Application-oriented Studies on Mg3Sb2 Thermoelectrics: Property, Stability, Contact, and Device

Thermoelectric technology has the potential to directly convert heat into electricity and vice versa, making it a promising solution for waste heat recovery and solid-state cooling applications. To achieve high conversion efficiency in thermoelectric devices, it is crucial to develop thermoelectric materials with a high figure of merit (zT) and to address thermal stability issues and engineering challenges during device manufacture. Recent research has shown that Mg3Sb2-based Zintl compounds exhibit high thermoelectric performance over a wide temperature range. Additionally, these compounds are low-cost, environmentally friendly, and mechanically robust, making them an attractive alternative to commercial Bi2Te3 alloys for near-room-temperature applications. To advance their practical applications, we conducted studies on Mg3Sb2-based thermoelectric compounds, including transport property optimization, thermal stability enhancement, contact layer design, and device preparation. We first attempted to improve the thermoelectric performance of n-type Mg3SbBi for low-grade heat recovery by incorporating multi-walled carbon nanotubes. The composite showed a significant reduction in lattice thermal conductivity (~21% lower with the incorporation of MWCNT at room temperature) due to the mismatched phonon spectra of the two constituents, which induced large interfacial thermal resistance. However, the high power factor was maintained due to the remarkable conductivity of carbon nanotubes, resulting in an overall enhancement of the thermoelectric performance (25% higher in peak zT than that of the MWCNT-free sample). Next, we investigated a simple strategy of doping Mn at the Mg site to address the thermal stability issue of n-type Mg3Sb2 at high temperatures. Our results revealed that the vi doped atom can improve the bonding with Mg and increase the formation energy of Mg vacancy, thereby preventing Mg loss and maintaining the material's performance during long-term measurements. We also found that the contact resistivity in the Fe/Mg3SbxBi2-x/Fe single leg was dependent on the composite, which could limit the performance of the device. To address this issue, we examined a multi-layered single leg design to obtain an overall low contact resistivity (5.3 μΩ cm2) and maximize the device's output performance. Lastly, we successfully enhanced the performance of p-type Mg3Sb2, enabling us to prepare and evaluate a low-cost all-Mg3Sb2-based unicouple. The unicouple showed a decent conversion efficiency of ~5.5% at the hot side temperature of 573 K. Given the similar thermodynamic properties of p- and n-type material, the device exhibited good stability.