| Summary: | Thermoelectric materials can convert heat into electricity and vice versa for use as power generator and cooler. Skutterudite, CoSb3-based materials are particularly promising thermoelectric (TE) material used in the intermediate temperature range (600～900K), due to its reasonably large Seebeck coefficient (α) and high electrical conductivity (σ), resulting in high power factor (α2σ), which are comparable to the state-of-art TE materials. However, its thermal conductivity (κ) is too high to make it an effective TE material. In this work, nanocomposite approach, in which nanoscaled structures (nano-inclusions) are incorporated into the matrix of a conventional micron-sized material, is attempted in order to reduce its thermal conductivity and improve the overall thermoelectric properties of skutterudite. The effects of nano-inclusion on the electrical properties of nanocomposites were also investigated. The work started with the synthesis of p-type pure CoSb3 nanoparticles via a modified polyol process. Systematic experimental results indicated a high purity of CoSb3 phase could be obtained at a relatively low temperature of 195 °C for 15 mins. N-type Ni-doped CoSb3 nanoparticles were also synthesized using the same reaction conditions with the doping amount optimized. It was found that the Ni1.25Co6.75Sb24 sample possessed the maximum power factor, and the highest ZT value of 0.33 was achieved at around 480 °C which is much higher than that of undoped CoSb3 of 0.067 at around 450 °C. The nanoparticles obtained were also used as dispersants to prepare both p-type and n-type nanocomposites. The p-type nanocomposite with undoped CoSb3 acting both as nano-inclusions and micron-sized matrix were prepared. The results showed that both the total and lattice thermal conductivity were reduced after introducing nano-inclusions due to its high density of grain boundaries. Besides thermal conductivity, the electrical properties of the nanocomposite were also found to be improved by the nano-inclusions due to the different electrical properties of these two phases, where the nano-inclusions have a much higher carrier concentration than that of micron-sized matrix. Furthermore, another two types of p-type nanocomposites whose nano-inclusions have much lower or comparable carrier concentration than that of micron-sized matrix were prepared and the effects of these nano-inclusions on both thermal and electrical properties were investigated.
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