Preparation Bi₂Te₃-based Nanofilms By Flash Evaporation And Their Thermoelectric Properties

Thermoelectric materials, which could realize the inter-conversion between heat and electricity, have attracted much attention in recent years. As one of the best thermoelectric materials near room temperature, Bi₂Te₃-based thermoelectric materials have been broadly applied in many fields, such as medical and opto-electrical, aerospace, military, and so on. However, thermoelectric efficiency of Bi₂Te₃-based bulk materials is not competitive than that of a domestic electrical refrigerator. Nanotechnologies bring a new era to the thermoelectric materials. It is reported that the performance of thermoelectric nano-materials could be greatly improved owing to their quantum size effect. On the basis of a broad survey to the research status of thermoelectric materials, I bring forward the research goal of this thesis.In order to get a stable and reproductive deposition, the vacuum deposition system was modified. The evaporation powder source was prepared by mechanical alloying plus plasma activated sintering (MA–PAS), P-type Bi_0.4Sb_1.6Te₃ thin films and N-type Bi₂Te_2.7Se_0.3 thin films were deposited by flash evaporation. XRD, FE-SEM, EDX were performed to characterize the structure and composition of thin films respectively, their thermoelectric properties were also measured at room temperature. Variation of the thermoelectric properties of N-type Bi₂Te_2.7Se_0.3 thin films with the vaccum degree, evaporating distance and the thickness of thin films have been examined. With decrease of the air pressure, thermoelectric performance of N-type Bi₂Te_2.7Se_0.3 thin films increases. When the air pressure is 3.2Pa, the film has the maximum Seebeck coefficient of -163μV/K, and the power factor is 1675μW/mK2; With increase of the evaporating distance, the thickness of N-type Bi₂Te_2.7Se_0.3 thin film reduces,the surface flaws of the films reduce, while the Seebeck coefficient and electrical resistiviy increase, therefore the power factor also increase. A maximum power factor of 1560μW/mK2 was obtained when the evaporating distance is 11cm. Both the Seekbeck coefficient and the electrical conductivity increase with the thickness of thin films.Variation of the thermoelectric properties of P-type Bi_0.4Sb_1.6Te₃ thin films with annealing temperature and film thickness have been examined. In the range of 150℃to 250℃, as the annealing temperature increasing, the grain size and Seebeck coefficient of P-type films increase, and the electrical resistivity reduces. The maximum power factor was obtained as 1250μW/mK2 when the film was annealed at 250℃for 1h. With increase of the thickness of thin films, both Seekbeck coefficient and electrical resistivity of P-type films increase.In chapter 5, the main conclusion of the thesis was drawn and some suggestions were given for the next stage research.