THERMAL-EXPANSION OF MULLITE

The thermal expansion coefficients of transition‐metal‐free sinter mullite and fused mullite, and of chromium‐doped (11.5 wt% Cr2O3) and iron‐doped (10.3 wt% Fe2O3) sinter mullites are measured between 25° and 900°C by high‐temperature Guinier X‐ray diffraction techniques. Most mullites display low and nonlinear thermal expansions below, but larger and linear expansion above, ∼300°C. Although the temperature‐induced c‐axis expansion coefficients seem to be less dependent on the compositional state x and on transition‐metal incorporation of the Al4+2xSi2–2xO10–x mullite‐type phases (α(c) = 5.6 × 10−6/°C to 6.1 × 10−6/°C), thermal a‐ and b‐axis expansion coefficients change more significantly (α(a) = 3.1 × 10−6/°C to 4.1 × 10−6/°C and α(b) = 5.6 × 10−6/°C to 7.0 × 10−6/°C, where the values were calculated between 300° and 900°C). The larger temperature‐induced b than c and a expansions probably are caused by intense lengthening of the relatively long and elastic octahedral Al(1)–O(D) bonds in mullite, which form at an angle of about 30° with b, but of about 60° with a. With increasing x value of the transition‐metal‐free mullites, the volume thermal expansion decreases, while the anisotropy of thermal expansion is reduced simultaneously. We believe that the variation of the thermal expansion coefficients is controlled by the Al* occupancy and by the number of O(C) vacancies in the mullite structure, and also by the disordering distribution of both structural elements. Transition‐metal incorporation into mullite has no distinct influence on thermal expansion anisotropy, but does reduce thermal volume expansion. A prestressing of the crystal structure by substitution of Al3+ by the larger Fe3+ and Cr3+ ions may be the main reason for the latter effect. Copyright © 1990, Wiley Blackwell. All rights reserved