Electrical conductivity and permittivity of ceramics and other composites

[摘要] Determining the properties of composites and how these relate to those ofthe components and the microstructure is extremely useful as it enables theunderstanding of existing materials and the design of new materials with avariety of applications. However, the link between the ac conductivity dataand the microstructure and composition of the composite is not a simpleone. Simulations of binary composites are presented which show that fromrelatively simple component properties arise complex composite properties.Accurate identi¯cation of the components of composites, using characteristicfrequencies, is demonstrated for simulated and actual experimental data.The Maxwell-Wagner and Brick Layer Models, which are often appliedbeyond the range of their original derivation, are found to consistently ¯tthe data of yttria-stabilized zirconia ceramics measured at various tempera-tures. The results from ¯tting single crystal and polycrystalline sample dataindicate that accurate modeling of the properties of the polycrystalline sam-ples requires more theoretical work on the conduction mechanisms in singlecrystals and the grains of the ceramics.Data from a polyester-resin/silicon system is found to be best ¯tted us-ing the Single Exponent Phenomenological Percolation Equation (SEPPE)with experimentally measured component properties as input. The percola-tion threshold obtained suggests a system where the insulator tends coat theconductor. The results show that, with the actual component properties asinput, the SEPPE can be used to qualitatively and semi-quantitatively modeland ¯t composite ac conductivity data.Analysis of the ac conductivity of liquid-phase-sintered silicon carbide ce-ramics showed that for this system, the features often observed in the imped-ance spectra are all due to a multi-component grain bondary/binder phaseand not the SiC grains. This multi-component grain boundary phase can be¯tted accurately to the Brick Layer Model, indicating a microstructure wherean insulating component coats a more conducting component.

[发布机构] University of the Witwatersrand