Low-power plasma torch method for the production of crystalline spherical ceramic particles

A low-power, atmospheric pressure, microwave plasma torch was used to make spherical alumina particles of controlled size from irregularly shaped precursor powders. Detailed studies of the impact of operating parameters, particularly gas identity (argon or air), gas flow rates, and applied power, showed that particle size changed in a predictable fashion. The most important factor in controlling particle size appears to be precursor particle density in the aerosol stream that enters the plasma hot zone. This and other facts suggest that particle collision rate is primarily responsible for determining ultimate particle size, although atomic addition also plays a role. Reproducible volume average particle sizes ranging from 97 to 1150 mum(3) were formed from precursor particles of order 14 mum(3). Moreover, for the first time we report the creation of an atmospheric pressure low-power air plasma (<1 kW).