The Study On Thermoelectric Performance Optimization Of CuFeS₂ Based Materials By Doping And Interface Controlling

In the past few decades,increased concerns about the environmental degradation and the rising energy cost have sparked a vigorous research activity aiming to identify alternative energy sources and develop novel energy materials.One of the most exciting clean energy conversion technologies is thermoelectricity that can,via the Seebeck effect,be used to harvest waste industrial heat and convert it by purely solid-state means into electricity with no moving parts involvedCurrently,there are several families of excellent thermoelectric materials available,such as Bi₂Te₃,PbTe^[1-3],CoSb₃,GeTe and SiGe.Although they all possess high thermoelectric performance,most of them contain toxic and rather very expensive elements.Moreover,synthesis processes used to make them consume a lot of energy and are time intensive.The above limitations constrain their large scale industrial applications.Thus,it is important to develop efficient,inexpensive and environmentally friendly thermoelectric materials that can be synthesized by simple,rapid and low cost routes.In the past few years,the diamond-like CuFeS₂ compound,consisting of earth-abundant,low-cost and non-toxic elements,and possess a high Seebeck coefficient has been considered as a promising thermoelectric material for intermediate temperature applications.To improve its thermoelectric performance,most of the research efforts focus on adjusting the carrier concentration by introducing element doping,controlling the chemical composition and introducing sulphur defects.However,now the thermoelectric performance of the CuFeS₂-based materials are still too low and there are some key issues about its transport property need further research.Revealing and understanding the intrinsic property of the CuFeS₂ compound and improving its thermoelectric performance are crucial to the study of CuFeS₂-based materials.Now there are some key issues for the CuFeS₂ compound:（1）The intrinsic carrier mobility is too low and the influence of Fe on the mobility is still unknown;（2）The carrier concentration need to be optimized;（3）The influence of chemical bonds and crystal structure on the thermal transport need to be studied;（4）Now the synthesis processes of CuFeS₂ used to consume a lot of energy and are time intensive.So it is imperative to developed a rapid,facile and low cost synthesis route for the CuFeS₂compound.According to these issues,in this study,focusing on the CuFeS₂-based materials.The influence of different dopants on the carrier concentration and mobility have been investigated,and the correlation between Fe content and mobility has been revealed;The formation mechanism of the ZnS in the Cu1-xZnxFeS2 compound has been studied and the effect of these nano-precipitate to the lattice thermal conductivity has been investigated;A novel thermoelectric material Cu_17.6Fe_17.6S₃₂ has been discovered and the connection between weak chemical bonding,the lattice dynamics and the intrinsically low lattice thermal conductivity for Cu_17.6Fe_17.6S₃₂ compound has been revealed;We develop a rapid,facile and low cost synthesis route that combines thermal explosion with plasma activated sintering（PAS）and used it to prepare the CuFeS₂-based materials.The main results of this study are summarized as follow:（1）The study of the carrier mobility.In this work,a series of CuFe_1-xIn_xS₂ and Cu_1-xFe_1+xS₂ compounds were synthesized by vacuum melting combined with the plasma activated sintering（PAS）process,and the effect of In substitutions and Fe doping on the carrier mobility and thermoelectric properties were investigated.The results show that:Although the presence of In introduces alloy scattering for charge carriers and decreases their carrier mobility and electrical conductivity,the reduction of the thermal conductivity in conjunction with the likely weakened magnetic scattering is more dominant and lead to the enhancement of thermoelectric performance.Moreover,the Fe doping can optimize the carrier concentration and improve the thermoelectric performance.However,when the matrix at the same alloying content,the higher Fe content,the lower mobility has been obtained.To the CuFeS₂-based materials,choosing some nonmagnetic dopant to substitute the Cu sublattice will be a effect way to optimize the transport property.（2）Optimized the carrier concentration and reducing the lattice thermal conductivity.In this work,a series of nominal compounds of the form Cu1-xZnxFeS2（x=0⁰.1）were synthesized by vacuum melting combined with the plasma activated sintering（PAS）process and thorough structural and transport studies were made to ascertain the role of Zn in the structure.We found that the solubility of Zn in CuFeS₂ is low,and when the solubility limit is exceeded,Zn does two things:it enters the sites of Cu and it forms in-situ ZnS which,in turn,leads to the formation of anti-site defects of Fe/Cu.Such anti-site defects relieve the lattice strain of the matrix and this enhances the solubility of Zn further.And the co-doping of Zn and Fe is effect to enhance the electrical transport properties for CuFeS₂.The presence of the ZnS nanophase has a strong influence on the lattice thermal conductivity which is treated using a model based on the Effective Medium Approximation（EMA）.The calculation shows that the radius of the ZnS nanoparticles has a crucial effect on the value of the lattice thermal conductivity of the composite CuFeS₂/ZnS structure.Only when the radius of the ZnS is less than 31 nm,the lattice thermal conductivity can be reduced due to the enhanced boundary scattering of phonons on ZnS nanoparticles.The thermoelectric performance of the CuFeS2 compound can be improved by optimizing the amount of Zn dopants and modifying the morphology of the ZnS second phase.（3）The influence of chemical bonds and lattice dynamics on the lattice thermal conductivity.In this work,CuFeS₂ and Cu_17.6Fe_17.6S₃₂ compounds,which possess the same constituent elements and similar crystal structures,have been taken as contrasting examples of the influence of chemical bonds on the thermal transport.At room temperature,the lattice thermal conductivity of Cu_17.6Fe_17.6S₃₂ of 0.6 Wm^-1K^-11 is about one-sixth of the value of the lattice thermal conductivity of CuFeS₂,and approaches the amorphous limitκmin at 625 K.Due to the difference between the anion-cation ratio,the Cu_17.6Fe_17.6S₃₂ compound has a more complex atomic coordination structure,in which Cu and Fe are randomly configured on the cation sublattice,and the bond length between anions and cations is longer,and the bond is therefore weaker,than that in CuFeS₂.Moreover,in the Cu_17.6Fe_17.6S₃₂ compound,some Wyckoff positions are only partially occupied by Cu and Fe,resulting in a high concentration of vacancies on the cation sublattice.Hence,compared to the CuFeS₂compound,the unique crystal structure and atomic coordination in the Cu_17.6Fe_17.6S₃₂compound introduces weak localized chemical bonds,and they,in turn,enhance the anharmonicity of the lattice vibrations and reduce the sound velocity.Consequently,an extremely low lattice thermal conductivity results.（4）Employing the thermal explosion technique to synthesize the CuFeS₂/Cu_17.6Fe_17.6S₃₂(Cu_1.1Fe_1.1S₂)composite.A single phase CuFeS2 compound was synthesized within 60 s using the thermal explosion process and its ZT value turned out to be superior to other traditional processing methods of CuFeS₂.In addition,the DSC results revealed that the phase transformation of CuFeS₂ during the thermal explosion synthesis proceeded in two steps:（1）Cu+S→CuS followed by（2）CuS+Fe+S→CuFeS₂.Moreover a series of composites contain different CuFeS₂ and Cu_1.1Fe_1.1S₂ can be synthesized by controlling the S content.The result show that,the presence of both phases gives rise to a phase boundary which,in turn,has a significant impact on the transport properties:phase boundaries scatter low frequency phonons and reduce the thermal conductivity of a composite structure.Moreover,large differences in the carrier concentration in CuFeS₂ and Cu_1.1Fe_1.1S₂ drive a redistribution of electrons in the composite and lead to an enhancement in the electronic transport properties.This demonstrated the critical role the phase boundary plays in improving the thermoelectric properties of the composite structure.