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

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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).

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