Glass matrix composites containing a conducting filler such as antimony tin oxide (ATO) or silicon carbide whiskers (SiCw) have the potential for applications such as transparent electrodes, heating elements, and electromagnetic shielding. For these applications, the composite performance is highly dependent on the microstructure of the composite and the interactions the added filler has with one another. In this research, borosilicate glass-matrix composites were fabricated using a processing method that creates segregated percolated networks at low concentrations of conducting fillers. The conducting fillers were hot pressed with the glass microspheres at temperatures near the glass transition temperature (550°C) using various pressures. Upon hot-pressing at these low temperatures, the glass microspheres deformed into faceted polyhedra and the fillers were displaced to the edges of the glass particles, resulting in percolation. The processing method used in this study was able to bypass many of the current composition and densification issues associated with the creation of percolated networks in glass composites. In some cases, the formation of these percolated networks resulted in a 12-13 orders of magnitude decrease in the resistivity. Using a non-destructive electrical measurement technique, ac impedance spectroscopy (IS), the changes in the electrical properties were tracked as the conducting networks developed. Using IS in conjunction with other techniques, correlations were made between the electrical properties, the filler interfaces, and the influence the processing parameters had on the development of the percolation networks within these composites.

The impedance response of silicon carbide whisker-alumina composites is investigated utilizing novel stereological techniques along with a microstructural simulation. The stereological techniques developed allow for a measurement of the trivariate length, radius and orientation distribution of whiskers in the composite from measurements made on two-dimensional sectioning planes. The measured distributions are then utilized in a Monte Carlo simulation that predicts connectivity in the composite for a given volume fraction. It is assumed in the simulation that connectivity factors dominate the electrical response, not interfacial phenomena. The results of the simulation are compared with impedance spectra taken from real samples, and conclusions are drawn regarding the nature of the impedance response.

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; the whisker aspect-ratio distributions were the same for all methods. At the electrode interfaces, Schottky barriers at the junctions of the electrically-percolating wide-bandgap semiconductor whiskers on the surface were responsible for a significant portion of the total measured impedance. The associated electrical response was studied on the microscopic and macroscopic level, and the gap between these different scales was bridged. Also, a modeling approach was developed for the non-linear behavior of the composite which results from these barriers. In regards to the whiskers within the composite bulk, the effects of various factors on the wide-band frequency dependence of the dielectric response and dc conductivity were explained and contextualized for the electromagnetic absorber application. Such factors include whisker preferred orientation, electrical percolation and cluster structure, the interfaces between electrically-connected SiC whiskers, and porosity. A quantitative correlation between the anisotropy of the microstructure and that of the conductivity was found, and was understood in terms of the interfacial SiC-Al2O3-SiC conduction mechanism. This behavior was shown to differ from the behavior commonly observed for other disordered mixtures of relatively conductive particles dispersed inside insulating polymer hosts. A description of this new mechanism was developed based on an observed correlation between the temperature dependencies of the static and radio-frequency electrical responses. Also, the aforementioned non-linear response model was expanded upon to describe conduction through and across electrically-percolated clusters. The model demonstrates how loading and interface behavior influence the topology and the strength of the non-linear response of the clusters.