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3-D Thermoelectrical Materials with Nano-, Micro- and Submicrometer Structure Elements

Объемные термоэлектрические материалы с нано- микро- и субмикронными элементами структуры.
Solid themoelectrical regenerators have a few advatages over traditional elctrical cooling and generating devices: simple construction, ecological compatibility, low noise level, high reliability, efficiency loss-free miniaturization. Yet for large-scale industrial application the efficiency of the thermoelectrical regenerator shall dramatically increase. There has been substantial research in the field of creating nanostructued thermoelectrical materials first of all based on solid solutions of (Bi,Sb)2Te3. There has been believed that higher thermoelectrical efficiency of such materials is caused by lower lattice heat conductivity due to increased phonon scattering at the borders of nanoscale grains. The smaller the grain is the stronger is the effect. Obtaining of the original nanopowder is a less complicated technological task than preserving the nanostructure of the sample in the compacting process by hot pressing enevitably leading to the growth of the original grains due to recrystallization. Grains size is impacted by a number of factors: temperature, hot pressing period, pressure, material composition as well as second phase nanoparticles presence in the composite. Consequently the purpose of the research is to investigate the possibility to preserve nano-sized grains in 3-D materials obtained by spark plasma sintering. In order to reach the target a mix of structured methods was developed to diagnose thermoelectrical nanostructured materials. It includes the powderb grain-size analysis, the material phase composition and homogenety analysis, analysis of the texture, fine structure and sponginess of the sintered samples. The research studied samples of 3-D nanostructured material based on BixSb1−xTe3 solid solution obtained by spark plazma sintering. The original powders with 10-12 nanometer grains were obtained by ball milling. X-ray diffractometry, scanning and transmission (including high resolution) electronic microscopy methods were used to investigate regularities of changes in the fine structure in relation to the sintering temperature between 250 and 550C. For the first time it was observed that the size of coherent scattering regions does not increase monotonically with the increase of sintering temperature but at the temperature over 400C refinement occures. Transmission electronic microscopy research results suggest that the decrease of an average size is caused by intensive formation of finely devided grains at the temperature of sps-sintering 4500C as a result of recurrent recrystallizing. Initiation of new grains is faster than the growth of the old ones. At the same time general defect concentration, including twins, falls down. Formation of new cetres of recurrent recrystallizing occur on the grains boundaries, dislocation defects as well as inside the grains, possibly on sub-grains. The discovered regularity affords to to obtain (depending on the kevel of recurring recristallizing) a structure with various degree of fineness of grains and addresses the challenge of preserving nanocondition at advanced temperatures of spark plazma sintering. the research results were used in developing the technology of obtaining thermoelectrical material in Giredmet.

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