Research On SnTe-based Semiconductor Thermoelectric Materials

Thermoelectric(TE)materials is a kind of energy semiconductor materials which can convert heat to electricity,and vice versa.It presents important application prospects in energy conservation,solid state refrigeration,temperature control and so on.SnTe compound is a potential medium temperature TE material,which has the same crystal structure and similar band structure with Pb Te and even more environmentally friendly since free of heavy metal lead.The current key problem of SnTe is the low TE performance,which is line in three main reasons:firstly,the intrinsic high hole concentrations in SnTe lead to low Seebeck coefficient;secondly,the narrow band gap and large energy split between the light and heavy valence bands resulting in poor electrical performance;thirdly,the high lattice thermal conductivity of SnTe also devalues its performance.Aimed at the above-mentioned problems,this paper combining doping,point defect engineering,nanocomposite,in-situ reaction as well as hot deformation process to synergistically improve the electrical and thermal properties of SnTe-based materials.The main contents and conclusions are as follow:1.The effects of anion and cation co-doping on the TE properties of SnTe was studied.The study found that by introducing Ca I₂ compound,low-valent iodide ions contribute electrons to reduce the hole concentrations,while the solid solution of Ca can converge the valance band of SnTe,thus effectively improved the electrical transport performance of the sample.Therefore,the Sn_0.97Ca_0.03Te_0.94I_0.06 sample obtained the maximum ZT value(?1.2@873 K)among differernt Ca I₂-doped samples.After adding Cu Cl compound in SnTe,two donors,low-valent chloride ions and interstitial copper ions,were introduced at the same time,which effectively reduced the hole concentrations,and as the doping content increased,the nanoprecipitates Cu₂Te formed,which are benefit to reduce the thermal conductivities,finally the Sn_0.96Cu_0.04Te_0.96Cl_0.04 sample obtained a ZT of?1.13 at 873 K.2.The effect of heterogeneous recombination of graphene and graphene oxide nanosheets on the TE properties of SnTe compound was studied.The results show that the addition of graphene nanosheets into SnTe can effectively reduce the hole concentrations of the material and enhance the scattering of carriers,thus significantly increased the electrical transport performance of the sample.However,the ultrahigh thermal conductivity of graphene lead to less reduction in the thermal conductivity of the composite samples.Finally,when the adding content of graphene is 0.3 wt.%,the composite sample obtained a maximum ZT value about 0.64 at 873 K.By adding graphene oxide into SnTe sample,the poor conductivity of graphene oxide make less contributions to reduce the hole concentrations,but due to the enhanced carrier scattering and energy filtering effect,the electrical properties of the composite samples also enhanced.Meanwhile,the thermal conductivity also reduced due to the enhanced carrier scattering after composite with graphene oxide.Therefore,a ZT value of?0.68 was obtained at 873 K in the SnTe-0.3 wt.%GO sample.3.The effect of N-type Mg₂Si semiconductor on the TE properties of SnTe was studied.The results show that the N-type Mg₂Si compound can effectively reduce the hole concentrations of the system and significantly increased the Seebeck coefficient of the samples.At the same time,the electronic thermal conductivity and the lattice thermal conductivity were also reduced by adding the N-type Mg₂Si compound.Finally,the sample of SnTe-3 wt.%Mg₂Si show a higher ZT value compared with other samples,the ZT value reached above 1.0 at 873 K in SnTe-3 wt.%Mg₂Si sample.4.The effects of the in-situ chemical reaction between nano-additives and SnTe on the microstructure and TE properties of the materials were studied.When?-Zn₄Sb₃ compound added to SnTe matrix,?-Zn₄Sb₃ will first decompose to Zn and Zn Sb compounds,then Zn will dissolve into SnTe matrix and formed a solid solution,while Zn Sb undergoes an in-situ solid-phase reaction with SnTe,and formed special Sb@Zn Te"core-shell"structures and Sn element.This reaction introduced point defects such as Zn and Sn,as well as multiscale microstructures at the same time,thereby effectively improved the electrical and thermal transport performance of the samples.Finally,a ZT value of?1.32(873 K)was obtained in the SnTe+1.5 at.%?-Zn₄Sb₃ sample.After adding nano-oxide Cu₂O to SnTe,an in-situ chemical reaction also happened between Cu₂O and SnTe matrix.Cu⁺will react with Sn and Te and formed Cu₄₁Sn₁₁ and Cu₂Te compounds,and oxygen ions will combined with Sn elements to generate Sn O₂ compounds.Benefit from the carrier and phonon scattering caused by Cu⁺interstitial,various lattice defects and secondary precipitates,the power factor of the sample was enhanced,and the thermal conductivity was also significantly reduced.On this basis,In element was further doped in SnTe,and the resonance energy level was introduced to further improve the electrical properties of the sample.Thereby,a ZT value of?1.5 was obtained in Sn_0.99In_0.01Te-4 wt.%Cu₂O sample at 823 K,the average ZT value of the sample was also improved by nearly 3 times.5.The influences of hot deformation(HD)processing on the microstructure and TE performance of SnTe materials were explored.Results show that the microstructure and grain orientation of the SnTe samples will be reorganized after repeatedly hot deformation processing.With increasing the HD times,the intensity of(220)diffraction peak of the in-plane direction which perpendicular to the applied pressure was gradually increased,and the orientation factor increased from 0.067 to 0.26.The enhanced orientation is beneficial to increase the carrier mobility,so the power factor of the samples were improved after hot deformation.In addition,repeatedly HD caused a large number of dislocations,stress distortions and other lattices defects,which enhanced the phonon scattering and reduced the thermal conductivity of the sample.Therefore,the figure of merit of the in-plane and cross-plane HD1 sample is increased by?45%and?58%,respectively.