The effect of Bi doping on thermoelectric properties of Mg2Si0.5Sn0.5

Mg₂Si_(1-x)Sn_x solid solution systems are an ecologically friendly semiconductor, and have been proposed to the probable materials for high-performance thermoelectric generators at temperatures range from 500 to 800 K. The single phase of this system at the compositions range of 0.4 < x < 0.6 has not been reported until now. The single phase of Mg₂Si_0.5Sn_0.5 has been successfully obtained by a liquid-solid reaction method and hot-pressing method. The minimum value of thermal conductivity was identified at around x=0.5. The high thermoelectric performance can be attained by the controlling of carrier concentration for Mg₂Si_0.5Sn_0.5. In this present work, the thermoelectric properties for the single-phase of Bi-doped Mg₂Si_0.5Sn_0.5 were investigated. Seebeck coefficient alpha, electrical resistivity rho and thermal conductivity kappa were measured from room temperature to 850 K. The carrier concentration n increased lineally with the amount of Bi-doping, and the Bi atom acts as a singly ionizable substitutional donor. The reduction Fermi energy xi increased with increasing Bi amount. The undoped sample (xi=-2.808) was non-degenerated state, and 15000 ppm-Bi doping sample (xi=2.304) was heavily degenerated state. The absolute values of alpha for all samples showed a pronounced maximum which shifts to a higher temperature with increasing n. The rho of non-doping sample shows semiconducting behavior, and Bi-doping samples indicated the same behavior as a metal, which increased monotonously to the intrinsic region with increasing temperature. The carrier component of thermal conductivity was increased, while the phonon component of thermal conductivity was decreased slightly with carrier concentration. The dimensionless figure of merit was showed markedly enhanced the maximum value of ZT=0.87 for 7500 ppm-Bi doping at 630 K.