Ceramics

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Introduction Until the 1980s, most people thought of ceramics as artware and artifacts. The recent development of advanced ceramics for high performance thermal insulation (space shuttle tiles), high temperature structures (heat engines), and electronics (superconductors) has dramatically changed perceptions about the utility of ceramics. High technology ceramics are related to "traditional" ceramics only to the extent that they are inorganic, nonmetallic materials. Traditional ceramics are derived from minerals. For example, dinnerware and bricks consist mostly of clay, while sand is the major ingredient in flat glass and containers. Abundant raw materials, simple processing, adequate performance at low cost, and technological evolution have kept these industries viable for several thousand years. But, for demanding electronic or structural applications, synthesizing ceramics from minerals is often unacceptable. The chemical variability of mineral deposits, the difficulty of obtaining a homogeneous mixture of powders, and the problems of consolidating the powders into a uniform ceramic with desirable grain morphology, chemistry and grain boundary phases have stimulated the development of chemically derived ceramic precursors. Electronic Ceramics The key to fabricating electronic ceramics lies in precisely controlling the microstructure and composition of the ceramic grains and grain boundaries. Solution processes offer several advantages for preparing these ceramics. Zinc oxide varistors offer an example. Varistors are solid state switches that consist of semiconducting zinc oxide grains in a matrix of insulating grain boundary material (Figure 1). When a sufficient electric field is applied across the ceramic, conduction takes place across the insulating grain boundaries through the semiconducting zinc oxide. The current voltage behavior is controlled by doping of the zinc oxide grains, the composition of the grain boundary phase, and the size of the grains (Figure 2). The dopants influence the rate at which current increases with increasing electrical field. The grain size controls the field at which the device switches. Control of the three critical elements — grain composition, grain size, and grain b o u n d a r y c o m p o s i t i o n — has been

Figure 1. High field varistors prepared from solution-derived powders. obtained by forming a hydrous precipitate from a zinc chloride salt solution containing dopants of cobalt chloride and manganese chloride, then converting it to oxalate particles which can be heated at relatively low temperatures to make an easily dispersed oxide powder. 1 By contacting the oxide powder with a bismuth nitrate solution, bismuth hydroxide is precipitated on the particles. Modest heating converts it to an oxide preferentially located in the grain boundaries. The resultant powders are small, less than one micrometer, and uniform in size. The uniformity and small size lead to relatively easy conversion to a dense ceramic with moderate heating. The ceramic has desirable electrical prop

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