Metastable Phases of Alumina
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METASTABLE PHASES OF ALUMINA
A. DAUGER, D.FARGEOT. AND J.P.LAVAL Laboratoire de Cdramiques Nouvelles - L.A. C.N.R.S. Avenue Albert Thomas 87065 LIMOGES CFDEX FRANCE
No 320
ABSTRACT Plasma sprayed pure alumina samples have been studied by Transmission Electron Microscopy and X-Ray diffraction. The main families of metastable forms (y, 6 and e) have been observed in as-sprayed or in annealed states. All derive from a spinel structure through a periodic antiphase boundaries mechanism, together with an ordering of vacant sites in the cationic sublattice.
INTRODUCTION The only well known structure of alumina is that of the stable form, the corundum. In most production or forming processes[1-3]other structures as metastable phases, are observed, and called y, n, 6 and 0. The main common feature of these structures is a cubic close packing of oxygen ions with a more or less ordered cationic sublattice, instead of the hexagonal close packing of the oxygen network in corundum. In most cases, the results concerning metastable phases have been product of calciobtained on products of hydroxides calcination. The first nation is the y or the n phase, depending on the nature of starting material These two phases have a rather similar structure closely related to the spinel lattice[4]. Further heating leads to intermediate phases, 6 and 0. The 6 structure has mainly been determined in the alumina rich part of various systems such as AI9O3 - Mg0[5], Al103 - Ni0[61, AI203 - AIN[7]. It has a tetragonal unit cell based on a spinet super lattice structure arising from a periodic antiphase mechanism. The 0 phase is known to be monoclinic and isostructural with ý - Ca 2 0 3 [8]. Among the large number of papers concerning these metastable phases[9], very few T.E.M. observations are described and most results are based on powder experiments, because of the lack of bulk specimens and of suitable single crystals.
RESULTS We have studied plasma sprayed alumina samples[IO]. The starting powder is 99.9 % pure alumina. The rapidly solidified deposits are dense and suitable for T.E.M. observations in spite of the very small grain size and the difficulties in obtaining large thin areas by ion thinning in this case. X-Ray powder diffraction patterns of as-sprayed material are typical of the y form[Il]with the main lines of a spinel lattice, some line broadening and an increased diffuse background. T.E.M. observations show more or less ordered domain structure[12]with roughly periodic arrays of antiphase boundaries (FIG. 1) of the {hoo}1/4 type. Important features are the very diffuse satellite scattering flanking the reciprocal lattice points of the cationic sublattice, and some weak superlattice reflections. Aluminum va-
Mat. Re.
Soc.Symp. Proc. Vol. 21 (1984) Published by Elsevier Science Publishing Co., Inc.
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cant sites, with respect to the Al located in the antiphase planes. FItC.
FIG.
13 -33 4(af .
04 spinel structure,
1. - As-sprayed alumina. Dark field micrograph of the (113) spots.
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