Preparation And Thermoelectric Properties Of N-type Bismuth Telluride-based Materials

Thermoelectric (TE) materials, which can achieve solid-state mutual direct conversion of electrical-thermal energy, are of great advantage for some special application. Bismuth telluride based materials are the most commonly used TE material, they are usually prepared by crystal growth methods (e.g. Bridgman method), but their machinabilily is quite low due to its brittleness. So powder metallurgy route is commonly used to overcome this demerit. Traditional powder metallurgical method needs to premelt to obtain an alloy ingot, and then pulverize the ingot and sinter to form an equiaxial microstructure. The whole process is complicated and of quite high cost. In the first part of this thesis, the development of thermoelectric materials is summarized first, and powder metallurgy of Bi₂Te₃-based materials is then introduced. Mechanical alloying (MA) is a powerful method in the preparation of powders, which can directly alloy the raw materials into target powders in a relatively low cost way. In this work, MA method is used to prepare n-type Bi₂Te₃-based alloy powders, and hot press (HP) method is used to consolide the powders into bulk material. In order to improve the TE properties of the as HP-ed materials, some exploring work of hot extrusion (HE) has been done. The MA process of the n-type Bi₂Te₃-based material is studied by using XRD and DTA analysis method. 4 hour MA is proved to be enough to obtain the single phase Bi₂Te₃ based alloy powders. With the obtained alloy powder, hot press sintering experiments are conducted. It shows that with the increase of sintering temperature, TE properties increase. SEM analysis shows that the 340℃HP-ed sample is not sintered well enough. Prolonging HP time is helpful to improve TE properties, but overtime HP sintering may cause abnormal growth of grains, which are not favorable for to TE properties. The 4-hour-HPed sample has the highest TE properties, which shows a figure-of-merit of Z=1.7×10-3/K. In order to modulate the TE properties of HP-ed samples, Ag and Sn doping are tried. It shows that, a doping content of 0.20wt.% Ag addition could improve the figure-of-merit from 1.3×10-3/K for the undoped sample to 1.7×10-3/K, while for Sn-doped materials, heavy addition of tin produces a negative effect, because some Te-rich second phase appears, which causes the deterioration of TE properties. Athough good mechanical property can be achieved through powder metallurgy by little grain size, yet the anisotropy of Bi2Te3-based materials is lost due to its polycrystalline microstructure. Thus, hot extrusion (HE) process of Bi2Te3-based material is conducted to improve the TE properties of the HP-ed samples. The applied force analysis, stress field analysis by using plastic molding simulation software and experiment results are used to find the source of the bad surface quality of HE-ed sample, and the HE process are mended. Microstructure observation and XRD analysis show that HE process can achieve an oriented microstructure, and the TE properties increase. A further effort to improve the TE properties is annealing treatment. In our experiment of annealing of the HP-ed and the HE-ed samples, in the case of a temperature higher than 450℃, air hole in the samples expands and the density decreases sharply, so do the TE properties. With an annealing temperature of 400℃, the TE properties of the annealed sample show a Λ-shaped change as the annealing time prolonged. For HE-ed sample annealed for 18 hours, the power factor shows an increase of 36%. Pulse activate sintering (PAS) has been tried to sinter the MA-ed powder also. The result shows that the samples'density is quite high even with a sintering time of 10 minutes, and the TE properties of the PAS-ed samples are close to that of the HP-ed sample. Micrograph analysis shows some degree of orientation has been aquired with a short sintering time.