Development of N-Type Mg3(Sb, Bi)2-Based Thermoelectrics Toward Applications



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Recent developments in thermoelectrics have made it more feasible to generate electricity from waste heat or obtain refrigerating output from a solid-state device. The n-type Mg3(Sb, Bi)2 material is one of the most popular and commercially promising thermoelectric materials due to its low raw material cost, simple synthesis technique, good repeatability, and wide working temperature range, particularly in terms of performance equivalent to commercial Bi2Te3 for room temperature cooling. Despite the effectiveness of many optimization strategies in increasing the figure of merit zT, device-level optimization for large-scale applications remains challenging. To address practical application difficulties, I focus on large-scale synthesis and device-level optimization of n-type Mg3(Sb, Bi)2-based thermoelectric materials and modules in the dissertation.

First, I proposed a scalable synthesis method for n-type Mg3(Sb, Bi)2 based on Simoloyer ball-milling, which can produce over 1 kg Mg3.1Sb1.5Bi0.49Te0.01 powder per batch. The cold-welding issue could be avoided by optimizing the ball-milling procedures. The as-prepared bulk samples not only maintained excellent thermoelectric performance but also exhibited good homogeneity. Importantly, the single leg exhibited a high energy conversion efficiency of ~12.9% at a temperature difference of ~480 K.

Second, I conducted stability testing on Mg3(Sb, Bi)2-based samples with different amounts of initial Mg to understand the evolution of their internal defects by analyzing the changes in the time-dependent electrical performance curves. Based on a thermodynamic point of view, I tried to rebalance these unsteady defects using a low-temperature annealing treatment, thereby reducing material property fluctuation and enhancing the entire compound structure. Finally, I used transition metals to fill vacancies in place of excess Mg, which slowed performance degradation at high temperatures.

Third, I focused on both the material- and device-level properties of n-type Mg3(Sb, Bi)2 and p-type GeTe. A multi-segmented n-type Mg3(Sb, Bi)2 leg with Ni buffer layers and good layer-to-layer contacts was successfully fabricated. Later, I used three-dimensional (3D) finite-element simulations to identify the appropriate module size. The constructed segmented-Mg3(Sb, Bi)2/cubic-GeTe module demonstrated high performance with a conversion efficiency reaching (12.8 ± 0.8)% under ΔT of 480 K.



Thermoelectrics, Power generation, Thermoelectric cooling, Thermoelectric devices


Portions of this document appear in: C. Xu, Z. Liang, W. Ren, S. Song, F. Zhang, Z. Ren, Realizing high energy conversion efficiency in a novel segmented-Mg3(Sb, Bi)2/cubic-GeTe thermoelectric module for power generation, Advanced Energy Materials 12 (2022) 2202392; and in: C. Xu, M. Jian, Z. Liang, B.-H. Lei, S. Song, F. Zhang, D.J. Singh, Z. Feng, Z. Ren Enhancing the thermal stability of n-type Mg3+xSb1.5Bi0.49Te0.01 by defect manipulation, Nano Energy 106 (2023) 108036; and in: C. Xu, Z. Liang, H. Shang, D. Wang, H. Wang, F. Ding, J. Mao, Z. Ren, Scalable synthesis of n-type Mg3Sb2-xBix for thermoelectric applications, Materials Today Physics 17 (2021) 100336; and in: C. Xu, Z. Liang, S. Song, Z. Ren, Thermoelectrics: From thermoelectric figure of merit to device design, in digital Encyclopedia of Applied Physics, eap872 (2022) 1.