Preparation And Thermoelectric Properties Of CoSb₃/SiO₂ Nano-composites

Recent years, Skutterudite compound as one of the most promising thermoelectric (TE) materials has attracted attention because of its "phonon-glass /electron-crystal" transfer characters. However, the figure-of-merit Z for evaluating the performance of TE materials is not ideal due to the larger thermal conductivity (k) of binary Skutterudite. Therefore, researchers focus on reducing k to improve the dimesionless figure of merit ZT value.In this research, micro- and nano- CoSb₃ compounds are synthesized by solid state reaction and Cross-coprecipitation. We introduced nano-Sio₂ second phase with nonelectric conduction and low thermal conductivity via mechanical mixture to obtain the CoSb₃/SiO₂ nano composite powders. Corresponding, we introduced nano-SiO₂ via chemical coating to obtain the CoSb₃/SiO₂ particles with core-shell structure. The bulk nano composites were fabricated by Spark Plasma Sintering (SPS). We investigated the effect of content and distributing of nano-SiO₂ on microstructure and thermoelectric properties of CoSb₃/SiO₂ composites. The mainly conclusions as follows:In the CoSb₃/SiO₂ nano-composites synthesized via mechanical mixture, the agglomeration of nano-SiO₂ became more and more serious when its content is increasing. The nano-SiO₂ conduced the decreasing of electrical conductivity (σ) of composites because of the nonelectric conductivity of nano-SiO₂, but the Seebeck coefficient (α) increased remarkably at the same time, especially to the nano-CoSb3 based composites, when temperature is 625 K, theαreached 261μVK^-1; The phonon scattering of abundant interfaces and low thermal conductivity(k) of nano-SiO₂ led to the declining of k for composites, and k is only 1.92 Wm^-1K^-1 when nano-SiO₂ content is 5% and temperature is 800 K. As to the composites of micro-CoSb₃/SiO₂, when the content of SiO₂ is 3%, the ZT_max is 0.19 at the temperature of 700 K. Under the same temperature, the increment of ZT reached 64% compared to micro-CoSb3 compound; As to the composites of nano-CoSb₃/SiO₂, when the content of SiO₂ is 5%, the ZT_max is 0.30 at the temperature of 700 K. Under the same temperature, the increment of ZT reached 118% compared to nano-CoSb₃ compound.In the micro-CoSb₃/SiO₂ composites synthesized via chemical coating, coating layers mainly concentrated the surface of the smaller CoSb₃ particles. In the nano-CoSb₃/SiO₂ composite powders, two coating methods have been investigated. The one is coating nano-CoSb₃ powders, the other is coating nano-CoSb₃ precursors followed reduction treatment, and the microstructure of the latter is excel apparently the former. The thicknesses of coating layers are increasing as the increment of SiO₂ content, and the coating layers are equality. After SPS, the coating layers partially melted, and distributed at the edge of the grain forming island-morphology enrichment areas, which distributed homogeneously in the composites.As to the composite of micro-CoSb₃/SiO₂ when content of SiO₂ is 3%, the ZT_max is 0.15 at the temperature of 675 K. Under the same temperature, the increment of ZT reached about 20% compared to micro-CoSb₃ compound; As to the composite of nano-CoSb₃/SiO₂ when content of SiO₂ is 1%, the ZT_max reaches 0.42 at the temperature of 675 K. Under the same temperature, the increment of ZT reached about 140% compared to nano-CoSb₃ compound.