Preparation And Performance Of Nanocrystalline Bi₂Te₃Bulks

Bi₂Te₃based alloys are the most important thermoelectric material forstate-of-art thermoelectric cooling and power generation devices near ambienttemperature. It belongs to the diamond crystal system and hexahedron layeredstructure. Its structure is similar to graphite with anisotropy, thus thethermoelectric performance is anisotropic. At the beginning of the study, Bulkthermoelectric alloys were studied, but no matter by what means, including doping,etc., the thermoelectric performance of materials cannot be well improved. Thencontinuous exploration discovered that thermoelectric materials of low dimensions,such as, superlattice film, etc., can greatly improve the thermoelectric optimalvalue. But the low dimension materials are hard to commercial production andpractical application. In recent years, people with the help of the theory basis ofsuperlattice, studied a kind of structure which not only improved thethermoelectric performance but also can be applied in reality, i.e. nanocrystallinebulk structure. The main purpose of this paper is the preparation of Bi₂Te₃nanostructures of bulk materials, and optimizing on the thermoelectric properties.First, this paper using coprecipitation and hydrogen reduction prepared Bi₂Te₃nanopowder. Generally ball grinding method is used to prepared powder Bi₂Te₃nanopowder, but this method is long cycle, high cost, and the synthesis ofnon-uniform particles. And the method we used is simple and low cost and canobtain the small size and uniform nanoparticles in a short period. We studied thereduction temperature and reduction time on the influence of the sample. Theresults of this work show that when the reduction temperature is400℃and thereduction time is3h, the synthesis of the samples do not contain any impurityphase and crystalline is good and the grain size is about41nm.Second, nanocrystalline Bi₂Te₃bulks were fabricated via ambient pressuresintering. The experiment shows that when the sintering temperature is400℃andthe sintering time is2h, the material has the highest conductivity. At the same time,this paper also analyzed the relationship between the conductivity andmicrostructure. Results show that when temperature is too low, the materialdensity is very low, which is obviously to cause a decline in conductivity. Whenthe sintering temperature is too high or when the time is too long, the particles inthe samples will grow up abnormal. In this time, the anisotropy of thermoelectricproperties of each big grain has showed clearly. Due to the nanostructure blockcontains a lot of mess arrangement of big grain, which can lead to drop in conductivity. Third, because high pressure sintering is a kind of technique whichcan both guarantee the high density of nanocrystalline Bi₂Te₃bulks and preventabnormal nanoparticles grew up, we fabricated nanocrystalline Bi₂Te₃bulks viahigh pressure sintering technique in this work. Through grope for pressure,sintering temperature and the holding time, the best preparation conditions wasobtained. They are respectively4GPa,350℃and1h. The main reason can beobtained by analyzing the microstructure, thermal conductivity and electricalconductivity of samples. It’s that due to the combination of the grains, carriertransmission will not be too big effected but phonon scattering is enhanced. So inthe case of guaranteed conductivity will not be reduced, the thermal conductivityhad a larger degree of reduced, which had the characteristic nanocrystalline bulkmaterials.Finally, nanocrystalline Bi₂Te₃/Sn bulks were fabricated by high pressuresintering technique and studied the effect of different Sn content on thethermoelectric properties. The experiment has shown that the addition of Sn didhelp to improve the thermoelectric performance of the material, and the bestmixing amount of Sn was10%.