Thermoelectric Properties of p-Type Clathrate Ba8.0Ga15.9ZnySn30.1 Single Crystals with Various Carrier Concentrations

Abstract

To match the excellent performance of n-type type-VIII clathrate Ba8Ga16Sn30 single crystals in thermoelectric (TE) generator design, synthesizing a large size p-type counterpart with a high TE figure of merit ZT continues to be a challenge for many years. In this work, we obtained 7-mm-diameter single crystals of Zn-substituted type-VIII clathrate Ba8Ga16Sn30 (Zn-BGS) with various carrier concentrations by Ga-flux method. Though the actual substituted compositions of the single crystals Ba8Ga15.9ZnySn30.1 (y = 0–0.019) are much lower than the nominal ones, the Zn atoms work as a “spur” and effectively suppress the nucleation rate of BGS in the Ga-flux, which is vital to achieving a similar large size crystal with the n-type counterpart. The y = 0.006 and 0.007 crystals share nearly the same or a slightly lower value of resistivity with a pristine crystal in the whole temperature. However, the Hall coefficient measurement revealed that Zn substitution triggers a profound transition from a “low-carrier-concentration high-carrier-mobility” regime to a “high-carrier-concentration low-carrier-mobility” one in p-type BGS. As a consequence, both the Seebeck coefficient and thermal conductivity benefit from the relief of the negative impact of minority carrier by shifting the “trigger temperature” of intrinsic excitation to a higher temperature. Figure of merit ZTs of 0.97 and 1.07 are observed for x = 0.006, 0.007 at 500 K, which are more than 25% and 40% higher than the figure of merit for pristine BGS at the same temperature zone, respectively. The improvement demonstrates the potential of making TE modules from n- and p-type single crystal BGS for waste heat recovery at medium temperatures.