Thermoelectric properties of La- and Sc-doped Mg3Sb2 synthesized via pulsed electric current sintering

Abstract

High-efficiency thermoelectric materials that convert heat into electricity are considered as a countermeasure against greenhouse gas emissions and global warming. In this regard, Mg3Sb2-based Zintl compounds, such as Mg3(Sb, Bi)2 and Mg3Sb2–Mg3Bi2, are inexpensive yet suitable candidates in the medium-temperature range due to low-toxicity and relative abundance of their constituent elements. However, n-type Mg3Sb2 without Bi possesses superior oxidation resistance. This study aimed to investigate the suitability of Sc and La as candidates for n-type Mg3Sb2 dopants. La-doped and Sc-doped Mg3Sb2 (Mg3Sb2Mx [M = La, Sc]; x = 0.000 to 0.05) were synthesized by a one-step, rapid pulsed electric current sintering using Mg, Sb, and a small amount of hydrate (La(OH)3) or oxide (Sc2O3). Electron backscatter diffraction revealed that the average grain sizes for the La-doped (x = 0.03) and Sc-doped (x = 0.05) Mg3Sb2 were 17 and 38 μm, respectively. The room temperature maximum electron concentration for the La-doped and Sc-doped Mg3Sb2 increased up to 2.3 × 1019 and 3.7 × 1019 cm−3, respectively, which is comparable to or higher than the maximum values obtained for the reported n-type Te-doped Mg3Sb2 (approximately 2 × 1019 cm−3). The maximum dimensionless thermoelectric figure of merit values for the La-doped and Sc-doped Mg3Sb2 at 770 K were 0.93 and 0.80, respectively, indicating that Sc and La are promising doping candidates for achieving higher ZT values in n-type Mg3Sb2 without Bi.