Improvement of Thermoelectric Properties of Iron Disilicide by Plasma Processing and Its Relation with Microstructure

为了提高FeSi2和SiGe陶瓷的热电性能，我们在烧结前对微晶进行了射频等离子体处理。用SiH_4或GeH_4作为等离子体气体进行处理。用常规方法对等离子体处理后的颗粒进行烧结，并对所得陶瓷样品的热电性能进行了测试。实验结果表明：(1)等离子体处理陶瓷比未处理陶瓷具有更高的电导率。这种电导率的增加是由SiH_4或GeH_4等离子体处理引起的。霍尔测量表明，电导率的增加主要是由于载流子迁移率的增加。(2)电子顺磁共振研究表明，一些顺磁中心被等离子体处理所湮没。电导率的增加可能是由于晶界势垒高度的降低。(3)在SiH_4等离子体处理的情况下，热导率没有明显的变化。(4)等离子体处理对Seebeck系数的影响比较复杂，但在大多数情况下，等离子体处理会增加Seebeck系数。(5)等离子体处理不仅对FeSi_2陶瓷，而且对SiGe陶瓷都有影响。在这些实验结果的基础上，我们可以总结出等离子体处理对FeSi_2和SiGe陶瓷的影响如下：(1)等离子体处理和由此产生的微结构改变使FeSi2和SiGe陶瓷的热电性能提高了2倍或3倍。(2)SiH_4等离子体处理对提高FeSi2和SiGe陶瓷的电导率是有效的，而GeH_4等离子体处理主要是导致导热系数降低的主要原因。因此，适当混合SiH_4和GeH_4气体的等离子体处理有望进一步改善材料的热电性能。

To improve the thermoelectric figure-of-merit of FeSi_2 and SiGe ceramics, we have employed rf-plasma processing of micrograins prior to sintering. Either SiH_4 or GeH_4 were used as plasma gas for processing. The plasma processed grains were sintered by the ordinary method and the thermoelectric properties of the resultant ceramic specimens were measured. The experimental results are shown in the followings :(1) Plasma-processed ceramics shows electrical conductivity higher than non-processed ceramics. This conductivity increase occurs by either SiH_4 or GeH_4 plasma processing. The Hall measurements revealed that the conductivity increase is mainly due to the increase of carrier mobility.(2) Electron paramagnetic resonance study indicates that some paramagnetic centers are annihilated by plasma processing. The conductivity increase is probably due to the decrease of potential barrier height at grain boundaries.(3) In case of SiH_4 plasma processing, no significant change was oberved in thermal conductivity. However, GeH_4 plasma processing reduces the thermal conductivity.(4) The effect of plasma procesisng on Seebeck coefficient is somewhat complicated, but in most cases plasma processing increases the Seebeck.(5) The effects of plasma processing described above are observed not only for FeSi_2 but also SiGe ceramics.On the basis of these experimental results, we can summarize the effect of plasma processing on FeSi_2 and SiGe ceramics as follows.(1) Plasma processing and resulting microstructure modification improve the thermoelectric properties of FeSi_2 and SiGe ceramics by factor 2 or 3.(2) SiH_4 plasma processing is effective to the electrical conductivity improvement, but GeH_4 plasma processing primarily contributes to the decrease of thermal conductivity. Therefore, it is expected that plasma processing with suitably mixed SiH_4 and GeH_4 gases further improves the themoelectric properties.