An important breakthrough has been made in the development of the next generation thermoelectric refrigeration module based on magnesium-based new materials.

The important function of thermoelectric materials is to realize the direct conversion between thermal energy and electric energy. Thermoelectric refrigeration technology is based on the principle of heat transfer when current passes through different conductors. The direction and size of heat flux density can be regulated by controlling the direction and size of direct current. Thermoelectric refrigeration has the characteristics of high regulation precision and fast response. compared with general mechanical refrigeration, it does not need moving parts such as compression transmission, let alone refrigerant, and does not produce any emission and environmental pollution. it is an ideal all-solid high-efficiency refrigeration and thermal management technology. In general, the key parameters to measure the refrigeration performance of thermoelectric devices include the maximum refrigeration temperature difference Δ Tmax, the maximum refrigeration capacity Qcamx and the maximum refrigeration energy efficiency COPmax. As shown in the following formula, these performance parameters are closely related to the geometric design of the thermoelectric materials, interface electrodes and thermoelectric devices that make up the device:

Among them, I, TC, S, R, K and others correspond to the parameters such as DC current of the device, cold end temperature of the device, Seebeck coefficient of the device, resistance of the thermoelectric arm, thermal conductivity of the thermoelectric arm and so on.

It can be seen from the formula that the performance of thermoelectric devices or thermoelectric modules mainly depends on the thermoelectric transport performance of their core thermoelectric materials. For a long time, Bi "Te" is the only commercial high performance (zT~1) material system in near-room temperature thermoelectric materials and thermoelectric refrigeration. Bi "Te" thermoelectric refrigeration devices are widely used in cold chain storage, medical devices, optical communication temperature control and other important industries. In the future, with the further improvement of the demand for chip temperature control in the field of communication and electronic information, the prospect of thermoelectric refrigeration industry is more broad. However, as the core thermoelectric refrigeration material at present, Bi "Te" has some significant defects, such as poor mechanical properties, high cost caused by the use of Te elements, poor zT of N-type Bi "Te" materials, and so on, which limit the further promotion and expansion of this kind of materials. Therefore, the discovery of a new near-room temperature thermoelectric material system has become a key scientific problem and task in the thermoelectric field. In recent years, magnesium-based thermoelectric materials such as MgAgSb, Mg ceramics (Si,Sn), especially N-type Mg alloys (Sb,Bi) have become hot material systems that have attracted much attention. Compared with traditional bismuth telluride commercial materials, Mg telluride (Sb,Bi) uses magnesium, antimony and bismuth as raw materials, abandons expensive and toxic tellurium elements, and can save material costs by about 90%. At the same time, Mg telluride (Sb,Bi) has both excellent mechanical properties and environmental protection advantages. For this material, the related research has made rapid progress at home and abroad, but most of them focus on the properties of basic materials, but there is still a blank in the construction and application of vital serviceable full-scale thermoelectric devices.

Figure 2. (A) according to theoretical prediction and literature reports, the dependence of room temperature ZT value on carrier concentration and composition in Mg3Sb2-xBix system, and the time-dependent relationship between electrical transport properties and time dependence of Mg3.2Bi1.4975Sb0.5Te0.0025 samples prepared by; (C) thermal deformation method with maximum refrigeration temperature difference predicted by (B). The electrical transport properties of Mg3.2Bi1.4975Sb0.5Te0.0025 samples before and after; (D) thermal deformation process; The TEM images of hot deformed specimens containing a large number of point defects and dislocation defects in (E). The scale of low magnification TEM image and high magnification TEM image is 500 nm and 10 nm; (F) Seebeck coefficient distribution on the Mg3.2Bi1.4975Sb0.5Te0.0025 sample after hot deformation (the sample is 20 nm in diameter).

In recent years, the research group of Zhao Huaizhou, Institute of Physics of the Chinese Academy of Sciences / Beijing National Research Center for condensed matter Physics, has aimed at the key scientific and technical problems in the future application of Mg "(Sb,Bi)" materials. A series of breakthroughs have been made in the improvement of thermoelectric properties, chemical and thermal stability, design and preparation of interface electrode materials, processing and welding assembly of thermoelectric arms, etc. Finally, the construction of commercial size thermoelectric refrigeration module with stable service performance is realized, which lays a foundation for the application of this kind of materials.

Around the Mg "(Sb,Bi)" materials and devices, the research group has obtained 3 domestic authorized invention patents (2017114542305 / 2019101353516 / 201910811895.X) and one American invention patent (PCT/CN2019/075799). Recently, the full-scale thermoelectric refrigeration module related paper "Next-Generation Thermoelectric Cooling Modules Based on High-Performance Mg Thermal (Bi,Sb) cooling material" based on N-type Mg3.2Bi1.4975Sb0.5Te0.0025 and P-type Bi0.5Sb1.5Te3 was published as a cover article (figure 1) in the top international energy journal Joule (Joule 2022,6,1-12). Philip Earis, editor-in-chief of Joule, said: "We look for breakthrough research that has real-world energy impact and influence, and we feel there is a case here, particularly in view of the high performance and robust stability that you report."

Fig. 3. (A) Mg-Cu-Bi ternary phase diagram and (B) Mg-Mg Cu-Mg3Bi1.5Sb0.5 room temperature pseudo-ternary phase diagram region the distribution of Cu elements near the; (C) Mg Cu transition layer and the Mg3.2Bi1.4975Sb0.5Te0.0025 boundary. The illustration is the measurement result of the; (D) interface resistivity of the SEM image in the selected boundary region. The test temperature curve of the thermal expansion coefficient of (E) Mg3.2Bi1.4975Sb0.5Te0.0025 and Mg Cu materials (CTE) is compared with the thermal expansion coefficient of Bi "Te", Fe, Ni and Cu; (F) reported in the literature, and the simulation results of thermal stress distribution of Mg3.2Bi1.4975Sb0.5Te0.0025 module under the optimal current are simulated.

In the work of this paper, the team first pre-designed the components of Mg "(Bi,Sb)" materials through the SPB model (figure 2A), and theoretically predicted the cooling temperature difference of the corresponding devices (figure 2B). At the same time, in view of the poor chemical and thermoelectric stability of the current Mg (Bi,Sb) system, the molded samples of Mg3.2Bi1.4975Sb0.5Te0.0025 were further treated by thermal deformation process (figure 2C), which significantly improved the thermoelectric stability of Mg3.2Bi1.4975Sb0.5Te0.0025 materials (figure 2D). In this process, high concentrations of dislocations and point defects are introduced into the sample through defect control (Fig. 2e). The analysis shows that the existence of these defects is helpful to realize the migration and diffusion of Mg ions, thus solving the problem of missing Mg elements in the local area of the material (directly causing the decrease of carrier density and conductivity), and laying a foundation for the practical application of this kind of materials.

Figure 4. (A) designed improved Mini-PEM thermoelectric device refrigeration energy efficiency and refrigeration capacity testing device; (B) is based on P-type Bi0.5Sb1.5Te3 and N-type Mg3.2Bi1.4975Sb0.5Te0.0025 thermoelectric refrigeration modules (corresponding to 7 pairs and 31 pairs of refrigeration device); (C) Mg thermoelectric (Sb,Bi)-based refrigeration modules, respectively, the dotted line is the theoretical calculation, and the illustration is a comparison with the temperature difference of commercial devices. The normalized cost of the two (D) materials and their corresponding modules, as well as the performance input ratio of the refrigeration capacity of the two modules.

Compared with material properties, the construction of thermoelectric devices is a complex and comprehensive problem. Interface material and fabrication process are one of the key factors. It is found that conventional electrode materials Fe and Ni are difficult to meet the requirements of stability and repeatability of full-scale Mg cathode (Bi,Sb) thermoelectric devices. In this regard, the team first designed and developed Mg2Cu transition layer materials, aiming at the introduction of magnesium-rich environment for Mg "(Bi,Sb)" thermoelectric arms, which helps to inhibit the loss of Mg elements. At the same time, due to the thermodynamic stability of the interface of Mg "Cu/Mg" (Bi,Sb), there is no obvious element diffusion reaction (Fig. 3A, 3B and 3C). In addition, the melting point and processing temperature of the Mg2Cu transition layer are low, which can further prevent the loss of the properties of the Mg (Bi,Sb) alloy during the preparation of the electrode. Tests show that the interface resistivity of Mg "Cu/ Mg" (Bi,Sb) is 12 μ Ω cm ²(figure 3D), which fully meets the ohmic contact requirements; while the coefficient of thermal expansion of Mg "Cu" is better matched than Fe, Ni, and Mg "(Bi,Sb)" thermal arm materials (figures 3e and 3F), avoiding thermal stress problems in device service. On the basis of the above material and interface optimization, the team successfully constructed 7 pairs and 31 pairs of Mg Bi,Sb-based refrigeration modules with excellent performance (figure 4B). The test results show that the maximum temperature difference of the module at room temperature is 59 K (figure 4C), and the maximum refrigeration energy efficiency COP is 8 when the temperature difference is 5 K. the performance of the new Mg (Bi,Sb)-based device is about 23% higher than that of commercial bismuth telluride refrigeration devices (figure 4D). Compared with commercial bismuth telluride refrigeration devices, the biggest advantage of the new device is that the performance-to-input ratio is increased by about 23% (figure 4D). In this work, the team carried out a complete characterization of the new material refrigeration module, and the cooling capacity, COP, temperature difference, and service stability all reached or close to the level of commercial Bi "Te" devices (figure 5), showing the great potential of Mg "(Bi,Sb)" materials and their full-scale thermoelectric refrigeration modules in the next generation thermoelectric refrigeration applications. At present, the related research is in continuous progress.

Figure 5. Relationship between cooling capacity and current of 7 pairs of modules with (A) thermoelectric arm length 2 mm under the condition of fixed heat flux, the function of cooling temperature difference with current of module (thermoelectric arm length 2 mm); (C) works at the temperature difference of 5 K, 10 K and 15 K respectively, and the dotted line is the theoretical prediction of 7 pairs of modules (thermoelectric arm length is 2 mm) COP). The illustration shows the COP measurement data of the commercial Bi "Te" module under the same working conditions.; (D) carries out 3000 min continuous service monitoring of the resistance of 7 pairs of modules (the length of the thermal arm is 2 mm) at a current of 3 A. At the same time, the module resistance is continuously monitored within 6 months, which shows that the service is excellent.

In this thesis, researcher Zhao Huaizhou and associate researcher Zhu Aerospace are co-authors, and Yang Jiawei, a second-year doctoral student in the Institute of Physics, is the first author. This work is supported by key R & D projects of the Ministry of Science and Technology.