Basic Classifications and Engineering Materials
Solid materials have been conveniently grouped into three basic classifications： metals，ceramics and polymers．This scheme is based primarily on chemical makeup and atomic structure, and most materials fall into one distinct grouping or another， although there are some intermediates. In addition，there are three other groups of important engineering materials—composites， semiconductor， and biomaterials． Composites consist of combinations of two or more different materials， whereas semiconductors are utilized because of their unusual electrical characteristics；biomaterials are implanted into the human body．A brief explanation of the material types and representative characteristics is offered next．
Metals： Metallic materials are normally combinations of metallic elements． They have large numbers of nonlocalized electrons；that is，these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons． Metals are extremely good conductors of electricity and heat，and are not transparent to visible light：a polished metal surface has a lustrous appearance． Furthermore， metals are quite strong，yet deformable， which accounts for their extensive use in structural applications．
Ceramics： Ceramics are compounds between metallic and nonmetallic elements： they are most frequently oxides, nitrides，and carbides．The wide
range of materials that falls within this classification includes ceramics that are composed of clay minerals， cement， and glass. These materials are typically insulative to the passage of electricity and heat， and are more resistant to high temperatures and harsh environments than metals and polymers．With regard to mechanical behavior，ceramics are hard but very brittle．
Polymers：Polymers include the familiar plastic and rubber materials． Many of them are organic compounds that are chemically based on carbon，hydrogen， and other nonmetallic elements； furthermore，they have very large molecular structures．These materials typically have low densities and may be extremely flexible．
Composites： A number of composite materials have been engineered that consist of more than one material type．Fiberglass is a familiar example，in which glass fibers are embedded within a polymeric material. A composite is designed to display a combination of the best characteristics of each of the component materials． Fiberglass acquires strength from the glass and flexibility from the polymer． Many of the recent material developments have involved composite materials．
Semiconductors： Semiconductors have electrical properties that are intermediate between the electrical conductors and insulators． Furthermore，the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms，which concentrations may be controlled over very small spatial regions．The semiconductors have made
possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries over the past two decades．
Biomaterials: Biomaterials are employed in components implanted into the human body for replacement of diseased or damaged body parts．These materials must not produce toxic substances and must be compatible with body tissue(i．e．must not cause adverse biological reactions)．All of the above materials--metals，ceramics，polymers，composites and semiconductors--may be used as biomaterials．For example，some of the biomaterials such as CF／C(carbon fibers／carbon)and CF／PS (polysulfone) are utilized in artificial hip replacements．
Advanced Materials
Materials that are utilized in high-technology (or high-tech) applications are sometimes termed advanced materials．By high technology we mean a device or product that operates or functions using relatively intricate and sophisticated principles；examples include electronic equipment(VCRs，CD players，etc．)，computers，fiberoptic systems，spacecraft，aircraft，and military rocketry．These advanced materials are typically either traditional materials whose properties have been enhanced or newly developed，high—performance materials．Furthermore，they may be of all material types(e．g．metals，ceramics，polymers)，and are normally relatively expensive．
Modern Materials Needs
In spite of the tremendous progress that has been made in the discipline of materials science and engineering within the past few years，there still remain technological challenges，including the development of even more sophisticated and specialized materials，as well as consideration of the environmental impact of materials production．Some comment is appropriate relative to these issues so as to round out this perspective．
Nuclear energy holds some promise，but the solutions to the many problems that remain will necessarily involve materials from fuels to containment structures and facilities for the disposal of radioactive waste．
Significant quantities of energy are involved in transportation．Reducing the weight of transportation vehicles(automobiles，aircraft，trains，etc．)，as well as increasing engine operating temperatures，will enhance fuel efficiency．New high strength，low—density structural materials remain to be developed，as well as materials that have higher—temperature capabilities，for use in engine components．
Furthermore，there is a recognized need to find new，economical sources of energy，and to use the present resources more efficiently．Materials will undoubtedly play a significant role in these developments．For example，the direct conversion of solar energy into electrical energy uses silicon materials．To ensure a viable technology，materials that are highly efficient in this conversion process yet less costly must be developed．
Additionally，environmental quality depends on our ability to control air and water pollution．Pollution control techniques employ various materials． In addition, materials Processing and refinement methods need to be improved so that they produce less environmental degradation，that is，less pollution and less despoilage of the landscape from mining of raw materials． Also， in some materials manufacturing processes，toxic substances are Produced，and the ecological impact of their disposal must be considered.
Many materials that we use are derived from resources that are noenewable，that is not capable of being regenerated．These include polymers， for which the prime raw material is oil， and some metals．These noenewable resources are gradually becoming depleted, which necessitates： (1)the discovery of additional reserves， (2)the development of new materials having comparable properties with less adverse environmental impact， and／or (3) increased recycling efforts and the development of new recycling technologies· As a consequence of economi
cs of not only production but also environmental impact and ecological factors，it is becoming increasingly important to consider the“cradle—to—grave’’life cycle of materials relative to the overall manufacturing process．
(Selected from Materials Science and Engineering：An Introduction，by William D Callister，2002)
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