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Thermal conductivity in Bi 0.5 Sb 1.5 Te 3+ x and the role of dense dislocation arrays at grain boundaries

2018; American Association for the Advancement of Science; Volume: 4; Issue: 6 Linguagem: Inglês

10.1126/sciadv.aar5606

2375-2548

Rigui Deng, Xianli Su, Zheng Zheng, Wei Liu, Yonggao Yan, Qingjie Zhang, Vinayak P. Dravid, Ctirad Uher, Mercouri G. Kanatzidis, Xinfeng Tang,

Physics of Superconductivity and Magnetism

Several prominent mechanisms for reduction in thermal conductivity have been shown in recent years to improve the figure of merit for thermoelectric materials. Such a mechanism is a hierarchical all-length-scale architecturing that recognizes the role of all microstructure elements, from atomic to nano to microscales, in reducing (lattice) thermal conductivity. In this context, there have been recent claims of remarkably low (lattice) thermal conductivity in Bi0.5Sb1.5Te3 that are attributed to seemingly ordinary grain boundary dislocation networks. These high densities of dislocation networks in Bi0.5Sb1.5Te3 were generated via unconventional materials processing with excess Te (which formed liquid phase, thereby facilitating sintering), followed by spark plasma sintering under pressure to squeeze out the liquid. We reproduced a practically identical microstructure, following practically identical processing strategies, but with noticeably different (higher) thermal conductivity than that claimed before. We show that the resultant microstructure is anisotropic, with notable difference of thermal and charge transport properties across and along two orthonormal directions, analogous to anisotropic crystals. Thus, we believe that grain boundary dislocation networks are not the primary cause of enhanced ZT through reduction in thermal conductivity. Instead, we can reproduce the purported high ZT through a favorable but impractical and incorrect combination of thermal conductivity measured along the pressing direction of anisotropy while charge transport measured in the direction perpendicular to the anisotropic direction. We believe that our work underscores the need for consistency in charge and thermal transport measurements for unified and verifiable measurements of thermoelectric (and related) properties and phenomena.