Synthesis of boron nitride powders

In the materials science community there is much interest in the development of new, efficient approaches for preparing ceramic powders having properties or performance characteristics not found with powders produced by traditional metallurgical synthesis methods. In this regard, aerosol-based syntheses are finding general acceptance for the preparation of non-metal and metal oxide powders. In contrast, much less effort has been given to aerosol-type syntheses for non-oxide powders despite potentially useful benefits. This dissertation describes the application of two chemical systems in aerosol assisted vapor phase synthesis (AAVS) for the preparation of spherical morphology boron oxynitride, BN_xO_y, powders that are subsequently converted to spherical morphology boron nitride in a second nitridation step.; Chapter 1 describes the AAVS synthesis of BN_xO_y powders using a reaction of an aqueous boric acid containing aerosol with ammonia at 1000°C. The effect of reactor tube material, total gas flow rate, ammonia concentration, boric acid concentration, and urea addition to the boric acid aerosol on the percent oxygen composition is described. The resulting BN_xO_y powders contain significant amounts of oxygen that require replacement in a second stage nitridation reaction at elevated temperature under ammonia. The influences of the reaction temperature profile, crucible geometry and transformation additive on final oxygen composition and powder crystallinity are described.; Chapter 2 outlines the formation of BN_xO_y powders from an AAVS reaction between the boron precursor (MeO)₃B and ammonia. The formation of the powders is studied as a function of total gas flow rate and ammonia concentration. In all cases the resulting powders contain lower levels of oxygen compared to powders produced from aqueous boric acid aerosols. The conversion of the BN_xO_y powders in the second stage nitridation reaction with ammonia is examined as a function of crucible geometry, temperature profile and ammonia flow rate. In support of this process, the molecular reaction between (MeO)₃B and NH₃ was reexamined. The adduct, (MeO)₃B·NH₃, was isolated and its molecular structure determined by single crystal X-ray diffraction techniques.; The results of these studies provide guidance for more detailed studies that should result in industrial scale synthesis of spherical morphology BN which currently is not formed by standard metallurgical syntheses. This new material has potential applications in several areas including the formation of BN loaded organic polymer composites.