| SnTe as one of enviremental friendly alloys is viewed as promising thermoelectric materials after PbTe.SnTe and PbTe all possess face center crystal structure.This crytal structure is benefit to transport carriers,thus high electrical conductivity is garenteed.Meanwhile,SnTe possess two valance band structure,which is similar with PbTe.The high degenerated heavery band could participate in carrier transportance via band convergence.Consequently,the carrier density of state effective mass is enhanced which leading to a high Seebeck coefficient.However,due to low formantion energy of Sn vacancy in instinct SnTe,there are plenty of Sn vacancies.The more Sn vacancies the higher carrier concentration for SnTe.High carrier concentration is detrimental for high Seebeck coefficient and leads to high electrical thermal conductivity.Along with narrow band gap(0.18 eV),large energy difference between light and heavy energy bands(0.30 eV),boosting the Seebeck coefficient of SnTe is challenging.Single doping Indium(In)is effective to enhance the Seebeck coefficient of SnTe.The resonant levels associated with In doping could increase the density of state near Ferimi level of SnTe,which leads to a high Seebeck coefficient.Single substituting Mg,Mn or Cd on Sn site could enhance the Seebeck coefficient of SnTe via band convergence.All these reports show that single doping on Sn site is usful to enhance the Seebeck coefficient of SnTe through band engeenering.As for decreasing the thermal conductivity,sigle substituting Se on Te site or substituting Sb on Sn site could increase phonon scattering via point defect or nano-precipitations.Based on the reports mentioned above,dual doping is utilized in this dissertation to enhance the Seebeck coefficient via band engeenering and to reduce the thermal conductivity of SnTe via point defect,seconde phase or higher-order anharmonicity based on the band engeenering.Synergystical optimization of electrical and thermal transport properties are achieved.In addition,hardness is another important factor for SnTe in real application except for figure of merit.As the hardness of SnTe increased,not only the service time are prolonged,but also easier to synthesis the samples.The hardness of Cu2S,PbSe,SnSe,BiSbTe and AgPbmSbTem+2 could be enhanced using SiC compositing.In addition,the figure of merits were maintained or even increased at the same time.Thus SiC compositing is an effective way to synergystilly enhance hardness and thermoelectric properties of materials.In this dissertation,the Viker hardness and thermoelectric performances of SiC composited SnTe was firstly investigated.Following achievements have been obtained:1.Substitute In at Sn site to enhance the electrical properties of SnTe via resonant levels.Next,doping Selenium(Se)at Te site to reduce the lattice thermal conductivity via increased point defect.Results show that the best doping concentration of In in SnTe is 1%under melting and quenching.Based on the best In concentration,Se was selected as second dopant to substitute Te.Results show that Se doping merely influences the Seebeck coefficient.The electrical resistivity becomes higher due to extra carrier scattering sources.But the increased electrical resistivity also indicates decreased electrical thermal conductivity.Moreover,the lattice thermal conductivity was reduced due to the increased point defect scattering.With 1%In and 10%Se doping,the lattice thermal conductivity is decreased from 1.3 Wm-1K-1 to 1.05 Wm-1K-1 at 873 K.Finally,the electrical and thermal transport properties are synergistically optimized with In and Se co-doping.The figure of merit for SnTe is improved to 0.87 at 873 K.2.Dual doped In and Antimony(Sb)at Sn site.The doping concentration of In and Sb are 0.8,1.0,1.2%and 8,10,12%respectively according to each solubility limit in the SnTe lattice.Results show that the Seebeck coefficient of SnTe is increased in the whole measurement temperature range via resonant level effect and the increased carrier effective mass.As for the thermal transport properties,the grain boundary scattering is got enhanced from the increased grain boundaries.Second phase was created because the doping concentration is near the solubility limit.The second phase increase extra phonon scattering sources.Combining with the point defect scattering induced by atom substituting,the phonons are scattered in all scales.Finally,the lattice thermal conductivity 0.96 Wm-1K-1 and total thermal conductivity 1.87 Wm-1K-1 are achieved.Due to the synergistically enhanced electrical and thermal transport properties,the figure of merit for In and Sb co-doped SnTe is 0.8 at 800 K.Additionally,the average figure of merit is 0.45,which is higher than both In or Sb single doped SnTe.3.Dual doped Copper(Cu)and Manganese(Mn)on Sn site.The first prinsple calculation results show that band convergence is responsible for the enhanced Seebeck coefficient with Cu and Mn doping.Meanwhile,the anharmonicity of heavily doped SnTe increased.In detail,the linear thermal expansion parameter is enhanced with increased doping concentration.And the soften variation parameter is negligible under high doping concentration.Combining the experimental results of Raman and Brillouin scattering parameter,we conclude that the interaction between softened transverse optical phonons and high frequency acoustic phonons could reduce the lifetime of acoustic phonons.Thus,the lattice thermal conductivity sharply decreased to 0.47 Wm-1K-1.And it is more easily to reach the minimum value of lattice thermal conductivity with increased temperature.In the end,the highest figure of merit 1.3 is obtained under 873 K,which increased 225%than undoped SnTe.As Umklapp scattering commonly exist in various solid-state thermoeletic materials,so the experimental and simulated methods could be popularized to other thermoelectric material systems.4.Compositing SiC with SnTe to enhance the hardness of SnTe.And the influence of interface on thermoelectric properties of SnTe is studied.In the real application,the hardness of thermoelectric materials is as same importance as the figure of merit.Thus in this part,we mainly concentrate on the synergistic optimization of thermoelectric and hardness of SnTe.According to many researches,SiC composting is widely used to optimize the mechanical properties of thermoelectric materials without influence the thermoelectric properties.Moreover,there is no research about the thermoelectric properties of SiC composted SnTe.Thus,the thermoelectric and mechanical properties of SiC composted SnTe are studied in this part.The results show that homogeneous dispersive microstructure of SiC obtained by hand milling and hot press in pristine SnTe alloys.Specially,with 1 vol%SiC compositing,more Sn vacancies are created due to the lattice mismatch,thus the electrical conductivity is got improved.SiC compositing suppressed grain size thus created more grain boundaries.Multiple mechanisms are participated in lowering the lattice thermal conductivity,such as interface scattering,point defect and grain boundaries.Moreover,the minimum value of lattice thermal conductivity is close to the results achieved by chemical doping.In addition,the Vickers hardness of SnTe also enhanced at least 1.7 times with SiC compositing.However,the figure of merit for SiC composited SnTe is inferior to chemical doped samples.This is due to the unchanged Seebeck coefficient with compositing.In this dissertation,the dual-doping method is used to improve the thermoelectric figure of merit of SnTe alloys.The reason for dual-element doping synergistic optimization of thermoelectric properties for SnTe is systematically studied.The results show that the thermal and electrical transport properties of dual-doped SnTe alloys can be synergistically optimized through energy band engineering,multi-scale phonon scattering and strong lattice vibration anharmonics,confirming that SnTe is a potential thermoelectric alloy material for real application.At the same time,the research results of this dissertation on hetero-composite SnTe show that hetero-composite is an effective method that can synergistically improve SnTe thermoelectric performance and hardness,which is conducive to improving the service performance of SnTe in real applications. |
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