Flash Sintering of Boron-Containing CeramicsWednesday (26.09.2018) 15:00 - 15:15 S1/03 - 283 Part of:
The aim of this study was to explore the feasibility of using flash sintering to densify boron-containing ceramics. Sintering of ceramic materials with dielectric properties often involves processing at high pressures and high temperatures, in some cases approaching temperatures at which volatilization occurs. Volatilization is a distinct concern for boron-containing ceramic materials. Flash sintering is an attractive consolidation method for the potential to reduce sintering times and temperatures at ambient atmospheric and sample pressures. However, a limited understanding of how an applied electric field impacts underlying sintering mechanisms can make this difficult. It is known that the kinetics of conventional solid-state sintering are driven by the reduction of surface energy. It is also supposed that flash sintering, and the associated flash event, are dictated by Joule heating at grain boundaries (GB) and resulting thermal runaway, respectively. This study was focused on exploring the impact of particle size, targeting an increase in specific surface area to (a) improve the sinterability of a powder compact during flash sintering and (b) increase the GB to bulk volume ratio, potentially increasing the number of current pathways. A variety of powder chemistries have been investigated, including commercial boron suboxide (B6O) and boron carbide (B4C) powders, and varying ratios of B4C blended in-house with silicon carbide (SiC) or titanium diboride (TiB2). While the majority of the flash sintering literature has been focused on densification of specially designed millimeter-sized parts (3–240 mm3), this work involved the fabrication of larger specimens (400–650 mm3) with regard to eventual scale-up. All powders were characterized prior to consolidation, with flash sintered specimens analyzed via x-ray diffraction, scanning electron microscopy, and pycnometry. All test runs were conducted in a horizontal, quartz tube furnace at ambient pressures in a reducing atmosphere, accompanied by in-situ characterization of furnace temperature, current flow, and electrical resistance of the specimens. Data has shown that the relationship between the conductivity of a dielectric sample is nonlinear with temperature and is heavily dependent upon powder properties. In order to avoid non-uniform heating, boron-containing ceramic powders are currently being investigated to improve the current pathway distribution.