Study On The Effect Of Porosity And Doping On Thermoelectric Properties Of Sb₂Te₃-based Materials

Thermoelectric materials have attracted much attention because they can realize the direct conversion of heat and electric energy through Seebeck effect and its inverse Peltire effect.The performance of thermoelectric materials is evaluated by the dimensionless figure of merit（ZT）.To realize the wide application of thermoelectric materials,the dimensionless figure of merit must be improved up to 3.Two common strategies to achieve high ZT are to explore the material with a higher power factor（S²σ）and a lower thermal conductivity（κ）.However,the strongly coupled relationship of the Seebeck coefficient（S）,the electrical conductivity（σ）and the thermal conductivity（κ）makes it difficult to reach this goal.A complete TE device must equip with several TE couples,which contain at least a matched pair of N type and P type TE materials.Sb₂Te₃ based materials are typical P type thermoelectric materials suitable near room temperature.However,compared with the commercially used N type Bi₂Te₃ materials,the pure phase Sb₂Te₃ thermoelectric materials have a lower ZT value,which becomes a bottleneck to restrict the application of thermoelectric devices.In order to solve the coupling between electron and phonon,this work has tried to adjust the porosity of the material,introduce the second phase,as well as dope non-equivalent elements,in order to increase the power factor while reducing or slightly increasing the thermal conductivity,thus improve the thermoelectric performance of Sb₂Te₃-based materials(including Sb₂Te₃ and Bi_0.4Sb_1.6Te₃ materials).The main research contents and results are as follows.1.Four groups of Sb₂Te₃ bulks with different porosity were compressed under different pressures（1 k N,4 k N,7 k N,10 k N）.One group has a sample fabricated by one-step sparking plasma sintering,and the other one first compressed into bulks and then sintered.The difference of porosity and the effect of pores on thermoelectric properties of samples were studied.The sample fabricated by two steps under 10 k N has suitable porosity,as a result,phonons are strongly scattered by pores and grain boundaries whereas the mobility of electrons is not reduced.Compared with the sample sintered by one step,it has better thermoelectric performance,and the maximum ZT value reaches 1.17 at 523 K.2.We have successfully grown in situ Pt nanoparticles on Sb₂Te₃ hexagonal plates prepared by solvothermal method through Xe light reduction method under full-spectrum conditions.The addition of Pt nanoparticles can significantly increase the electrical conductivity without degrading the thermal conductivity.The results show that the maximum ZT value of the Sb₂Te₃-based matrix coated 0.2 wt%Pt nanoparticles reaches1.67 at 573 K,and the average ZT value reaches 1.32 within 300-573 K.3.MXenes Ti₃C₂ nanosheets were prepared from Ti₃Al C₂ by etching method,and then were successfully mixed into commercial P-type Bi_0.4Sb_1.6Te₃ materials to fabricate Bi_0.4Sb_1.6Te₃-Ti₃C₂ composite.The introduction of Ti₃C₂ increases Seebeck coefficient,which compensates for the decrease in electrical conductivity,and significantly reduces thermal conductivity.Hence,the thermal performance can be effectively optimized without degrading the electrical properties remarably.Finally,the maximum ZT value of 1.13 is obtained around 393 K.4.Sn_xBi_0.4-xSb_1.6Te₃（x=0,0.04,0.06,0.08）quaternary compound nanosheets were synthesized by one-step hydrothermal method.As the ratio of Sn element to Bi element increases,the carrier concentration in the matrix increases significantly,contributing to the significant increase of weighted mobility（μ_W）,which optimizes the electrical conductivity while suppressing the bipolar thermal conductivity.Finally,the Sn_0.08Bi_0.32Sb_1.6Te₃thermoelectric bulk reaches the maximum ZT value of 0.91 at 393 K,which is 1.54 times that of the pure sample at 363 K.