Study On Performance Optimization Of SnTe And Bi₂Te₃-based Thermoelectric Materials

With the growing contradiction between the development and the energy demand,balancing economic growth,energy consumption and envirmomental protection has been a long-standing worldwide chanllege,which makes the development of the renewable energy materials and technology an urgent need.In particular,thermoelectric materials can directly convert heat to electricity or conversely and thus to be used for power generation and solid-state cooling,which is the potential energy materials to mitigate the energy crisis.Narrow bandgap tellurides constitute a major portion of state-of-the-art thermoelectric materials,such as SnTe and Bi2Te3.The lead-free SnTe possesses the same crystal structure and the similar two-valley valence band structure as in PbTe,has been considered as a promising and ecofriendly materials alternative to PbTe.However,the thermoelectric performance of SnTe is inferior to PbTe,which is mainly due to the higher lattice thermal conductivity,as well as the native Sn vacancies act as acceptors and contribute holes,resulting in the high carrier concentration and thereby the lower Seebeck coefficient.Bi2Te3-based thermoelectric materials has been the benchmark materials for power generation and solid-state cooling around room temperature.However,the potential of Bi2Te3 in power generation is not yet fully developed,further research is needed to optimize the thermoelectric performance in the temperature range of 300K～500K.Aiming at the above-mentioned problems,we herein prepared the SnTe and Bi2Te3 thermoelectric materials by a growth-from-the-melt spark plasma sintering or high-pressure sintering procedure,respectively.In the meantime,the point defect engineering and the nanostructuring engineering were employed to optimize the thermoelectric performance.The research contents and the conclusions are as follows:(1)We adopted a synergistic charge-balanced compensation-doping approach derived from the interplay between heterovalent Sb ions and Sn vacanvies,designed a composition series of Sn1-xSb2x/3Te(0 ≤ x ≤0.20),in which every three Sn2+ are substituted by two Sb3+ and one Sn vacancy.To the first order,such chemical composition does not contribute net charge carriers in the context of electron counting and also ensures that the composition of the primary phase is not altered by the formation of secondary phase Sb2Te3.Such a scenario is certainly over-simplified,the compensating effect between the Sb3+as dornors and the Sn vacancies as acceptors fine turning the carrier concentration.The carrier concentration gradually reduced with the increasing Sb content and in turn increased the electrical transport property.Meanwhile,the coexistence of Sb dopants and Sn vacancies effectively scattered the heat-carring phonons,thus restrained the lattice thermal conductivity.Consequently,a peak zT value of～1.1 at 873K was achieved in the Sn1-xSb2x/3Te with x=0.12.(2)Based on the composition of Sn1-xSb2x/3Te(x=0.10),we adopted the Pb substitutes on Sn site to investigate the effects of Pb-Sb co-doping on the thermoelectric performance of SnTe thermoelectric material.We found that the lattice thermal conductivity was reduced due to the Pb doping induced the mass and strain field fluctuations strengthened the phonon scattering.In addition,the carrier concentration decreased with the increasing Pb doping content,thereby enhancing the Seebeck coefficient.As a result,a peak zTvalue of～1.02 at 773K was achieved in the 18 at%Pb doping sample.(3)We investigated the effect of Sm doping on the thermoelectric performance of p-type(Bi0.2Sb0.8)2(Te0.97Se0.03)3 thermoelectric material.In conjunction with micro-morphology and compositional analyses,the electrical transport property and the thermal conductivity were measured as a function of temperature between 298K and 473K.We found that there was no discernible crystal lattice symmetry change upon Sm doping and Sm doping generally increased the electrical conductivity while practically retained the Seebeck coefficient,thereby yielding higher power factors.Meanwhile,Sm doping reduced the lattice thermal conductivity,which derives from the point defects scattered the phonons.Consequently,a minimum thermal conductivity of～0.92Wm-1K-1 at 423K was achieved in the 0.4 at%sample,resulting in a maximum zT value～1.22,which is-25%higher than the maximum zT value of the pristine sample.Furthermore,contrary to the customary view that Sm dopant adopts a trivalent state Sm3+,the XRD analysis and the variations of lattice parameters pointed toward a lower valence state of Sm.(4)We prepared the Gd doping p-type(Bi0.2Sb0.8)2(Te0.97Se0.03)3 thermoelectric materials via a growth-from-the-melt high-pressure sintering procedure.The thermoelectric performance measurements were conducted before and after annealing,respectively.We found that Bi2Te3 thermoelectric materials with high pack density,high crystallinity,as well as multi-orientation nanocrystalline were obtained by the high-pressure sintering.In the meantime,high-pressure induced the modulated structures into the crystal.The coexistence of nanocrystalline and the modulated structures effectively scattered phonons and thus reduced the lattice thermal conductivity.Moreover,annealing regulated the intrinsic point defects,thereby optimizing the carrier concentration and in turn increasing the Seebeck coefficient.Finally,Gd doping further increased the electrical conductivity.As a result,a maximum zT value of～1.1 at 348K,as well as the average zT value of-0.90 between 298K and 473K were achieved in the 0.4 at%Gd doping sample.(5)The effect of Co doping on the thermoelectric performance of n-type Bi2(Te0.97Se0.03)3 was investigated and the existence state of Co in the matrix was analyzed.We found that the micron secondary phase CoTe2 was embedded into the matrix due to the low solubility of Co in the n-type Bi2(Te0.97Se0.03)3.The micron-scale secondary phases with increasing grain boundaries scattered long wavelength phonon and the Co occupied Bi sites introduced the point defects scattered short wavelength phonons effectively restrained the lattice thermal conductivity.Meanwhile,the charge carrier interacted with the local magnetic moment carried by Co ions improved the density of state effective mass,thereby yielding the higher Seebeck coefficient.Cosequently,a peak zT value of～0.77 at 398K was achieved in the 8 at% Co doping sample.