Microstructure Modulation And Doping Modification For Sb₂Te₃ Based Thermoelectric Materials

The low temperature（<673 K）waste heat which is difficult to be recycled restrict further improvement of the efficiency of energy utilization.Thermoelectric mateirals with the ability of directly converting heat to electricity have shown great potential on the low temperature waste heat harvesting.However,the narrow working temperature of the thermoelectric mateirals cannot meet the requirement of recycling the waste heat in wide temperature range.In this work,high-performance mid-and low-temperature thermoelectric materials are prepared from a single Sb₂Te₃ material system through microstructure modulation control and element doping.Based on that,a high-performance segmented thermelectric leg is fabricated by simple one-step sintering method.Further using advanced microstructure characterization technologies,thermoelectric performance testing equipment,and mechanical property measuring platforms,the influence of nano twins,second phases,and acceptor doping on the structure,thermoelectric and mechanical properties is systematically studied for the prepared alloys,which could promote the further development of thermoelectric materials in the field of low-temperature waste heat power generation.The thermoelectric and mechanical properties are simultaneously optimized by adjusting the Te content of Bi_0.4Sb_1.6Te₃ alloy.On one hand,adding a tiny amount of Te can increase the carrier concentration and suppress the intrinsic excitation,thus contributing to a significant power factor improvement.Meanwhile,uniformly dispersed micropores are constructed due to the volatilization of the Te-rich precipitates,which enhances the phonon-scattering and reduces the lattice thermal conductivity.With adding 0.01 Te,the z T_max and z T_ave reach 1.55@373 K and 1.39（303～473 K）,respectively.On the other hand,reducing the Te content builds dense nano-twins with interstitials located in the twin boundary gaps,which strengthens the material.As the Te content decreases,the compressive strength first increases,and then remains unchanged.When the Te is reduced by 0.03,the compressive strength increases from 184.6 MPa to 248.2 MPa,and the hardness increases from 102.4 MPa to 159 MPa.In addition,the decrease of Te content is conducive to the formation of negatively charged anti-site defect Sb（Bi）_Te,which greatly increases the carrier concentration and suppresses intrinsic excitation,thus broadening the optimal working temperature.Subsequently,through the Bi/Sb ratio adjustment,the z T_max and z T_ave of Bi_0.5Sb_1.5Te_2.98 reach 1.32@423 K and 1.2（303～473 K）,respectively.Then,through assembling a thermoelectric device with n-type Cu_0.01Bi₂Te_2.7Se_0.3 leg,high conversion efficiency of 4.8%is obtained at a temperature difference of 220～270 K.Mn doping is used to increase the carrier concentration of In doped Sb₂Te₃ alloys which possess larger bandgap and expected to be used at mid temperature.The increased carrier concentration suppresses the bipolar effect,and pushes the Fermi level deep into the valance band,which forces the second valence band to participate in the electrical conduction,thus improving the density-of-state effective mass.As a result,the power factor is improved in the whole measuring temperature range.Meanwhile,dense grain and twin boundaries are generated through the ball miling followed by sintering process to enhance the phonon-scattering and reduce the lattice thermal conductivity.Ultimately,a high z T value of 0.99 is obtained in Mn_0.02In_0.15Sb_1.83Te₃ at 673 K.B doping could impede the migration and deformation of grain boundary to suppress the high-temperature softening of In_0.1Sb_1.9Te₃ without deteriorating the thermoelectric performance.Based on that,Cu doping is employed to increase the carrier concentration and in situ construct nanoprecipitates,suppressing the bipolar and reducing the lattice thermal conductivity,respectively.Furthermore,some Cu atoms enter into the van der Waals gaps,which work as an electrical connection between two adjacent twin variants to accelerate the electron transfer.Hence,compared to the In-Sb-Te alloys doped with other acceptors,the Cu doped alloys exhibit a higher Seebeck coefficient when possessing similar electrical conductivity.Ultimately,the z T value of Cu_0.015B_0.05In_0.1Sb_1.885Te₃ reaches 1.05 at 673 K.Adding Mg B₂ to realize B-Mg co-doping in In_0.1Sb_1.9Te₃ could form uniformly dispersed nanoprecipitates,which not only improve the mechanical strength（increasing the compressive strength from 183.0 to 98.8 MPa,and the hardness from230.6 to 131.8 MPa）but also enhance the phonon-scattering to reduce the lattice thermal conductivity.Combining with the acceptor effect of Mg,a high z T value of1.01 at 673 K is achieved.Then,a segmented leg composing of this material and high-performance low-temperature Bi_0.4Sb_1.6Te_3.01 alloy is fabricated and reaches a high conversion efficiency of 7%under a temperature difference of 375 K,showing great potential to recycle the waste heat below 673 K.