Phase selection in electrohydrodynamic atomization of alumina
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I. INTRODUCTION The evolution of metastable structures in rapid solidification processing (RSP) is intimately tied to the thermal history, which is primarily determined by the competition between the solidification kinetics and the heat transfer processes.1 While countless efforts have been made to improve the cooling efficiency of atomization techniques, it has been shown that the largest deviations from equilibrium are linked with the achievement of high supercoolings prior to nucleation rather than high cooling rates. For example, the limitations to heat transfer in atomization and the fast growth kinetics characteristic of metallic systems result in practically adiabatic recalescence upon nucleation of supercooled metal droplets. Since the deviations from equilibrium are asst dated with the interfacial velocity and supercooling, the best approach to extend metastability in metal pc\ 'ders is to enhance the initial supercooling of the droplets.2 While rapid solidification studies in metals and ceramics started in the same time period, the application of RSP to ceramics has been limited, among other things, by the assumption that the low thermal conductivity of these materials, in general, makes rapid solidification less feasible.3 One could argue, however, that the Gibbsite
X
higher structural complexity of ceramics should result in more sluggish kinetics enhancing both the undercoolability and the relative importance of the external cooling in slowing down interfacial recalescence. For example, numerous oxide compounds have been known to form glasses under cooling rates as low as those characteristic of natural convection in air ( < 10 K/s). In contrast, fewer glass-forming compositions are available in metals, and they typically require severe quenches ( > 105 K/s) in order to suppress nucleation of the crystalline phase. Aluminum oxide exhibits a single stable crystallographic form known as corundum or a-Al 2 O 3 , which consists of close-packed planes of oxygen anions stacked in an hep sequence with the Al cations occupying two thirds of the octahedral interstices between the anion layers. From the stacking sequence it is easily shown that the unit cell of a-Al 2 O 3 involves six close-packed anion planes.4 Alumina has a wealth of metastable crystalline phases, some of which are of considerable commercial importance as catalysts or carriers of catalysts.5 They are generally produced by calcination of aluminum hydroxides following the decomposition reactions summarized by Wafers and Bell:6 a
\ Boehmite Bayerite
-> Diaspore
a rj
a a
J. Mater. Res. 3 (5), Sep/Oct 1988
http://journals.cambridge.org
0003-6951 /88/050969-15$01.75
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Levi eta/.: Electrohydrodynamic atomization of alumina
Gibbsite and Bayerite are polymorphs of A1(OH) 3 whereas Boehmite and Diaspore are monohydroxides, A1OOH. The particular decomposition path for the trihydroxides depends on the particle size, pressure, humidity, and heating rate of
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