Microwave heating offers the unique potential for selectively heating a strongly microwave-absorbing material within a nonabsorbing matrix, providing the mixture is homogeneous and the absorbing phase is diluted enough to retain a penetration depth sufficient for volumetric heating. Mainly for this reason, microwave processing opens up a way to produce entirely new materials and microstructures 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 reliability of ceramic materials. A novel approach to synthesizing such powders is based on microwave heating of metalorganic precursor compounds. In contrast to conventional precursor-based methods for powder syntheses, which use diluted solutions of precursor compounds, 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.
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