REFRACTORY

any material that has an unusually high melting point and that maintains its structural properties at very high temperatures. Composed principally of ceramics, refractories are employed in great quantities in the metallurgical, glassmaking, and ceramics industries, where they are formed into a variety of shapes to line the interiors of furnaces, kilns, and other devices that process materials at high temperatures. In this article the essential properties of ceramic refractories are reviewed, as are the principal refractory materials and their applications. At certain points in the article reference is made to the processing techniques employed in the manufacture of ceramic refractories; more detailed description of these processes can be found in the articles traditional ceramics and advanced ceramics. The connection between the properties of ceramic refractories and their chemistry and microstructure is explained in ceramic composition and properties. material not deformed or damaged by high temperatures, used to make crucibles, incinerators, insulation, and furnaces, particularly metallurgical furnaces. Refractories are produced in several forms: molded bricks of various shapes (see firebrick); bulk granular materials; plastic mixtures consisting of moistened aggregate that are rammed into place; castables composed of dry aggregates and a binder that can be mixed with water and poured like concrete; and mortars and cements for laying brickwork. Refractories may be chemically acid, basic, or neutral, depending on the application. Silica (made from sand or quartzite), zircon (for extreme heat resistance), and fireclay (made by baking kaolin) are acid; magnesite and dolomite are alkaline; high-alumina refractories, mullite, chromite, silicon carbide, and carbon are neutral. Carbon is an excellent refractory in places where no oxygen can contact it, as in the hearth of a blast furnace. A smelting-furnace lining must be acid if the slag is acid and basic if the slag is basic, so that it will not react and be eroded. Magnesite and dolomite are the most important refractory materials; their oxides are used in open-hearth steel furnaces and in portland-cement kilns. In fact, most refractories are oxides; others include silica, alumina, chromite, and zirconia. Fireclay is a mixture of silica and alumina, with impurities that soften the bricks at high temperatures. Silicon carbide (not an oxide) has a high strength at elevated temperatures, but both silicon and carbon burn in oxidizing atmospheres if the protective skin of silica flakes off. The ideal refractory material has a high compressive strength at furnace temperatures, a low thermal conductivity, and a low coefficient of expansion. Refractory insulating bricks approach this ideal. Special types of refractories are required in nuclear power plants. Various materials that have outstanding chemical and physical stability at high temperatures have been tested and adopted. They include high-melting oxides, carbides, sulfides, and nitrides, none of which has been widely employed as refractories. Additional reading Useful resources on refractories include Harbison-Walker Refractories, Modern Refractory Practice, 5th ed. (1992); and Refractories: Significance, History, Classifications, Manufacturing Processes, Forms, Applications, Future (1987), published by the Refractories Institute. 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 traditional ceramics is described in F.H. Norton, Elements of Ceramics, 2nd ed. (1974); James S. Reed, Introduction to the Principles of Ceramic Processing (1988); George Y. Onoda, Jr., and Larry L. Hench, Ceramic Processing Before Firing (1978); and four sections of Theodore J. Reinhart (ed.), Engineered Materials Handbook, vol. 4, Ceramics and Glasses, ed. by Samuel J. Schneider (1991): 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