CONDUCTIVE CERAMICS

advanced industrial materials that, owing to modifications in their structure, serve as electrical conductors. In addition to the well-known physical properties of ceramic materialshardness, compressive strength, brittlenessthere is the property of electric resistivity. Most ceramics resist the flow of electric current, and for this reason ceramic materials such as porcelain have traditionally been made into electric insulators. Some ceramics, however, are excellent conductors of electricity. Most of these conductors are advanced ceramics, modern materials whose properties are modified through precise control over their fabrication from powders into products. The properties and manufacture of advanced ceramics are described in the article advanced ceramics. This article offers a survey of the properties and applications of several electrically conductive advanced ceramics. The causes of resistivity in most ceramics are described in the article ceramic composition and properties. For the purposes of this article, the origins of conductivity in ceramics may be explained briefly. Electric conductivity in ceramics, as in most materials, is of two types: electronic and ionic. Electronic conduction is the passage of free electrons through a material. In ceramics the ionic bonds holding the atoms together do not allow for free electrons. However, in some cases impurities of differing valence (that is, possessing different numbers of bonding electrons) may be included in the material, and these impurities may act as donors or acceptors of electrons. In other cases transition metals or rare-earth elements of varying valency may be included; these impurities may act as centres for polaronsspecies of electrons that create small regions of local polarization as they move from atom to atom. Electronically conductive ceramics are used as resistors, electrodes, and heating elements. Ionic conduction consists of the transit of ions (atoms of positive or negative charge) from one site to another via point defects called vacancies in the crystal lattice. At normal ambient temperatures very little ion hopping takes place, since the atoms are at relatively low energy states. At high temperatures, however, vacancies become mobile, and certain ceramics exhibit what is known as fast ionic conduction. These ceramics are especially useful in gas sensors, fuel cells, and batteries. Additional reading Materials on conductive ceramics may be found in A.j. Moulson and J.M. Herbert, Electroceramics: Materials, Properties, Applications (1990); Larry L. Hench and J.K. West, Principles of Electronic Ceramics (1990); and the section titled Electrical/Electronic Applications for Advanced Ceramics, in Theodore J. Reinhart (ed.), Engineered Materials Handbook, vol. 4, Ceramics and Glasses, ed. by Samuel J. Schneider (1991), pp. 110566.A good introduction to ceramics in general is provided by David W. Richerson, Modern Ceramic Engineering: Properties, Processing, and Use in Design, 2nd ed., rev. and expanded (1992). The processing of both traditional and advanced ceramics is described in James S. Reed, Introduction to the Principles of Ceramic Processing (1988); I.J. McColm and N.J. Clark, Forming, Shaping, and Working of High Performance Ceramics (1988); George Y. Onoda, Jr., and Larry L. Hench, Ceramic Processing Before Firing (1978); and four sections of the Reinhart book cited above: Ceramic Powders and Processing, pp. 41122; Forming and Predensification, and Nontraditional Densification Processes, pp. 123241; Firing/Sintering: Densification, pp. 242312; and Final Shaping and Surface Finishing, pp. 313376. Thomas O. Mason