Analytical transmission electron microscopy and surface spectroscopy of ceramics: The microstructural evolution in titanium-doped chromia polycrystals as a function of sintering conditions

Abstract

Titanium-doped chromium oxide has been successfully employed in the form of thick film gas sensing devices where the porosity and surface conditioning are key aspects in providing a measurable gas response. Under normal gaseous atmospheres, where the partial oxygen pressure (p(O₂)) is approximately 0.2 atm, sintering of the host material (α-Cr₂O₃) to high densities is not possible, instead, significant grain growth occurs through evaporation-condensation transport mechanisms owing to the volatility of non-sesquioxide phases formed at high temperatures. The doping of α-Cr₂O₃ with Ti does not significantly affect the sintering behaviour of the host oxide under atmospheric conditions, but instead tends to form a nanodimensional, surface-segregated ternary phase of a nominal composition: Cr₂Ti₂O₇, whilst the composition of grain interiors is close to pure α-Cr₂O₃. By reducing the p(O₂) to ∼10⁻¹⁵ atm during sintering, thereby reducing the formation of volatile phases, solid state diffusion mechanisms have been encouraged allowing the densification of green bodies to a density >99% of the theoretical value. Ceramic bodies obtained by sintering in reduced p(O₂) atmospheres display a single phase solid solution, isostructural with α-Cr₂O₃ (space group R3c).