Development of New Fluidized-bed processes for Si-based Materials

为了开发新的颗粒处理，例如颗粒涂覆，可利用等离子体条件下颗粒的流化现象。用直流等离子体喷射流化床反应器，以SiH_4和CH_4为原料，在颗粒表面包覆SiC。SiH_4和CH_4通过端口引入，在等离子体射流周围形成涡流。比较了预混法和扩散混合法两种气体喷射方法。与预混法相比，扩散混合法具有较高的SiH_4和CH_4转化率。虽然在出口管道上安装的过滤器中捕集了细小的β-SiC粉末，但X射线衍射没有证实SiC在床层颗粒上的沉积，这可能是由于非晶或很薄的SiC涂层。沿涂层颗粒横截面的元素分析沿着表明，大部分Si沉积在颗粒内部，而不是靠近表面。可以推测，在等离子体中激发的反应气体扩散到颗粒中，并在颗粒内部发生沉积。微波等离子体流化床反应器的基本特性进行了研究颗粒处理。氧化铝或多晶硅用作床材料。流化现象和等离子体状态的稳定性似乎取决于等离子体气体的种类和颗粒的介电损耗水平。光学观察表明，等离子体在床中的气泡内产生。甲烷转化作为模型反应进行。虽然在颗粒的存在下的甲烷转化率的水平低于在没有颗粒的情况下，甲烷被有效地转化为碳在微波等离子体流化床。

For the development of novel particle processing such as particle coating, utilization of fluidizing phenomena of particles under plasmatic conditions is available. Coating of SiC on the surface of particles from SiH_4 and CH_4 was studied using a D.C.plasma jetting fluidized bed reactor, consisting of fluidized bed superimposed on a D.C.plasma generator. SiH_4 and CH_4 were introduced through the ports forming vortex flow around the plasma jet. Two types of gas injection methods, premix and diffusion-mix methods, were compared. The diffusion-mix method gave high conversions of SiH_4 and CH_4, compared with the premix method. Though fine beta-SiC powder was trapped in the filter installed on the way of exit line, deposition of SiC on the bed particles was not confirmed by X-ray diffraction, probably due to the coating of amorphous or very thin SiC.The elemental analysis along the cross section of the coated particles showed that most silicon was deposited inside the particles and not near the surface. It can be speculated that the reaction gases excited in plasma were diffused into the particles and deposition occurred inside the particles.Fundamental characteristics of a microwave plasma-fluidized bed reactor for particle processing were also investigated. Alumina or polycrystalline silicon was used as a bed material. Fluidizing phenomena and stability of plasma state seem to depend on the kinds of plasma gas and the level of dielectric loss of particles. Optical observation showed that plasma was generated within bubbles in the bed. Methane conversion was carried out as a model reaction. Although the level of methane conversion in the presence of particles was lower than that in the absence of particles, methane was effectively converted to carbon in the microwave plasma-fluidized bed.