Reliable metal alloy contact for Mg_3+δBi_1.5Sb_0.5 thermoelectric devices

Abstract

Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg_3+δBi_2-xSb_x exhibits very good TE performance near room temperature, which makes Mg_3+δBi_2-xSb_x-based compounds highly promising candidates to replace the Bi₂Te_3-ySe_y 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 Mg_3+δBi_1.5Sb_0.5. 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/Mg_3+δBi_1.5Sb_0.5 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 Mg_3+δBi_1.5Sb_0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg_3+δBi_1.5Sb_0.5/Bi_0.4Sb_1.6Te₃ unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K.

Keywords

Thermoelectric power generator / ohmic contact / Mg_3+δBi_1.5Sb_0.5 / NiFe alloy / energy conversion

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