Restructuring of alumina particles using a plasma torch
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Restructuring of alumina particles using a plasma torch H. Shim and J. Phillipsa) Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802
I. S. Silva FCT/UNL, Dep. QUIMICA, Quinta Da Torre, 2825 Monte Caparica, Portugal (Received 3 November 1997; accepted 8 July 1998)
A method to modify ceramics using a low power microwave plasma torch is described. The size, shape, surface area, and phase of alumina particles were dramatically modified by passage through an atmospheric pressure argon plasma, operated at 1 kW or less power. Specifically, irregularly shaped particles of g-alumina with an average diameter of 11 mm were converted to smaller (ca. 4 mm) spherical particles primarily consisting of d- and a- (corundum) phases. Also notable was the finding that modifications of the particles, such as changes in surface area, correlate to applied plasma energy. The plasma torch was operated with an argon flow rate of 5 slpm, power of between 400 and 1000 W, and average particle residence time in the plasma of 0.1 s.
I. INTRODUCTION
There is considerable interest in the use of plasma treatments to sinter micron scale oxide particles to form high density solids. The impetus was the finding by Bennett and co-workers1 that in plasmas, alumina compacts sinter more rapidly, and at significantly lower temperatures, than compacts treated thermally. Since that time several groups have confirmed that plasma processing accelerates the sintering of alumina and other oxides.2 – 4 Another standard use of plasmas in the ceramics industry, in particular, high energy plasma torches, is to densify and spheroidize particles during the process of creating ceramic coatings/thermal spraying.5 Surprisingly, there is little literature regarding phase transformations resulting from plasma treatments. Even in studies of the impact of alumina passage through an atmospheric pressure plasma torch, phase modification is generally not studied.6 Indeed, the only thorough study of alumina phase transformation after passage through a plasma torch focused on the fraction of the original alumina which vaporized and renucleated as it passed through a high power radio frequency torch.7 Moreover, the impact of power level on the structural and phase transformation of alumina is not understood. Previous work focused exclusively on high power (.25 kW) systems, implying that high power is a necessary ingredient for spheroidization. The present work focuses on aspects of the transformation of alumina in a plasma previously given little attention. Specifically, it was found that alumina particles carried in an aerosol through a low power (1 kW or less) microwave generated 1-atmosphere argon plasma a)
II. EXPERIMENTAL A. Torch
An ASTEX quartz microwave torch and model S-1500i microwave power generator were employed. Using this system it is possible to apply up to 1500 W
Address all correspondence to this author. J. Mater. Res., Vol. 14, No. 3,
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