High-Performance Mg ₃ Sb _{2- x} Bi _x Thermoelectrics: Progress and Perspective

Abstract

Since the first successful implementation of n-type doping, low-cost Mg3Sb2-xBix alloys have been rapidly developed as excellent thermoelectric materials in recent years. An average figure of merit zT above unity over the temperature range 300–700 K makes this new system become a promising alternative to the commercially used n-type Bi2Te3-xSex alloys for either refrigeration or low-grade heat power generation near room temperature. In this review, with the structure-property-application relationship as the mainline, we first discuss how the crystallographic, electronic, and phononic structures lay the foundation of the high thermoelectric performance. Then, optimization strategies, including the physical aspects of band engineering with Sb/Bi alloying and carrier scattering mechanism with grain boundary modification and the chemical aspects of Mg defects and aliovalent doping, are extensively reviewed. Mainstream directions targeting the improvement of zT near room temperature are outlined. Finally, device applications and related engineering issues are discussed. We hope this review could help to promote the understanding and future developments of low-cost Mg3Sb2-xBix alloys for practical thermoelectric applications.

Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho

“…The carrier mobility reduction is commonly believed to be induced by grain boundary scattering, impurity, or Mg vacancy. ,,− However, unlike previous reports on n-type chalcogen-doped Mg 3 Sb 2– x Bi x samples, the carrier mobility reduction for the Gd- or Ho-doping is very sensitive to the carrier concentration, doping content, or Fermi level position. The carrier mobility reduction here is only obvious for samples with lower carrier concentrations and doping levels (i.e., Mg 3.5 Gd 0.01 Sb 2 , Mg 3.5 Ho 0.01 Sb 2 , and Mg 3.5 Ho 0.02 Sb 2 ).…”

In‐Situ Loading Bridgman Growth of Mg3Bi1.49Sb0.5Te0.01 Bulk Crystals for Thermoelectric Applications

“…[ 21 ] The Seebeck coefficient was found to increase with increasing temperature (Figure 3a), and the negative values indicate the successful growth of n‐type materials, which is consistent with the Hall measurements below. The substantial reduction on resistivity is mainly due to the significantly improved Hall mobility, and a value of 220 cm 2 V −1 s −1 at room temperature due to the high Bi/Sb ratios [ 22 ] and less grain boundaries is much higher than most of reported single crystalline and polycrystalline materials in the literatures (Figure 3b), e.g., ≈146 cm 2 V −1 s −1 of Mg 3 Bi 1.25 Sb 0.75 single crystal, [ 15 ] ≈161 cm 2 V −1 s −1 of Se‐doped Mg 3.2 Bi 1.4 Sb 0.6 polycrystalline material [ 23 ] and ≈139 cm 2 V −1 s −1 of Y‐doped Mg 3.05 BiSb polycrystalline material. [ 24 ] The power factors is ≈24 µW cm −1 K −2 around room temperature (Figure S4, Supporting Information).…”

Understanding the High Thermoelectric Performance of Mg3Sb2‐Mg3Bi2 Alloys

“…Alloys of n-type Mg 3 Sb 2 -Mg 3 Bi 2 (Mg 3 X 2 where X is a group 15 element, often a mixture of Sb and Bi) are receiving increasing attention as some of the most promising thermoelectric materials with high performance in the range of room temperature (≈300 K) to mid-temperature (≈700 K). [1][2][3][4][5][6][7][8] Synthesizing n-type Mg 3 X 2 -based compounds has been a great challenge due to the intrinsic defects, until recently n-type Mg 3 Sb 1.5 Bi 0.5 was reported by introducing excess Mg coupled with Te doping. [1,2] As long as some essential conditions are fully filled, various synthesis methods are effective to synthesize n-type materials, including ball milling [3,9] and melting method.…”