Vaporization and condensation of some refractory oxides in plasmas derived initially from argon in expanded dc arcs

Abstract

Experiments are described in which aluminium(III), silicon(IV) and titanium(IV) oxides were vaporized in and re–condensed from plasmas derived initially from argon. These were carried out in a rotating-wall, dc-arc plasma reactor. Conditions were found under which the oxides vaporized completely and at high rates. The crystalline structure and morphology of materials which condensed were examined by optical microscopy, electron microscopy, and X-ray diffraction, and were found to depend upon the location in the reactor from which they were collected. Qualitative mechanisms are proposed by which crystalline materials may be expected to nucleate and grow from cooling plasma vapours. It is suggested that the crystalline modification and morphology of solid condensates may depend upon the degree of supersaturation of vapours fromwhich they form. It is shown that the properties of the materials recovered during these studies, and themetal oxide smokes recovered by earlier workers from the plasmas of free-burning arcs and plasmatorches, may be accounted for by the mechanisms put forward here. It is also shown that, by suitableselection of conditions, the rotating-wall, dc-arc plasma reactor may be used to condense a polymorphicmetal oxide predominantly in a crystalline modification characterized by either a large or a small primitiveunit cell. Consequences of the nucleation mechanism proposed here are supported by experimentalobservations, and some of these predictions and observations are at variance with predictions ofthe Ostwald Rule.