Development of a Large-Diameter Thermal Plasma Processing Device and its Application to Synthesis of Thermoelectric Materials

减压热等离子体射流具有速度快、气体温度高、等离子体体积大等优点，在合成新材料方面具有巨大的潜力。为此，我们设计了由强制收缩式等离子体射流发生器和进料环组成的等离子体反应器。为了阐明该反应器应用于材料精细等离子体加工的可行性，对等离子体射流的行为进行了研究，结果表明：（1）在低压下，等离子体射流的高温区和高速区扩大;通过使用这种膨胀等离子体射流，获得了无囊泡的致密膜，并且膜厚度易于控制。(2)在低压下，梯度功能材料的硬度和结合强度较高。(3)在常压和低压条件下，由α-Al_2 O_3、Na_2CO_3和MgO混合粉末成功地合成了β”-Al_2O_3薄膜。(4)合成纯金刚石膜时，CH_4流量与H_2流量之比低于某一临界值。也就是说，衬底附近的CH/H_2强度<alpha>对纯金刚石薄膜的沉积有很大的影响。(5)计算机层析成像技术应适用于非对称等离子体射流的诊断。

Thermal plasma jet operated under reduced pressure has a great potential for synthesizing new materials, because of its high speed, high gas temperature and large plasma volume. Therefore, we have newly designed the plasma reactor composed of the forced constricted type plasma jet generator and the feed ring. In order to clarify the application feasibility of this reactor to fine plasma processing for materials, the behavior of the plasma jet has been discussed.Findings are as follows :(1) Under low pressure, high temperature and high velocity regions of the plasma jet are expanded. With the use of this expanding plasma jet, the dense film without vesicles is obtained and the film thickness is easily controlled.(2) Under low pressure, the hardness and the adhesion strength of functionally gradient materials are found to be high.(3) Under not only atmospheric pressure but also low pressure, thin-films of beta" -alumina are successfully synthesized from the powder mixtures, i.e.alpha-Al_2O_3, Na_2CO_3 and MgO.Synthesis of beta" -alumina depends on jet power and the distance from the feed ring exit to the substrate.(4) When pure diamond films are synthesized, the ratio of CH_4 flow rate to H_2 flow rate is lower than a certain critical value. Namely, the intensity of CH/H_<alpha> in the vicinity on the substrate has strong effect on pure diamond film deposition.(5) The computer tomography technique should be applicable to the diagnostics of asymmetric plasma jets.