The Shanghai Institute of Ceramics has achieved significant innovative progress in device research on liquid-like thermoelectric materials

Advanced thermoelectric conversion technologies hold broad application prospects in fields such as semiconductor cooling, industrial waste heat recovery, and power generation from exhaust heat of automobile engines. For a long time, research on thermoelectric materials has primarily focused on crystalline compounds. In 2012, the thermoelectric team at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, discovered a new class of high-performance, quasi-liquid thermoelectric materials with "phonon liquid–electron crystal" characteristics by introducing ions with "quasi-liquid-like" properties into solid-state materials to weaken or even eliminate certain transverse acoustic phonon modes (Nat. Mater. 2012). The thermoelectric team at the Shanghai Institute of Ceramics further studied and elucidated the mechanisms underlying the migration and precipitation of mobile ions in quasi-liquid materials under external field effects, and proposed a thermodynamic criterion for determining whether a material can be used reliably (Nat. Commun. 2018).

High-performance thermoelectric materials are the prerequisite and foundation for advanced thermoelectric conversion technologies, while high-efficiency thermoelectric devices represent the core and key to bringing these technologies into practical applications. For conventional thermoelectric materials, the development and research of devices typically focus solely on achieving maximum energy conversion efficiency or maximum output power through the design and optimization of structural dimensions and interfaces. However, for quasi-liquid materials, the mobility of quasi-liquid ions poses new challenges to the operational stability of thermoelectric devices. Therefore, the design and integrated manufacturing of such devices must also incorporate targeted optimizations aimed at enhancing operational stability, thereby simultaneously achieving both high energy conversion efficiency/output power and excellent operational reliability.

Recently, Researcher Shi Xun, Researcher Chen Lidong, and Associate Researcher Qiu Pengfei from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, in collaboration with Professor G. Jeffrey Snyder from Northwestern University in the United States, have gone beyond conventional design approaches and schemes for thermoelectric devices. They have proposed a new design principle for thermoelectric devices based on quasi-liquid materials—achieving both high energy conversion efficiency and operational stability through optimized device structural dimensions and interfaces. They successfully applied this principle to the development of thermoelectric devices made from quasi-liquid materials such as Cu2Se and Cu1.97S, resulting in novel thermoelectric devices that combine high energy conversion efficiency (9.1%) with excellent operational stability. The related research findings have been published in the journal Joule (DOI: 10.1016/j.joule.2019.04.010).

The study found that Cu2Se exhibits a relatively high critical voltage (~0.05 V) under different temperature differences, whereas Cu1.97S has a much lower critical voltage, only about ~0.01 V at a temperature difference of 275 K. The voltage carried by the quasi-liquid material is directly related to the ratio of the cross-sectional areas of the p-type and n-type thermoelectric legs (Ap/An). Assuming the quasi-liquid material is p-type, the larger the Ap/An ratio, the lower the voltage carried by the quasi-liquid material, and the better the stability of the device will be.

Based on the above-mentioned model and mechanism, the research team designed two types of liquid-like material thermoelectric devices. For the p-type thermoelectric legs, they selected the classic liquid-like materials Cu2Se and Cu1.97S, while for the n-type thermoelectric legs, they chose the conventional Yb0.3Co4Sb12-filled skutterudite. At a temperature difference of 200 K, the voltages carried by Cu1.97S exceeded the critical voltage threshold under all Ap/An conditions, rendering both the material and the device unstable. However, for Cu2Se, if the Ap/An ratio exceeds 4, both the material and the device remain stable at a temperature difference of 675 K. Meanwhile, three-dimensional topological structure simulations indicate that to achieve the theoretical maximum efficiency of 90% for the Cu2Se/Yb0.3Co4Sb12 thermoelectric device, the Ap/An ratio should be in the range of 2 to 8. Therefore, when the Ap/An ratio is between 4 and 8, the device can simultaneously attain high operational stability and high energy conversion efficiency. The research team experimentally fabricated Cu2Se/Yb0.3Co4Sb12 thermoelectric devices. The test results show that at a temperature difference of 680 K, the device's energy conversion efficiency can reach up to 9.1%, significantly higher than that of SiGe-based and half-Heusler-based high-temperature thermoelectric devices. When the temperature difference is 520 K, the device's performance showed no significant degradation even after prolonged exposure to constant-temperature aging. The experimental results are consistent with the theoretical model, demonstrating that the new design principle for thermoelectric devices proposed in this study can be effectively applied to the research and fabrication of devices based on novel liquid-like thermoelectric materials.

The research was funded and supported by the National Key Special Project, the National Natural Science Foundation of China, the Key Deployment Project of the Chinese Academy of Sciences, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Shanghai Young Science and Technology Star Program.

(a) Schematic diagram of the Cu2Se/Yb0.3Co4Sb12 thermoelectric device structure; (b) Thermoelectric figure of merit for representative liquid-like thermoelectric materials; (c) Physical image of the Cu2Se/Yb0.3Co4Sb12 thermoelectric device; (d) Changes in energy conversion efficiency and output power of the Cu2Se/Yb0.3Co4Sb12 thermoelectric device under long-term isothermal aging.

Link: https://www.cell.com/joule/fulltext/S2542-4351(19)30208-9