AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Numerical Study of Growth Process of Binary Alloy Nanopowders in Thermal Plasma Synthesis
MASAYA SHIGETA, Takayuki Watanabe, Toyonobu Yoshida, Tohoku University
Abstract Number: 51 Working Group: Synthesis of Functional Materials using Flames, Plasmas and other Aerosol Methods
Abstract The growth processes of binary alloy nanopowders in thermal plasma synthesis are investigated numerically using our unique model. The model makes it possible to compute the collective and simultaneous combined process of binary nucleation, binary co-condensation of high temperature metallic vapors, and coagulation among nanoparticles even with different compositions successfully. The freezing point depression due to nanoscale particle diameters is also taken into account. This study analyzes two representative systems. One is the Nb-Si system which has a large difference of the saturation pressures between the two materials, and the other is the V-Si system which presents a small difference. In the Nb-Si system, at the early stage, Nb-rich nuclei are generated by nucleation followed by condensation of Nb vapor on the nuclei, which produces a number of Nb-rich nanoparticles. Subsequently, Si vapor condenses on the Nb-rich nanoparticles. The finally obtained nanopowder consists of larger Nb-rich nanoparticles and smaller Si-rich nanoparticles. Furthermore, the present model predicts that several kinds of intermetallic compounds (Nb$_3Si, Nb$_5Si$_3, and NbSi$_2) are produced, which agrees with our previous experiment. On the other hand, in the V-Si system, a large number of V-Si-mixed nuclei are generated by the binary nucleation and immediately V vapor and Si vapor co-condense on those nuclei simultaneously. Almost the same saturation pressures of V and Si cause such a co-condensation process. This type of systems tends to produce a single phase alloy nanopowder, which also agrees with the experiment results. Moreover, the effect of the cooling rate on the final profile of a nanopowder is examined. In both systems, at the higher cooling rate, the mean sizes and the standard deviations of sizes become smaller. The standard deviation of Si content in the nanopowder becomes larger in the Nb-Si system, while that in the V-Si system is less affected.