Grain Size Dependence of Transverse Thermoelectric Transport in the Weyl Semimetal NbP*

Weyl semimetals are excellent candidates for transverse thermoelectric transport via the Nernst effect. The Nernst effect is a thermoelectric phenomenon in which a temperature gradient is applied orthogonal to an applied magnetic field, resulting in a voltage in the mutually perpendicular direction. Single-crystal NbP has shown a large Nernst thermopower, exceeding 800 mV K-1 at 109 K and 9 T [1]. Published work in bulk polycrystalline NbP, with an average grain size of ~100 microns, maintains a large Nernst thermopower, albeit decreased by a factor of ~8 [2]. In this work, we present a grain size study on bulk polycrystalline samples of NbP, where different annealing times result in varying grain sizes. Nernst thermopower continues decrease with decreasing grain size. However, thermal conductivity is also greatly reduced, which is an advantage for thermoelectric transport applications. Via microstructural characterization and thermomagnetic transport measurements, we can determine the optimal grain size for transverse thermoelectric transport applications.
[1] S. J. Watzman et al. Phys. Rev. B 97(16), 161404(R) (2018).
[2] C. Fu et al. Energy Environ. Sci. 11(10), 2813-2830 (2018).