Complex permittivity measurements and mixing laws of ceramic materials and application to microwave processing

The complex permittivity of alumina composites was examined with respect to its dependence on the volume fraction of constituents, microstructure, processing temperature, and processing method. In addition, the effective permittivity of these composites was quantitatively modeled based on the permittivities, volume fractions, and microstructures of the constituents.; The studies focused on the complex permittivity of alumina composites, which contained the lossy additives silicon carbide and copper oxide. Two composite systems were prepared by physically mixing alumina and one of the additives. A third composite system was produced by chemically precipitating copper oxide onto alumina. The two synthesis methods produced composites with different microstructures and complex permittivities. The imaginary part of the complex permittivity was generally larger in the chemically precipitated composites than in the physically mixed composites.; The dependence of the complex permittivities of the composites on volume fraction and microstructure were compared with several algebraic mixing laws and with three dimensional, electrostatic numerical simulations. The algebraic mixing laws do not take into account for the dependence of the imaginary part of the complex permittivity on absorbed water and microstructure, which is affected by composite synthesis. By incorporating general physical characteristics of the composites, the electrostatic simulations were able to accurately predict their permittivity.; Heating some selected alumina composites in conventional and microwave furnaces demonstrate several interesting results. The densification and dielectric proper-ties of the alumina/copper oxide composites varied due to processing temperature. The changes in these properties depended upon preparation method and not on heating method. The density and real part of the complex permittivity of alumina/silicon carbide also varied due to processing temperature and not on heating method. Interestingly, the imaginary part of the complex permittivity of alumina/silicon carbide did depend on heating method. The electrostatic simulations were found to be of limited value in predicting the permittivity when there is a lack of data of the volume fraction or permittivity of minor constituents, which contribute significantly to the overall effective permittivity.; Several dielectric measurement techniques were specifically developed for this research. A stainless steel open-ended coaxial probe accurately measured the complex permittivity of solid dielectric materials up to 1000C and over a broad frequency range of 0.3 to 6 GHz. The probe's insensitivity to low loss materials constrained accurate dielectric measurements of materials with a loss tangent greater than 0.05. A nondestructive resonant cavity was developed to measure the dielectric properties of low loss materials with variable dimensions.