(Bi,Sb)₂（Te,Se）₃ Thermoelectric Materials For Cooling And Power Generation:Preparation And Performance Optimization

Zone melted(Bi,Sb)2(Te,Se)3 alloys have been the most widely applied commercial thermoelectric materials for past decades due to their high zT values～1 near room temperature.However,on a large scale,the zone melted ingots have lower zT values about 0.8～0.9.Bi2Te3 based TE devices have been well understood by scientists and manufacturers.Hence it is of significant importance to improve the TE performance of commercial zone melted(Bi,Sb)2(Te,Se)3 ingots in a simple process.On the other hand,shifting the peak zT temperature to higher zone makes(Bi,Sb)2(Te,Se)3 alloys more suitable for power generation.We improved the TE performance of commercial zone melted ingots via optimized preparing process and composition content.Meanwhile,we shifted the peak zT temperature of both p-type and n-type Bi2Te3 based alloys to higher temperature via a synergistic optimization procedure of Indium doping and hot deformation that combines intrinsic point defect engineering,band structure engineering,and multi-scale microstructuring.The main results are listed as below:1.We improved the TE performance and uniformity of zone melted(Bi,Sb)2(Te,Se)3 alloys through the optimized preparing process.All the raw materials and instruments are the same as that used in industry.We just improved the tube sealing process and added the annealing process after zone melting.As a result,the deviation of TE performance in the whole ingot is less than 15%,which is much better than the commercial ingot.Then we optimized the carrier concentration of both p-type and n-type ingot by changing the content of Sb and SbI3,respectively.And the zT value was improved to 1.1 for p-type ingot and 1.05 for n-type ingot.In order to confirm the reproducibility of the present results,five ingots of p-type and n-type respectively had been re-prepared using the same procedure.And the deviation is less than 4%.Based on these materials,we prepared TE devices and tested the performance.And our devices performed better than the commercial ones.2.Based on commercial zone melted ingot,we shifted the peak zT temperature to 400 K of p-type Bi-Sb-Te based alloys by increasing hole concentration.A noticeable increase in Seebeck coefficient was obtained due to the donor-like effect induced by hot deformation.Meanwhile,nanostructures and high-density lattice defects were created by the HD process,which significantly reduced the lattice thermal conductivity.As a consequence,the hot deformed Bi0.4Sb1.6Te3 alloy showed a highest zT of 1.36 at 400 K and the average zTav value about 1.2 from 300 K to 525 K.Different from traditional zone melted Bi0.5Sb1.5Te3 alloy,the maximum zT value has been shifted to higher temperature because of the higher Sb content,which makes Bi2Te3 based alloy feasible for low temperature power generation.It is worth mentioning that mechanical properties have been obviously enhanced after hot deformation.3.We developed a synergistic optimization procedure of Indium doping and hot deformation that combined intrinsic point defect engineering,band structure engineering,and multi-scale microstructuring.In particular,indium doping modulated the intrinsic point defects,broadened the band gap and thus suppressed the detrimental bipolar effect in the mid-temperature regime;in addition,hot deformation treatment rendered a multi-scale microstructure favorable for phonon scattering while the donor-like effect helps optimize the carrier concentration.As a result,a peak value of zT～1.4 was attained at 500 K,along with a state-of-the-art average zTav of～1.3 between 400 K and 600 K in Bi0.3Sbi.625In0.075Te3.These results demonstrate the efficacy of the multi-synergies,which can also be applied to optimize other thermoelectric materials.4.We successfully shifted the service temperature of n-type Bi2(Te,Se)3 to the mid-temperature range by suppressing the intrinsic conduction.In doping broadened the band gap of Bi2Te2Se and thus suppressed the detrimental bipolar effect that would otherwise present in the mid-temperature range.SbI3 and In dopants,along with the donor-like effect induced by hot deformation,optimized the carrier concentration.In addition,the In doping induced extrinsic and intrinsic point defects,along with the multi-scale microstructures created by hot deformation,helped suppress the lattice thermal conductivity.The integration of all these effects in Bi1.85In0.15Te2Se + 0.25 wt%SbI3 HD sample led to a zT～1,1 at 625 K and a state-of-the-art zTav of～1.03 between 500 K and 700 K.