Thermoelectric material is a kind of functional material that can convert thermal energy and electrical energy. The Seebeck effect discovered in 1823 and the Peltier effect discovered in 1834 provided a theoretical basis for the application of thermoelectric energy converters and thermoelectric refrigeration. The thermoelectric refrigerator with the Peltier effect has the advantages that the mechanical compression refrigerator is difficult to match: small size, light weight, no mechanical rotating parts, no noise, no liquid or gaseous medium, so there is no environmental pollution problem. It can achieve precise temperature control, fast response speed and long service life. It can also provide a low-temperature environment for the use of superconducting materials. In addition, the use of thermoelectric materials to prepare micro-elements for the preparation of micro-power, micro-zone cooling, optical communication laser diode and infrared sensor temperature adjustment system has greatly expanded the application of thermoelectric materials. Therefore, thermoelectric materials are a kind of materials with a wide range of application prospects. In today's increasingly serious environmental pollution and energy crisis, research on new types of thermoelectric materials has strong practical significance.

Figure 1. High-performance flexible thermoelectric materials.

Applications:

• Thermoelectric generation：Typical thermoelectric materials in this application field are filled with skutterudite materials CoSb₃. CoSb₃ has good electrical properties, but its thermoelectric performance is not ideal due to its thermal conductivity. Filling the center of the hole with suitable impurity atoms can optimize the performance of CoSb₃. Because the filled atoms are weakly bound to the surrounding atoms, they can vibrate in the pores and scatter phonons, thereby greatly reducing the thermal conductivity of the material. When the pores are partially filled, the thermal conductivity of CoSb₃ may drop to 1/10 to 1/20 of the original coefficient, showing a typical "electronic crystal-phonon glass" (that is, high-performance thermoelectric materials should have crystal-like conductivity and glass-like thermal conductivity) model transport characteristics.

Figure 2. Crystal structure of skutterudite CoSb₃

• Thermoelectric cooling: Thermoelectric refrigeration technology is a new pollution-free and noise-free refrigeration technology. Compared with conventional compression refrigerators, thermoelectric refrigerators do not use refrigerants that are harmful to the environment, and operate reliably and without noise. They are particularly suitable for a small load and small volume refrigeration occasions. Compared with metal compound thermoelectric materials, oxide materials have the advantages of not being easily oxidized, cheap, non-toxic, and high temperature resistant. Recent research have shown that the introduction of the nano-second phase can effectively improve the thermoelectric properties of oxide materials. Some researchers have used solid-state reaction combined with SPS sintering method to prepare rare-earth Lu-doped nano-Ag composite Ca₃Co₄O₉ material with a maximum ZT value of 0.61, which is the polycrystalline oxide thermoelectric material with the highest thermoelectric performance. And it can also be used in thermoelectric refrigeration technology.

Figure 3. Ca₃Co₄O₉ crystal structure

Classification:

The thermoelectric materials can be divided into the following three categories according to their operating temperature.

• Bismuth telluride and its alloys: This is a material widely used in thermoelectric coolers, and its optimal operating temperature is<450°C.
• Lead telluride and its alloys: This is a material widely used in thermoelectric generators, and its optimal operating temperature is about 1000°C.
• Silicon-germanium alloy: This type of material is also commonly used in thermoelectric generators, and its optimal operating temperature is about 1300°C.

References:

1. Zhao L D, Tan G, Hao S, et al. (2016) "Ultrahigh power factor and thermoelectric performance in hole-doped single-crystal SnSe." Science, 351(6269): 141-144.
   Zhao L D, Lo S H, Zhang Y, et al. (2014) "Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals." Nature, 508(7496): 373-377.