The Effects Of The Thermal And Mechanical Loading On The Structure And Performances Of Skutterudite-based Thermoelectric Materials

The thermoelectric materials is looked as an environment-friendly materials, it can convert thermal energy into electrical energy directly. Among them, skutterudite-based materials have attracted great attention for their potential applications because of their unique crystal structure and excellent thermoelectric properties. It is well known that the thermoelectric materials will suffer the thermal and mechanical loadings inevitably, when they are used in the actual power generation system. So what changes will happen on the structures and properties of the skutterudite-based materials under the thermal and mechanical loadings? This will impact the application prospects of skutterudite-based materials directly. This paper researches the evolutions of the structures and properties of the skutterudite-based materials, which include the binary skutterudite CoSb3, the filled skutterudite Yb0.1Co4Sb12, the nanostructured CoSb3skutterudite, and micro-nano composites Co4Sb11.5Te0.5+TiN.Effects of the cyclic thermal loading (changing from room temperature to500℃and then to room temperature) and the cyclic mechanical loading(changing from0-150-0MPa) on the microstructure and the properties of CoSb3were investigated. Under the cyclic thermal loading, the material’s surface morphology changes, the grain boundary crack appears and the partial surface bulges. Especially, this change is significantly after1500cycles. X-ray diffraction shows that the phase transition does not occur after2000cycles, although the antimony volatilizes to some extent. The average grain size increases a little. The material’s density obviously decreases. Its value decreases by4.38%after1500cycles. After the CoSb3material has experienced cyclic thermal loading, its electrical conductivity becomes greater. Especially, the electrical conductivity changes significantly in the low temperature region, but it varies little when the temperature is higher than600K. The material’s Seebeck coefficient decreases slightly, compared with before cycling. However, the thermal conductivity decreases remarkably. Thus the ZT value instead a small increase after2000cycles. The compressive strength and bending strength of the material decrease, but the compression modulus and flexural modulus change slightly after2000cycles. In addition, under the cyclic mechanical loading, the structures and the properties do not change almost.Effects of the cyclic thermal loading (changing from room temperature to500℃and then to room temperature) and the cyclic mechanical loading(changing from0-150-0MPa) on the microstructure and the properties of Yb0.1Co4Sb12were investigated. Under the cyclic thermal loading, the grain boundary crack appears and the partial surface bulges on the material’s surface. The XRD results suggest that the major phase of the sample is still skutterudite after1000cycles, except for a trace amount of YbSb phase. After1000thermal cycles, the material’s density decreases, the electrical conductivity decreases, the absolute value of Seebeck coefficient increases, the thermal conductivity decreases. The maximum value of ZT is0.74for the sample before thermal cycling, it decreases to0.69after1000cycles. It is shown that the ZT value is stable after1000thermal cycles. In addition, under the cyclic mechanical loading, the material’s structures and the properties do not change almost.Effects of annealing treatment on the microstructures and the thermoelectric properties of nano structured CoSb3were investigated. The XRD results suggest that the major phase of the150-h-annealed sample is still skutterudite, except for a trace amount of Sb phase. However, annealing has a very significant impact on the grain size of the samples. The grain size of the100-h-annealed sample is three times greater than before annealing. Compared with the100-h-annealed sample, the average grain size of the150-h-annealed sample is almost unchanged. This shows that the average grain size become s stable after100h of annealing. Compared with the sample before annealing, the thermoelectric properties of the annealed sample change, the electrical conductivity increase, the Seebeck coefficient decrease, and the thermal conductivity decrease, the maximum value of ZT is0.35for the sample before annealing, but it decreases to0.24after150h of annealing. The ZT value of the150-h-annealed sample is almost the same as the ZT of the100-h-annealed sample. This indicates that the ZT value is almost stable after100h of annealing.Effects of annealing treatment on the microstructures and the thermoelectric properties of micro-nano composites Co4Sb11.5Te0.5+TiN were investigated. The influence of the annealing on the material’s density is very significant, the density is 7.46g/cm3before annealing, but it decreases to6.81g/cm3after90h of annealing. The particle size of the nano-TiN changes little after annealing, however, the aggregation of nano-particles at the grain boundaries is more evident. The EDX results show that the Sb element content increases on the material’s surface, after annealing, which content is in accord with that in the nominal composition of the material. X-ray diffraction shows that the phase transition does not occur after120h annealing. After annealing, the electrical conductivity decreases to some extent, the absolute value of Seebeck coefficient increases a little, the thermal conductivity decreases observably. Thus, the ZT value increases after annealing, the ZT value reaches1.17at800K after120h annealing, which has a nearly20%increase than that before annealing.