Thermoelectric Properties of N-type Mg₃La_0.005Mn_xSbBi Materials Doped with La and Mn

Abstract

Mg₃Sb₂-based n-type materials are consisted of earth-abundant elements and possess comparable thermoelectric properties with n-type Bi₂Te₃ at low temperatures, which make them promising candidates for cooling and power generation applications in terms of cost and performance. Substitution of Sb atom with chalcogen elements (Te, Se S) is a conventional method for n-type doping, but doping cations such as rare-earth elements and transition metals is also widely studied for its unique advantages. In this study, La and Mn were selected for co-doping of Mg3SbBi, and the thermoelectric performances of the doped materials were investigated. Mg₃La_0.005Mn_xSbBi (0  <i>x</i>  0.015) polycrystalline samples were made by sintering the fine powders of the mother alloy after arc melting, in which elemental Mn and LaSb compound were included for n-type dual doping. Considering the loss of Mg at elevated temperatures by vaporization, the molar ratio of Mg, Sb, and Bi in the mixture for arc melting was set to 4 : 1 : 1 with excess Mg. Analysis shows that all the samples are n-type, and the electrical conductivity of Mg₃La_0.005Mn_0.015SbBi increased by 62% from the Mn-free Mg₃La_0.005SbBi at 298 K. In addition, the lattice thermal conductivity (<i>_lat</i>) decreased with increasing Mn content in the measured temperature range of 298-623 K. The minimum value of <i>_lat</i> was about 0.60 W m^-1K^-1 in Mg₃La_0.005Mn_0.015SbBi at 523 K, which is about 19% smaller than that of the Mn-free sample. As a result of these enhancements in thermoelectric performance, the maximum figure of merit (<i>zT_max</i>) of 1.12 was obtained in Mg₃La_0.005Mn_0.01SbBi and Mg₃La_0.005Mn_0.015SbBi at 573 K, and the <i>zT</i> at 298 K increased by 73% to 0.35 in Mg₃La_0.005Mn_0.015SbBi compared to Mn-free Mg₃La_0.005SbBi, which is beneficial to room-temperature applications.