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Original Publication Date: 2000-Apr-01
Included in the Prior Art Database: 2003-Jun-18

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Described is a new integrated material synthesis methodology for a special class of ceramic/ceramic composites. Composites made by this approach will exhibit more precision on tolerance control from machining stress. There are two phases in this integrated method: (1) synthesis and (2) further selection of material with the right microstructure for targeted machining distortion. In phase one, there are several requirements for selection of the raw ceramic constituents which make up the composite materials. The raw material for the two major constituent phases should be in powder blend form. They should be mutually chemically stable up to the sintering temperature. The two major phases at any time must not be allowed to form a complete solid solution; one constituent phase should be electrically conductive and be a minority phase near its percolation limit, usually between 18 to 28v%, with respect to the insulating phase (IP). The percolation requirement is to ensure, if kinetically favorable, a formation of interpenetrating microstructure. To provide driving force for conductive phase (CP) interconnection during consolidation, it is commonly determined that, kinetically, the in-situ inter-phase energy should be less than 60% of grain boundary energy of IP. The prescribed electrical property relationship among phases should not be modified by the consolidation process of the powder set. Powder consolidation is achieved through solid state sintering, a process relying on solid state mass transfer to consolidate powder into near pore-free solids. As a necessary condition, powder should be mechanically mixed/milled to achieve homogeneous dispersion prior sintering. The most critical step in sintering is the selection and control of sintering temperature. Sintering should be chosen below solidus temperature, and as low as possible without degrading density requirements. The sintering condition is considered to be best when the sintered body possesses the following characteristics: (1) in-situ grain growth and particle coarsening is not excessive (it is considered not excessive when, by intentional lowering of sintering temperature by 5 to 10%, the variation of the average particle size and the distribution of CP stays within a typical engineering SPC control limits); (2) the statistical size distribution of the CP particles in the IP matrix should be consistent across, as well as between, sintered bodies; (3) the grain size of the IP should be comparable to the inter-particle spacing of CP particles. Sintering condition includes heating rate, sintering temperature, isothermal dwell during sintering, and cooling rate.