Thermelectric Properties Of Bi₂Te₃ Thin Film Tailorted By Grain Boundary Complexion

With the severely increasing of energy shortage,improving the energy efficiency as one of the strategic development of energy technology.Thermoelectric devices as a thermoelectric energy conversion device have gained increasing interstate.The performances of thermoelectric materials are the key to thermoelectric energy conversion device.Bismuth telluride（Bi₂Te₃）with the best thermoelectric properties at room temperature,as the earliest thermoelectric materials has been the focus and has broad application prospects and practical significance.In this thesis,Bi₂Te₃ was used as the object of study.Two series of different crystallinity were prepared by magnetron sputtering.The microstructures and morphology of the two series were characterized by XRD and SEM.The performances of the samples were characterized by Hall tester and Seebeck tester.The samples prepared at 573 K were more denser and stable than those prepared at 623 K.But the crystallization of 623 K samples is high and the surface is more rough,and the pores with diameters of about 200 nm are produced,which facilitates the rapid diffusion of grain boundaries.Electron beam evaporation technique was used to deposit Sn on the 573 K and 623K films with a thickness of about 10 nm.The microstructures,morphology and properties of two samples after Sn plating were characterized by XRD,SEM and Hall tester.The test results show that the performance of the sample are not affected by the addition of Sn.The sample of 473 K/523 K/573 K for 2 h and at 573 K for 2 h/3 h/4 h/5 h were prepared by self-built protective gas annealing furnace.The effects of annealing temperature and annealing time on the properties of Bi₂Te₃ and Sn-Bi₂Te₃ were investigated.The microstructures,morphology and thermoelectric properties were characterized by XRD,SEM,XPS,Hall test and Seebeck test.The experimental results show that the incorporation of Sn can increase the carrier concentration and mobility of Bi₂Te₃（623 K）thin films and increase the power factor by 1-2 times.The reason can be attributed to:Sn is along the grain boundary into the Bi₂Te₃ thin film,reducing the barrier height barrier,so that low-energy carriers can jump over the barrier,involved in transmission.Calculated by theoretical formula found Sn diffusion on the membrane barrier height is about 5-6 times the pure annealing effect.However,the main reason that the above phenomenon is not observed in the Bi₂Te₃（573K）thin film is that the annealing has a major effect on the sample performance due to the low degree of crystallinity of the573K thin film.The annealing time has little effect on the properties of the samples with respect to the annealing temperature.When the annealing time is more than 3 hours,the properties of annealed and diffused samples reach the steady value.