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

为实现表面包覆亚微米颗粒的批量生产，研究了CVD法制备AlN包覆Si_3N_4颗粒的反应器设计和操作条件。用筛网对种子颗粒进行团聚，以提高流动性。首先，采用带悬浮导流管的流化床，在悬浮导流管下注入AlCl_3气体使团聚体浸没。它们被溢出到环形区域与NH_3气体反应。接触时间通过床内循环来控制。但是，该产品在运行过程中堵塞了分配器，因此短期内进行了改进无效。二是采用逆流式移动床，不设分配器。利用反应器底部的孔口调节团聚体的停留时间。虽然单道沉积量少，但通过合理的内部结构设计，反应区大，压降小，可以实现连续运行。结果表明，SEM对团聚体形貌没有明显改变，XRD识别出AlN的偏转线，EPMA对团聚体内部沉积了Al。较高的反应气体浓度、较高的反应温度和较长的反应时间可以提高AlN的包覆率，但较低的温度和较小的团聚体有利于其均匀分布。这是由于反应速率和扩散速率与温度和扩散距离的综合作用而得到的。因此，建立了基于一级反应速率的核壳模型来分析表观包覆比的轴向变化，然后引入了多孔团聚体内部扩散控制模型来描述沉积物的径向分布。

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.