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Influence of chromium doping on thermal expansion of mullite between 480 and 940^oC

Mullite is an aluminosilicate with general composition Al^(VI)_2(Al^(IV)_(2+2x)Si^(IV)_(2-2x))O_(10-x) (0<x<1), where x denotes the number of oxygen atoms missing per average unit cell, and IV and VI are the coordination states of the cations. It is a major component of many traditional and advanced ceramics. Because of its excellent thermomechanical properties like high thermal shock resistance, small thermal expansion, small heat-conductivity and high chemical resistance, mullite is important for technical applications at high temperatures. Mullite is orthorhombic with space group Pbam. The mean thermal expansion of mullite is low. Incorporation of Cr^(3+) ion in mullite can further reduce it. A goal of this work is to examine the thermal expansion of Cr-doped mullite on Cr content in a temperature range between 480 and 940^oC. 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 1400^oC for two hours. Unit-cell parameters of the samples were determined using Si as an internal standard, and refined by the whole-powder-pattern fitting method. according to the unit-cell parameters of undoped sample: a=7.5474(3), b=7.6894(3), c=2.8837(1)A, prepared mullite corresponds to structural state close to the #/2 mullite (61.1 mol% Al_2O_3). Unit-cell parameters of as-prepared Cr-doped samples increased linearly with chromium content up to 6 mol%. Experimental X-ray diffraction data for determination of mullite thermal expansion were collected in-situ by a high-temperature diffractometer. It was found that thermal expansion of prepared samples was anisotropic, being higher parallel to b than to c and a axes. Thermal expansion anisotropies were calculated according to Schneider and Eberhard. Both thermal expansion coefficient and expansion anisotropy decreased with chromium content up to 6 mol% Cr_2O_3, and increased at 8 mol% Cr_2O_3.

Cr-doped mullite; Thermal expansion; X-ray diffraction

Poster je osvojio prvu nagradu na znanstvenom skupu Ninth International Workshop of Crystallography: Recent Advances in X-Ray Powder Diffraction, Assiut, Egipat, 27.11.-02.12.2004.