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

点击翻译按钮获取中文摘要

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.