Thermal expansion of chromium-doped mullite

Abstract

Mullite (nominally 3Al_2O_3.2SiO_2) is an important engineering ceramic material due to its excellent thermomechanical properties like high thermal shock resistance, high temperature shape stability and low thermal expansion. It possesses an orthorhombic structure in the space group Pbam. Incorporation of Cr^3+ ion in mullite is of great interest because the low thermal expansion of mullite can be further reduced in this way. A major goal of this work is to understand dependence of the thermal expansion of Cr-doped mullite on the Cr content. The samples of pure mullite and of mullite with 2, 4, 6 and 8 mol% Cr_2O_3 were derived from single-phase precursors and sintered at 1673 K for two hours. Prepared powder samples were examined by in-situ high temperature X.ray powder diffraction up to 1213 K. Unit-cell parameters of the samples were determined using Si as an internal standard ; and refined by the whole-powder-pattern fitting method. It was noticed that the unit-cell parameters of as-prepared samples linearly increased with chromium incorporation up to 6 mol% Cr_2O_3, and nonlinearly above this content. The samples generally displayed strongest thermal expansion parallel to the b axis, followed by c and a. Linear thermal expansion coefficients along a, b, and c axes were determined in the temperature range between 753 and 1213 K. The thermal expansion coefficients decreased with Cr content up to 6 mol% Cr_2O_3, and then pronouncedly increased at 8 mol% Cr_2O_3. Such behaviour could be attributed to the change of chromium incorporation mode into mullite structure at higher Cr content.