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Srinivasarao Popuri1 Michael Pollet2 Ingo Loa3 Dominic Fortes4 Jan-Willem Bos1

1, Heriot-Watt University, Edinburgh, , United Kingdom
2, ICMCB-CNRS, Bordeaux, , France
3, University of Edinburgh, Edinburgh, , United Kingdom
4, ISIS Facility, Harwell, , United Kingdom

Thermoelectric devices are promising clean energy technologies that use waste heat to generate electricity. SnSe has recently attracted attention due its large peak thermoelectric figure of merit, ZT ~ 2.5 at 923 K in single crystals and large temperature average ZTdevice~1.3 in Na doped single crystals.1-3 Here, ZT = S2T/ρk, where S is the Seebeck coefficient, ρ is the electrical resistivity, k is the sum of the lattice (klat) and electronic thermal conductivity (kel) and T is the absolute temperature. The outstanding thermoelectric performance of SnSe is largely based on its low klat, which has proved controversial with large variations in reported values.

In this contribution, we present our results on the link between microstructure and ZT in polycrystalline ingots and on our investigation into the link between the crystal structure and thermal properties.4-6

Polycrystalline samples were synthesized using solid-state reactions and hot pressing. These samples showed strong “V-shape” texturing of the SnSe platelets with marked differences in measured thermal conductivities depending on the ingot (0.6 ≦ klat ≦ 1.6 W m-1 K-1).4 Ingots with larger and more oriented SnSe platelets afford thermoelectric power factors (S2/ρ) = 0.9 mW m-1 K-2 at 750 K, and ZT>1 at ~850 K in p-type polycrystalline SnSe. Callaway fitting suggests that lower klat values are linked to an increased amount of disorder in the ingots, which we attribute to changes in the microstructure.4

A variable temperature neutron powder diffraction (4-1000 K) was undertaken to investigate the evolution of the crystallographic structure. Distortion mode analysis was used to reinvestigate the Pnma-Cmcm phase transition.5 This reveals significant Sn motions perpendicular to the SnSe layers, which broaden the phase transition. This was complemented by heat capacity measurements to probe the lattice dynamics.6 The data could be satisfactorily fitted using two Debye modes with ΘD1 = 345(9) K and ΘD1 = 154(2) K. The energies of these modes are found to scale with the bond strengths of the short and long bonds in the crystal structure. The presence of two lattice energy scales is reminiscent of the classical Phonon Glass Electron Crystal materials with weakly bound rattling atoms. This suggests that searching for materials with widely diverging bond distances is another possible route towards discovering good thermoelectric materials.

References:
1. L. D. Zhao et al., Nature, 2014, 508, 373.
2. L. D. Zhao et al., Science, 2016, 351, 141-144.
3. K. L. Peng et al., Energy & Environmental Science, 2016, 9, 454-460.
4. S. R. Popuri et al., Journal of Materials Chemistry C, 2016, 4, 1685-1691.
5. S. R. Popuri et al., in-preparation, 2017.
6. S. R. Popuri et al., Applied Physics Letters, 2017, 110, 253903.

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