New insight into the material parameter B to understand the enhanced thermoelectric performance of Mg[subscript 2]S[subscript n1−x−y]Ge[subscript x]Sb[subscript y]

• Main Authors: Liu, Weishu (Author), Zhou, Jiawei (Contributor), Jie, Qing (Author), Li, Yang (Author), Kim, Hee Seok (Author), Bao, Jiming (Author), Chen, Gang (Contributor), Ren, Zhifeng (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Mechanical Engineering (Contributor)
• Format: Article
• Language: English
• Published: Royal Society of Chemistry, 2017-05-01T18:22:56Z.
• Subjects: Article
• Online Access: Get fulltext

 Historically, a material parameter B incorporating weighted mobility and lattice thermal conductivity has guided the exploration of novel thermoelectric materials. However, the conventional definition of B neglects the bipolar effect which can dramatically change the thermoelectric energy conversion efficiency at high temperatures. In this paper, a generalized material parameter B* is derived, which connects weighted mobility, lattice thermal conductivity, and the band gap. Based on the new parameter B*, we explain the successful tuning of the electron and phonon transport in Mg[subscript 2]S[subscript n1−x−y]Ge[subscript x]Sb[subscript y], with an improved ZT value from 0.6 in Mg[subscript 2]Sn[subscript 0.99]Sb[subscript 0.01] to 1.4 in Mg[subscript 2]Sn[subscript 0.73]Ge[subscript 0.25]Sb[subscript 0.02]. We uncover that the Ge alloying approach simultaneously improves all the key variables in the material parameter B*, with an ∼25% enhancement in the weighted mobility, ∼27% band gap widening, and ∼50% reduction in the lattice thermal conductivity. We show that a higher generalized parameter B* leads to a higher optimized ZT in Mg[subscript 2]Sn[subscript 0.73]Ge[subscript 0.25]Sb[subscript 0.02], and some common thermoelectric materials. The new parameter B* provides a better characterization of material's thermoelectric transport, particularly at high temperatures, and therefore can facilitate the search for good thermoelectric materials.