Study On Thermoelectric Transport Properties Of Tetrahedrite And Chalcopyrite Based Compounds

Thermoelectric conversion technology is capable of converting heat into electricity in solid state,which provides an effective method for relieving the problems caused by energy crisis and environment pollution.As thermoelectric phenomenon is closely related to both electrical and thermal transport,it is also an important platform to study basic problems in solid state physics.Tetrahedrite and chalcopyrite are two types of natural minerals and thermoelectric compounds,and they are of great interest worldwide due to the features of environment friendly and earth abundant.In this study,we focus on the tetrahedrite and chalcopyrite based compounds,which are synthesized via traditional solid state method.The intrinsic characteristics and inherent problems of these compounds are investigated,among which physical transport mechanisms especially those abnormal phenomenona are highlighted.Our results can lay the theoretical foundation for further boosting the thermoelectric performance of tetrahedrite and chalcopyrite.The derails are summarized briefly as follows:（1）The electrical and thermal transport properties of tetrahedrites are balanced by synergistic integration of exsolution process and band engineering,which significantly promotes the thermoelectric performance.Exsolution process usually ocurrs in the mineralogy,which describes the occurrence of phase separation when solubility limit is exceeded.By adding excess Cu in tetrahedrite Cu₁₂Sb₄S₁₃,which exhibits intrinsically low lattice thermal conductivity,exsolution process can be induced.Uniquely,the exsolution process in tetrahedrite enables the coexistence of two phases with close compositions and isostructures,in which the lattice contants show little difference.The results reveal that the intrinsically low thermal conductivity of tetrahedrites can be further reduced by 59%to 0.25 W m^-1 K^-1 with the aid of exsolution process,as compared to pristine Cu₁₂Sb₄S₁₃.Meanwhile,the band engineering strategy is employed to increase the band degeneracy by forming solid solutions with Se.As a result,a substantial power factor enhancement is attained.By properly balancing electrical and thermal transport properties,a maximum zT of 1.1 at 723 K for Cu_13.5Sb₄S₁₂Se is achieved.The method could shed light on the optimization strategy of electrical and thermal transport properties for other mineral based compounds with intrinsically low thermal conductivity.（2）With the help of transport coefficient theory based on single parabolic band（SPB）model,the electrical transport mechanism is fully comprehended,despite of the difficulty in obtaing the accurate carrier concentration.It is concluded firstly that the dominant scattering mechanism of carriers in tetrahedrite based compound is acoustic phonon scattering by analyzing the experimental data with a variety of substituting site,doping element and temperature.Meanwhile,it is found that the transport coefficient of tetrahedrties increases with temperature,leading to the conclusion that the conducting mechanism in tetrahedrite is a mixture between Hopping and band conducting at room temperature and finally evolves to fully band conducting at high temperature.Moreover,the experimental data indicate that Sn doping on Sb sites is an effective method to boost the electrical transport coefficient and thermoelectric performance of Cu₁₂Sb₄S₁₃3 as well as Cu_13.5Sb₄S₁₃.（3）The lattice thermal conductivity is greatly reduced and thermoelectric performance is greatly enhanced via rational defect engineering.Cation vacancies are successfully generated in CuInTe₂ system by forming solid solutions with differnent content of In₂Te₃ with a smaller cation-to-anion ratio,which can introduce the maximized mass and strain fluctuations.It is found that cation vacancies can intensively scatter phonon with high frequency,and significantly decrease the lattice thermal conductivity.At room temperature,the lattice thermal conductivity is reduced to 1.97 W m^-1 K^-1 from 6.67 W m^-1 K^-11 when the content of In₂Te₃ reaches 0.15.Based on the Debye-Callaway-Klemens model,the variation tendency of lattice thermal conductivity with respect to temperature and In₂Te₃ content is investigated,and the result further confirms the contribution of vacancies to the reduction of lattice thermal conductivity.In addition to alloying with In₂Te₃,Ag/Cu substitutional point defect is further created to impede the phonon transport.As a result,the attained lowest lattice thermal conductivity is 1.43 W m^-1 K^-1 for(Cu_0.85Ag_0.15InTe₂)_0.88（In₂Te₃）_0.12 sample at room temperature,a 79%reduction as compared to the pristine CuInTe₂.Combining the optimization of carrier concentration and the reduction of thermal conductivity,a peak zT value of 1.1 is obtained at 840 K,which achieves a 94% enhancement compared to the pristine CuInTe₂.Furthermore,the average zT is effectively enhanced up to¹92%,which is superior to the reported values for other CuInTe₂ based single phase compounds.（4）Our primary experimental data indicate that lattice thermal conductivity of compounds I-III-VI₂（I=Cu;III=Ga,In,Al;VI=Te,Se）exhibits abnormal behavior.Namely,the lattice thermal conductivity increases with melting temperature which apparently does not agree with the classical conclusiton in solid state physics textbook.By excluding other possible factors,it is concluded that by U process in other words lattice anharmonicity is the reason for abnormal behavior assisted by theoretical calculation.However,it is found that such anharmonicity is not the tranditional macroscopical anharmonicity but the local anharmonicity,which barely has influence on the rigid framework of these compounds.This work makes useful modification for the existing theoretical model for lattice thermal conductivity as well as melting point and elucidates the influential impact of local anharmonicity on lattice thermal conductivity.More importantly,this study indicates that it is possible for a material to have both high melting point and low lattice thermal conductivity,which shed light on new anvenue for development of novel thermoelectric materials and other functional materials for thermal management.