Layer-Structured Zintl Phases as Novel Thermoelectric Materials



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Thermoelectric materials have been fascinated extensive interest in the last two decades due to the potential applications in waste-heat recovery from industrial processes, automobiles, and renewable energy sources. In pursuit of potential thermoelectric candidates, Zintl phases have recently gained interest because of their characteristics desired for use in thermoelectric devices for power generation: complex structures, extensive opportunities to tune transport properties, narrow band gaps, and thermal and chemical stability.

In this huge pool of Zintl compounds, the two-dimensional layered CaAl2Si2-typed Zintls, especially antimony-based Zintl compounds, have been demonstrated to be promising TE materials for middle- to high-temperature applications. Here, we report the rarely studied bismuth-based Zintl phases (Ca,Yb,Eu)Mg2Bi2 with competitive thermoelectric performance. This thesis first describes the thermoelectric characterization and optimization of these phases, including eliminating the Bi impurity, Na substitution to boost power factor, as well as as understanding the band engineering and strain field fluctuation in solid solutions. In addition, since the fabrication method for synthesizing the Zintl compounds here is different from the most reported melting method, the comparison between these two methods based on the classical Sb-based Ca1-xYbxZn2Sb2 compounds will be included, which allows us to further understand the defect-controlled p-type Zintl phases.

For application, thermoelectric generators should consist of both n- and p- type legs with equivalent performance. However, through several decades’ efforts, it still remains very difficult to make competitive n-type Zintl materials until the discovery of n-type Mg3+xSb2-based Zintl phase. Given the reported low electrical conductivity and low average ZT, it motivates us to further improve it through Nb partial substitution, resulting in enhanced average ZT value to about 1 across the entire measured temperature. Moreover, doping with the ideal hole dopants (Na+) on its corresponding p-type Mg3Sb2 Zintls has also been investigated, allowing for effectively increasing the carrier concentration and optimization of the thermoelectric performance. Besides the Zintl phases, the final part introduces another excellent p-type thermoelectric materials MgAgSb with potential power-generation application near room temperature. The works highlight the discovery of complex bulk thermoelectric materials with intrinsically low lattice thermal conductivity, which would stimulate many of the recent advances in thermoelectrics.



Thermoelectrics, Zintl phases


Portions of this document appear in: Shuai, Jing, Huiyuan Geng, Yucheng Lan, Zhuan Zhu, Chao Wang, Zihang Liu, Jiming Bao, Ching-Wu Chu, Jiehe Sui, and Zhifeng Ren. "Higher thermoelectric performance of Zintl phases (Eu0. 5Yb0. 5) 1− xCaxMg2Bi2 by band engineering and strain fluctuation." Proceedings of the National Academy of Sciences 113, no. 29 (2016): E4125-E4132. And in: Shuai, Jing, Zihang Liu, Hee Seok Kim, Yumei Wang, Jun Mao, Ran He, Jiehe Sui, and Zhifeng Ren. "Thermoelectric properties of Bi-based Zintl compounds Ca 1− x Yb x Mg 2 Bi 2." Journal of Materials Chemistry A 4, no. 11 (2016): 4312-4320. And in: Shuai, Jing, Yumei Wang, Zihang Liu, Hee Seok Kim, Jun Mao, Jiehe Sui, and Zhifeng Ren. "Enhancement of thermoelectric performance of phase pure Zintl compounds Ca1− xYbxZn2Sb2, Ca1− xEuxZn2Sb2, and Eu1− xYbxZn2Sb2 by mechanical alloying and hot pressing." Nano Energy 25 (2016): 136-144. And in: Shuai, Jing, Hee Seok Kim, Zihang Liu, Ran He, Jiehe Sui, and Zhifeng Ren. "Thermoelectric properties of Zintl compound Ca1− x Na x Mg2Bi1. 98." Applied Physics Letters 108, no. 18 (2016): 183901. And in: Shuai, Jing, Hee Seok Kim, Yucheng Lan, Shuo Chen, Yuan Liu, Huaizhou Zhao, Jiehe Sui, and Zhifeng Ren. "Study on thermoelectric performance by Na doping in nanostructured Mg1-xNaxAg0. 97Sb0. 99." Nano Energy 11 (2015): 640-646. And in: Shuai, Jing, Yumei Wang, Hee Seok Kim, Zihang Liu, Jingying Sun, Shuo Chen, Jiehe Sui, and Zhifeng Ren. "Thermoelectric properties of Na-doped Zintl compound: Mg3− xNaxSb2." Acta Materialia 93 (2015): 187-193.