Recently, the School of Materials Science and Engineering at Shanghai Jiao Tong University (SJTU) has made progress in the plastic processing of semiconductor materials. The research team discovered that Bi_0.85Sb_0.15, a classic thermoelectric semiconductor material used in the liquid-nitrogen temperature range, undergoes a brittle-to-ductile transition at temperatures between 448-473 K (175-200 °C). Utilizing “warm metalworking” methods, the team achieved efficient shaping of the material while preserving its outstanding low-temperature thermoelectric transport properties. This work, titled “Warm Metalworking for Brittle Liquid-Nitrogen-Temperature Thermoelectric Materials,” has been published online in Advanced Energy Materials and was selected as the back cover image for the issue (Figure 1). Fu Ling, a PhD student at the School of Materials Science and Engineering, SJTU, is the first author. The co-corresponding authors are Dr. Pan Zhenyu (Postdoctoral Fellow) and Professor Wei Tianran from the School of Materials Science and Engineering, SJTU, alongside Researcher Shi Xun from the Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS).

Paper Link: https://doi.org/10.1002/aenm.202503241

Figure 1. This work was selected as the back cover image for the current issue of Advanced Energy Materials.

Refrigeration and temperature control technologies in the liquid-nitrogen temperature range are crucial for the stable operation of equipment such as electronic devices and quantum computing systems. These applications require thermoelectric materials to possess not only excellent low-temperature thermoelectric performance but also good mechanical processability to accommodate complex structural requirements. However, most current high-performance thermoelectric materials are brittle inorganic semiconductors that are difficult to shape using conventional processing methods. This represents a major bottleneck restricting the miniaturization and integration of liquid-nitrogen temperature thermoelectric cooling technology.

To address this challenge, and building on the research group’s previous work (Nature Materials 2025, 24: 1538), this study systematically investigated the thermoelectric and mechanical properties of the classic thermoelectric cooling material Bi_xSb_1-x (x = 0 ~ 1). The researchers discovered that Bi_0.85Sb_0.15  exhibits unique advantages.

In terms of thermoelectric performance, the material achieves an average thermoelectric figure of merit (zT) of 0.27 in the 50-200 K range (covering liquid-nitrogen temperatures). At 123 K, the peak thermoelectric parameter z reaches 2.4×10^-3 K^-1, positioning it at the top tier of low-temperature thermoelectric materials. Regarding mechanical properties, its brittle-to-ductile transition temperature (T_trans) is as low as 448-473 K (175-200 °C), significantly lower than other high-performance thermoelectric materials (Figure 2). Near this temperature, the compressive strain of Bi_0.85Sb_0.15 can exceed 60%, and its bending strain surpasses 14%. Even at room temperature, it achieves a compressive strain of approximately 21.5%, far exceeding traditional brittle thermoelectric materials.

The team explored the mechanism of temperature-induced plasticity in Bi_0.85Sb_0.15 using transmission electron microscopy (TEM) analysis and molecular dynamics (MD) simulations. On one hand, at elevated temperatures, grains undergo significant elongation and orientational reconstruction, dissipating strain energy through grain boundary migration to avoid brittle fracture. On the other hand, increase temperature significantly amplifies the vibration amplitude of Bi and Sb atoms, drastically lowering the slip barrier energy.

Figure 2. (a) zTavg between 50 K and 200 K v.s. brittle-to-ductile transition temperature (T_trans) for typical thermoelectric materials.Bi_0.85Sb_0.15 stands out with high zTavg and low Ttrans. The data of Bi_0.85Sb_0.15 are obtained in this work while others are taken from literature. (b) Temperature dependence of z from 50 to 300 K forBi_0.85Sb_0.15 (this work) and other typical low-temperature thermoelectric materials. (c) Photos and schematics of warm stamping for Bi_0.85Sb_0.15.

Leveraging the excellent plasticity of Bi_0.85Sb_0.15 at relatively low temperatures, the team developed metal-like warm processing technologies suitable for this material, including warm rolling, warm extrusion, and warm stamping. By rolling a disk (20 mm in diameter, 0.6 mm in thickness) at 423 K, a flexible foil with a thickness of only 65 μm was obtained, achieving a thickness reduction ratio of 89%. At 473 K, a cylinder (24 mm in diameter, 8 mm in height) was extruded into a rod-shaped material (8 mm in diameter, 60 mm in length) with no significant material loss during the process. Clear “SJTU” characters were also successfully formed on the material surface via warm stamping at 473 K.

Notably, the Bi_0.85Sb_0.15 processed via warm extrusion retained excellent thermoelectric performance. The room-temperature zT value reached 0.32, and the average zT from 50-200 K was 0.29, comparable to that of unprocessed samples. Simultaneously, the compressive strength increased by approximately 11%, and the compressive strain improved by approximately 16%, successfully achieving both “shaping” and “property control.” This study provides a potential strategy and method for resolving the contradiction between high performance and difficult processability in low-temperature thermoelectric materials.

This work was supported by the National Natural Science Foundation of China and other projects. The SJTU High Performance Computing Center provided technical assistance for data analysis.