Microwave Processing of Metalorganics to Form Powders, Compacts, and Functional Gradient Materials
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that cannot be processed by conventional methods. By proper selection of raw materials and microwave heating conditions, processes can be tailored to produce desired materials and structures, such as powders, compacts, or functional gradient materials, as described in this article. Synthetic powders with controlled morphology, agglomerate structure, and composition are necessary to improve the
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reliability of ceramic materials. A novel approach to synthesizing such powders is based on microwave heating of metalorganic precursor compounds.1 In contrast to conventional precursor-based methods for powder syntheses, which use diluted solutions of precursor compounds,2 the main objective of microwave processing is to develop a method for direct pyrolysis of the precursor compound into a ceramic powder, produced by simultaneous decomposition of the precursor within the whole volume of the reaction mixture. Besides a simple pyrolysis of a single precursor compound, decomposition of precursor mixtures is also possible, eventually followed by further reaction of the components. Furthermore, an inert or reactive powdery matrix-material can be impregnated with the precursor and then converted to an "alloyed" ceramic powder. A new method for microstructural modeling of ceramic materials combines precursor compounds with heating by microwave radiation to introduce a second phase and to control grain growth.3 The precursor compounds are introduced into the ceramic material by infiltrating a porous compact or by impregnating a classical powder with the precursor prior to compaction. Microwave radiation is used for decomposition of the precursor and for sintering. Decomposition-rate controlled pyrolysis of the precursor is possible only by microwave heating. Ceramic-metal composites play an important role as structural materials as well as in functional devices. Improvement of reliability and performance can be expected, when such materials are designed with a continuous compositional, and therefore functional, gradient. Synthesis of ceramic-metal composites with a functional gradient material (FGM)4 character
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Figure 1. Conventional (oven) vs. microwave (MW) pyrolysis of alumina green parts infiltrated with (a) Al(O-i-C3H?)3, abbreviated as ATIP, and with (b) Ti(O2C 5H 7)2(OC 3H 7)2, abbreviated as TACP. Squares are for weight loss upon decomposition and circles are for temperature.
MRS BULLETIN/NOVEMBER 1993
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Microwave Processing of Metalorganics
is possible utilizing infiltration of a porous metallic matrix with ceramic precursors and thermal processing using microwave heating.5 Infiltration of the metal matrix with a ceramic precursor facilitates bonding between metal and ceramic. Repeated infiltration and decomposition achieves complete filling of the pores with a ceramic phase. The key step of this processing route to FGM is the development of a selective heating method that combines a high penetration depth and a
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