The thermal expansion coefficients of transition-metalfree sinter mullite and fused mullite, and of chromiumdoped (11.5 wt% CrZO3) and iron-doped (10.3 wt% Fez03) sinter mullites are measured between 25" and 900°C by hightemperature 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 A14+2rSi2-2r010-r mullite-type phases ( (~( c ) = 5.6 X 10-6/OC to 6.1 X 10-6/"C), thermal a-and b-axis expansion coefficients change more significantly (a(u) = 3.1 X 10-6/"C to 4.1 X 10-6/oC and a(b) = 5.6 X 10-6/"C to 7.0 X 10-6/oC, where the values were calculated between 300" and 900°C). The larger temperatureinduced b than c and a expansions probably are caused by intense lengthening of the relatively long and elastic octahedral AI(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 vaiue 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 AP 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 A13+ by the larger Fe3+ and Cr3+ ions may be the main reason for the latter effect. [
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 he due to possible deformations, rotations, and tiltings of the [AIO,] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [AI*O,] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400" and 500°C.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.