Role of Plasma Temperature and Residence Time in Stagnation Plasma Synthesis of c-BN Nanopowders.

摘要

The synthesis of cubic boron nitride (c-BN) nanoparticles is examined experimentally by introducing borane ammonia precursor into a thermal plasma oriented in a stagnation point geometry, where nanoparticles are formed in the flow field upon reaching a cold substrate. The quasi-one dimensional flow field allows for correlating the plasma temperature and residence time to the final particle phase, morphology, size, and purity. Constant temperature and residence time cases are studied to assess the parameter s affect on the resulting particle characteristics. The as-synthesized nanoparticles are characterized by high-resolution transmission electron microscopy (HRTEM) and x- ray diffraction (XRD). Cubic structured particles are synthesized at plasma temperatures of 3000-8000K and precursor decomposition times greater than or equal to 0.030s. The highest purity samples are produced at a plasma temperature and residence time of 6500K and 0.075s, respectively. Samples with lower c-BN content are observed with higher percentages of hexagonal and amorphous phases. The particle morphology shifts from spherical agglomerates to faceted shapes as c-BN purity increases. Also, particle size undergoes an increase in nominal size. The resulting phase and purity is proposed to be governed by growth mechanisms that result in high-energy particle-particle interactions where the energy transferred is sufficient for atomic re-alignment into a denser phase.