Effects of interfaces and preferred orientation on the electrical response of composites of alumina and silicon carbide whiskers
Ceramic-matrix composites of alumina and silicon carbide whiskers have recently found novel commercial application as electromagnetic absorbers. However, a detailed understanding of how materials issues influence the composite electrical response which underpins this application has been absent until now. In this project, such composites were electrically measured over a wide range of conditions and modeled in terms of various aspects of the microstructure in order to understand how they work. For this purpose, three types of composites were made by different methods from the same set of ceramic powder blends loaded with different volume fractions of whiskers. In doing so, the interfaces between whiskers, the preferred orientations of whiskers, and the structure of electrically-connected whisker clusters were varied. In Chapter 3, it shown that Schottky energy barriers form at the junctions of the wide-bandgap semiconductor whiskers when metal electrodes are applied for measurements. These barriers were characterized on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the loading dependence of the composite non-linear response which results from the barriers. In Chapter 4, the effects of significantly different types of preferred orientation are elucidated and a strong structure-property correlation is established. The effects of other structural issues on the electrical response are uncovered as well, such as those pertaining to porosity in the ceramic and the interfaces between electrically-connected SiCw. In Chapter 5, the non-linear response model of Chapter 3 is adapted in the development of a new model for electrically-percolated clusters. This model demonstrates how loading and interfacial issues influence the cluster topology and may result in the cluster having a non-linear electrical response. In Chapter 6, the effects of various factors on the broadband frequency dependence of the electrical response are explained and then contextualized for the electromagnetic absorber application. Such factors include whisker orientation, percolation, cluster structure, and interfaces. Finally, in Chapter 7, it is demonstrated that the interwhisker interfacial conduction mechanism differs from those which have been previously reported for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A new model for the interfacial conduction mechanism is then developed, which connects the temperature dependencies of the static- and radio-frequency electrical response and helps explain the aforementioned structure-property correlation.