggest that amorphous LiNbO 3 and LiTaO 3 are amorphous ferroelectrics, b u t ferroelectric domains have n o t been observed. Furthermore, amorphous LiNbO3 and LiTaO 3 exhibit room temperature ionic conductivities of 10 -5 ~2-1 cm -1, six orders of magnitude greater than their crystalline counterparts. Because of this, films of amorphous LiNbO 3 and LiTaO 3 are being considered for application as solid electrolytes in lithium-based secondary electrochemical cells. It should also be noted that solid electrolytes based on amorphous polyolefin alkali thiocyanates are being considered by the French. Experience with ordinary NaSiO 3 and borosilicate glasses is the foundation for our understanding of the amorphous state. Until recently these glasses have been adequate for making containers and for optical applications. With the advent of the nuclear waste disposal problem and laser fusion research, new demands are being placed on glass. Instead of acting as an impenetrable barrier (i.e. a bottle), for corrosive chemicals, glass is being asked to retain within its chemical structure transmuting radioactive atoms for m a y b e 10 000 years. Glasses that can last indefinitely as lenses in the highly concentrated light of commercial motion picture projectors suffer structural damage on exposure to a nanosecond laser pulse. It has recently been learned that borosilicate glasses devitrify in a matter of weeks

when subjected to the action of water and steam at high pressures (300 kbar) and temperatures (300 °C) [ 1 3 8 ] . This results in a large increase in leachable surface area from which radioactive cesium easily escapes. The mechanism of devitrification is complex, nonlinear with time and not well understood. More research is needed on the kinetics of nucleation and growth of the crystalline phases both above and below the glass transition temperature and on the partitioning of radionuclides between residual glass and the crystalline phase during devitrification with a view toward developing new complex glasses. With regard to glass lasers and other optical components for laser fusion systems, it was quickly recognized that impurities and surface imperfections had to be reduced to prevent localized optical absorption and that glasses with low indices of refraction were preferable because self-focusing and other non-linear effects scale with the index of refraction [ 139, 140]. Laser-induced damage to thin film optical coatings (antireflection, mirror, polarizer) frequently limits the performance of high power laser systems. These coatings are prepared from multilayer stacks of high and low index materials such as SIO2, TiO2, MgF2 and ZnS. Multicomponent glassy films such as BaSiO3 and barium-aluminum borosilicate have been prepared and found to exhibit low optical attenuation in integrated optical circuit applications [141]. If compositions can be caused to phase separate after deposition, then, by etching the coating to produce a refractive index gradient, a coating with very low reflectivity can be achieved [ 142]. In the glass lasers themselves, the host glass has an important influence on the ability of a lasing ion such as n e o d y m i u m to absorb light from the optical pumping source, to store this energy and to release it on stimulation. Empirical relations have been developed from which laser amplification as a function of incident p h o t o n flux can be calculated [ 1 4 3 ] .

5.2. Research opportunities and needs Because of immediate energy-related applications, certain amorphous materials such as F e - B alloys for transformer cores, a-Sill x for photovoltaics and borosilicates for radioactive waste containment have received considerable support from both industry and government. Hopefully it will continue until the principles

174 governing the properties of these materials are understood. In the area of synthesis there is n o w an opportunity to extend the composition ranges of glass formability by more rapid quenches. New graded-index non-reflective coatings for laser optical systems are needed. In the area of characterization there are three recommendations. The newer characterization techniques should be applied to determine the nature and extent of short-range order and to characterize the surfaces of amorphous materials. Careful measurements are needed to compare amorphous solids of the same composition prepared by different techniques. Crystal nucleation rates in alloy glass formers are very important b u t very difficult to measure and characterize (homogeneous versus heterogeneous). Many experimental and theoretical opportunities exist. Radiation damage is still a good probe for understanding structure and the role of defects on properties. More research is needed in alloy glasses to be used in radiation environments, such as in fusion energy superconducting magnets. Determination of the microscopic parameters which lead to superconductivity and studies of amorphous or amorphous-crystalline composite superconducting materials are of importance both for basic understanding and applications. There should be a further exploration of liquid-liquid phase separation in alloy glasses as a part of measuring atomic transport coefficients. The non-linear interaction of laser light with glasses and coating materials is a very fertile area of research. On the theoretical side there is a need for a truly comprehensive theory of electronic transport, especially in d o p e d semiconductors and oxide glasses and other non-metallic solids. Too little is known both theoretically and experimentally a b o u t magnetic interactions in alloy glasses containing rare earth elements. Also the phenomenon of magnetostriction needs theoretical research. There is an opportunity to develop a comprehensive theory of chemical passivation on surfaces by using the contrast in the behaviors of amorphous and crystalline materials as a guide. Similarly the contrast in the mechanical strengths and ductilities of amorphous and ordinary alloys demands a theoretical understanding on the microscopic scale. A theory, also on the atomic

scale, is required for the bonding levels of metal ions and ionic conduction to aid the search for new materials useful in nuclear waste containment, solid electrolytes and lasers. 5.3. Technological opportunities Although at first sight it may seem to be a disadvantage that alloy glasses are only available as wires or ribbons instead of bulk materials, it should be recognized that the ease and speed with which they can be made and incorporated into composites, woven into braids and laminated into plate form opens new vistas for application. One of the first commercial products was a ductile brazing foil made from a normally brittle nickel-based alloy. Soft magnetic materials for electrical power applications and for magnetic memory devices made from ribbon are being commercially developed. Temperature-stable wirewound resistors are another application. The future looks promising for high strength fibrous and lamellar composite structural materials, corrosion- and wear-resistant coatings for bearings and valves, high strength formable conductors f o r superconducting magnets, and radiation-resistant structural and electrical components. Finally it should be recalled that there is an opportunity to use the liquid-liquid phase separation in some alloys to make porous metal membranes for filtration, osmosis and catalysis.

6. THIN FILMS 6.1. Evaluation The field of thin film research [144, 145] has progressed extremely rapidly in the past 20 years, mainly because of the incentive of technological applications, and particularly electronic circuits n o w demonstrating a great complexity of functions on a single substrate. It is only realistic to assume that the future of the field will again be influenced strongly by the progress expected in semiconductor and magnetic-bubble-integrated circuitry, with their associated techniques of pattern generation by photo, electron-beam and X-ray resist lithography. Strides in thin film research have occurred primarily in materials, techniques and instrumentation being strongly influenced by the

175 semiconductor industry; other aspects of the field remain at a stage where ample opportunity exists for research at a fundamental level. Preparative methods n o t c o m m o n l y used by industry have been neglected b u t are interesting from a research point of view and may be more valuable for other emerging applications. F o r these reasons, although the greatest progress in the field of thin films in the future will probably be associated with semiconductor circuitry, it will only be dealt with briefly here. In a recent publication [146] the research that will be required to support the future of the electronics industry is outlined. The application of thin films in energy technology comes mainly in the areas of photothermal solar absorbers, photovoltaics, coatings in laser fusion systems and corrosionerosion-resistant coatings for fossil and nuclear energy systems. There is also a connection with superconductors for magnetohydrodynamic generators, electric power-conditioning equipment, transmission lines and magnetic confinement fusion. At the m o m e n t , thin films cannot be used directly for superconducting magnets. Practical considerations dictate the need for multifilamentary wire. The next generation o f high T¢ high He2 superconductors are being produced and studied in thin film form. Ways will be found later to make them into usable wires. To help in later discussion, the c o m m o n m e t h o d s of thin film preparation are summarized, and the peculiarities o f each are noted.

6.1.1. Evaporation Using a single source, heated resistively or with an electron beam, this m e t h o d is primarily useful only for elements and certain molecules such as CdS. Using multiple sources, c o m p o u n d s and alloys can be prepared. Molecular beam epitaxy [147] is the growth of single-crystal material b y multiple-source evaporation. In addition to evaporation of the elements comprising the c o m p o u n d , impurities can also be introduced as dopants [ 1 4 8 ] . Multilayer structures, with each layer being a few monolayers of a given c o m p o u n d , have been grown [ 1 4 9 ] . Thus molecular beam epitaxy can be regarded as the most carefully controlled thin film growth to date. Because evaporation is a high vacuum process, all the tools developed for surface science can be utilized and in situ surface studies of the growing film can be

carried out, b u t in practice this is rarely done except in molecular beam epitaxy.

6.1.2. Sputtering By bombarding a target with energetic ions from an electrical discharge, films can be prepared whose composition generally matches that of the target. Thus this m e t h o d is ideal for alloys and compounds. A d.c. discharge is often enhanced by magnetic field confinement (magnetron heads) [150] or by an electron source (triodes). Insulators can be sputtered with r.f.-driven ion sources. Typical gas pressures are 0.1 - 10 Pa; thus gas incorporation is very likely and high vacuum surface diagnostics cannot be used during deposition. The use of focused ion beams [150] allows some improvement in the vacuum level. Getter sputtering [ 1 5 1 ] , in which a liquid-nitrogencooled container is placed around the discharge target and substrate, reduces the partial pressure of active impurity gases. 6.1.3. Plasma deposition A glow discharge is set up as in r.f. sputtering, b u t the substrate is shielded from the sputtered electrode material. A volatile film-forming material is introduced into the plasma. This gas is decomposed by the glow discharge mainly at surfaces (substrate, electrodes or walls), leaving the condensable species as a thin solid film. Reactive carrier gases can be added to synthesize c o m p o u n d s in situ [ 1 5 0 ] . There are a large number of possible gases and glow conditions and hence the types of films that can be produced are almost endless. This m e t h o d has the advantage of low substrate temperatures and high energy efficiency. If the carrier gas is oxygen, very thin protective amorphous oxide coatings can be produced on such metals as aluminum, tantalum, titanium and zirconium. Some organic inlet gases produce polymerized films. 6.1.4. Chemical vapor deposition Chemical vapor deposition [152] is carried o u t by reacting gases over a substrate, usually in a heated tube. Different chemical vapor deposition processes, such as the halide or organometallic methods, depend on the composition of the metal-bearing gas. In situ surface analysis is n o t possible, b u t growth rates are high and thick (greater than 10 p m ) layers can be prepared.

176

6.1.5. Organometallic vapor deposition It has recently been shown [153] that the thermal decomposition of organometallics to form solid solution lattice-matched submicron semiconductor layers will result in devices exhibiting properties comparable with those produced by liquid phase epitaxy and chemical vapor deposition. The use of organometallics should be applicable to the synthesis of thin films of many materials, particularly intermetallic compounds. In addition, it has recently been demonstrated that dissociation of organometallic c o m p o u n d s can be accomplished by laser-induced photodissociation and this offers another possibility of accomplishing the controlled growth of thin films [ 1 5 4 ] . It may also be possible to produce alloy films from t w o or more volatile compounds, e.g. an alkyl c o m p o u n d in interaction with a chloride to produce the alloy with elimination of alkyl chloride. 6.1.6. Liquid phase epitaxy Liquid phase epitaxy [155] is growth from a melt saturated with elements of the material of interest. It has been used particularly for semiconductors such as GaAs and Gal_xAlxAs. Doping of the individual layers can be carried out. 6.1.7. Electroplating Electroplating can take place from aqueous solutions at room temperature, or from molten salts at quite high temperatures, say 600 °C. The substrate to be plated becomes the cathode of the electrolytic cell. Metal ions from solution are deposited at a rate dependent on the current through the cell. In general, evaporation and sputtering are the means most commonly used to prepare thin metallic films; the other techniques discussed above are more specifically applied to certain materials such as semiconductors. For one material, variations in substrate temperature, deposition rate and gas pressure can result in films with a surface which is optically smooth or so granular that it appears black. There are a number of ways in which the surface of a bulk material or even a thin film can be modified to produce more desirable properties such as resistance to corrosion, erosion and wear, or selective optical absorption. The formation of a surface c o m p o u n d by the reaction o f a deposited layer with the bulk is

a relatively simple process and is commonly used to produce transition metal silicides on silicon and protective oxides on metals. By codeposition of metals and insulators or of a metal in oxygen, films have been prepared which exhibit electrical behavior ranging from metallic to insulating [ 1 5 6 ] . Considerable work has been carried o u t on such systems b u t recent interest in the general nature of localization in one, two and three dimensions is already giving new insight into their behavior. The optical properties of such materials are also of interest, in that their absorption can be made to peak at different wavelengths depending on the volume fraction of metal. A promising class of selective solar absorbers derives its absorption characteristic from a two-phase metal-metal oxide structure. The particle size and thickness of these so-called black chrome or black cobalt films lie below optical wavelengths, and their structure and arrangement are n o t known in detail. Therefore the basis is missing for a theoretical effort directed toward improving such films and toward preventing their degradation at desired operating temperatures. Ion implantation is a way of introducing impurity ions into the surface of a material, with the depth of implant extending to 1 pm and being variable with ion energy. It is widely used to dope semiconductors, and c o m p o u n d formation can occur. The implantation of hydrogen into palladium produces a metastable c o m p o u n d whose superconducting Tc is greatly increased over that of pure palladium. In addition to implantation, sputtering and damage of the surface occurs. It has been reported [ 157] that this strongly changes the corrosion resistance and oxidation rates of the implanted-damaged part of the surface. Much further work is required in this field to understand the relative importance of the nature of the implanted ion, its energy, its compounds and sputter damage on surface properties. This is an area where surface tools can be used to advantage, and in situ studies in high resolution electron microscopes (including energy loss spectrometers) should be possible. In some situations, e.g. a fusion reactor first wall, materials will be at elevated temperatures and subjected to intense neutron or electron radiation. The interaction of this displacementproducing radiation with metallic alloys gives rise to the formation of thin layers o f non-

177 equilibrium phases or phase distributions at the surface. The kinetics and morphology of this microstructural evolution can now be studied using high resolution analytical microscopy. Since the process is phenomenologically understood (see refs. 158 and 159 (and particularly ref. 159, pp. 2 - 23, 98 - 1 0 8 and 208 - 213 }) as being the preferential binding of one alloy c o m p o n e n t to one of the mobile radiation-produced defects that tend to annihilate preferentially at defects sinks, this phenomenon can be used to understand d e f e c t - d e f e c t , d e f e c t - s o l u t e and defect-sink interactions. The fresh clean surface produced by thin film formation affords an opportunity for preparing a clean interface between the already deposited layer and other thin film layers. In fact, multiple interfaces can be formed by successive deposition. Epitaxial growth of a film on a fresh underlying film or on a substrate has been demonstrated in a limited number of cases, molecular beam epitaxy of GaAsGaz-x Alx As layers being the primary example. In other cases, lattice matching but n o t singlecrystal growth is achieved. By deposition of N b s G e on the stable c o m p o u n d Nb~Ir it was shown that the A15 phase of Nb3Ge could be extended b e y o n d its equilibrium composition [ 1 6 0 ] . An amorphous silicon layer, deposited onto a grooved silica substrate and subsequently laser annealed, became single crystal with an orientation determined by the grooves [ 161]. Since the grooves were 2 ~m wide, which is huge on an atomic scale, this suggests that the initial nucleation and growth of thin films might be controlled in a predictable way by suitable preparation of substrate surfaces. The use of so-called diffusion couples to measure interdiffusion and c o m p o u n d formation is well known. However, multiple-source deposition allows the formation of films with alternating c o m p o n e n t s where the individual layers become v e r y thin (a few atomic layers) ] 1 6 2 ] . These structures allow the study of very slow diffusion rates, but, more significantly, the interface regions can become a large fraction of the total film. Thus the chance for observing unusual c o m p o u n d s stabilized by interfacial energies is optimized. Again, in situ studies (by X-ray or electron scattering and Rutherford backscattering at low angles) o f such films as a function of annealing temperature should be encouraged.

A clean film surface is ideal for the preparation of tunnel junctions, which are useful probes of superconductors and semiconductors [ 1 6 3 ] , and also for the creation of Schottky barriers and metal-insulator--semiconductor barriers n o w of interest as photovoltaic devices. An interesting new class of layered structures consisting of t w o metals grown epitaxially on one another has recently been reported [164 166]. Compositional modulation has been achieved for b.c.c. N b ( l l 0 ) sputter grown on f.c.c. C u ( l l l ) surfaces in alternate layers of thickness down to 0.5 nm [ 1 6 7 ] . Cross-over behavior characteristic of a transition from two- to three-dimensional superconductivity has been observed in critical magnetic fields parallel to the layers for temperatures below Tc and in fluctuation conductivity above To. This can be explained qualitatively by Josephson coupling in N b - G e composites ] 1 6 8 ] . Work on magnetic-non-magnetic multilayers has also created much interest [ 169]. In addition to structural and electrical properties, thin films have important mechanical properties. There are t w o aspects to the mechanical properties of thin films. The first aspect is the mechanical stability and adhesion of the film to the substrate. The second aspect is the enhancement of mechanical properties of the film over those of the bulk. There is a discontinuity in the stress at the interface between the film and the substrate although the strain is continuous [ 170 ]. If the mechanism that gives rise to the internal stress is homogeneous, only the planar stress components are present in the film. Within a region of the order of a few film thicknesses from the edges, however, both shear stresses and normal stress perpendicular to the plane of the film are present. These additional components, which may be an order of magnitude larger than the homogeneous stress, are important in the failure near film edges or other discontinuities such as pinholes or other large defects. Adhesion is understood in a qualitative way, especially for materials used in the microelectronics field. Slow progress is being made b y the application of surface analytic tools such as electron spectroscopy for chemical analysis and high resolution Auger electron spectroscopy to the interface chemistry. In addition, mechanical techniques for measuring the adhesion are also receiving more attention

178 [ 1 7 1 ] . It is often stated that sputtered films are highly adherent. Presumably this is because sputter cleaning of the surface and the high energy ions give rise to some implantation. The detailed mechanism of this improved adhesion is n o t so clear. It may be that the stress concentration is more widely distributed as a result of intermetallic compounds and the gradual change in composition giving the equivalent of the graded seal which occurs when one metal is coated on another with a thin oxide layer between them. The question of carrier gas entrapment in sputtered films and its effect on mechanical properties has y e t to be answered. Stresses in films are generally of three types. There are homogeneous stresses arising from differential thermal expansion, external stresses and what have been termed "growth or intrinsic stresses" arising in some cases from lattice mismatch. From the point of view of understanding these stresses, it can be said that differential expansion is well understood. Growth stresses are well enough understood in I I I - V compounds and garnets in terms of misfit dislocations. A better understanding is needed in amorphous systems. In general, metallic and single-crystal systems are far better understood than dielectric materials and amorphous or polycrystalline systems. Even in metallic systems, when multiple-source vapor deposition is used, unexpected results sometimes occur. For example, it was found by chance that films of M o s R u 3 being made for superconductivity studies [ 172] exhibited unusual hardness and corrosion resistance [ 173 ]. Another example occurs in the multilayer materials mentioned above [ 1 6 4 ] . Very surprisingly, these multilayer films have a significantly higher modulus than homogeneous foils of the same average composition. In fact some C u - N i films have the highest modulus reported for any material, including diamond. 6.2. Research opportunities and needs As a means of material preparation, thin film methods are unusual in that much of the required equipment, both for attaining vacuum and for film deposition, can be purchased commercially, as can the surface analysis tools that may be added to the same system. There ap~pears to be a danger that basic research in thin films will partially be dictated by what

equipment is most easily available, and thus the field will lack variety and novelty. Unusual approaches to pumping systems, deposition methods and film characterization should be encouraged whenever possible. An example is the work of Thompson [ 1 7 4 ] , who cools an entire vacuum chamber to liquid helium temperatures. The operating pressure is orders of magnitude lower than in conventional systems at room temperature. In the area of synthesis, several ideas suggest themselves for the formation of new structures and also for gaining an understanding of the growth process itself. Deliberate changes could be made in the vapor molecular structure and effects on film growth studied, e.g. atomic As versus As4. Other rich areas involve looking at the production of films from low energy ions, from very complex high temperature vapors where there are unusual molecular aggregates, and under conditions where molecules are excited, either optically or perhaps by electron impact, while they are on the surface, e.g. by focusing a laser on the growing film and comparing film formation in the illuminated region with that outside. At high laser powers, this process would become an in situ annealing of the growing film, which might allow improvements of crystallographic order at low overall substrate temperatures. Laser annealing of completed films, in materials other than semiconductors, should also be studied as a m e t h o d of producing metastable phases. The conventional evaporation and sputter techniques can be utilized in novel ways to prepare materials or to search rapidly for new phases and physical properties within a given alloy or c o m p o u n d system. By controlling the rate of t w o evaporation sources and placing substrates above and parallel to the line between them, a fraction o f the phase diagram of the t w o evaporating constituents can be produced. Similarly, a gradient in substrate temperature can be introduced. Amorphous films can be prepared at lower temperatures. Thus the structural, mechanical and electrical properties of every part of the phase diagram can be surveyed rapidly. This m e t h o d has been used with success recently for A15 superconductors; the structure, the lattice constant, Tc and the energy gap were determined as functions of composition [ 1 7 5 , 1 7 6 ] . It should be noted that in some cases the phase diagram

179 will differ from that of the bulk, in that film deposition can produce metastable phases, NbsGe being one example. Sputtering from multiple sources can be used similarly and, as the substrates can often be placed closer to the sources, almost the whole phase diagram can be produced in a single deposition [ 1 7 7 ] . This m e t h o d was employed to search for magnetic films for bubble circuits [ 178 ]. Use o f magnetron heads appears very desirable, in that their deposition rate is high, the rate can be set by the sputtering current, and unwanted heating of the substrates is small. This multiple-source deposition method, although demonstratedly powerful in the production of materials, is being used only in isolated cases but its use should become much more widespread. Film deposition is often regarded as a rapid quench process to the temperature of the substrate. This has allowed the preparation of metastable phases which cannot be made by conventional bulk methods, as for Nb3Ge where Tc in films has reached 23 K [179] compared with an onset of 17 K in bulk arcmelted, quenched and annealed samples. Also, by the use of a cooled substrate temperature, down to 4 K if necessary, amorphous materials can be readily produced in a form ideal for in situ electrical and structural characterization. With a change in substrate temperature the amorphous-crystalline transition can be studied in a controlled manner. Turning from new synthesis to film characterization, we note that thin film research has been, and still is, plagued by non-reproducibility of films made under supposedly the same conditions. This is because many parameters affect the film, some of which m a y n o t be controlled or even appreciated. Gas impurities can be incorporated in the film from the source, from the vacuum system or from the substrate; metallic impurities from the source, from the chamber walls or from the masks near the substrate can also enter in. The film structure is sensitive to the substrate temperature, the gas pressure in sputtering systems and the nature of nucleation on the substrate. If full understanding of the materials is to be achieved, a much better characterization of the completed films must be carried o u t and their properties must be related to deposition parameters. A more widespread use of in situ surface analysis tools would be a major step

forward. At present, such studies are fairly c o m m o n only in semiconductor molecular beam epitaxy work where high vacuum exists. In the glow discharge environment it is very difficult to use electron probes to obtain in situ characterization of the growing film. Optical absorption is being used to characterize the atomic and molecular species in the glow discharge and perhaps X-ray or low angle very high energy electron scattering can be used to study film formation. Clearly, new tools are needed. In addition to the control of the deposition process and characterization of the resulting film by existing surface and bulk methods, it is often of interest to look at the electronic properties, the impurity content and the structure at a given depth within a film. Many methods accomplish chemical analysis of the film by ion milling. There is a need to identify and develop non-destructive tools which accomplish this and give structural and electronic information as well. Rutherford backscattering is of value b u t the depth resolution is generally a b o u t 10 nm unless low angles are used. Specific nuclear reactions can be used to profile hydrogen and oxygen. Electron microscopy has good lateral resolution but is not very specific in depth. The scanning acoustic microscope should be valuable for film and interface studies [ 180]. Compared with other material forms, thin films have some unique advantages for the study of material properties. To utilize these fully, thin film growth itself should first be understood in more detail. There is a need for fundamental investigations of the detailed atomic-scale mechanisms by which films, both crystalline and amorphous, form and grow. Such understanding will allow greater control over thin film composition, morphology, microstructure, mechanical properties and electronic properties. It could also lead to new selective techniques for producing films of unusual chemical composition and atomic structure. Research of this type is closely allied to other problems in modern surface physics and chemistry. Interesting possibilities for research include combining kinetic measurements with detailed observation and characterization o f incipient films during their early growth stages. Such studies could be carried out as a function of vapor chemical constituents, of surface

180 temperature and composition and of overall incident flux. Another aim would be to characterize the limitations and advantages of each deposition method so that for a given material property the m e t h o d could be chosen to optimize this characteristic. T w o examples, both poorly understood at present, are that sputtering seems to yield Nb3Ge with a higher Tc than that obtained by evaporation, b u t sputtering of polycrystalline silicon yields material which is unsuitable for semiconductor device fabrication. The success of silicon integrated circuitry relies heavily on the insulating properties of silicon oxide. However, many other useful semiconductors {such as GaAs) do n o t grow a thermal oxide of a quality suitable for device use. When integrated circuitry utilizes linewidths of 1 pm, oxide insulators will become thinner than at present. There is at present little basic understanding of the limitations of evaporated dielectrics as insulators. Research is required into the effects of non-uniformity and defects, non-stoichiometry, ionic and electronic conduction and interface reactions with metals and semiconductors. These studies should n o t be restricted to inorganic compounds such as oxides, fluorides, nitrides etc., b u t polymeric thin films should also be considered. The formation of single-crystal polymer monolayers should be possible using techniques of epitaxial growth, plasma polymerization, vapor deposition and liquid crystal orientation using spin coating. Basic questions to be answered include the effect of the substrate on orientation, the limit of film thickness to achieve insulating properties, m o r p h o l o g y and the nature of m e t a l - p o l y m e r interactions, and interdiffusion. In addition to insulating properties, the corrosion resistance, wear resistance, adhesion strength, photosensitivity and radiation sensitivity should be determined. As was pointed o u t above, films with surprising mechanical properties are being found almost serendipitously. This points to the need to study mechanical properties. The problems in this area are those of obtaining wellcharacterized samples and of making adhesion and stress measurements in situ on epitaxial films. Surface physics theory has mainly dealt with a very limited number of solid-vacuum interfaces, particularly the crystal faces of silicon. Thin film research deals with less ideal

materials and interfaces. To contribute to many of the research problems outlined above, theory should begin to consider semiconductordielectric and metal-dielectric interfaces, and the contact between dissimilar metals, particularly electrical resistance. In many cases there is no lattice matching at such interfaces; perhaps an understanding of the defects and grain boundaries generated in such cases will even lead to prediction of interface and surface compounds.

6.3. Technological needs and opportunities In a field which has been stimulated mainly by technology, the division between basic and technological research is somewhat arbitrary and there is strong overlap with the recommendations made above. This section has been arranged in order of decreasing importance to energy needs. Selective absorbers are films and surfaces whose absorbance ~ for solar radiation is high and whose emittance e in the IR is low. One group of selective absorbers are composites of an IR-reflecting metal substrate coated with a thin absorber layer of semiconductor, metaldielectric interference stack or granular metal-oxide film. The second important group comprises simple metal films with high to e ratio, or metal substrates covered with small particles of the same material. Particularly for this latter group, practical limits to the ~ to e ratio requirement need to be determined [ 1 8 1 ] . Photovoltaics will be discussed in Section 9. Although the semiconductor-semiconductor crystalline junction is most efficient at present, other considerations such as the cost of materials, energy cost of production etc. make it worthwhile to examine other heterojunctions. Already the semiconductor-metal and semiconductor-oxide--metal junctions are receiving attention. It would be worth determining, perhaps from carrier lifetime estimations alone, whether the m e t a l - o x i d e - m e t a l junction can ever be a useful photovoltaic cell. In photovoltaic cells, and in all thin film circuitry, interfaces between different materials are subjected to electric field and current flow. The stability of these interfaces, in particular their composition profile and defect distribution, should be investigated. Again, unusual c o m p o u n d formation at the interface might be possible.

181

Given the number of preparative methods for films and surfaces of alloys, intermetallic compounds and amorphous materials mentioned so far, it should be possible to produce surfaces which have desirable properties for a given application. Many films based on transition or rare earth metals are very hard and exhibit corrosion, erosion and wear resistance and some do not oxidize to produce an insulating coating. However, interest in these films is often for another characteristic, such as superconductivity or magnetism, and the mechanical properties are generally not measured or published in any detail. Some way needs to be found to encourage those in the thin film field to be more aware of the importance of such characteristics. Similarly, dielectrics, which have been mentioned above, should be considered as passivating and protecting layers for photovoltaics. A self-replenishing low Z coating for a fusion reactor first wall is required, as are corrosion-resistant structural materials for fission and fusion reactors. Although the physics and applications of microcircuitry will probably be more important commercially than all the above, we c o m m e n t only briefly in view of an earlier publication [ 1 4 6 ] . The thin film geometry can be made as small as desired in one dimension, although uniformity of the film may be hard to achieve. When combined with m o d e m lithographic techniques, it is possible to make two or even all three of the dimensions of a film small, i.e. 100 rim, and thus to approach the electronic wavelength, at least in semiconductors and semimetals. This offers an o p p o r t u n i t y for fundamental studies, particularly of electronic properties of materials, and also for preparing new materials by "molecular engineering" or at least "molecular layer engineering". The use of molecular beam epitaxy in the GaAs-Gal_xAlxAs system, where repetition of monolayers or placement of dopant impurities into specific layers has been achieved, can serve as an example of what might also be done in m a n y other systems, given sufficient understanding and control of the growth process. The technological implications are just being recognized. However, mechanical properties such as diffusion, electromigration, creep and strength have to be considered before the possibilities are known. Thermal boundary resistances

between different materials will also be important both in semiconductor circuitry and in solar power applications.

7. SOLID STATE IONICS 7.1. Evaluation Solid state ionics can be defined as the area of research related to ion transport in a solid and how it affects other properties of the solid. In a general way a fair a m o u n t is k n o w n about the basic mechanism of diffusion in solids. Open crystal structures, high amplitude thermal vibrations, a small ionic radius and a small ionic mass all promote rapid movement of ions through a solid. In relatively simple systems such as AgI, molecular dynamics calculations [182] show the criticality of ionic size and agree with the temperature dependence of the Ag + diffusion constant and density map of Ag ÷ ions as seen by neutron scattering. Good progress is being made on relatively complicated materials such as NaK-/~-A12Oa using model calculations [ 183] and equilibrium statistical mechanics [184]. The influence of intercalated species on the electrical conductivity of graphite can be predicted to some extent from electronic band theory [185]. Not enough is known, however, to engineer new materials for electrochemical applications or even to understand the cooperative effects responsible for the so-called stages in intercalation compounds. For some materials it is even difficult to determine the charge state of the transported ions. Ionic mobility is critical in a number of technologically important areas, including electrochemical cells, hydrogen storage, corrosion of metals, oxidation catalysts, semiconductor junction devices, oxygen sensors for controlling automobile air pollution devices and electrochromic display panels. Except for oxidation catalysts and semiconductor devices, which will be discussed in Sections 8 and 9, the above applications will be described briefly below. In a society relying increasingly on electrical energy, there is a need for storing this energy, particularly in a mobile form. Batteries are one way of doing this. A battery consists of a set of electrochemical cells. A cell has three active components: the anode (reducing elec-

182 trode), the electrolyte (the medium through which charged ions, b u t no electrons, are transferred from one electrode to the other) and the cathode (oxidizing electrode). In addition, there are various inert components, e.g. current collectors, separators and containment materials. In batteries of interest for advanced energy technologies, solid materials play differing roles depending on the configuration of the particular system. High discharge rates preclude solid-solid interfaces so that these cells have either both electrodes solid with a liquid electrolyte, e.g. LiAl-molten salt-FeS, Liorganic-TiS2, or both electrodes liquid with a solid electrolyte, e.g. Na-~-A12Oa-S. New materials can have a marked impact on the energy storage capacity of batteries and on the numbers of discharge-charge cycles that can be obtained. For lithium cells, development has concentrated on TiS2 [ 186 188] and on transition metal oxides, such as V601 a [189]. Although several new compounds come close to TiS2 in actual storage capacity, none shows significant improvements in terms of total requirements for capacity, cost, materials availability and electronic conductivity over the whole range of discharge, reversibility and rate of discharge. A whole new class of cathode materials that are n o w being investigated are those that are amorphous in structure, e.g. MoS2, V2S5, MoS3 [190, 191] and V6Oz3 [ 1 8 9 ] . It should be emphasized, however, that the cathode is no longer the rate~ontrolling step in the development of advanced lithium batteries, and the fundamental understanding of their mode of operation is in good shape. Work is needed on other parts of the system such as the redeposition of the anode material during recharging and the long-term chemical properties of the electrolyte, which are n o t immediately related to solid state ionics. Intercalation-type anodes, e.g. lithium intercalated in aluminum or silicon, are of interest because dendrite formation during charging is prevented because the lithium is dissolved in the host material. Much excellent work has been reported on ~-A1203 for N a - S cells since their ion-conducting properties were reported in 1967 [ 1 9 2 ] . Subsequently, the Lincoln Laboratory [ 193] has discovered, through a basic understanding of the structural needs of ionic mobility, a

number of comparable materials, e.g. Nasicon, which are isotropic in their properties. Such isotropic materials help both the ionic conductivity and the mechanical properties. Ideally, higher conductivities are needed at lower temperatures to reduce corrosion problems associated with the. present high temperature Na-S cells. Also, it would be desirable to operate cells at temperatures sufficiently low (about 200 °C) that plastic components can be used. However, if such electrolytes are discovered, then an alternative cathode will be needed because of the solidification temperature and high viscosity of sulfide melts, which restricts the use of the present N a - S cells to a lower temperature limit of about 300 °C. Another way of storing energy is in the form of metal hydrides from which hydrogen gas can be released by raising the temperature [194, 1 9 5 ] . Hydrogen forms stable hydrides with all the metals in the first two columns of the periodic table, transition metals in the columns starting with scandium, titanium and vanadium, rare earth metals and actinide metals. The specific needs for hydrogen storage media are high and reversible incorporation of hydrogen coupled with light weight and low cost; the temperature and pressure at which this should occur depends on the application in mind. The material should be able to take up and release hydrogen for many cycles before becoming deactivated by impurities or other changes. These criteria eliminate all elementary metals except perhaps magnesium. More success in meeting the criteria has been obtained with intermetallic compounds such as FeTi, LaNi 5 and MgNi. A more basic understanding of hydride formation and decomposition will be needed in order to suggest improved materials. Metal hydride systems are not only of technical interest b u t also of basic interest. For example, the reversible hydriding of LaNi5 was discovered while studying magnetic properties. Some hydrides are superconducting and show unusual effects such as an inverse dependence of T¢ on isotopic mass. A considerable amount of experimental and theoretical work is in progress at this time [ 1 9 6 ] . It should be noted that the problem of hydrogen embrittlement is the reverse of the problem of hydrogen storage. The requirements are opposites to one another. Thus, to minimize hydrogen embrittlement both the

183

solubility and the diffusion coefficient need to be minimized, whereas for storage these need maximizing. What is learned in studies in the one will surely help in advancing the other. One of the more successful applications of solid state ionics to date is the oxygen gauge used in metal processing and in automobiles for control of the air-fuel mixture [ 1 9 7 ] . The sensor in automobiles is normally a tube closed on one end, sticking into the exhaust gas stream. The tube is made of stabilized ZrO2 coated on the inside and outside with a porous inert metal, typically platinum, that acts as electrodes. The difference in oxygen partial pressure across the ZrO2 generates a voltage between the electrodes. Because no significant current flows, the working temperature can be as low as 400 °C. In specialized applications oxygen pressures as low as 10 -2° atm can be measured. Gauges for detection of other gases and for oxygen at lower pressures are under development [ 1 9 8 ] . Another application of ion-conducting materials is the electrochromic flat panel display [ 1 9 9 ] . In an electrochromic material such as WO3 or anodically grown IrO2 films, a persistent b u t reversible color change can be induced by an electric field or current [ 2 0 0 ] . The electrochromic mechanism in WO3 involves the injection of hydrogen into the solid from a liquid or solid electrolyte to form the tungsten bronze HxWO3 [ 2 0 1 ] . Bleaching occurs when protons and electrons are withdrawn from the surface simultaneously. The electrochromic mechanism of anodically grown IrO 2 films is still under investigation b u t appears to involve ionic transport of h y d r o x y l ions. Many of the solid state ionic materials mentioned above, such as the bronzes and the hydrides, form intercalation compounds. An intercalation c o m p o u n d may be defined as a

c o m p o u n d formed reversibly between guest and host species in which the host structure is essentially maintained; it may be a one-, twoor three-dimensional conductor. Typical examples of these are shown in Table 6. The c o m p o u n d Hg3_ ~AsF 6 is an example of a one-dimensional ionic conductor; the transport of mercury is along mercury chains [202, 2 0 3 ] . Graphite and TiS2 form prototypical layered or two-dimensional intercalates. Some of the transition metal oxides contain an extended network of intersecting channels which may be intercalated b y atoms such as hydrogen, lithium and sodium [186, 204] ; examples include ReO3, WO2, V205 and M o O 3. The properties of t w o of these classes of c o m p o u n d s will be discussed in detail to illustrate the state of our understanding. The intercalation c o m p o u n d s of graphites and of the transition metal dichalcogenides were first synthesized in Europe, as is the case with so many of the materials n o w the subject of research in the U.S.A. The alkali metal intercalates of graphite have been known since 1926 [ 2 0 5 ] , whereas those of the dichalcogenides were not reported until 1959 [ 2 0 6 ] . Both classes of c o m p o u n d s were rapidly enlarged b y U.S. efforts driven by an interest in novel superconducting c o m p o u n d s and phase transitions in two-dimensional materials. The initial hopes of finding higher temperature superconductors or evidence for non-phononmediated superconductivity were n o t realized. However, research on the simple binary compounds and their alkali metal intercalation c o m p o u n d s has been extensive because of the discovery of the highly energetic electrochemical intercalation of lithium in TiS2 and its resulting potential for high energy density reversible ambient temperature batteries.

TABLE 6 Examples of intercalation compounds Host

Guest

Sulfides: TiS2, Vl_xFexS2, NIPS3, MoS 3 Oxides: WO3, MOO3, FeOC1, V6013 Metals: LaNis, Al Clays: zeolites Graphite-BN

Metals; organic electron donors Metals; "oxygens"; molecules H; Li Organics (+ ion exchange) Metals; halogens; Lewis acids

184 The existence of independently variable interpenetrating interacting sublattices offers not only technological opportunities b u t also scientific opportunities to discover new physical phenomena or to study well-known phenomena in controllable systems. An example of this is found in the c o m p o u n d s LixTiS2 (0 ~< x < 1) where the lithium array acts as a two-dimensional lattice gas with a tendency to order at certain rational values of x [ 2 0 7 ] . An understanding of this phenomenon on a fundamental level appears possible and is n o w the subject of research at a number of universities. Such an understanding will be important b e y o n d the LixTiS2 compounds and should be useful in identifying some of the key features which control the freedom of ions in ionically conducting solids. Perhaps the most significant implication of the Li-TiS2 cell discovery is that reversible rapid highly energetic redox chemistry is possible at ambient temperature. The search for compounds which undergo similar reactions, i.e. for compounds containing transition metals in a reasonably high oxidation state within a structure which can accommodate alkali ions (lithium, sodium) without the necessity of breaking covalent bonds or radically altering the host structure, has been scientifically fruitful b u t no candidates for battery cathodes have been identified which are significantly better than TiS2. A need exists for a clearly enunciated theory or a set of principles to guide this research. Recent work [191] suggests that a profitable turn in the search for high energy systems might be found in amorphous transition metal sulfides such as M o S s . Intercalated graphite is another interesting solid state ionic material. The two principal applications being discussed are anisotropic electronic conductors and catalystswith a large surface-to-volume ratio. Graphite intercalated with AsF 5 shows a conductivity parallel to the graphite planes which is comparable with that of copper, while the conductivity perpendicular to the plane can be 106 times smaller. There has not been an improvement in the basal plane conductivity for several years. In the meantime, however, a great deal has been learned about the electronic band structure and about the thermal and magnetic properties [208]. The details of the charge transfer between the intercalate and the host are under intense investigation.

7.2. Research opportunities and needs Clearly, a good deal of research could profitably be directed toward the search for other examples of a similar nature. A particularly interesting subset of these materials is one which results from the insertion of the metallocenes dicyclopentadienyl cobalt and dicyclopentadienyl chromium between the layers of the dichalcogenides. The resulting compounds, TaS2-1/4 (CP2C0)i/4+ and TaS2-1/4 (CP2 Cr)i/4+ (Cp = cyclopentadienyl), are superconductors even though the cobalt and chromium ions carry a local magnetic moment. Such complexes m a y have interestingcatalytic implications. Another subclass of molecular compounds of interestisthose in which a long-chain amine is inserted between the layers. These yield compounds containing a molecular bilayer specifically similar to those in cellwalls. These layers are undoubtedly liquid-likeand undergo "crystallization"as the temperature is decreased. Thus we have, sandwiched between metallic layers 6 A thick, a solvent where solution chemistry and diffusion should be of some interest. The liquid-like character of this region should be enhanced by the introduction of disorder via admixture of chains with double bonds and side groups. Overall, such compounds appear to offer an interesting, unusual and variable electrochemical environment which could prove fascinating to explore, but with as yet no k n o w n technological application. Part of the difficulty associated with the study of intercalation is the failure of researchers to prepare crystals suitable for structure work of any intercalation c o m p o u n d , whether it is of the layered dichalcogenides, graphite or other host. This has been of lesser concern for the structurally simpler metallic intercalation compounds than it has been for the molecular c o m p o u n d s where it tends to preclude progress in elucidating the nature of the guest host bond. Electron diffraction and extended X-ray absorption fine structure should help to solve this problem. In order to generate new materials exhibiting the desired ionic motion there is a need for a broadly applicable theory of ion transport at the atomic level. Such a theory should be able to explain which lattice defects are important for diffusion within a solid of its constituent ions and also the probability of incorporating foreign ions or molecules (guest species) into the host lattice. Data are now

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being collected on diffusion in solids in the bulk state, on surfaces and at grain boundaries, so that the time is ripe for a coordinated theory or model of ion transport to be generated. An understanding of the driving force behind intercalation-type reactions would be closely related to such a model. What determines the thermodynamics of this very energetic redox chemistry? H o w can the bonding between the host and the intercalate be described? In particular, will the description be different for metallic hosts compared with insulators or for molecular intercalates compared with single ions? As the intercalate moves, it sees a different electronic environment from instant to instant. H o w does this influence the ionicity of the moving particle? A sufficient number of c o m p o u n d s have n o w been studied that it should be possible to elucidate the role of the crystal structure in determining the magnitude of the ionic mobility. Particularly, what is the relation between the ionic size of the mobile species, the diffusion path size and the ionic mobility; what role does the dimensionality of the lattice play? in addition, does electron delocalization in the host lattice favor diffusion, i.e. reduce the potential wells, over that in an electronic insulator? An understanding here might allow the synthesis o f materials with predetermined properties and in any event will give guidance to the synthetic chemist or the device engineer. Diffusion resulting in the formation of an intercalation c o m p o u n d may possibly be the precursor to a whole new set of composites at the molecular level; an example might involve the incorporation of a m o n o m e r into a structure followed by a polymerization step. Growth of novel zeolites is also being accomplished through construction of the inorganic lattice around long-chain molecules; these molecules are subsequently removed, leaving channels of predetermined size a n d with minimum defects [ 2 0 9 ] .

7.3. Technological opportunities and needs From the above discussion it can be seen that n o t all the technological applications of solid state ionic materials have been explored. In order to take advantage of the opportunities an interdisciplinary approach to this broad class of c o m p o u n d s should be adopted in order to uncover unusual properties. F o r example, materials synthesized in the pursuit

of the electrochemical applications should also be studied for superconducting, catalytic and optical properties etc. The following are t w o specific technological opportunities that should be explored. Graphite intercalates such as CsAsFs, CsAsF5 etc. might be combined with suitable electrolytes to give a battery approximating an alkali-halogen system. Various fluoride-type materials containing non-bonding valence pair species such as bismuth exhibit high anionic conductivity and could conceivably lead to electrochemical devices involving oxide- or halide-containing cathodes or batteries of the type alkali-alkali + and fluoride--fluorine material.

8. H ETERO G EN EO U S CATALYSTS

8.1. Evaluation A catalyst is a substance that accelerates a chemical reaction without itself being consumed. A heterogeneous catalyst is a solid in contact with a gas or liquid. Even though the chemical reactions occur at the catalyst surface, the bulk properties of the catalyst and those of the substrate on which it may be supported are often very important. Catalysts occur in many compositions and structures. They can be simple metals, alloys, intermetallic compounds, intercalates, semiconductors, sulfides, oxides, nitrides, carbides or mixtures of these. They can be crystalline or amorphous. Frequently, defect structures such as single ion vacancies, steps on shear planes are important. A commercial catalyst usually has several different stages of life. The phases present in a material may change dramatically when it is used as a catalyst. For example, C02{M004) 3 is converted to a C02Ss-MoS mixture when used as a hydrodesulfurization catalyst. All catalysts eventually degrade with use, b u t sometimes they can be regenerated. Zeolite catalysts used for cracking petroleum have an effective lifetime of only a few minutes and must be regenerated continuously. The number of different chemical reactions which require catalysts is t o o large to enumerate here. There are hundreds of such reactions, and all can be related to energy in various ways. Furthermore, catalysts can serve critical roles in terms of solving pollution problems

186 and conserving raw materials. In coal conversion, catalysts are important for liquefaction, gasification and the CO-H 2 reaction. Photocatalysts may play an essential role in the utilization of solar energy. Electrocatalysts are important both for producing electricity in fuel cells and for efficiently using electricity in electrolysis cells of various kinds. Heterogeneous catalysts currently play a major role with energy in many different ways, and it can confidently be predicted that this role will become increasingly more important. Improvements in catalysts which might appear minor have already had dramatic impacts on the energy situation. For example, the replacement of part of the amorphous A12 O3-SIO2 cracking catalyst with a crystalline alumina silicate (a zeolite) saves the U.S.A. more than t w o billion dollars per year by converting a larger fraction of crude petroleum into gasoline. Rather than surveying the field, a few specific examples will be given to indicate the state of part of the amorphous AI2Os-SiO2 used for both hydrogenation and oxidation b u t in the latter case the surface may be mainly metal oxide [ 2 1 0 ] . Valuable metals such as platinum are frequently dispersed almost in submonolayers on a support such as SiO2, A120 a or carbon. In this case the support plays an important part in the catalysis. It has been found that the morphology and pattern of metal particles on the support may be controlled by the structure of the support. A metal may wet some supports b u t not others. The metal may interact chemically with the support, as for instance the formation of Pt-A1 alloys when platinum is supported by AI203 at high temperatures in a reducing atmosphere. When the catalyst does not completely cover the support, the reactant, e.g. dissociated hydrogen, may spill over onto the support and increase the rate of hydrogenation per catalyst atom [ 211 ]. The contact potential between the catalyst and the support is important. The charge transfer which occurs is difficult to measure or predict and depends n o t only on surface electronic states but also on the particle size. By using modern characterization tools, such as X-ray photoelectron spectroscopy and extended X-ray absorption fine structure, it has been shown, for example, that platinum on SiO 2 becomes positive. This effect can be changed by chemical reactions with the substrate, e.g. as

for platinum on TiO 2 . After a low temperature reduction (200 °C), this shows typical platinum chemisorption properties~ after higher temperature reduction, with hydrogen ( a t about 400 °C), the TiO2 is significantly reduced to TiO2-x and the contact potential changes so that platinum becomes negatively charged and catalytic activity is reduced [212 - 2 1 4 ] . This effect is currently being explored as a way of controlling catalytic activity. The removal of sulfur from fossil fuels is catalyzed by a so-called Co2(MO4) 3 catalyst. After activation by sulfur, the actual working catalyst is a mixture of Co9Ss and MoS2. In the presence of hydrogen, the catalyst removes sulfur from organic compounds and forms hydrogen sulfide. The activity is highest when the ratio of Co:Mo is a b o u t 1:3. Many models have been proposed to explain the promoter effect of cobalt on MoS2 [215]. It has been suggested that cobalt dissolved in MoS2 changes the electronic structure, b u t the solubility of cobalt in MoS 2 is inadequate to explain the observed dependence on composition. Although MoS2 is a layered structure like TiS2, cobalt does not intercalate to a significant extent. Ternary c o m p o u n d s of cobalt, m o l y b d e n u m and sulfur exist b u t have never been identified in conditioned catalysts. Recently, a spill-over mechanism has been advanced to explain the synergism. Recently, Mobil Oil announced a catalyst called ZSM-5 which fosters the conversion of methanol to gasoline [ 2 1 6 ] . Pilot plants are being built. ZSM-5 is a shape-selective zeolite catalyst. Zeolites are crystalline aluminosilicates with open framework structures so large that organic molecules can diffuse in and out. By various ion exchange processes it is possible to incorporate hydrogen into the structure as interstitial ions. These hydrogen sites are very active (acid) in promoting chemical reactions. The shape selectivity results because certain molecules are t o o large for the pores of the zeolite structure. Many different cations such as transition metals may be ion exchanged into zeolites and p r o m o t e different kinds of reactions [ 2 1 7 ] . The task of evaluating zeolites has just begun. There are 35 naturally occurring zeolites and over a hundred others have been synthesized during the past 30 years [ 2 1 8 ] . The last examples to be considered are selective oxidation catalysts, such as the

187 bismuth molybdates which, for example, convert propylene CH~--CH----CH2 to acrolein CH2-~-CH--CHO in the presence of oxygen [219 - 2 2 1 ] . Experiments using 180 have shown that the oxygen incorporated into acrolein comes from the catalyst. Molecular oxygen is converted to lattice oxygen at the bismuth sites. Electron transfer from molybdenum to bismuth sites is possible because of an overlap of the electron structure as confirmed by band calculations. Oxygen mobility on the surface and in the bulk is apparently facile. 8.2. Research needs and opportunities From the above examples it can be seen that our knowledge of catalysts is patchy. Heterogeneous catalysts have become very advanced materials by a mainly empirical approach. A large problem in suggesting future directions for improved or new catalysts is that we do n o t k n o w where we are now, i.e. we know very little a b o u t some of the most successful and important catalysts that are at present in use. Therefore an area which should receive special emphasis is the application of state-ofthe-art materials characterization techniques to important catalysts. A critical characterization tool would be electron microscopy with microprobe and microdiffraction. Examples of other useful techniques are extended X-ray absorption fine structure, X-ray and UV photoelectron spectroscopies, Auger electron spectroscopy, nuclear magnetic resonance, electron spin resonance and IR and Raman spectroscopies. Since catalysts frequently have several different stages of life, the characterization of catalysis should include the fresh, activated, deactivated and regenerated stages. Once current catalysts become better characterized, ideas will naturally emerge for new and improved catalysts. Surface studies are of obvious importance to catalysis; however, they may not be relevant unless they are performed on the phases and the crystallographic faces actually present in real catalysts. Frequently, such information is lacking even for very important catalysts. There is good reason to believe that the highly empirical approach to new catalysts is no longer successful enough to be encouraged. Further advances are most likely to occur if a systematic approach is stressed. Such studies would combine and correlate careful synthesis,

characterization and reactor studies. For the synthesis part, the composition, structure and synthesis technique would be varied systematically. Materials prepared would then be characterized by techniques such as X-ray diffraction, adsorption measurements and electron microscopy. Reactor studies should be carried out over a wide variety of conditions, and an attempt should be made to derive kinetic expressions. There are many possibilities for both materials and reactions. An example would be the study of Chevrel phases (AxMo6Ss) where A --- Pb, Sn, RE etc. for hydrodesulfurization. The electronic mechanism of oxidationreduction deserves special study. Selective oxidation catalysts are continually being reduced at one type of surface site and being reoxidized at a different site. There must be an effective m e t h o d of passing electrons from the site of catalyst reduction to the site of 'its reoxidation. Valence degeneracy is involved, and detailed knowledge of the electronic structure of the catalyst may be very important in certain types of catalysis. Bulk, as well as surface, diffusion is of great importance in catalysis. It is believed that the high mobility of oxygen anions is a characteristic of some of the best catalysts for selective oxidation. However, there are no reliable data on oxygen mobility in such materials as MoO 3 and bismuth molybdates. The selectivity obtained with zeolite catalysts is generally assumed to be related to the diffusion of molecules in the zeolite pores. More and better diffusion data for molecules in zeolites would improve our understanding and point the way to better catalysts and processes. Many catalysts deactivate because of sintering. A study of diffusion in such catalysts could suggest methods of improving catalyst life. Many important catalysts consist of several phases. The catalytic properties of these mixtures are clearly superior to those of any of the individual phases. One example is the Co2S3-MoS mixture, mentioned above. The nature of the synergistic mechanism is n o t known in such catalysts, and there should be increased effort in this area, particularly to look for replacements for cobalt since it could become scarce in the future. Many oxides have so-called acid sites which are critical to their catalytic properties. There have been many attempts to characterize such

188 sites by chemical adsorption with molecules such as ammonia and by spectroscopic techniques such as IR. However, none of these techniques has been completely satisfactory, and there is a need for a better method to characterize surface acidity. Just as in the case of making new ceramics, a rational synthesis of a catalyst generally depends on an understanding of the physics and chemistry of particulates; also, defects in materials can strongly influence catalytic properties and lifetimes of catalysts. Recent advances in catalysis have come about primarily from research activities in industrial laboratories. Frequently, information is n o t made available on the actual composition or the optimum synthesis m e t h o d for advanced catalysts. This presents a problem for those outside a particular company who desire to carry o u t research on a relevant catalyst. However, this problem cannot be allowed to inhibit progress in this critical field. Industrial laboratories will generally welcome more research on catalytic materials by scientists in academic and government laboratories, and they would certainly be willing to identify many of the important materials relevant to catalysis.

8.3. Technological needs and opportunities There are many ways in which a catalyst advance could be successful from a technological point of view. Generally the more important consiclerations are higher activity, better selectivity to a desired product and longer life. Also it is becoming increasingly important to find replacements for the very costly catalysts such as platinum. Potentially, the most dramatic advance would be the discovery of a catalyst which would catalyze a reaction which had never before been effectively catalyzed, e.g. the oxidation of methane to methanol. Catalysis is at the heart of petroleum refining and will become more critical as the feed stock becomes heavier, since such crudes, whether from traditional sources or from shale, tar sands, coal etc., will require much greater conversion. The ability both to break d o w n large carbonaceous molecules and to remove undesirable species such as sulfur, nitrogen, oxygen and various metals, in particular vanadium and nickel, is especially important. It is likely that petroleum crndes

will become the nation's largest single source of vanadium.

9. SEMICONDUCTORS

9.1. Evaluation Semiconductors are the key materials of the second industrial revolution. They are able to act as switches, and as detectors and emitters of light at high functional densities. Thus semiconductors make possible the inexpensive intelligent machines that have begun to pervade nearly all aspects of industrial economies. Improved information technology contributes to energy conservation and can be considered energy technology in a broad sense. Since studies of future research needs in these areas, including pertinent materials aspects, have been covered elsewhere [ 2 2 2 ] , in this section we shall only deal with general aspects of bulk semiconductors and emphasize some devices such as solar cells and thermoelectric converters [ 2 2 3 ] . Because of more economic alternatives for producing energy, the potential of semiconductors in terrestrial photovoltaic conversion did n o t receive adequate attention until the mid-1970s. There are now substantial applied research and engineering programs under way in the U.S.A. [224, 225] and to a lesser degree in other countries. These programs are justified through economic analyses that indicate there is a realistic chance that photovoltaics may replace some conventional electric-generating capacity [ 2 2 6 ] . Three types of technology are pursued currently to obtain economic photovoltaic converters: concentrators; flat-plate silicon; thin film arrays. In concentrators the semic o n d u c t o r cell area may be ten to ten thousand times smaller than the system aperture. Cells can be comparably expensive (approximately U.S. $1000 m -2 ) b u t must be highly efficient (more than 30%) [ 2 2 6 ] . The best research results to date are 20.5% in silicon cells [ 2 2 7 ] , 25% in cells based on GaAs [ 2 2 8 ] , 28% in a converter that employed spectral splitting for more efficient conversion by a pair of siliconand GaAs-based cells [229] and 26% for a silicon thermophotovoltaic cell designed to convert radiation from a black b o d y at 2000 °C, ultimately to be heated by sunlight [ 2 3 0 ] . Device theory, materials and cell fabrication

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are closely related to those employed in conventional semiconductor technology. Flat-plate silicon arrays represent the only current commercial technology, drawing on 20 years of development for small-scale applications primarily in spacecraft. Conventional single-crystal silicon is used as the starting material but alternatives, such as coarsely polycrystalline (multigrain) cast materials, are in sight. One principal barrier to the rapid introduction of cast and other non-single-crystal silicon is the lack of understanding of growth, electronic properties and processing of polycrystalline semiconductors and junction devices. For silicon cells a specific fabrication technology, distinct from those employed in microcircuit production, has been developed over the years. Owing to the small size of the market, processing steps have evolved incrementally. The challenge n o w is for the rapid introduction of efficient and highly automated production of pure silicon, its crystallization, device processing and module fabrication. Requirements on cost (less than 70 U.S. ¢ Wp-1 generating capacity), performance (a 10% efficiency) and operating life (20 years) of thin film modules have been well defined, but no such cell has been demonstrated in the laboratory. This lack of an obvious candidate that combines minimal materials consumption with acceptable performance in a device structure that is highly amenable to automation explains the very broad screening programs being carried o u t at present. These programs are severely hampered by the absence of elementary information a b o u t optical and electronic properties of the majority of materials of interest. The structure of these materials ranges from amorphous to polycrystalline, but adequate information on device theory, processing characteristics or aging behavior does n o t exist. Thin film modules more expensive than 70 U.S. ¢ Wp-1 or less efficient than 10% can address limited b u t still substantial markets. Commercial production is imminent for Cu28-Cd8 modules for solar applications. The production of small cells based on amorphous Si-H alloys for incorporation in watches began in late 1980. Emphasis on inexpensive thin film devices similarly will provide the motivation for the

screening of candidates for solar thermoelectric converters. Here new materials such as the amorphous Si-H alloys or polyacetylene will be characterized for the three properties that contribute to the figure of merit: the Seebeck coefficient, the electrical conductivity and the thermal conductivity.

9.2. Research opportunities and needs Opportunities may be brokeh d o w n into six items, three of which are related to synthesis or modifications of materials and three of which concern evaluation and understanding existing material.

9.2.1. Preparation and understanding of perfect crystals The group IV semiconductors silicon and germanium have been prepared as more perfect crystals and with a higher purity than any other material. This quest for materials with ideal properties that can be compared with theoretical predictions has generated advanced purification, crystallization and measurement techniques. These techniques and the better theoretical understanding of crystalline solids have been valuable in controlling the properties of a wide range of materials of use in the energy field, Moderately funded b u t longrange work in the growth of silicon or germanium crystals with a zero dislocation density and with a minimum c o n t e n t of low Z impurities (carbon, nitrogen, oxygen) is needed.

9.2.2. Nucleation and growth Structures with a preferred grain orientation and also coatings are needed in a number of energy technologies to perform a variety of tasks. Polycrystalline and thin film cells depend on controlled nucleation and growth with specified grain dimensions. Guidance from a better theoretical understanding of nucleation and growth could shorten the recurring and wasteful search for grain growth conditions. It has been shown recently that nucleation and growth phenomena also affect the structure, and apparently the electrical characteristics, of a-SiHx. In this c o n t e x t the question of directional solidification in a low technology environment merits attention. A n u m b e r of techniques for the preparation o f polycrystalline bulk or sheet silicon would benefit from quantitative guidelines.

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9.2.3. Interaction o f energy beams with semiconductors Another area that is receiving much attention from physicists, materials scientists and electrical engineers is the processing of semiconductors with laser or electron beams. For instance, heavily damaged ion-implanted semiconductor layers have been recrystallized with a single laser pulse. Ion-implanted solar cells have been rendered electrically active with one electron pulse covering a wafer 7.6 cm in diameter. The mechanisms of recrystallization, the distribution of impurities in the temporarily liquid phase and the nature of residual defects under conditions of extremely rapid cooling should be studied. Basic understanding of these issues is likely to lead to new techniques of treating and processing surface layers. 9.2.4. Interaction between defects Numerous properties of solids {plastic behavior, electronic conductivity in semiconductors, impurity diffusion) can be dominated by defects rather than by the matrix material. Since more methods for analysis can be brought to bear on semiconductors than on most other classes of materials, defect studies can attain a scope and a degree of complementarity difficult to reach elsewhere. Efforts should be undertaken to promote comprehensive understanding of interactions between point defects, line defects, and surfaces or interfaces. These studies should cover structural, electronic, thermodynamic and kinetic aspects. Examples of these needs are theoretical and experimental investigations of the energetics of point defect interaction, or the correlation of electronic with metallurgical properties at high concentrations of impurities. 9.2.5. Relation o f bulk to surface properties Much of the current surface research focuses on aspects of surfaces that differ from the bulk. It would be desirable, however, to define guidelines that permit prediction o f surface properties from known bulk data. Clearly, this must be a theoretical effort carried out with a large base of reliable experimental data. There are several questions that might be asked. How do surface layer space charges and the associated field superpose on bulk properties? Are surface and bulk electron affinities identical? Are bulk thermodynamic data applicable to surface reactions?

9.2.6. Electronic structure and optical properties of polycrystalline semiconductors With a few notable exceptions, fundamental research on polycrystalline semiconductors for junction devices has been carried out only during the past 2 or 3 years [231]. Electronic equilibrium and transport properties, the segregation of impurities and the formation of phases in the grain boundary, grain boundary diffusion and passivation, the reduction of grain boundary effects on carrier transport, and the effects of intersecting grain boundaries and p - n junctions need basic understanding. Very fruitful studies could be carried out in the following areas: (1) explanation of the known differences between group IV, III-V and II-VI semiconductors, in majority carrier transport across grain boundaries; (2) measurement of the electronic band structure at grain boundaries and comparison with the existing models; (3) use of high lateral and depth resolution surface analysis methods to study the mechanism of formation and segregation of impurity phases within grain boundaries; (4) application of controlled preparation techniques in conjunction with methods for structural studies to identify the relative uptake o f strain by grain boundary and grain. Another important area of opportunity is the study of amorphous semiconductors such as a-SiHx. Since this topic was covered in Section 5 it will not be discussed here. Research needs are related to material synthesis, characterization and theoretical understanding. Seven items have been identified. 9.2.6.1. Classification o f preparation techniques. A large number of preparation techniques have been used to produce device-quality semiconductors. These include crystallization from the melt or from liquid solution, evaporation, chemical vapor deposition, sputtering and related techniques, ion or neutral beam deposition, electrodeposition, spraying, screen printing etc. Patterns have begun to emerge on the applicability of these techniques. For instance, II-VI semiconductors can be screen printed while group IV or III-V compounds do not yield useful material. Sputtering has shown little promise with conventional semiconductors (as opposed to a-SiHx ), apparently

191

on account of concomitant radiation damage and of difficulty in controlling the grain growth. A survey of published literature on this question could provide very useful guidelines to the experimenter and could avoid unnecessary duplication.

9.2.6.2. Refractory materials. Since the performance of semiconductor devices is highly susceptible to impurities, non-reactive materials for containers, reactors and substrates are important. However, for fabrication temperatures above 500 °C, the number of available refractories is so limited as to be restrictive. In essence, fused SiO 2 and graphite are used for crucibles and shaping dies, fused SiO2 for reactors, and A1203, mullite, carbon, tungsten and m o l y b d e n u m as substrates. A program on new refractory materials as well as on the mechanisms of wetting and reaction would be useful not only to semiconductor and solar cell research but also to other energy conversion technologies. 9.2.6.3. Exploratory preparation o f new semiconductors. Particularly in the area of thin film photovoltaics, a continuous search for new semiconductors would be welcome to broaden the choice of available materials. This search need n o t be carried out in the c o n t e n t of a separate program. Instead, support could be provided to all pertinent synthesis groups for the measurement of the optical absorption edge, the conductivity type and the conductivity of all new compounds. Guidelines for exploring new semiconductors or analogue compounds of known semiconductors could be of value. Such guidelines, possibly derived from semiempirical theory, should indicate the nature (direct or indirect) and the magnitude of the energy gap. 9.2.6.4. Theories for mass and heat transfer during solidification and condensation. Two areas in semiconductor preparation require improved mass and heat transfer theories: crystallization of sheet silicon and deposition of films from vapors. A drawback, c o m m o n to current silicon sheet growth techniques, is the need for extensive instrumentation and highly trained operators to prevent or rectify instabilities that develop easily at the solidification front. Improved theoretical support in the design of film-shaping parts and of crucible,

heater and heat shield geometries could contribute substantially to cost reduction. In chemical vapor deposition the problems of non-uniform deposition over the length of a reactor and of the efficient use of reactant gases need to be addressed, the former in the interest of improved device yield and the latter as a means of materials conservation.

9.2.6.5. Methods for simultaneous structural, electrical or chemical analysis. The most promising techniques for characterizing defects in semiconductors appear to be those combining structural or chemical analysis with electrical measurements. For instance, electron-beam-induced currents produced in a scanning transmission electron microscope have provided the first direct observation of the electrical activity of semiconductor interfaces [232]. Other techniques that could produce useful fundamental information are (1) characterization of electrical and chemical properties on a lateral scale of 10 nm and (2) direct measurement of space charge or its associated field with an energy resolution of better than 0.1 eV. 9. 2. 6.6. Theories o f deep levels and in terfaces and o f carrier recombination. Although the performance of most junction devices is limited by carrier recombination or generation at deep levels within the energy gap or at surfaces, no adequate predictive theories exist. At present it is n o t possible to calculate the energy levels of a given impurity, the density of states within the gap at a surface or interface, or the rate of carrier generation and recombination that proceeds by these defects. Therefore the effect of introducing impurities or of an interface cannot be predicted quantitatively, nor can limits be placed on the ultimate performance of devices. 9.2.6. 7. Photoexcitation and charge separation. Photovoltaic processes different from charge excitation across a band gap and charge separation by a space charge layer do exist. Examples can be found in the photosynthetic apparatus which employs molecular levels and tunneling, and in m e t a l - i n s u l a t o r metal structures that can exhibit internal photoemission, albeit with low efficiency [233, 234]. Tunneling barriers (1 - 5 nm) are typically more than one order of magni-

192

tude narrower than semiconductor space charge layers (100 nm~. Hence the materials saving for tunneling converters would be significant. Provided that very highly absorbing materials (a ~> 10 -~ cm -1 ) can be employed, fundamental research on alternative excitation and separation processes could lead to less expensive photoconversion technologies.

9.3. Technological needs and opportunities In Section 9.2 we provided some technological justification for the research needed. In this section we shall provide more detail on materials and processing needs. Semiconductor technology depends on continued advances in the preparation of crystals and crystalline films of known and well-characterized materials. Silicon, GaAs, InP and alloys of III-V and of I I - V I compounds with high purity and crystalline perfection are needed for high performance devices such as concentrator cells and other optoelectronic devices, and for microcircuits. Preparation and purification techniques need to be explored for "solar-grade" silicon. This is a yet undefined material containing more impurity than silicon used in microelectronics. Solar-grade silicon should be inexpensive b u t exhibit adequate photovoltaic performance. Demands on purity may vary with the method employed for crystallization. For instance, polycrystalline material may offer better opportunities for impurity rejection into grain boundaries and grain boundary phases than single crystals. Thin small-grain polycrystalline films of direct gap materials for photovoltaic conversion have been plagued with non-uniform crystallite size and porosity with the effect that these films have to be grown much thicker (20 - 30 ~m) than the thickness thought to be economically acceptable (about 2 - 5 pm). Technology needs to encompass the selection of proper substrates, control of nucleation and growth and the development of growth techniques that make efficient use of the starting material. In this context, thought should be given to the recycling of unused reactants. A separate category of semiconductors demanding material development is electrodes for photoelectrochemical conversion. The need is for stable electrodes inert against dark corrosion and photocorrosion with band gaps low enough for substantial solar absorption.

Inexpensive or reusable substrates and containers should be explored with emphasis on the principles governing the introduction of impurities, materials losses and reactivity (in the case of reuse). New transparent, electrically and thermally conducting materials such as S n O 2 , indium tin oxide and C d S n O 3 need to be developed for application in contacts or in connecting layers, with emphasis on low contact resistance. A clear need exists for inexpensive contact metallization to reduce the demand for silver, and possibly for gold. Stable intermetallic compounds that do n o t release impurities into the semiconductor, or metals that form a diffusion barrier on initial reaction with the underlying semiconductor, are required. Non-conventional potentially cheaper methods are needed for the fabrication of device-grade materials. Methods that have not been explored adequately are spraying, screen printing, casting of sheets, melt spinning, horizontal growth and electrodeposition. Processing techniques that are amenable to automation need to be explored. Examples of such techniques are sheet growth, sputtering techniques and energy beam techniques for annealing or recrystallization. The problem of grain boundary passivation should be examined in a systematic fashion. In polycrystalline silicon, promising results have been obtained by introducing hydrogen and by in-diffusion of phosphorus (the dopant for the top layer in the solar cell). The mechanisms of passivation are under dispute. Preferential oxidation at grain boundaries has been demonstrated in polycrystalline GaAs films. Two device-oriented materials problems that also fall in the domain of thin film research are the preparation of reproducible and stable tunneling barriers and the very general quest for cell structures that are compatible with polycrystalline materials. With polycrystalline semiconductors, junction formation at low temperatures by producing a Schottky barrier may be preferable to high temperature techniques, such as in-diffusion of a dopant, to avoid shorting or shunting of cells along grain boundaries.

10. SUMMARIZING REMARKS AND CONCLUSIONS

The Panel that produced the material for this paper was given a broad charter and

193 TABLE 7 Analysis of papers published in the Journal of Solid State Chemistry a Country of origin

Publications (%) 1970- 1972

1976- 1978

1979 (9 months)

Australia England France Germany Japan Netherlands U.S.A. All others

8.4 2.8 15.6 3.1 1.1 7.8 47.2 14.0

3.3 5.9 25.7 5.1 7.4 4.2 25.6 22.8

4.4 3.7 25.7 9.6 7.4 8.1 19.1 22.0

Total number of papers

358

544

136

published

aRef. 235. responded by producing short treatises on eight different subjects, any one of which could easily be expanded into a book. There are several important points that can be made on the basis of what has been written. The first of these points is t h a t a large n u m b e r of new compounds and new phases of wellknown compounds can be made w i t h o u t a massive effort, simply by imaginative application of a variety of state-of-the-art techniques. New techniques, beyond the traditional routes of synthesis, that have the potential of vastly increasing the number of k n o w n compounds were discussed in Sections 2, 4 and 6. Transport of the reactants through liquid, vapor or vacuum can be highly controlled with modern instrumentation and feedback techniques. Film synthesis and characterization make it possible to survey phase space over wider ranges of temperature and composition and in much finer detail than heretofore. The second point is that the close coupling of synthesis and characterization efforts with investigations of physical and chemical properties is highly desirable. The ideas that flow around a closed loop of integrated research involving synthesis, characterization, investigation of properties, modification of synthesis etc. generate a stimulating and vigorous environment, conducive to creative research. An example of this was discussed in Section 4 where unusual magnetic and superconducting responses to chemical substitution on certain sites are f o u n d in ternary phases. The use of model systems to learn about relationships

between structure, composition and properties is advocated for polymers, catalysis and semiconductors. The third point is that new phases of wellknown materials, e.g. amorphous materials and thin films, offer an opportunity to increase our basic understanding of these materials and at the same time promise solutions to technical problems. Basic concepts need to be established. What is a defect in an amorphous material? If that concept makes sense, what role do defects play in plastic deformation, diffusion, atomic transport or ionic conduction? What is the origin of the extreme corrosion resistance of certain amorphous alloys? How can electron structures and transport phenomena in amorphous alloys be described? Finally, it should be pointed out that most innovative inorganic synthesis has been accomplished in Europe with the U.S.A. playing a minor and perhaps diminishing role. Until recently, Europe has provided the world with well-trained solid state chemists and has been a source of new compounds. In Table 4 the first synthesis of some p r o t o t y p e compounds are referenced. The relatively small contribution from the U.S.A. is evident. The diminishing role of the U.S.A. has recently been d o c u m e n t e d by the editor of the Journal o f S o l i d S t a t e C h e m i s t r y in an analysis of the number of papers published in that journal with respect to c o u n t r y of origin (Table 7). The relative percentages, comparing the 3 year periods 1970 - 1972 and 1976 - 1978, are given. Also included are the results for

194

the first 9 months of 1979. The apparent gainers are France, Germany and Japan. The enhanced publication rate of the French laboratories is a direct result of increased government support for solid state chemistry research in France. The actual explosion of French publication in this area is even larger than the table indicates since the French have started two materials journals in the French language since 1972. The long time intervals between the first synthesis of the prototype compound and recognition of special properties was a consequence of the fact that compounds were synthesized in Europe for their own sake and not closely coupled to property measurements, phenomena or theory. Recent trends in Europe and Japan show more interest and effort in evaluating newly synthesized materials. The need of the U.S.A. for solid state scientists skilled in synthesis and for sources of compounds to help in overcoming materials problems in energy-related technology will require more university participation in the types of research discussed in this review. Creating the proper climate in the U.S.A. is a challenge to both university faculties and funding agencies.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

ACKNOWLEDGMENTS

We would like to thank the members of the Panel on New Materials not only for the original effort of producing their report but also for reading and correcting the edited version which appears here. We also want to thank M. J. Sienko [235], editor of Journal of Solid State Chemistry, for the use of his analysis of publication trends. We are deeply indebted to Linda Twenty for her painstaking efforts in typing the manuscript. Finally, we would like to acknowledge the support of the Division of Materials Sciences, U.S. Department of Energy, in the preparation of this paper.

21

22 23 24 25

26 27 28

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