Preparation And Properties Of Bi₂Te₃-based Thermoelectric Nanocrystalline Thin Films By Vacuum Thermal Evaporation

As a new type of energy material that can directly couple and convert thermal energy to electrical energy,thermoelectric materials have a wide range of application prospects.Low-dimensional structures can reduce the thermal conductivity of the material by enhancing the interface phonon scattering of materials,thereby enhancing the thermoelectric performance of materials.Bi₂Te₃ is the most widely used traditional thermoelectric material working at near room-temperature,which can be significantly optimized through nanostructuring route.The existing studies have focused on nanocrystalline bulk materials and have little knowledge of thin films.This dissertation focuses on the vacuum thermal evaporation growth,structure control and physical properties of Bi₂Te₃-based thermoelectric thin films.The main results are as follows:?1?The Bi₄Te₃ nanopillars array films with highly preferred?00l?-orientation have been successfully prepared on SiO₂ substrates by a vacuum thermal evaporation deposition technique for the first time.The detailed structures and electrical transport properties of the samples have been characterized.The effects of evaporation temperature and substrate temperature on the phase and the growth of microstructures of Bi₄Te₃ films were discussed.The results show that the obtained films are composed of well-aligned nanopillars array with?00l?-orientation.The formation of the Bi₄Te₃ phase is attributed to the large difference in the saturated vapor pressures of Bi and Te atoms,and the growth of the nanopillars array is due to the large deposition rate and the lower substrate temperature.The test on the electrical transport properties of Bi₄Te₃ films shows that highly?00l?-oriented nanopillars array films have a improved electrical transport properties compared with the non-oriented ones,and the PF value at 339 K is 0.032 mWm^-1·K^-2,which is close to twice the non-oriented films.The enhancement of the electrical properties for the Bi₄Te₃ films can be addressed by the high crystallization and highly?00l?-orientation.?2?Sb-doped Bi₂Te₃(Bi_2-xSb_xTe₃,x=0,0.7,1.1,1.5,2)ternary films were prepared by a vacuum thermal evaporation method.The microstructures and morphologies of the obtained films were characterized.The results show that the well-alloyed Bi_2-xSb_xTe₃oriented films have been prepared,with an ordered microstructure of nanopillars array,and the doped Sb atoms are uniformly distributed in the film.The examination on the electrical properties shows that the conductivity of Bi_2-xSb_xTe₃ films transform from n-type to p-type when x is over 1.2.In addition,the metal-insulator transition phenomenon was observed due to Sb doping.And the Bi_0.5Sb_1.5Te₃ nanopillars array film exhibits the best electrical properties,with the maximum conductivity?of 2.95*10⁵ S·m^-1 at T=300 K,which is approximately ten times more than the undoped Bi₂Te₃ film and about three times larger than the reported date for Bi_0.5Sb_1.5Te₃ film and bulk materials.The maximum power factor PF is about 3.83?Wcm^-1·K^-2 at T=475 K.The work obtained some new material forms,realized the control for microstructures of films,and optimized the thermoelectric properties,which extend and enrich the study of Bi₂Te₃-based materials.Moreover,high vacuum thermal evaporation technique used in the work is a facile and low-cost route that has promising application prospects.