Structural transformations of alumina by high energy ball milling
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R. Schulz Technologie des Materiaux, Institut de Recherche d'Hydro-Quebec, Varennes, Quebec J3X1S1, Canada
S. Kaliaguine Departement de Genie Chimique, Universite Laval, Quebec G1K 7P4, Canada
A. Van Neste Departement de Mines et Metallurgie, Universite Laval, Quebec G1K 7P4, Canada (Received 16 April 1993; accepted 26 July 1993)
Room temperature, high energy ball milling was applied to various transition aluminas (y, K, and x)> producing thermodynamically stable a-alumina—a phenomenon that could otherwise be achieved only by high temperature (1100-1200 °C) heat treatment. The transformation proceeds in two steps. The first one consists of rapid microstructural rearrangements with continuously increasing a-transformation rate. In the second step (1-2 h from the start), only relatively small changes in morphology are observed with a constant a-transformation rate. The rate is influenced only by the milling intensity. The presence or the absence of oxygen in the milling atmosphere has a large influence on the final surface area of a-alumina.
I. INTRODUCTION
750-850 "C
The extensive research on the various crystallographic phases of alumina is closely related to its wide range of application. The general use of AI2O3 spreads at the present time into many areas of modern industry.1 Alumina has been used for aluminum production, as an abrasive, in heterogeneous catalysis, as an adsorbent, and as a ceramic material. Presently, there are seven known metastable phases of alumina also designated as transition aluminas (£?> Xi K-: V' #> 7> a n d S), and only one is thermodynamically stable (a-form). The usual way to obtain aA12O3 is by high temperature calcination of hydroxides, oxyhydroxides, or transition aluminas. Temperatures as high as 1100-1200 °C are necessary to transform these aluminas in the stable form.2 The sequence of transformations is illustrated by the following scheme2"5:
600 °C
750-850 °C
->
-> 800-900 °C
1100-1200 °C -> a e
1050-1200 °C ->
K
> a
900-1000 °C 1100-1200 °C -> 9(+S) -> a
(1) (2) (3)
1200 °C 1050 °C (4) -> d + a -> a
(*) = more crystalline than the usual y form. With respect to applications in catalysis, it is unfortunate, however, that the high temperature heat treatments of metastable aluminas also produce sintering and grain growth, and therefore a loss of the specific surface area. It is usually found that the a-alumina obtained by this technique has a specific surface area of about 0 . 1 5 m 2 /g. 6 High energy ball milling is a technique that produces new materials.7 It circumvents many of the limitations of conventional alloying and creates alloys or composites that are difficult or impossible to produce by other means. In some cases, this technique produces supersaturated solid solutions at room temperature that could otherwise be formed only in thermodynamic equilibrium at relatively high temperatures.8 The possibility of transforming at room temperature metastable transition aluminas into thermodynamically stable a-Al 2 O 3 by means of high energy ball milling is the
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