Regulation Of Thermoelectric Properties For Telluride With Low Thermal Electrical Conductivity

Thermoelectric materials are attractive because of their ability in realizing reversible conversion between heat and electricity.Thermoelectric materials have very important application value and prospect in the fields of solid-state refrigeration and thermoelectric power generation,which is one of the potential materials to solve the increasingly serious energy crisis.The conversion efficiency is represented by the dimensionless figure of merit z T,and the z T value is proportional to the product of the square of the Seebeck coefficient and electrical conductivity(power factor),and inversely proportional to the thermal conductivity,which is mainly composed of the lattice thermal conductivity and the electron thermal conductivity.And the electron thermal conductivity is proportional to the electrical conductivity.Currently,the two broad strategies for improving z T values are to enhance the power factor and reduce the lattice thermal conductivity of thermoelectric materials.However,the reduction of thermal conductivity has many limitations.For example,the reduction of electron thermal conductivity also means the reduction of electrical conductivity,thus limiting the power factor.Synergistic optimization of power factor and thermal conductivity is very difficult due to the strong coupling effect between the thermoelectric transport coefficients(electrical conductivity,Seebeck coefficient and electron thermal conductivity).However,some novel materials with intrinsically low lattice thermal conductivities have also attracted great interest.The optimization of electrical transport performance for materials with intrinsically low lattice thermal conductivities may be a better research direction.In Te and Cu In Te₂ are two Te-based thermoelectric materials with intrinsically low lattice thermal conductivity.This paper firstly takes In Te as the research object and improves its thermoelectric performance through doping and process optimization.Secondly,taking Cu In Te₂ as structural prototype,a new group of quaternary A₂Cu₃In₃Te₈(A=Cd,Zn,Mn,Mg)compounds have been designed and prepared by traditional solid-state reaction.The thermoelectric performance of A₂Cu₃In₃Te₈(A=Zn,Cd)compounds can be improved through in-situ nanodomain regulation,and main conclusions are as follows:(1)The samples of In_1-xCd_xTe(x=0,0.002,0.003,0.005,0.01,0.02)were synthesized by melt-annealing.Owing to the presence of strongly anharmonic phonons originating from the rattling vibrations of In⁺cations,the lattice thermal conductivity(?_L=0.87 W m^-1 K^-1)of pristine In Te is extremely low at room temperature.Firstly,the band gap of In Te is narrowed and the carrier concentration can be effectively increased with the substitution of Cd for In in In Te,leading to the increase of the electrical conductivity.After the doping limit x=0.005,the presence of Cd Te nanoparticles provides extra phonon scattering centers for mid-to-long wavelength phonons,resulting in the reduction of the lattice thermal conductivity.The maximum thermoelectric figure of merit z T of 0.57 at 623 K has been achieved for In_0.98Cd_0.02Te,which is 84%higher than the value of In Te.Secondly,although the carrier concentration is optimized,the electrical conductivity of the In_1-xCd_xTe compounds are limited by the low carrier mobility.The scattering mechanism in pristine In Te sample varies from ionization scattering(In Te-2days)into the mixed scattering(In Te-7days)at the lower temperature range via prolonging the annealing time,leading to the noticeably enhanced carrier mobility.The carrier mobility is 16cm²V^-1 s^-2 for the In_0.98Cd_0.02Te-7days sample,which increases?264%in comparison with that of 4.4 cm²V^-1 s^-2 for the In Te-2days sample.Finally,the maximum z T value of In_0.98Cd_0.02Te-7days sample is 0.87 at 823 K.(2)A series of novel quaternary diamond-like structural materials with intrinsically low lattice thermal conductivity have been predicted through complex structure design.Taking classical ternary Cu In Te₂ as structural prototype,inserting equal number of divalent cations into the 4a and 4b sites,leading to cation disordering.Among them,the new compounds A₂Cu₃In₃Te₈(A=Cd,Zn,Mn,Mg)were successfully synthesized by different experimental processes.Solution and refinement of the crystal structure of Cd₂Cu₃In₃Te₈ obtained from synchrotron radiation X-ray powder diffraction and transmission electron microscopy.The room temperature?_L of the Cd₂Cu₃In₃Te₈ sample is approximately 1.9 W m^-1K^-1,which is 163%lower than its prototype Cu In Te₂.Finally,the maximum z T value,average z T value and engineering z T value of Cd₂Cu₃In₃Te₈ reached 0.90,0.38 and 0.23 respectively,which are 20%,58%and 109%improved compared with Cu In Te₂.(3)A₂Cu₃In₃Te₈ system possess low lattice thermal conductivity but low carrier concentration and mobility.We revealed the existence of in-situ nanodomain with interstitial atoms result from the composition fluctuation in A₂Cu₃In₃Te₈(A=Zn,Cd).Cations enter into the gap to form the in-situ nanodomains,leaving more vacancies and interstitial atoms,enhancing the scattering of high frequency phonons by point defects,thus reducing the lattice thermal conductivity.Based on above findings,the nanodomains can be successfully tailored by further composition fluctuation with the substitution of Cu⁺for A²⁺,resulting in a significantly improved carrier concentration and reduced lattice thermal conductivity.The lattice thermal conductivity?0.23 W m^-1 K^-1 of the Zn_1.6Cu_3.4In₃Te₈ sample at 873 K is comparable to the minimum theoretical thermal conductivity.Finally,the maximum z T value at 873 K increased from 0.6 for Zn₂Cu₃In₃Te₈ and 0.7 for Cd₂Cu₃In₃Te₈ to 1.0 for Zn_1.6Cu_3.4In₃Te₈ and1.2 for Cd1.6Cu3.4In3Te8,respectively,which are beyond the hitherto existing z T values among quaternary diamond-like semiconductors.