High temperature thermal expansion of mullite: an in situ neutron diffraction study up to 1600°C

Structural thermal expansions of undoped and Cr-doped 3/2 mullites (10.8 wt.% Cr2O3) were measured with in situ neutron (25–1600°C) and X-ray synchrotron (25–1000°C) diffraction techniques. Heat treatments between 25 and 300°C cause little and non-linear structural expansions. Above 300°C and up to 1000°C the mullites display linear and low increase of the lattice constants. The mean values of undoped mullite are slightly higher (αav≈ 5.45×10−6) than those of Cr-doped mullite (αav≈ 4.95×10−6), thus confirming earlier in situ high-temperature diffraction data. Above about 1000°C the expansion behaves discontinuously. Structural expansions of undoped and Cr-doped mullite strongly increase yielding similar mean expansion coefficients (αav≈ 7.50×10−6) for both materials. The highest increase of lattice constants has been determined in b axis direction throughout 1600°C in undoped and Cr-doped mullite. High temperature diffraction studies under heating-up and cooling-down conditions yielded reversible expansion effects, including the anomalous behaviour at about 1000°C.