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Phase selection in electrohydrodynamic atomization of alumina

Published online by Cambridge University Press:  31 January 2011

C. G. Levi ,
V. Jayaram ,
J. J. Valencia  and
R. Mehrabian
C. G. Levi
Affiliation:
Materials Department and Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106
V. Jayaram
Affiliation:
Materials Department and Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106
J. J. Valencia
Affiliation:
Materials Department and Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106
R. Mehrabian
Affiliation:
Materials Department and Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, California 93106
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Abstract

Electrohydrodynamic atomization'has been adapted to produce Al2O3 powders ranging in size from 10 nm to 300 μm. Microstructural characterization using x-ray diffraction, scanning, and transmission electron microscopy reveals changes in phase selection as a function of particle size, hence supercooling. Amorphous powders are common below 100 nm in diameter. Cubic spinel γ is found in single phase form between 100 nm and 2μm, and partially transformed to δ between 2 and 20 μm. There is also evidence of σ and θ or a precursor of θ forming directly from the liquid above 5 μm. The stable corundum structure is consistently found above 20 μm but exhibits three different morphologies: faceted, dendritic, and cellular. Phase selection is examined on the basis of fundamental thermodynamic and kinetic considerations and results from computer models predicting the thermal history of the powders. It is concluded that metastable phases require the elimination of catalytic sites for the nucleation of a and are thus more likely to form in the smaller powders. Furthermore, submicron powders achieve sufficiently high cooling rates to preserve the metastable phases formed (γ), but those higher than ∼ 1 μm experience a thermal excursion long enough to transform γ to more stable forms of Al2O3.

Type
Articles
Information
Journal of Materials Research , Volume 3 , Issue 5 , October 1988 , pp. 969 - 983
Copyright
Copyright © Materials Research Society 1988

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