Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

The growing awareness of the environmental impacts of overdependence on fossil energy and increased greenhouse gas emissions has compelled the world to develop alternative and efficient strategies to reduce energy consumption across different sectors. Among these strategies, recovering waste heat has drawn significant research attention. Although it is much easier to recover high-quality waste heat at high temperatures in the industry setting, efficient harvesting of low-grade heat, meaning heat at temperatures lower than 300 °C, has remained a big challenge. This can be attributed to its low quality and low energy density.

Thermoelectric technology offers a direct conversion between electricity and heat. Unlike other conversion methods, thermoelectric technology is a clean and noise-free solution, hence suitable for recovering low-grade heat. At present, Bi₂Te₃-based thermocouple modules dominate the commercial market for thermoelectric cooling applications, including the potential on recovery of low-grade heat owing to its excellent figure of merit at around room temperature. The figure of merit is the primary determinant of the energy conversion efficiency of thermoelectric modules. Unfortunately, their narrow operating temperature range and high materials cost limit their potential applications in converting low-grade heat to electric power.

In an effort to promote the use of thermoelectric technology for low-grade heat recovery, low-cost, highly efficient and high-stability thermoelectric modules are desirable. To this end, non-toxic n-type Mg₃Sb₂-based compounds with remarkable performance across a wide range of temperatures have been developed. Owing to its numerous benefits, n-type Mg₃Sb₂-based compounds have emerged as a viable alternative for conventional Bi₂Te₃ modules for low-grade heat recovery. However, there are limited studies on constructing and analyzing all-Mg₃Sb₂-based modules because p-type Mg₃Sb₂ have a substantially lower figure of merit than n-type counterparts. Therefore, it is important to enhance the performance of p-type Mg₃Sb₂ to match n-type counterparts to develop high-performance all-Mg₃Sb₂-based modules for low-grade heat recovery.

Herein, the University of Houston researchers: PhD candidate Zhongxin Liang, PhD candidate Congcong Xu, Dr. Shaowei Song, PhD candidate Xin Shi, and Dr. Wuyang Ren led by Professor Zhifeng Ren reported the enhancement of thermoelectric performance of p-type Mg₃Sb₂ to facilitate the development of all-Mg₃Sb₂-based thermoelectric devices for efficient low-grade heat recovery. To achieve this objective, the authors performed a comprehensive regulation of the carrier concentration, carrier mobility and lattice thermal conductivity via Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Their work is currently published in the peer-reviewed journal, Advanced Functional Materials.

The research team reported a significant improvement in the thermoelectric performance of p-type Mg₃Sb₂ through co-doping Na and Yb. The resulting p-type Mg₃Sb₂ was reliable, low-cost, and also exhibited an improved figure of merit. For instance, the figure of merit of approximately 0.7 was obtained at a temperature of 573K, which was a significant improvement compared to values previously reported. Na doping played a big role in regulating carrier concentration, while Yb alloying achieved synergistic optimization of lattice thermal conductivity and carrier mobility. It was worth noting that the construction of the reliable all-Mg₃Sb₂-based device was also attributed to the similarity between the p- and n-type Mg₃Sb₂ in terms of their coefficient of thermal expansion and stability performance.

In summary, University of Houston scientists successfully improved the thermoelectric performance of p-type Mg₃Sb₂ compounds by optimizing the carrier mobility and concentration and lattice thermal conductivity. This allowed the construction of all-Mg₃Sb₂-based thermoelectric devices. Given its high conversion efficiency of 5.5% at a heat source temperature of 573K, good performance stability, and reduced material and fabrication costs, the proposed device is a promising candidate for practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery. In a statement to Advances in Engineering, M. D. Anderson Chair Professor Zhifeng Ren explained that the device would supplement or potentially replace the longstanding Bi₂Te₃-based devices for low-grade heat recovery.