ELECTROCERAMICS

category of advanced ceramic materials that are employed in a wide variety of electric, optical, and magnetic applications. In contrast to traditional ceramic products such as brick and tile, which have been produced in various forms for thousands of years, electroceramics are a relatively recent phenomenon, having been developed largely since World War II. During their brief history, however, they have had a profound impact on the so-called electronics revolution and on the quality of life in developed nations. Electroceramics that have low dielectric constants (i.e., low electric resistivity) are made into substrates for integrated circuits, while electroceramics with high dielectric constants are used in capacitors. Other electroceramic materials exhibit piezoelectricity (the development of strain under an applied field, or vice versa) and are employed in transducers for microphones and other products, while some possess good magnetic properties and are suitable for transformer cores or permanent magnets. Some electroceramics exhibit optical phenomena, such as luminescence (useful in fluorescent lighting) and lasing (exploited in lasers), and still others exhibit changes in optical properties with the application of electric fields and are therefore used extensively as modulators, demodulators, and switches in optical communications. All the applications listed above require electric insulation, a property that has long been associated with ceramics. On the other hand, many ceramics are suitable for doping by aliovalent materials (that is, materials with other charge states than the ions of the host crystal). Doping can lead to electrically conductive ceramics, which appear in products such as oxygen sensors in automobiles, heating elements in toaster ovens, and transparent oxide films in liquid crystal displays. In addition, ceramics have been developed that are superconducting; that is, they lose all electric resistivity at cryogenic temperatures. Because their critical temperatures (Tc's; the temperatures at which the transition occurs from resistivity to superconductivity) are much higher than those of conventional metallic superconductors, these ceramic materials are referred to as high-Tc superconductors. Most electroceramics are truly high-tech materials, insofar as they are made into high value-added items. Starting materials of high purity are employed, often in clean-room processing facilities. Because grain size and grain size distribution can be the deciding factors in the quality of the electroceramic being produced, strict attention is given to the steps of powder processing, consolidation, and firing in order to achieve the desired microstructure. The structure and chemistry of grain boundaries (the areas where two adjacent grains meet) must often be strictly controlled. For example, the segregation of impurities at grain boundaries can have adverse effects on ceramic conductors and superconductors; on the other hand, some ceramic capacitors and varistors depend upon such grain boundary barriers for their operation. Outline of Coverage Electroceramic products are described in a number of articles in , including electronic substrate and package ceramics, capacitor dielectric and piezoelectric ceramics, magnetic ceramics, optical ceramics, and conductive ceramics. For a directory to these articles within the overall coverage of industrial ceramics, see Industrial Ceramics: Outline of Coverage. Additional reading Works specifically on electroceramics include 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