How to increase the efficiency of thermoelectric generators
The Institute of Advanced Materials and Technologies of MIET studies the properties and new manufacturing technologies of thermoelectric materials based on lead telluride. This is one of the materials for thermoelectric generators, which is a promising area of alternative energy. Recently, scientists have completed a study on the role of strategies for lead telluride nanostructuring to increase its thermoelectric quality factor.
The thermal to electrical energy conversion using thermoelectric generators has already become one of the rapidly developing fields of modern science and technology. Generators like that are used as reliable sources of maintenance-free electricity, with a long service life and high power density. They are used in devices of different scales: from microelectronics and wearable electronics to spacecraft and nuclear power plants. Waste heat can also act as a heat source for thermoelectric generators.
The AMT Institute is looking for a way to increase the thermoelectric quality factor of such generators in order to increase their efficiency. The scientists investigated the possibility of changing one of the parameters of the thermoelectric quality factor of the material by reducing its thermal conductivity. They proposed to use for this purpose the technology of obtaining new volumetric nanostructured thermoelectric materials, which is currently in development.
"To minimize thermal conductivity, it is important that phonons with free path lengths are scattered. A large range of size distribution of structural heterogeneities in bulk nanostructured thermoelectric materials can provide effective scattering of phonons with medium and long free path lengths. Such phonons make a major contribution to the heat transfer process," says Maxim Stern, associate professor at the MIET's AMT Institute, PhD. "However, the ratio of structure and properties in correlation of synthesized thermoelectric material, nanopowder and nanostructured thermoelectric material is currently not fully understood. In this regard, a comprehensive study of the structure, its changes and properties of thermoelectric materials at each stage of their production is an urgent task."
The proposed technology was developed in the laboratory. There, a new nanostructured thermoelectric material was manufactured using a synthesized method of direct fusion of lead telluride, grounded in a planetary ball mill to nanoscale sizes, with subsequent compaction of nanopowders by spark plasma sintering.
"The purpose of our work was a comprehensive study of a nanostructured thermoelectric material based on lead telluride at various stages of its production: synthesized thermoelectric material, nanopowder, bulk nanostructured thermoelectric material," explains Maxim Stern.
The scientists studied the materials obtained using this technology and they identified the optimal combination of the manufacturing parameters, at which the lattice thermal conductivity is reduced at most. It turned out that this happens with grain sizes in the range of 100-350 nanometers, while the thermoelectric efficiency of the material increases by 14%.
A comprehensive study of the materials' properties was carried out by colleagues from the MIET's Research Laboratory of Electron Microscopy.
"Our task was to understand how the parameters of the formation of bulk nanostructured thermoelectric materials affect their structure," says a senior researcher at the laboratory, Ph.D. Julia Zaitseva. "For this purpose, we have prepared a series of samples for transmission electron microscopy using the In-Situ Lift-Out method. Using electronographic analysis, high-resolution and transmission scanning electron microscopy, we found that the thermoelectric materials under study have a granular structure. To analyze the features of their structure and, in particular, to determine the lateral grain sizes, it was necessary to accurately identify their boundaries on electron microscopic images. To do this, we carried out a digital processing of a series of microphotographs obtained at different angles of incidence of the electron beam on the sample. As a result, we were able to identify the correlation between the granular structure (grain sizes) and the thermoelectric quality factor of materials, as well as determine the optimal parameters for their production. A 14% increase in thermoelectric efficiency is a significant result, so our study can be called successful."
To summarize their research, the scientists from the AMT Institute said that the value of the developed technology lies in the fact that it is suitable for wide practical application . This technology can be used for the industrial production of nanostructured thermoelectric materials and the thermoelements based on them.
The article was prepared with the assistance from the Ministry of Education and Science using the materials of this article.