Effect of transition-metal-ion doping on high temperature thermal expansion of 3:2 mullite—An in situ, high temperature, synchrotron diffraction study

The effect of Ti4+, Cr3+ or Ga3+ doping on thermal expansion of 3:2 mullite was examined from 20 °C to 1400 °C by high temperature XRD using synchrotron radiation. The room temperature mullite lattice expanded with metal-ion incorporation and the change in the b-axis was larger than that along the a-axis (Δb > Δa) for all ion types. Upon heating, the largest elongation was along the b-axis (Δb/bRT) and was similar to that in the pure mullite (∼0.5%). Least expansion was observed along the a-axis (0.28% ≤ Δa/aRT ≤ 0.41%). The thermal expansion of mullite was not constant in the temperature range from 20 °C to 1400 °C, and had a linear dependence on temperature (α(T) = A0 + A1ΔT). The αmean values increased with temperature (20 °C ≤ T ≤ 1000 °C), but were always within 3.0–6.7. Cr3+ and Ga3+ doping decreased the anisotropy in mullite lattice expansion, however, Ti4+ doping showed a sharp increase. Trends associated with type and the quantity of dopant are discussed.