ABSTRACT. One of the most critical technical issues associated with the microwave processing of ceramics is understanding the propagation of electromagnetic waves within the physical structure and the interaction mechanisms of the fields with the materials that make up the green body. These factors directly determine the types of components and possible new materials that can be effectively and uniquely processed. The physical size scales of interest for this problem span a very large range. On the macroscopic level one is dealing with the overall part shape, and the principle concern is modeling the energy deposition distribution and thermal flow inside a complex shaped part under very high heating rates. The goals of such procedures are to optimize the absorption of energy and simultaneously minimize thermal gradients. Methods and examples of this large-scale modeling, using finite difference EM codes and time-dependent thermal codes, will be described.

However, during the vast majority of the sintering cycle, the ceramic is porous and the dielectric properties (which determine the behavior of microwaves within the structure, including the power absorption) are quite different from those of a fully dense body. Furthermore, the properties of a fully dense polycrystalline ceramic are considerably different from those of a single crystal material. In order to understand these issues and develop a predictive capability, one must know the electromagnetic behavior of the types of small-scale structures present in ceramics. This includes mesoscale structures such as the individual ceramic particles that make up the green body prior to full sintering, through microscopic scale structures such as grain boundaries, and even down to molecular scales associated with the rotation of dipoles and migration of charge carriers under the influence of the high frequency field. The presentation will emphasize new research in these areas, including the application of finite difference EM codes to a variety of percolating and nonpercolating ceramic-air microstructure geometries. A new model that describes the boundaries between particles in porous ceramics in terms of fractal-geometry surfaces separated by a dielectrically inactive or glassy layer, and its implications for microwave sintering, will be described in detail.

Work supported by the U.S. Department of Energy.

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