Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals

摘要

热电材料有望成为将余热转化成电能的一种实用手段，但现有材料的能量转化效率往往相当低。提高一种材料的热电效率的主要障碍，来自导电性和导热性之间常见的相互依赖性。热电效率要求高的导电性和低的导热性，也许能提供这种组合的一条途径是生成纳米结构。现在，Li-Dong Zhao及同事介绍了一种简单的分层晶体材料"硒化锡"（SnSe),它似乎具备这些特性。作者识别出了该材料的键合结构中他们认为造成这些特性的特点，并且提出这些也许有助于具有高热电性能的其他候选材料的发现。%The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Zis the figure of merit and Tis absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the caxis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grueneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m~(-1)K~(-1) at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.