The material figure of merit Z is an important parameter to measure the cooling capacity of TEC and its expression is
. In order to improve Z value, conductivity σ and Seebeck coefficient α should be improved and thermal conductivity λ should be reduced. λ consists of phonon thermal conductivity λp (about 90%) and electronic conductivity λe. These three parameters are not independent of each other, which all are the function of carrier concentration n and temperature T. In the condition of different environment temperature, the value of Z is different. In general, the dimensionless factor ZT is used to describe material’s performance. Bi
2Te
3 is the most widely used thermoelectric material and the value of ZT can be up to 0.9. However, the cooling efficiency of
TEC made by Bi
2Te
3is only 30% of compression refrigeration.
In recent fifty years, the performance of thermoelectric materials has been increased by roughly 20%. At present, there are many kinds of developing thermoelectric materials, such as non-oxide semiconductor thermoelectric materials, oxide thermoelectric materials, low dimensional thermoelectric materials, super lattice thermoelectric materials and so on.
Semiconductor thermoelectric materials of the earliest and most mature is (Bi,Sb)
2(Te, Se)
3, which is a solid solution material. Most of the current TEC are using this material, which can increase α and reduce λ.
Currently, the semiconductor oxide material is one of oxide thermoelectric materials, which has many advantages of no pollution, stable performance and can work for a long time at high temperature. This material mainly includes Na-Co-O and Ca-Co-O.
The left research progress of thermoelectric refrigeration technology will be introduced in the next article.
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