Thermal Expansion Discontinuities of Mullite

Abstract

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.

Interactions between heavy metals and clay matrix in fluid catalytic cracking catalysts

“…Mullite phase emerged at not less than 1200 • C through silica and alumina reaction while it arose at more than about 1000 • C in the phase transformation of kaolin [6,7]. The temperature of mullite formation dropped to 750 • C in the transformation of kaolin loaded with a suitable amount of vanadium, which showed that vanadium reacted significantly with kaolin.…”

“…Mullite formation through kaolin phase transformation had been investigated early [7], while the process of low temperature phase transformation by addition of vanadium has not yet been found in the literature. The effect of vanadium load on mullite content formed through kaolin transformation is shown in Fig.…”

Thermal expansion between 480 and 940 C of Cr-doped mullites derived from single-phase precursors

“…For pure 2:1 fused-mullite (x = 0.40), thermal expansion along the c-axis was greatest followed by b and a. Further studies (Schneider et al 1993) indicated that mullites displayed discontinuities in thermal expansion between about 400 and 500 °C. Expansion discontinuties were large along a, but much smaller along b and c. The slopes of a-and volume-expansion curves, respectively, were steeper above the expansion discontinuity than below it, but this tendency was not observed for those of b and c. It was supposed that such behavior in thermal expansion possibly could be caused by deformation, rotation, or tilting of the AlO 6 octahedra, or by deformation of T*O 4 tetrahedra of the mullite structure.…”

Temperature feature on synthesizing mullite whiskers from coal fly ash and andalusite-sericite phyllite

“…Mullite (3Al 2 O 3 ·2SiO 2 ), as the only binary compound in the SiO 2 -Al 2 O 3 system [1], is a promising material for advanced structural and refractory ceramic because of its promising thermo-mechanical properties such as low thermal expansion, low thermal conductivity, excellent creep resistance, good chemical properties and oxidation resistance [2][3][4][5][6][7]. Recently, considerable work has been done to characterize the composition and microstructure of mullite [8].…”