Sintering studies on ordered monodisperse silica compacts: Effect of consolidation

Abstract

Monodisperse silica spheres were synthesized by TEOS hydrolysis, then consolidated by three different processes. Spheres of 270 nm were dispersed in water and centrifuged to form highly ordered compacts with packing efficiencies of 67–68% of the powder density. Dilute sphere suspensions were filtered to form relatively thin dense cakes. Pellets were also prepared from randomly agglomerated powders by uniaxial pressing. Green bodies were sintered at temperatures between 800–1 100 °C for 2–24 h. Pore size distributions of the ordered compacts were determined by mercury porosimetry. No major changes in the pore size distributions were observed for samples sintered between 800 and 950 °C. The sintered ordered samples were not translucent and reached approximately 96–97% of theoretical density at 1 000 °C. In contrast to the centrifugally ordered samples, the filtered cakes became translucent at 1 000 °C. Temperatures of almost 1 100 °C were required to reach high density for compacts made from agglomerated powders. SEM micrographs of the sintered and ordered compacts show the presence of narrow void regions with sizes on the order of the sphere size. These areas are most likely the remnants of the domain boundaries which existed in the ordered green compact. The results of this study show the interference of the domain boundaries in the green compacts to be detrimental to densification, limiting the achievable final densities and degrading the properties of the sintered ceramics. This is despite the fact that the centrifugally-ordered samples had the highest densities in the green state.