How to increase the efficiency of conversion of heat into electricity
Baku, December 25 (AZERTAC). Materials, converts heat into electricity, are used in thermoelectric generators, thermoelectric coolers, and in thermometers widest operating range (from absolute zero to thousands of degrees). Despite the high demand for such substances, they greatly lack of effectiveness. If thermoelectrics better to convert the heat, they could well generate electricity from the heat dissipated by the computers, machines, and in particular the factory chimneys.
Chinese scientists have discovered a new way of restructuring the well-known thermoelectric material, which increases its effectiveness once in eight times. Results of the study are presented in the Journal of the American Chemical Society.
o be able to compare the effectiveness of different thermoelectric materials, introduced a synthetic parameter ZT. To achieve a high ZT material should conduct electricity well, while possessing low thermal conductivity. Unfortunately, in the case of bulk materials to achieve such alignment is almost impossible. One solution to the dilemma is complicated transition to nanomaterials (nanowires, nanocrystals, and so on), which contain a lot of boundaries, which dissipate the heat-carrying particles (photons), due to which there is a significant reduction in thermal conductivity. However, those same boundaries can dissipate not only phonons and electrons, and usually this ability is expressed to a much greater extent than we would like.
Scientists from the Chinese University of Science and Technology have found an alternative approach to increase the parameter ZT. As a starting point they chose bismuth selenide, featuring a layered structure. Earlier theoretical studies have shown that each layer of material has twice higher conductivity than the whole crystal.
To obtain the individual layers of bismuth selenide researchers vigorously stirred suspension with its lithium carbonate. Lithium ions are intercalated between the layers of bismuth selenide five atoms thick. Subsequent ultrasound exposure caused by the splitting of the intercalated crystal layers that are gathered and dried after removal of lithium.
Then the layers are pressed together and heated to 350 ˚ C - to obtain dense granules. As expected, multiple interfaces in the resulting composite scatter phonons, effectively reducing the thermal conductivity. And here’s the result: a comparative efficacy parameter ZT new granular material has reached 0.35, which is 8 times (!) higher than that of crystalline bismuth selenide.
Unfortunately, despite the considerable success of the composite shown efficacy parameter was still too small to rely on large-scale use. The limits of applicability thermoelectrics is the value of ZT = 0.7. Yet, the very strategy for improving the efficiency of conversion of heat into electricity, which was demonstrated in this study, is noteworthy.
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