Generation of Metal Ultra-Fine-Particles via Gas-Phase Chemical Reaction

Ultra fine particle/powder (UFP) of metal is generated by hydrogen reduction of MCl2 (M=Ni, Co, Fe) vapor. The effect of reaction temperature, hydrogen flow rate, residence time and total flow rate are discussed and it is concluded that the partial pressure of MCl2 (g), PMCl2, has the greatest influence on the average particle size. Smaller PMCl2 gives smaller median diameter.The particle size distributions are almost log-normal and their geometric standard deviations distribute between 1.36 and 1.60 independently on the median diameters.A simple model without any adjustable parameters is proposed based on the following assumputions;1. the reaction is instantaneous irreversible one and the initial reaction product is metal atom or cluster (called monomer), 2. because of very high supersaturation, monomers can start coagulation/coalescence among themselves.The numerical computation was done and compared with the experimental result. It is concluded though this model is very simple it explains the experiments facts fairly well.