Surface Coating of Submicron Particles by Fluidized/Moving Bed-CVD

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For the purpose of mass-production process of surface-coated submicron particles, reactor designs and operational conditions were investigated for Si_3N_4 particles coated with AlN by CVD method. The seed particles were agglomerated with a screen to enhance flowability without binder. Firstly, a fluidized bed was used with a suspended draft tube below which AlCl_3 gas was supplied to submerge the agglomerates. They were overflown to the annular region to react with NH_3 gas. The contact time was controlled with circulation inside the bed. But, the product blocked the distributor during operation, so improvements were made in vain shortly. Secondly, a moving bed in counter-current was adopted without any distributor. The residence time of the agglomerates was adjusted with an orifice at the ractor bottom. Though the deposit amount was a little with single pass, the continuous operation becomes possible with proper design of the inside structure to yield a large reaction zone and small pressure drop.As a result, the agglomerate shape did not change substantially with SEM,AlN defraction line was recognized with XRD,and Al was deposited inside the agglomerate with EPMA.Higher concentration of reaction gas, higher temperature and longer reaction time produced higher coating ratio of AlN,but lower temperature and smaller agglomerate were preferable to its uniform distribution.These are obtained due to combined effects of reactio rate and diffusion rate with temperature and of diffusion distance. Thus, a core shell model based on the first order reaction rate was developed to analyze the axial change in apparent coating ratio, and then a diffusion-control model inside a porous agglomerate was introduced to describe the radial distribution of the deposit.