Effect of process variables and sintering method on the microstructure and properties of ITO-borosilicate glass composites
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Composites consisting of indium tin oxide, the predominant transparent conducting oxide, as a filler within a borosilicate glass matrix have been fabricated via hot pressing (HP) and spark plasma sintering (SPS). Both sintering methods are shown to allow a grain-like glass matrix structure to form without incorporating the ITO into the glass particles, leading to the formation of segregated ITO networks and an accompanying drop in impedance of up to 12 orders of magnitude. In doing so, these networks imparted electrical conductivity to the composites while requiring far less filler material (under 1 vol% ITO under all studied conditions) to achieve percolation than other fabrication methods. It was found that the HP process achieved consolidation of the powder mixes at a temperature 70°C lower than that required by the SPS process, despite many previous reports in the literature that SPS generally imparts superior properties and requires lower temperatures. While investigating the reasons behind this apparent contradiction, it was found that the mechanisms at play in the SPS process were more numerous and complicated than those in HP, thus requiring a more detailed study. To date, it has not been possible to accurately predict the sintering processing parameters necessary to achieve a specific microstructure or properties for a given material due to the complexity of the underlying mechanisms. The SPS process introduces additional complications due to the application of an applied current. A systematic investigation found that interactions between the temperature, pressure, applied current and resulting voltage, hold time, heating rate, and material variables make it difficult to isolate the effects of changing a single variable. Furthermore, there is a widespread lack of understanding regarding the significance of the applied current and resultant electric field on the densification process, as well as the codependence between these parameters and other processing variables. Throughout this study, many conventional characterization methods such as SEM and XRD were employed to characterize the fabricated composites. Additionally, a detailed ac impedance spectroscopy study on both powder compacts and sintered samples was undertaken. Results showed how the SPS applied current and voltage are related to other variables such as the heating rate, applied pressure, maximum temperature, and equipment/tooling geometry. By enabling separation of the electrical responses of the different microstructural features present in the sample via equivalent circuit fitting, the impedance data provided extensive insight on the effects of changing sintering process variables and showed strong potential for facilitating further understanding and advancement in this field.