MATERIALS SCIENCE

study of the properties of solid materials and how these properties arise from a material's structure. The study encompasses the entire range of properties, including mechanical, thermal, chemical, electric, magnetic, and optical behaviour. The optimal use of materials in applications such as packaging, construction, magnets, batteries, engines, automobile bodies, insulation, catalytic cracking, electronics, and computers depends on the intelligent exploitation of these properties. The properties of materials are determined by their internal structurethat is, the way in which the fundamental parts of the materials are put together. Thus, the atomic structure is the arrangement of the atoms in space, the electron structure is the distribution of the electrons in space and in energy, the defect structure is the distribution of crystal flaws (such as impurities, vacant atomic sites, and dislocations), and the microscopic structure is the size and arrangement of microscopic grains and precipitates. These structures, and their interactions, are responsible for the behaviour of materials. For example, the combination of atomic and electronic structures controls the ease with which electrons can move in or through a solid and therefore determines whether it will be an insulator, a conductor, or a semiconductor; the atomic and defect structures control the ease with which a mechanical disturbance can move through a solid and therefore determine its degree of ductility or brittleness; and the distribution of spinning electrons gives rise to magnetic properties. After World War II economic progress and national defense needs required the development of sophisticated materials, and it was soon apparent that an integration of the knowledge and methods of metallurgy, chemistry, and physics was essential for their development. The field of semiconductor electronics was a prime example of this. The basic work was originally done by physicists, who were oriented toward the analysis of electronic properties of pure, simple solids. But the successful production of good semiconductor devices required a knowledge of defect structure, traditionally the province of the metallurgist, and the importance of impurity control was in many respects a problem of chemistry. By 1960 the integration of the three fields into a new activity was well under way. In the late 1950s the Advanced Research Projects Agency of the U.S. Department of Defense, in cooperation with research universities, sponsored an open competition to establish government-supported research laboratories at a limited number of universities to pursue the integrated study of materials and to educate graduate students in the new field. A dozen such facilities were set up in the United States. The methods of materials science have been extended to the study of polymers, glasses, ceramics, amorphous metals, and even biological materials such as bone. The simple concept of relating properties to structure has resulted in an astonishing variety of advanced materials of great utility. Louis A. Girifalco the study of the properties of solid materials and how those properties are determined by a material's composition and structure. It grew out of an amalgam of solid-state physics, metallurgy, and chemistry, since the rich variety of materials properties cannot be understood within the context of any single classical discipline. With a basic understanding of the origins of properties, materials can be selected or designed for an enormous variety of applications, ranging from structural steels to computer microchips. Materials science is therefore important to engineering activities such as electronics, aerospace, telecommunications, information processing, nuclear power, and energy conversion. This article approaches the subject of materials science through five major fields of application: energy, ground transportation, aerospace, computers and communications, and medicine. The discussions focus on the fundamental requirements of each field of application and on the abilities of various materials to meet those requirements. The many materials studied and applied in materials science are usually divided into four categories: metals, polymers, semiconductors, and ceramics. The sources, processing, and fabrication of these materials are explained at length in several articles: metallurgy; elastomer (natural and synthetic rubber); plastic; man-made fibre; and industrial glass and ceramics. Atomic and molecular structures are discussed in chemical elements and matter. The applications covered in this article are given broad coverage in energy conversion, transportation, electronics, and medicine. Additional reading General works Overviews of the properties and production of all engineering materials can be found in the following texts: James F. Shackelford, Introduction to Materials Science for Engineers, 3rd ed. (1992); William D. Callister, Jr., Materials Science and Engineering: An Introduction, 2nd ed. (1991); Richard A. Flinn and Paul K. Trojan, Engineering Materials and Their Applications, 4th ed. (1990); Donald R. Askeland, The Science and Engineering of Materials, 2nd ed. (1989); and Michael F. Ashby and David R.H. Jones, Engineering Materials: An Introduction to Their Properties and Applications (1980), readable even for those with no previous materials science background and providing clearly described examples of innovative ways to use materials. Materials Science and Engineering for the 1990s (1989) comprehensively describes new directions to be taken in materials science; it is written in a readily comprehensible manner by members of committees of the National Research Council (U.S.). An entire issue of Advanced Materials & Processes, vol. 141, no. 1 (January 1992), is devoted to a forecast of developments in various materials, trends in materials processing, and advances in testing and characterization of materials. Michael B. Bever (ed.), Encyclopedia of Materials Science and Engineering, 8 vol. (1986), with supplementary vols., is a comprehensive reference work. Materials for energy Three journal articles on the topic are Richard S. Claasen and Louis A. Girifalco, Materials for Energy Utilization, Scientific American, 255(4):102104, 109112, 117 (October 1986); Richard S. Claasen, Materials for Advanced Energy Technologies, Science, 191(4227):739745 (Feb. 20, 1976); and Bernard L. Cohen, The Disposal of Radioactive Wastes from Fission Reactors, Scientific American, 236(6):2131 (June 1977). Materials for ground transportation A lucid account of the shift away from conventional steels in modern automobiles is found in the excellent introductory article by W. Dale Compton and Norman A. Gjostein, Materials for Ground Transportation, Scientific American, 255(4):92100 (October 1986). Karen Wright, The Shape of Things to Go, Scientific American, 262(5):92101 (May 1990), projects the effect of advanced technology on automobiles of the future. Materials for aerospace An overview is found in Morris A. Steinberg, Materials for Aerospace, Scientific American, 255(4):6672 (October 1986). An entire issue of Advanced Materials & Processes, vol. 137, no. 4 (April 1990), is devoted to aerospace materials and applications. A special section on frontiers in materials science in Science, 255(5048):10771112 (Feb. 28, 1992), discusses polymers and aircraft engine materials, among other subjects.Works on composites include Tsu-Wei Chou, Roy L. McCullough, and R. Byron Pipes, Composites, Scientific American, 255(4):192203 (October 1986); Roy L. McCullough, Concepts of Fiber-Resin Composites (1971); Stephen W. Tsai and H. Thomas Hahn, Introduction to Composite Materials (1980); and Jack R. Vinson and Tsu-Wei Chou, Composite Materials and Their Use in Structures (1975). Materials for communications A useful introduction is by John S. Mayo, Materials for Information and Communication, Scientific American, 255(4):5866 (October 1986). Discussions of electronic and photonic materials may be found in the following essays, all from AT&T Technical Journal: in vol. 69, no. 6 (November/December 1990), see C. Kumar N. Patel, Materials and Processing: Core Competencies and Strategic Resources, pp. 28; Kenneth E. Benson, Lionel C. Kimerling, and Peter T. Panousis, Reaching the Limits in Silicon Processing, pp. 1631; Elsa Reichmanis and Larry F. Thompson, Challenges in Lithographic Materials and Processes, pp. 3245; and James W. Mitchell, Jorge Luis Valdes, and Gardy Cadet, Benign Precursors for Semiconductor Processing, pp. 101112; in vol. 68, no. 1 (January/February 1989), see Jim E. Clemans et al., Bulk III-V Compound Semiconductor Crystal Growth, pp. 2942; and W. Dexter Johnston, Jr., Michael A. Diguiseppe, and Daniel P. Wilt, Liquid and Vapor Phase Growth of III-V Materials for Photonic Devices, pp. 5363; and in vol. 71, no. 1 (January/February 1992), see John L. Zyskind et al., Erbium-Doped Fiber Amplifiers and the Next Generation of Lightwave Systems, pp. 5362 . Materials for medicine Robert A. Fuller and Jonathan J. Rosen, Materials for Medicine, Scientific American, 255(4):118125 (October 1986), offers an overview of the subject. S.A. Barenberg, Abridged Report of the Committee to Survey the Needs and Opportunities for the Biomaterials Industry, Journal of Biomedical Materials Research, 22(12):126792 (December 1988), surveys the applications of materials in medicine and highlights projected areas of clinical need. Joon Bu Park, Biomaterials Science and Engineering (1984), provides a qualitative university-level introduction to the field of biomaterials. Michael Szycher (ed.), Biocompatible Polymers, Metals, and Composites (1983), is a collection of detailed review articles that covers materials in medicine and biocompatibility and contains a pragmatic assessment of clinical and commercial aspects, including how to sterilize and package biomaterials. Harry R. Allcock and Frederick W. Lampe, Contemporary Polymer Chemistry, 2nd ed. (1990), is a basic textbook of polymer science, providing a university-level introduction to synthesis and characterization of polymers, including biomedical polymers. Advanced biomaterials texts with emphasis on research include Michael Szycher (ed.), High Performance Biomaterials: A Comprehensive Guide to Medical and Pharmaceutical Applications (1991), research articles covering orthopedic and cardiovascular biomaterials as well as most other areas of materials in medicine; Joseph D. Andrade (ed.), Surface and Interfacial Aspects of Biomedical Polymers, vol. 1, Surface Chemistry and Physics (1985), articles on surface characterization methods applied to biomaterials, including a quantitative presentation of the interactions of blood components (especially proteins) with biomaterial surfaces; Howard P. Greisler, New Biologic and Synthetic Vascular Prostheses (1991), a biological perspective on blood interactions, wound healing, and tissue integration with biomaterials and surface modified materials; D.F. Williams, Blood Compatibility, 2 vol. (1987), detailed review articles covering blood interactions with biomaterials and prosthetic devices and methods of modifying the surface of biomaterials; and H.L. Goldsmith and V.T. Turitto, Rheological Aspects of Thrombosis and Hemostasis: Basic Principles and Applications, Thrombosis and Haemostasis, 55(3):415435 (1986), a detailed and quantitative review article that describes and models blood flow and rheology in the vascular system, including the effects of different blood components. Louis A. Girifalco John D. Venables R.L.McCullough Diane S. Kukich C. Kumar N. Patel Roger Eric Marchant Materials for aerospace The primary goal in the selection of materials for aerospace structures is the enhancement of fuel efficiency to increase the distance traveled and the payload delivered. This goal can be attained by developments on two fronts: increased engine efficiency through higher operating temperatures and reduced structural weight. In order to meet these needs, materials scientists look to materials in two broad areasmetal alloys and advanced composite materials. A key factor contributing to the advancement of these new materials is the growing ability to tailor materials to achieve specific properties. Metals Many of the advanced metals currently in use in aircraft were designed specifically for applications in gas-turbine engines, the components of which are exposed to high temperatures, corrosive gases, vibration, and high mechanical loads. During the period of early jet engines (from about 1940 to 1970), design requirements were met by the development of new alloys alone. But the more severe requirements of advanced propulsion systems have driven the development of novel alloys that can withstand temperatures greater than 1,000 C (1,800 F), and the structural performance of such alloys has been improved by developments in the processes of melting and solidification. Materials for computers and communications The basic function of computers and communications systems is to process and transmit information in the form of signals representing data, speech, sound, documents, and visual images. These signals are created, transmitted, and processed as moving electrons or photons, and so the basic materials groups involved are classified as electronic and photonic. In some cases, materials known as optoelectronic bridge these two classes, combining abilities to interact usefully with both electrons and photons. Among the electronic materials are various crystalline semiconductors; metalized film conductors; dielectric films; solders; ceramics and polymers formed into substrates on which circuits are assembled or printed; and gold or copper wiring and cabling. Photonic materials include a number of compound semiconductors designed for light emission or detection; elemental dopants that serve as photonic performance-control agents; metal- or diamond-film heat sinks; metalized films for contacts, physical barriers, and bonding; and silica glass, ceramics, and rare earths for optical fibres. Electronic materials Between 1955 and 1990, improvements and innovations in semiconductor technology increased the performance and decreased the cost of electronic materials and devices by a factor of one millionan achievement unparalleled in the history of any technology. Along with this extraordinary explosion of technology has come an exponentially upward spiral of the capital investment necessary for manufacturing operations. In order to maintain cost-effectiveness and flexibility, radical changes in materials and manufacturing operations will be necessary. Materials for ground transportation The global effort to improve the efficiency of ground transportation vehicles, such as automobiles, buses, trucks, and trains, and thereby reduce the massive amounts of pollutants they emit, provides an excellent context within which to illustrate how materials science functions to develop new or better materials in response to critical human needs. For the automobile industry in particular, the story is a fascinating one in which the desire for lower vehicle weight, reduced emissions, and improved fuel economy has led to intense competition among aluminum, plastics, and steel companies for shares in the enormous markets involved (40 million to 50 million cars and trucks per year worldwide). In this battle, materials scientists have a key role to play because the success of their efforts to develop improved materials will determine the shape and viability of future automobiles. Just how seriously suppliers to the industry view the need either to protect or to increase their share of these enormous markets is demonstrated by their establishing of special programs, consortia, or centres that are specifically designed to develop better alloys, plastics, or ceramics for automotive applications. For example, in the United States a program at the Aluminum Company of America (Alcoa) called the aluminum intensive vehicle (AIV), and a similar one at Reynolds Metals, were established to develop materials and processes for making automobile space frames consisting of aluminum-alloy rods and die-cast connectors joined by welding and adhesive bonding. Not to be outdone, another aluminum company, Alcan Aluminium Limited of Canada, in a program entitled aluminum structured vehicle technology (ASVT), began to investigate the construction of automobile unibodies from adhesively bonded aluminum sheet. The plastics industry, of course, has a powerful interest in replacing as many metal automobile components as possible, and in order to help bring this about a centre called D&S Plastics International was formed in the Detroit, Mich., area of the United States by three corporations. The specific aim of this centre was to develop materials and a process suitable for forming several connected panels or components (e.g., body panels and bumper fascias) simultaneously out of different types of plastics. The centrepiece of the operation was a 4,000-ton co-injection press that could lead to cost reductions as great as 50 percent and thereby make the use of plastics for automotive applications more attractive. In programs such as these, and in many more carried out by vendors and within the automobile companies themselves, materials scientists with specialized training in advanced metals, plastics, and ceramics have been leading a revolution in the automotive industry. The following sections describe specific needs that have been identified for improving the performance of automobiles and other ground-transportation vehicles, as well as approaches that materials scientists have taken in response to those needs. Metals Aluminum Since aluminum has about one-third the density of steel, its substitution for steel in automobiles would seem to be a sensible approach to reducing weight and thereby increasing fuel economy and reducing harmful emissions. Such substitutions cannot be made, however, without due consideration of significant differences in other properties of the two materials. This is one important facet of the materials scientist's jobto help evaluate the suitability of a material for a given application based on how its properties balance against load and performance requirements specified by the design engineer. In this case (aluminum versus steel), it is instructive to consider the materials scientist's approach to evaluating the use of aluminum in automotive panelssuch components as doors, hoods, trunk decks, and roofs that can make up more than 60 percent of a vehicle's weight. Two primary properties of any metal are (1) its yield strength, defined as its ability to resist permanent deformation (such as a fender dent), and (2) its elastic modulus, defined as its ability to resist elastic or springy deflection like a drum head. By alloying, aluminum can be made to have a yield strength equal to a moderately strong steel and therefore to exhibit similar resistance to denting in an automobile panel. On the other hand, alloying does not normally affect the elastic modulus of metals significantly, so that automotive door panels or hoods made from aluminum alloys, all of which have approximately one-third the modulus of steel, would be floppy and suffer large deflections when buffeted by the wind, for example. From this point of view, aluminum would appear to be a marginal choice for body panels. One might attempt to overcome this deficiency by increasing the thickness of the aluminum sheet stock to three times the thickness of the steel it is intended to replace. This, however, would simply increase the weight to roughly that of an equivalent steel structure and thus defeat the purpose of the exercise. Fortunately, as was elegantly demonstrated in 1980 by two British materials scientists, Michael Ashby and David Jones, when proper account is taken of the way an actual door panel deflects, constrained as it is by the door edges, it is possible to use aluminum sheet only slightly thicker than the steel it would replace and still achieve equivalent performance. The net result would be a weight savings of almost two-thirds by the substitution of aluminum for steel on such body components. This suggests that understanding the interrelationship between materials properties and structural design is an important factor in the successful application of materials science. Another important activity of the materials scientist is that of alloy development, which in some cases involves designing alloys for very specific applications. For example, in Alcoa's AIV effort, materials scientists and engineers developed a special casting alloy for use as cast aluminum nodes (connecters) in their space frame design. Ordinarily, metal castings exhibit very little toughness, or ductility, and they are therefore prone to brittle fracture followed by catastrophic failure. Since the integrity of an automobile would be limited by having relatively brittle body components, a proprietary casting alloy and processing procedure were developed that provide a material of much greater ductility than is normally available in a casting alloy. Many other advances in aluminum technology, brought about by materials scientists and design engineers, have led to a greater acceptance of aluminum in automobiles, trucks, buses, and even light rail vehicles. Among these are alloys for air-conditioner components that are designed to be chemically compatible with environmentally safer refrigerants and to withstand the higher pressures required by them. Also, alloys have been developed that combine good formability and corrosion resistance with the ability to achieve maximum strength without heat treating; these alloys develop their strength during the forming operation. As a consequence, the list of vehicles that contain significant quantities of aluminum substituted for steel has steadily grown. A milestone was reached in 1992 with a limited-edition Jaguar sports car that was virtually all aluminum, including the engine, adhesively bonded chassis, and skin. Somewhat less expensive and in full production were Honda's Acura NSX, containing more than 400 kilograms (900 pounds) of aluminum compared with about 70 kilograms for the average automobile, and General Motors' Saturn, with an aluminum engine block and cylinder heads. These vehicles and others took their place alongside the British Land Rover, which was built with all-aluminum body panels beginning in 1948a choice dictated by a shortage of steel during World War II and continued by the manufacturer ever since. Materials for medicine The treatment of many human disease conditions requires surgical intervention in order to assist, augment, sustain, or replace a diseased organ, and such procedures involve the use of materials foreign to the body. These materials, known as biomaterials, include synthetic polymers and, to a lesser extent, biological polymers, metals, and ceramics. Specific applications of biomaterials range from high-volume products such as blood bags, syringes, and needles to more challenging implantable devices designed to augment or replace a diseased human organ. The latter devices are used in cardiovascular, orthopedic, and dental applications as well as in a wide range of invasive treatment and diagnostic systems. Many of these devices have made possible notable clinical successes. For example, in cardiovascular applications, thousands of lives have been saved by heart valves, heart pacemakers, and large-diameter vascular grafts, and orthopedic hip-joint replacements have shown great long-term success in the treatment of patients suffering from debilitating joint diseases. With such a tremendous increase in medical applications, demand for a wide range of biomaterials grows by 5 to 15 percent each year. In the United States the annual market for surgical implants exceeds $10 billion, approximately 10 percent of world demand. Nevertheless, applications of biomaterials are limited by biocompatibility, the problem of adverse interactions arising at the junction between the biomaterial and the host tissue. Optimizing the interactions that occur at the surface of implanted biomaterials represents the most significant key to further advances, and an excellent basis for these advances can be found in the growing understanding of complex biological materials and in the development of novel biomaterials custom-designed at the molecular level for specific medical applications. This section describes biomaterials that are used in medicine, with emphasis on polymer materials and on the challenges associated with implantable devices used in the cardiovascular and orthopedic areas. General requirements of biomaterials Research on developing new biomaterials is an interdisciplinary effort, often involving collaboration among materials scientists and engineers, biomedical engineers, pathologists, and clinicians to solve clinical problems. The design or selection of a specific biomaterial depends on the relative importance of the various properties that are required for the intended medical application. Physical properties that are generally considered include hardness, tensile strength, modulus, and elongation; fatigue strength, which is determined by a material's response to cyclic loads or strains; impact properties; resistance to abrasion and wear; long-term dimensional stability, which is described by a material's viscoelastic properties; swelling in aqueous media; and permeability to gases, water, and small biomolecules. In addition, biomaterials are exposed to human tissues and fluids, so that predicting the results of possible interactions between host and material is an important and unique consideration in using synthetic materials in medicine. Two particularly important issues in biocompatibility are thrombosis, which involves blood coagulation and the adhesion of blood platelets to biomaterial surfaces, and the fibrous-tissue encapsulation of biomaterials that are implanted in soft tissues. Poor selection of materials can lead to clinical problems. One example of this situation was the choice of silicone rubber as a poppet in an early heart valve design. The silicone absorbed lipid from plasma and swelled sufficiently to become trapped between the metal struts of the valve. Another unfortunate choice as a biomaterial was Teflon (trademark), which is noted for its low coefficient of friction and its chemical inertness but which has relatively poor abrasion resistance. Thus, as an occluder in a heart valve or as an acetabular cup in a hip-joint prosthesis, Teflon may eventually wear to such an extent that the device would fail. In addition, degradable polyesterurethane foam was abandoned as a fixation patch for breast prostheses, because it offered a distinct possibility for the release of carcinogenic by-products as it degraded. Besides their constituent polymer molecules, synthetic biomaterials may contain several additives, such as unreacted monomers and catalysts, inorganic fillers or organic plasticizers, antioxidants and stabilizers, and processing lubricants or mold-release agents on the material's surface. In addition, several degradation products may result from the processing, sterilization, storage, and ultimately implantation of a device. Many additives are beneficialfor example, the silica filler that is indispensable in silicone rubber for good mechanical performance or the antioxidants and stabilizers that prevent premature oxidative degradation of polyetherurethanes. Other additives, such as pigments, can be eliminated from biomedical products. Indeed, a medical-grade biomaterial is one that has had nonessential additives and potential contaminants excluded or eliminated from the polymer. In order to achieve this grade, the polymer may need to be solvent-extracted before use, thereby eliminating low-molecular-weight materials. Generally, additives in polymers are regarded with extreme suspicion, because it is often the additives rather than the constituent polymer molecules that are the source of adverse biocompatibility.