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.

For polymer-matrix composites (PMCs) that contain insulating matrices and conducting fillers, the electrical properties of the PMCs are especially sensitive to the local concentration of the fillers in the matrix. For PMCs that have phase-segregated microstructures, better prediction of the properties is possible since enhanced control over the distribution of the filler in the matrix can be achieved. In this research, PMMA-ITO nanocomposites were chosen as the composite system in which to explore alternative microstructures, specifically highly phase-segregated microstructures. The microstructures were primarily controlled by varying the ITO particle size and concentration, and the fabrication parameters used to form the nanocomposites. The motivation for this research was to develop correlations between the microstructure and non-destructive measurements in order to improve the predictability of properties in percolating PMCs. As a result of this work, a novel phase-segregated microstructure was discovered, where ITO aggregate-wire structures self-assembled during the composite forming process. Structural analysis of the specimens was conducted primarily using transmission optical microscopy and scanning electron microscopy (SEM). Impedance spectroscopy and optical spectroscopy were the primary NDE characterization tools used for analyzing the variations among the specimens. Ultra-small angle x-ray scattering (USAXS) and stereological techniques were also used to describe the dimensions of the ITO aggregate-wire structures that self-assembled in the PMMA-ITO nanocomposites.