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

Structural thermal expansions of undoped and Cr-doped 3/2 mullites (10.8 wt.% Cr2O3) were measured with in situ neutron (25-1600 degreesC and X-ray synchrotron (25-1000 degreesC) diffraction techniques. Heat treatments between 25 and 300 degreesC cause little and non-linear structural expansions. Above 300 degreesC and up to 1000 degreesC the mullites display linear and low increase of the lattice constants. The mean values of undoped mullite are slightly higher (alpha (av) approximate to 5.45 x 10(-6)) than those of Cr-doped mullite (alpha (av) approximate to 4.95 x 10(-6)), thus confirming earlier in situ high-temperature diffraction data. Above about 1000 degreesC the expansion behaves discontinuously. Structural expansions of undoped and Cr-doped mullite strongly increase yielding similar mean expansion coefficients (alpha (av) approximate to 7.50 x 10(-6)) for both materials. The highest increase of lattice constants has been determined in b axis direction throughout 1600 degreesC 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 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.