A COMBINATION of POINT DEFECTS and NANOSIZED GRAINS to MINIMIZE LATTICE THERMAL CONDUCTIVITY of Sn and Se CO-DOPED CoSb3 VIA MIXED BALL MILLING and SPARK PLASMA SINTERING

The efficient strategies to minimize thermal conductivity in skutterudite materials are creating point defects along with nanosized grains. In this report, Sn and Se co-doped CoSb3 materials were synthesized through mixed-ball milling and spark plasma sintering techniques to utilize this strategy. Their phases, microstructure and thermoelectric properties were investigated under the content variation of Sn and Se in CoSb3 samples. The experimental results revealed that the Sn and Se were substituted at Sb sites in CoSb3 crystal structure and grain sizes were restricted to a hundred nanometer. The lattice thermal conductivity was reduced to 2.4W/mK at 298K. Interestingly, increasing Sn and Se doped content could further minimize the lattice thermal conductivity. The lowest value at room temperature is 1.79W/mK for CoSb2.70Sn0.150Se0.150 which was dramatically lower than pure CoSb3. Moreover, the increment of Sn and Se content also increased the electrical conductivity of doped samples, while the negative Seebeck coefficient sign tended to decrease. As expected, low electrical conductivity and substantial reduction in the Seebeck coefficient of doped samples at high measurement temperature, resulting in low power factor and low ZT values. It was clearly seen that the highest power factor of 880μW/mK2 was found at 516K in CoSb2.65Sn0.175Se0.175. Furthermore, it also dominated the highest ZT value of 0.29 at 565 K, compared to the other Sn and Se co-doped samples. From these results, ball milling under dry conditions followed by wet conditions not only allowed a longer milling process but also generated a small fraction of pore which was a part of the reduction in thermal conductivity. Especially, the advantage of the existence of Sn and Se point defects and nanosized grains from this work will be escalated when it was applied to prepare materials that have high power factor values.