Melting and spheroidization of hexagonal boron nitride in a microwave-powered, atmospheric pressure nitrogen plasma

We have developed a method for producing spherically-shaped, hexagonal phase boron nitride (hBN) particles of controlled diameter in the 10-100 micron size range. Specifically, platelet-shaped hBN particles are passed as an aerosol through a microwave-generated, atmospheric pressure, nitrogen plasma. In the plasma, agglomerates formed by collisions between input hBN particles, melt and form spheres. We postulate that this unprecedented process takes place in the unique environment of a plasma containing a high N-atom concentration, because in such an environment the decomposition temperature can be raised above the melting temperature. Indeed, given the following relationship (V. L. Vinogradov and Kostanovskii, Teplofizika Vysokikh Temperatur 29 (1991) 1112): BN(condensed) <----> B-(gas) + N-(gas). Standard equilibrium thermodynamics indicate that the decomposition temperature of hBN is increased in the presence of high concentrations of N atoms. We postulate that in our plasma system the N atom concentration is high enough to raise the decomposition temperature above the (undetermined) melting temperature. (C) 2002 Kluwer Academic Publishers.