Study on Thermoelectric and Mechanical Properties of n and p-type Skutterudites, and Contact Alloy for Si-Ge



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Thermoelectric devices that consist of both n and p-type thermoelectric materials connected electrically in series and thermally in parallel is regarded as one of the alternative technologies to improve the system efficiency. Some of the unique features such as silence in operation, extreme reliability, no moving parts, low cost, and environmentally friendliness make thermoelectric devices attractive. The performance of these materials depends on the dimensionless figure-of-merit, ZT (=(S^2 )/ T), where S, , , and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Most of the commercialized thermoelectric materials have ZT value ~1. To realize commercially competitive devices, a higher ZT value is required. Skutterudites compounds allow tuning both electrical and thermal conductivity simultaneously to improve their ZTs. A peak ZT 1.4 at 550 oC obtained in Yb0.35Co4Sb12 is higher than the previously reported value of 1.2 for the same nominal composition. The enhancement in ZT is mainly due to the increased electrical conductivity, which may be due to the fewer defects in the sample made by pre-alloying the elements. By bringing a perturbation in Sb-planner ring replacing small amount of Sb by Sn and Te, a peak ZT above 1.1 at 500 oC is achieved in n-type CoSb2.8Sn0.005Te0.195. The thermoelectric devices based on skutterudites requires higher ZT in p-type material to maximize the device efficiency. Experiments were carried out to improve the performance of p-type skutterudites. By tuning the (Fe/Ni) ratio and fillers concentration, a peak ZT 1.1 at 475 oC in Ce0.4Nd0.4Fe3.7Ni0.3Sb12 is achieved. A peak ZT 1.1 at 425 oC in rare earth-based misch metal-filled Mm0.9Fe3.1Co0.9Sb12 (Mm=La0.25Ce0.5Pr0.05Nd0.15Fe0.03), and a peak ZT in excess of 1.1 at 500 oC in La0.68Ce0.22Fe3.5Co0.5Sb12 are achieved. An output power density of 8 W cm-2 for T = 475 oC is estimated in a 2212 mm3 sample of La0.68Ce0.22Fe3.5Co0.5Sb12. The mechanical property studies reveal that the hardness and Young’s modulus of elasticity of skutterudites are better than that of Bi2Te3- and PbTe- based thermoelectric materials. Mismatch in thermal expansion coefficient and a weaker bonding between the contact alloy and Si-Ge material led to broken samples. Further experiment is required to resolve this issue.



Thermoelectrics, Nanostructures, Dimensionless figure-of-merit


Portions of this document appear in: Dahal, Tulashi, Qing Jie, Giri Joshi, Shuo Chen, Chuanfei Guo, Yucheng Lan, and Zhifeng Ren. "Thermoelectric property enhancement in Yb-doped n-type skutterudites YbxCo4Sb12." Acta Materialia 75 (2014): 316-321. And in: Dahal, Tulashi, Yucheng Lan, Qing Jie, Weishu Liu, Keshab Dahal, Lu Tang, Chuanfei Guo, and Zhifeng Ren. "Substitution of antimony by tin and tellurium in n-type skutterudites CoSb 2.8 Sn x Te 0.2− x." JOM 66, no. 11 (2014): 2282-2287. And in: Dahal, Tulashi, Qing Jie, Yucheng Lan, Chuanfei Guo, and Zhifeng Ren. "Thermoelectric performance of Ni compensated cerium and neodymium double filled p-type skutterudites." Physical Chemistry Chemical Physics 16, no. 34 (2014): 18170-18175.