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Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg3+δBi2-xSbx exhibits very good TE performance near room temperature, which makes Mg3+δBi2-xSbx-based compounds highly promising candidates to replace the Bi2Te3-ySey alloys, but ideal contacts that can match their TE performance have not yet been well studied. Here we investigate different metal (Ni and Fe) and metal alloy (NiFe, NiCr, NiCrFe, and stainless steel) contacts on n-type Mg3+δBi1.5Sb0.5. It is first shown that the low Schottky barrier and narrow depletion region resulting from the band degeneracy and high carrier concentration of a heavily doped TE material are beneficial for the formation of a low-resistivity ohmic contact with a metal or a metal alloy. Most fully optimized TE materials can take advantage of this. Second, it is found that the NiFe/Mg3+δBi1.5Sb0.5 contact exhibits excellent thermal stability and the lowest ohmic contact resistivity among those studied after aging for over 2100 h, which is attributed to the formation of metallic NiMgBi between the NiFe and Mg3+δBi1.5Sb0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg3+δBi1.5Sb0.5/Bi0.4Sb1.6Te3 unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K.

Bi2Te3‐based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low‐grade heat into electric power. The recently developed n‐type Mg3Sb2‐based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi2Te3 module with a reliable and low‐cost all‐Mg3Sb2‐based thermoelectric device if a good p‐type Mg3Sb2 material can be obtained to match its n‐type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p‐type Mg3Sb2 is significantly improved through Na and Yb doping in Mg1.8Zn1.2Sb2. Moreover, p‐ and n‐type Mg3Sb2 are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all‐Mg3Sb2‐based unicouple. The decent conversion efficiency (≈5.5% at the hot‐side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg3Sb2‐based thermoelectric generators for low‐grade heat recovery.

p‐type CaMg2Sb2 has not been favorably considered a promising thermoelectric material in comparison to other Zintl phases. However, a series of meticulously designed tactics is successfully implemented here to analyze and improve the thermoelectric performance of a CaMg2Sb2‐based material. Band structure calculations show that the pristine CaMg2Sb2 has a wide indirect band gap of ≈1.11 eV, which results in its less optimal carrier concentration. To improve its consequently inferior thermoelectric performance, Na doping is first deployed to increase the carrier concentration to an optimal level. Subsequently, the ideal bandwidth and enhanced carrier mobility are simultaneously obtained through Zn alloying. Finally, partial Yb substitution for Ca effectively restrains the bipolar effect in this CaMg2Sb2‐based material and augments its power factor while suppressing its lattice thermal conductivity to a minimum. As a result, a 20‐fold‐higher peak figure of merit (zT) of ≈0.85 at 773 K and an average zT value of ≈0.50 from 298 to 773 K are achieved in Ca0.69Yb0.3Na0.01Mg1.1Zn0.9Sb2, the highest for both values among all [Ca,Mg,Eu,Yb]Mg2Sb2‐based materials reported to date. Furthermore, such a combination of rationally designed tactics as proposed here may assist in exploring the improvement of other thermoelectric materials that have long been overlooked.