engleski

Investigation of the thermal properties of single-crystalline SnSe at low temperatures

Thermoelectric materials as energy harvesters are a hot spot in research community due to their ability to directly convert waste heat into electrical energy. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit ZT. For high thermoelectric conversion efficiency large Seebeck coefficient, large electrical conductivity and low thermal conductivity are required. Unfortunately, last two requirements are often hard to realize simultaneously in the same material because charge carriers usually carry heat efficiently as well. Thus systems with relatively low charge carrier concentration are usually good thermoelectrics. In those systems phonons are dominant heat conducting channel and their contribution can be further reduced through material architecturing. But, it was recently reported that SnSe in single crystalline form, exhibits high thermoelectric figure of merit (2.6 at 973 K) mainly due to its intrinsic extremely low thermal conductivity [1]. This makes SnSe one of the most promising thermoelectric materials. Although physical properties of SnSe above room temperature are well investigated, there are very few studies of low-temperature single-crystalline phase [2]. Motivated to determine the possible origins of these remarkable properties, we measured thermoelectric properties of SnSe down to low temperatures. Here we present investigation of anisotropic thermoelectric transport properties of SnSe at low temperatures, along the three crystallographic directions. We find that the thermal conductivity features a pronounced umklapp maximum near 12K. Our results indicate that the lattice thermal conductivity of single-crystalline SnSe at room temperature is higher than this reported in the literature [1]. We further discuss possible reasons for the differences observed between our results and those previously obtained by Zhao et al. [1]. Wei et al. have reported that the lower densities of the samples measured by Zhao et al. (5.4 g/cm3) may be correlated with the lower thermal conductivity values of their samples (our sample density is close to theoretical density of 6.18 g/cm3) [3]. This implies that their reported thermal conductivities are not intrinsic to high quality SnSe single crystal. Possible routes for optimizing low- temperature thermoelectric efficiency will be discussed. [1] Zhao, L. D., Lo, S. H., Zhang, Y., Sun, H., Tan, G., Uher, C., Wolverton, C., Dravid, V. P., Kanatzidis, M. G. (2014). Nature 508, 373. [2] Ibrahim, D., Vaney, J.-B., Sassi, S., Candolfi, C., Ohorodniichuk, V., Levinsky, P., Semprimoschnig, C., Dauscher A., Lenoir, B. (2017). Appl. Phys. Lett. 110, 032103. [3] Wei, P.C., Bhattacharya, S., He, J., Neeleshwar, S., Podila, R., Chen, Y.Y., Rao, A. M. (2016). Nature 539, E1-E2.

thermal properties, single-crystalline SnSe

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