The Study Of Thermoelectric Performance Of N-type Bi₂Te₃ Based Compounds For Commercial Applications

Due to the development of modern industry and human society,a series of environmental and energy resource problems have become a major problem hindering social development and threatening human health.Thermoelectric materials can achieve a reversible conversion between heat and electricity,which have attracted worldwide attentions.Since the first report in 1954,Bi₂Te₃based compounds have always been the focus of researchers.However,up to now,traditional zone melting（ZM）materials are still the first choice in industrial production.In order to overcome the poor mechanical properties of zone melting materials,most research was focused on poly-crystallizing Bi₂Te₃based compounds by powder metallurgy method.However,the donor-like effect generated in the fracturing process makes the carrier concentration increase dramatically,deviating from the optimal carrier concentration of the service temperature of the material.Besides,the scattering of grain boundaries as well as the weakened orientation of grains in polycrystalline samples after the refinement of grain sizes,leads to a sharp decrease in carrier mobility,and the significantly degraded electrical transport properties,especially in room temperature.In addition,the band gap of Bi₂Te₃-based compounds is small.When the temperature is higher than 400 K,intrinsic excitation will lead to serious degradation of thermoelectric properties in the materials.All these pose great challenges to the synthesis and development of n-type polycrystalline Bi₂Te₃based compounds with excellent mechanical and thermoelectric performance.In view of the above problems,in our work,n-type polycrystalline Bi₂Te₃-based compounds were chosen to systematically study the influence of donor-like effect on the thermoelectric properties of Bi₂Te₃-based compounds and its underlying mechanism;The influence of S solid solution on the crystal structure,phonon spectrum,bond strength and transport properties of Bi₂Te₃and Bi₂Te₂S compounds was studied,and the origin of intrinsic low lattice thermal conductivity of Bi₂Te₃and Bi₂Te₂S compounds was revealed;The effects of extrusion ratio,extrusion angle,extrusion temperature,extrusion rate and holding time on texture,microstructure,thermoelectric transport and mechanical properties of Bi₂Te_2.79Se_0.21compound were investigated by hot extrusion process.The main conclusions obtained in this paper are summarized as follow:（1）Mechanism and regulation method of donor-like effect in n type Bi₂Te₃-based compounds.The effects and underlying mechanism of donor-like effect in n type Bi₂Te₃-based compounds were systematically studied.The results show that the generation of donor-like effect is closely related to oxygen in the air and the Te and Bi vancancies generated during the crushing process.Sample synthesized in atmospheric ambient presents an obvious donor-like effect with the carrier concentration increased significantly to 7.5×10¹⁹cm^-3.On the contrary,the carrier concentration and Seebeck coefficient of the sample prepared under inert atmosphere are 2.5×10¹⁹cm^-3which are consistent with those of the pristine zone melting ingot.Combined with theoretical calculation and experimental study,a new mechanism of donor-like effect is proposed.Te and Bi vacancies generated by deformation process would react with oxygen and antisite defects to promote Bi atoms which occupy Te sites to return to Bi sites,leaving O occupy Te vacancies and generate electrons to increase carrier concentration.In addition,a new method of eliminating Te and Bi vacancies generated by crushing process through thermal annealing is proposed to eliminate donor-like effect.（2）Regulation of carrier concentration and the exploration of intrinsic low lattice thermal conductivity of Bi₂Te₂S and Bi₂Te₃compounds.In this section,the phase composition,microstructures and thermoelectric properties of S solid solution and Cl doping samples were systematically discussed.There are two solid solution regions in Bi₂Te_3-xS_xsamples.When S content is low,the sample presents as single Bi₂Te₃phase.With the further increasing content of S,there exists a single-phase region of Bi₂Te₂S compound.Rietveld refinement suggests that Bi₂Te_2-xS_1+xcompounds possess the same crystal structure as Bi₂Te₃and S atoms preferentially fully occupy the Te^（2）site,while any extra S atoms randomly occupy the Te^（1）site.Compared with the pure Bi₂Te₃sample,the intrinsic excitation is inhibited in Bi₂Te_2-xS_1+xcompounds after S solution,and the thermoelectric properties of the material are significantly improved.Bi₂Te_1.93S_1.07sample obtained the highest ZT of0.58 at 500 K.Furthermore,the intrinsically low lattice thermal conductivity mainly comes from the strongly coupled low frequency optical branch and acoustic phonons which is donated by Bi and Te^（1）atoms.Although the chemical bonding becomes stronger and the sound velocity increases slightly after S solution,substitutions of S atoms on the Te site intensifies phonon alloy scattering that dramatically suppresses the thermal conductivity of Bi₂Te_2-xS_1+xcompounds.Doping with Cl further increases the room temperature carrier concentration and suppresses the intrinsic excitation at higher temperatures.As a result of the optimization of the electronic transport properties and the reduction in the lattice thermal conductivity because of the enhanced point defect scattering,the Bi₂Te_1.93S_1.065Cl_0.005sample achieved the highest figer of merit ZT of 0.67 at 625 K.（3）Influence and underlying mechanism of die parameters on microstructure,thermoelectric transport and mechanical properties of hot extruded Bi₂Te_2.79Se_0.21compound.In this section,the effect of extrusion angle and extrusion ratio on microstructure,thermoelectric and mechanical properties was systematically studied.The orientation of the（1 1 0）crystal plane perpendicular to the pressing direction increases significant with the decreasing extrusion angle and ratio,resulting in the increased carrier mobility.The highest carrier mobility are 233cm²V^-1s^-1and 230 cm²V^-1s^-1,respectively,for samples with the extrusion angle of G°and extrusion ratio of D,making the sample retain the same power factor of the zone melting sample at room temperature,more than 4.0 m W m^-1K^-2.In addition,the grain refinement caused by hot extrusion process effectively strengthens the scattering of phonons by grain boundaries,reducing the lattice thermal conductivity of the materials.The lattice thermal conductivity of 0.8-0.9 W m^-1K^-1is obtained at350 K,which is about 30%lower than that of commercial zone melting ingot which improves the ZT value.At room temperature,the samples with extrusion angle of40°and extrusion ratio of D possess the highest ZT values of 0.90 and 0.92,respectively,while at 325 K,the samples with extrusion angle of I°and extrusion ratio of D presents the highest ZT values of 1.01 and 1.04,separately.In addition,grain refinement greatly increases the mechanical properties of the material.The highest compressive strengths are increased from 14 MPa in ZM sample to 95 MPa in the sample with an extrusion angle of J°and the sample with extrusion ratio of F,while the highest bending strength is 102 MPa and 90 MPa in the sample with an extrusion angle of H°and the sample with extrusion ratio of F,respectively,which are increased by 9 times and 8 times compared with the bending strength of ZM sample.（4）Influence and underlying mechanism of extrusion parameters on microstructure,thermoelectric transport and mechanical properties of hot extruded Bi₂Te_2.79Se_0.21compound.In this work,the investigation of extrusion temperature,extrusion rate and holding time on the microstructure,thermoelectric and mechanical properties was carefully carried out.When the extrusion temperature,extrusion rate and holding time decrease,the orientation of（0 0 L）crystal plane parallel to the extrusion pressure is significantly enhanced which leads to the increasement of carrier mobility.Sample with A K extrusion temperature,sample with the extrusion rate of L mm min^-1and sample with O h holding time achieve the highest carrier mobility of 238 cm²V^-1s^-1,230 cm²V^-1s^-1and 234 cm²V^-1s^-1respectively.Benefiting from the enhanced texture,most samples possess a PF higher than 4.0 m W m^-1K^-2at 300 K.Moreover,the lattice thermal conductivities of extruded samples obtain relatively lower values because of the significantly enhanced phonon scattering on grain boundaries with the lowest lattice thermal conductivity of0.8-0.9 W m^-1K^-1obtained at 350 K,about 30%lower than that of ZM ingot.At room temperature,the sample with B K extrusion temperature,the sample with the extrusion rate of L mm min^-1and the sample with holding time of Q h achieve the highest ZT value of 0.92,0.92 and 0.90,separately.The highest compressive strengths of 80 MPa,80 MPa and 95 MPa are obtained at the extrusion temperature of A K,extrusion rate of L mm min^-1and holding time of Q h,respectively.The highest bending strengths of 80 MPa,80 MPa and 90 MPa are obtained in the sample with extrusion temperature of B K,extrusion rate of Nmm min^-1and holding temperature of Q h,respectively and are increased by 7 to 8 times compared with ZM samples.