4 years ago

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials
Chenguang Fu, Xinbing Zhao, Tiejun Zhu, Umut Aydemir, G. Jeffrey Snyder, Kaiyang Xia, Yintu Liu, Junjie Yu, Thomas C. Chasapis
NbFeSb-based half-Heusler alloys have been recently identified as promising high-temperature thermoelectric materials with a figure of merit zT > 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta-alloyed (Nb1−xTax)0.8Ti0.2FeSb (x = 0 – 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single-phase Ta-alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying. The successful synthesis of single-phase Ta-alloyed (Nb1−xTax)0.8Ti0.2FeSb (x = 0 – 0.4) thermoelectric materials by levitation melting is reported. Refractory Ta alloying dramatically reduces the thermal conductivity but maintains the good electrical properties due to the similar chemical nature between Ta and Nb, resulting in a high figure of merit, zT, of 1.6.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/aenm.201701313

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