Experimental Determination of Phonon Thermal Conductivity and Lorenz Ratio of Single Crystal Bismuth Telluride at Intermediate Temperatures*

A magneto-thermal resistance method is used to extract the phonon thermal conductivity (TC) of single crystal bismuth telluride from 5 - 60 K. Phonon TC is calculated by extrapolating the total TC vs. electrical conductivity curve to a zero electrical conductivity value. Knowledge of the phonon and electron components of TC in thermoelectric materials allows us to understand if either component dominates and what particular strategy may lead to an increased thermal conversion efficiency (ZT). Our results show that the phonon thermal conductivity follows the e^{Δ_min/T} temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. Our low-temperature data and analysis of bismuth telluride are a complement to previous measurements of Goldsmid (Proc. of the Phys. Soc. Sec. B 69, 1956) and theoretical calculations by Hellman, et al. (Phys. Rev. B 90, 2014) at higher temperatures (100 - 300 K). In addition to our results for bismuth telluride, data for a parallel study on single crystal Cu, Al and Zn will be presented.