Thermal Expansion Discontinuities of Mullite

Abstract

Thermal expansion data of mullite were collected between 25° and 800°C with high accuracy by means of an X‐ray high‐temperature Guinier powder camera, and high‐resolution single‐crystal Bond techniques. 3/2‐type mullites, synthesized from chemically coprecipitated precursors and 2/1‐type fused‐mullite single crystals, were used for the measurements. Mullite displays low and nonlinear thermal expansions along the crystallographic a, b, and c axes below about 200°C, and linear expansions between 200° and 400°C. Between about 400° and 500°C, expansion curves are discontinuous. Above 500°C, a, b, and c expansions are linear again. The temperature‐induced expansion discontinuities are relatively strong parallel to a, but are much weaker parallel to b and c. The slopes of the a and volume expansion curves are higher above the expansion discontinuity than below, while there is no significant tendency for those of b and c. Thermal expansion coefficients and discontinuity effects are reversible without any hysteresis. The increase of the a and volume expansion curves above the expansion discontinuities may be due to possible deformations, rotations, and tiltings of the [AlO6] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [Al⋆O4] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400° and 500°C.