A shear-type allotropie transformation in alumina
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I.
INTRODUCTION
THE dehydration
of bayerite produces the metastable y (gamma) and/or "0 (eta) allotropes. 1,,_ Structurally these are of cubic close-packed symmetry. The oxide-ion sublattice may contain a small fraction of hydroxides. The aluminum cations are distributed over both octahedral and tetrahedral sites, approximating a defective spinel structure. Delta alumina, a transition phase in the 3/-6-O sequence, is more spinel-like with respect to cation positions, but the oxide ion sublattice (now essentially free of hydroxides) is distorted tetragonally. The theta allotrope, though its X-ray diffraction (XRD) powder pattern is very similar to that of & cannot be indexed in the spinel system] Still it consists of a "distorted cubic-close-packed" sublattice of oxides, but about three-fourths of the aluminum ions are in tetrahedral positions. The only thermodynamically stable phase is c~ (alpha) alumina, into which theta transforms above 1100 ~ Its crystal structure is hexagonal close-packed, with aluminum cations filling two-thirds of the octahedral sites. Although the O --~ c~ transformation involves a fundamental symmetry change, structurally this can be accomplished by a modest cooperative shift of the totality of anions and cations, s The transformations 7-6-0 and/or r / ~ O occur over a wide temperature range above 500 ~ and are accompanied by a substantial decrease in specific surface. The particles of gamma or eta are pseudomorphous with the precursor bayerite, except that they have an extremely fine substructure of alternating solid lamellae and slit-like pores, both of the order of 0.7 to 1.0 nanometers in thickness, aligned with the 111 planes of the lattice. 45 The atom movements associated with the allotropic transformations are intricately coupled with atom movements of the sintering, recrystallization, or structure-consolidation type. 4 Although unequivocal evidence is lacking, the allotropic transformations (i.e., shifts in the lattice positions of the ions) do not seem to occur until recrystallization has created crystallites (i.e., regions of long-range order of significant magnitude) larger than a minimum size, and vice versa. J R. WYNNYCKYJ Is Professor. Department of Chemical Engineering, Umverslty of Waterloo. Waterloo, ON N2L 3G1, Canada C G. MORRIS, formerly Undergraduate Student, Department of Chemical Engineering. Umversity of Waterloo, is now Vice President, Hyprotech Ltd , Calgary, AB, Canada. Manuscript submitted December 23, 1983. METALLURGICAL TRANSACTIONS B
In spite of a voluminous literature on the preparation, structure, and definition of temperature ranges where the allotropes are inert (for a review see Reference l ), there has been little systematic stud)' of the transformation rates. This paper is concerned with the kinetics and the mechanism of the O ~ c~ transformation. Relevant previous work is that of Steiner et al.,6 Dethy and Durigneaud, 7 and of Bye and Simpkin. ~ The latter report that the rate data for the y ~ o: two-step process could be correlated by the Avrami
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