Powder Formation Process Research Articles

Gas-to-Particle Conversion in the Particle Precipitation–Aided Chemical Vapor Deposition Process I. Synthesis of the Binary Compound Titanium Nitride

Particle precipitation-aided chemical vapor deposition (PP-CVD) is a modification of the conventional CVD process, where an aerosol is formed in the gas phase at an elevated temperature, and particles are deposited on a cooled substrate. The synthesis of titanium nitride (TiN), using titanium tetrachloride vapor (TiCl4), nitrogen (N2), ammonia (NH3), and hydrogen (H2), by the PP-CVD process is studied. TiN is formed by a heterogeneous reaction, using TiCl4, N2, H2, whereas simultaneously TiCl4 and NH3 react to form an aerosol. The activation energy of this homogeneous reaction is on the order of 100 kJ/mol. The powder formation process is determined by the dissociation of a titanium containing intermediate species. At low temperature differences between substrate and gas phase (i.e., < 2 K), only dense columnar microstructures, with growth rates of around 20 μm/h, are observed. At these temperature differences no particle deposition is observed. The layers are formed by a molecular diffusion controlled CV...

Textures in Ceramic Superconductors

Textures in bulk Y–Ba–Cu–O superconducting materials were studied. Texturing methods are based on the paramagnetic anisotropy, grain-dimension anisotropy and surface-energy anisotropy of the crystals. Five incomplete X-ray pole figures including four superimposed ones were measured for the ODF calculation. The ODFs were calculated with the series expansion method including the positivity method by means of an iteration process.The results show that a strong (001)-fibre texture can be achieved by magnetic alignment but the density of the materials after sintering is still too low. The grain-dimension effect which exists in nearly all powder-forming process, is very weak for texturing of the materials. The surface texture depends on recrystallization of the powder particles.