Electron And Phonon Transport And Property Optimization Of P-type SnTe-based Thermoelectric Materials

IV-VI group elements-based materials are the earliest and most widely investigated thermoelectric materials,among which SnTe-based thermoelectric materials have attracted intense interests due to their similar energy band structure with PbTe and non-poisonous,eco-friendly properties.The pristine SnTe usually exhibits as heavy doped p-type semiconductor as a result of high concentration of Sn vacancy.However,the over-high carrier concentration significantly inhibits its Seebeck coefficient.And the large energy offset between light and heavy valence bands increases the difficulty to promote Seebeck coefficient by means like band convergence.In addition,the over-high lattice thermal conductivity and small band gap also influence the thermoelectric efficiency of pristine SnTe.This work focused on SnTe-based thermoelectric materials by using high temperature melting and hot-pressed process.We enhanced the electrical properties by means of resonant doping,carrier concentration optimization and band convergence,and reduced the lattice thermal conductivity by introducing multiple scattering mechanisms including point defects and second phases.The improved properties were investigated by phase analysis,microstructure analysis and physical modeling.Besides,the new layered thermoelectric material SnTe·Sb2Te3 and its thermoelectric property were also investigated.The results can be summarized as follows:1)The theoretical Pisarenko curve of SnTe was calculated by using a two-band model.By first principle calculation,Indium doping was proved effectively to introduce resonance level,increase density of state near Fermi level and therefore improve the Seebeck coefficient at room temperature.Based on Sn0.995In0.005Te and(SnTe)2.88(In2Te3)0.04,the carrier concentration optimization was further carried out.The introduction of Sb effectively suppressed the over-high carrier concentration in pristine material and increased mobility as well as Seebeck coefficient,giving an optimized overall electrical property.Eventually,Sn0.915In0.005Sb0.08Te obtained a high zT value of 1.1 at 825 K,illustrating that In-Sb dual-doping can significantly improve thermoelectric performance of SnTe-based materials.In addition,thermal deformation of SnTe was achieved due to its good thermoplasticity.The deformed sample has layered structure with more holes,but the thermoelectric property didn't change too much.It implied that thermal deformation is not an effective technology to improve thermoelectric property of SnTe-based materials.2)Antimony has higher solubility in Sn 1.03Te comparing with SnTe.The alloying with Sb not only optimized carrier concentration,but also achieved strong phonon scattering by introducing second phase,and therefore significantly decrease the lattice thermal conductivity.From the results of EPMA and phase analysis,it was found that there was second phase in mesoscale in addition to the previously reported nanoscale phase.The influence of point defect scattering was investigated by further alloying Mg in Sn0.85Sb0.15Te.Due to the relatively large difference of atom radius and mass between Mg and Sn,Mg alloying induced strong mass and stress fluctuation,resulting in more intense phonon scattering and further decreased its thermal conductivity,which was close to the lowest theoretical lattice conductivity.Eventually,the sample gave a zT value around 1.0 at 775K with highly improved device zT in all temperature range.3)The influence and mechanism of band convergence in SnTe was investigated.Based on Sn0.96Mg0.07Te,the influence of Sb and I doping in different sites were investigated.Sb doping optimized carrier concentration and increased Seebeck coefficient as high as 213 ?V·K-1.By using SPB model,the effective mass was calculated.Compared with the matrix,the effective mass of Mg alloying sample increased 60%as result of band convergence,and a further 40%improvement was achieved by introducing Sb.Sb induced decrease of the offset of light and heavy valence band by first principle calculation,indicating that Sb also has a band convergence effect.Combining the optimized carrier concentration,the dual band convergence by Mg and Sb achieved an improved zT value of 1.3 at 825 K.4)The anisotropy of SnTe·Sb2Te3 was investigated by various analysis.It was found to be a uniform single phase inside and the anisotropic zT value was determined by the Seebeck coefficient anisotropy.Due to Hall analysis,it was found that density-of-state effective mass are different along two directions.The energy band was calculated to be anisotropic,resulting in Seebeck coefficient anisotropy.Eventually,manual grinded polycrystal SnSb2Te4 had a maximum zT value of 0.42 along the out-of-plane direction,implying the potential thermoelectric property of this material.