Tetragonal to orthorhombic transformation during mullite formation

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INTRODUCTION

Mullite was identified over sixty years ago by Bowen and Greig1 to have an orthorhombic crystal structure with a chemical composition of 3A12O3 • 2SiO2. Since this early study, variations in mullite crystal structure and chemical composition have been reported.2"7 Cameron8 determined that a relationship existed between the chemical composition and the cell dimensions of mullite; namely, the a and c lattice parameters of mullite increase and the b lattice parameter decreases as the A12O3 content increases from 57 mol% to 76 mol%. Cameron9 also showed that mullite solid solutions can be expressed by the formula A\l\A\2+2x " Si2_2.t)lvOio-*, where (vi) and (iv) are the coordination states of the atoms and x is the number of oxygen atomic vacancies per unit cell (the stoichiometric 3:2 mullite composition corresponds to x = 0.25). In principle, x ranges from 0 to 1, corresponding to structures from sillimanite (Al2SiO5) to the iota polymorph of alumina. The range of composition is based on the substitution scheme 2Si4+ + O2 -* 2A13+ on the tetrahedra sites. By extrapolation from the relation between lattice parameters and composition, Cameron9 predicted that an orthorhombic to tetragonal phase transformation (a = b) should occur in mullite near x = 0.63, or 79 mol% A12O3. However, mullite solid solutions are known to be metastable above 63 mol% A12O3 and unstable above 75 mol% Al2O3.7>10u Attempts to synthesize tetragonal mullite by rapid solidification from the melt achieved 77.3 mol% A12O3, but this was still too low to produce tetragonal symmetry.12 ''Current address: Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792. J. Mater. Res., Vol. 6, No. 4, Apr 1991

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The occurrence of a mullite phase with a tetragonallike symmetry from co-precipitated (single phase) gels has been reported at temperatures between 900 °C and 1200 °C.31314 All these reports indicated that the tetragonal-like phase eventually transformed into the normal, orthorhombic structure at a higher temperature. Schneider and Rymon-Lipinski14 also reported that the structural states of mullite were highly sensitive to the processing conditions of the precursors. Because of the lack of information on the composition of tetragonal mullite, a pseudo-tetragonal structure was proposed by Schneider and Rymon-Lipinski,14 where twinning of orthorhombic units gave tetragonal reflections. The objective of this work was to examine single phase (co-precipitated) mullite gels that were processed under various conditions as well as gels with different mixing characteristics in order to determine the conditions under which such tetragonal-like phases form. The transformation of tetragonal to orthorhombic mullite (T —> O) was followed with dynamic x-ray diffraction (DXRD) to determine the temperature when the transformation takes place. II. EXPERIMENTAL PROCEDURE

All the samples are listed in Table I. Mullite sol-gel precursors have been characterized into single phase