DEPOSITION FROM THE VAPOR-PHASE DURING INDUCTION PLASMA TREATMENT OF ALUMINA POWDERS

A study was carried out of the induction plasma melting of alumina powders (particle mean diameter, d(p)BAR = 24.5 mum), (Ar/H-2 or Ar/N2 plasma, plate power, 40 kW) under reduced pressure conditions (400 torr). The results reveal that in the process, partial vaporization of the alumina powders takes place in the hot region of the discharge. As the molten particles cool down and solidify, the deposits from the vapour phase was formed with the spheroidized particles. In all treatments with the Ar/H-2 and Ar/N2 plasmas, a condensate of ultrafine alumina fume (d(p) < 200 nm) was obtained. The fine particles consisted essentially of metastable gamma-, delta- and theta-phases. Needle-like crystals(0.1-0.3 mum diameter, by 5-15 mum long) were observed when operating with an Ar/N2 plasma at powder feed rates exceeding 10 g min-1. Electron diffraction analysis revealed that the needles were whiskers, whose structure was very similar to kappa- or chi-aluminas with an hexagonal close-packed oxygen lattice. The change of morphology is related to the degree of supersaturation in the vapour phase.