Formation mechanism of transition metal boride nanoparticles in induction thermal plasma processing

Summary form only given. Boron-rich compounds are attractive materials because of their high melting point, hardness, high electrical conductivity, and high corrosion resistance to molten material. Therefore, their nanoparticles have received great attention for the applications of polishing material, neutron shield material, and catalyst of fuel cell [1]. The purpose of this paper is to prepare nano-sized boron-rich compounds and to investigate the formation mechanism of boron-rich compounds in induction plasma processing. For aluminum-based boride synthesis, AlB₁₂ and AlB₁₀ were successfully synthesized at boron excess conditions in the case of feed powders of crystalline boron (45 μm) and aluminum (15 μm), which were fed with the carrier gas into the induction thermal plasma. The average diameter of the prepared particles is 20 nm. The nucleation rate expression proposed by Girshick et al. [2] was used for investigation of the nanoparticle formation mechanism. Particle formation can be observed when the nucleation rate exceeds 1.0 cm-3 s-1. The corresponding value of saturation ratio is defined as the critical saturation ratio. The nucleation temperature of boron is higher than that of aluminum, accordingly the starting process is the boron nucleation, followed by the aluminum condensation onto the boron cluster to form aluminum boride. Combination of the boron cluster and aluminum leads to form the boron-rich compound structure. For the comparison, titanium-based boride was synthesized with the same manner. The raw materials were titanium (45 μm) and crystalline boron (45 μm). The nanoparticles of titanium-based boride have the average diameter of 15 nm. The mass fraction of TiB had the range from 0% to 99.58%. The powder mixture of titanium, aluminum, and boron was also used as the raw materials. TiB₂ was the main product under the condition of insufficient boron in the raw materials. On the othe- hand, AlB₁₂ was the main product under the boron-rich conditions. These results suggest that compositions of metal boride nanoparticles.