Preparation And Properties Of Cu-X-S(X=Fe,Sb) Thermoelectric Compounds

Thermoelectric conversion technology is a new energy conversion technology that uses carriers as the working medium to realize the direct conversion of thermal energy and electrical energy.This technology has great potential in the field of energy recovery of low-density heat sources such as solar energy,industrial waste heat,and vehicle exhaust waste heat.The large-scale application of thermoelectric technology can greatly alleviate the current thorny problems of energy shortage and environmental pollution.However,due to the unfavorable factors such as high price,high toxicity and poor mechanical properties of materials with high thermoelectric conversion efficiency,the application of thermoelectric technology in the field of medium and low temperature waste heat power generation has not yet been commercialized.The Cu-X-S（X=Fe,Sb）thermoelectric compounds have the advantages of high element abundance,adjustable transport properties,low toxicity,easy preparation and good mechanical properties,and are currently one of the candidate material systems in the field of high-performance thermoelectricity.Among them,Cu₃SbS₃ and CuFeS₂ ternary compounds are typical representatives,and these two materials have been widely studied internationally.In addition to the three known definite structures,Cu₃SbS₃ compounds also have another cubic phase structure with a special valence bond.Due to the existence of special structural units such as SbS₃ and Cu S₃（2）,the lattice vibration of this cubic phase structure is strongly anharmonic.At present,studies have shown that sub-group elements Fe,Co,and Ni replace the Sb site,which can stabilize the cubic phase by removing the lone pair of electrons from the Sb site.CuFeS₂ compound has a unique energy band structure and high carrier concentration,but its strong chemical bond and local magnetic moment have a strong inhibitory effect on electron transport.The simple crystal structure and the relatively small average relative atomic mass of the constituent elements result in high thermal conductivity of its intrinsic lattice,which seriously limits the further improvement of its thermoelectric performance.Although researchers have carried out a lot of research on CuFeS₂ compounds,the exploration experiments of Fe site exchange are relatively scarce.In order to investigate the similarities and differences between Fe substitution at Cu sites and Sb sites,(Cu_1-xFe_x)₃SbS₃ series samples were prepared by mechanical alloying combined with hot pressing process.The replacement of Cu sites by Fe can effectively suppress the phase separation and the appearance of Cu_7.2S₄ impurities in the Cu₃SbS₃samples,which is similar to the replacement of Sb sites.The lattice constants of the samples replaced by the Cu position increase gradually with the increase of the substitution amount,which is obviously different from the trend that the lattice constants of the samples replaced by the Sb positions first increase and then decrease.The DSC test results show that during the heating process,the Fe-substituted sample at the Cu site may recombine or adjust a certain structural unit.We speculate that it is a highly anharmonic SbS₃ or Cu S₃（2）structural unit.Due to the strong scattering of phonons from grain boundaries and point defects introduced by Fe-substituted Cu sites,the lattice thermal conductivity of the substituted samples is lower than that of the undoped samples.No increase in thermal conductivity related to the removal of lone pair electrons was found in this experimental sample.This result is similar to ordinary alloying substitution,implying that Fe preferentially occupies four-coordinate Cu（II）sites in Fe-substituted Cu samples.Although Fe entering the lattice may preferentially occupy the Cu（II）site,a small amount of Fe atoms may also occupy the Cu（I）site of the Cu S₃（2）structural unit under high substitution amount,and the changing trend of the resulting anharmonicity is still inconclusive.In order to compare the similarities and differences of the substitution of different groups of elements at the Sb position（such as the Ⅲ main group and the Ⅷ subgroup）,the Cu₃SbS₃ series of compounds in which the Sb position was replaced by the main group element Ga were prepared using binary compounds as starting materials.The undoped Cu₃SbS₃ compound obtained by hot-pressing sintering contains two cubic phase components,which are combined into a single cubic phase by heat treatment,and the Cu₃SbS₃ pure cubic phase bulk sample is obtained for the first time.Compared with the subgroup elements Fe,Co and Ni substituted for Sb sites,the enthalpy change of the samples substituted by Ga is the smallest,which is more conducive to stabilizing the Cu₃SbS₃ cubic phase structure.A small amount of Cu_2-xS impurities appeared in the pure cubic phase structure after long-term storage/annealing at room temperature,and the impurities could not be eliminated by re-annealing/annealing-quenching（HT/HT-QU）.The replacement of Sb sites with Ga reduces the carrier concentration of the material,which is obviously different from that of subgroup elements Fe,Co,and Ni replacement of Sb sites to significantly increase the carrier concentration.The lattice thermal conductivity of the Ga-substituted samples first increases and then decreases with the increase of the substitution amount,which is determined by the two mechanisms of lone pair electrons and alloying,which contradicts the traditional alloying model.The evolution law of the lattice thermal conductivity of the samples in this study is consistent with that of the Ⅷ subgroup element replacement samples,which further confirms the important role of the Sb-site lone pair electrons in the thermal transport properties of the compound.In order to elucidate the effect of replacing Fe sites with elements containing lone pair electrons on the thermoelectric transport properties of CuFeS₂ compounds,Cu Sn_xFe_1-xS₂（x=0～0.50）series samples were prepared in this study.When x=0 and 0.50,pure phase CuFeS₂ and Cu₂Fe Sn S₄ samples can be obtained in this experiment.When the Sn content is 0.05<x<0.50,the sample is in the coexistence state of the above two pure phases.Except for the Cu₂Fe Sn S₄ sample,the thermodynamic properties of the samples are relatively stable below 673 K.The introduction of Sn can significantly increase the nucleation rate of CuFeS₂ samples,suppress the grain growth,and help to enhance the phonon grain boundary scattering.At the same time,Sn substitution can introduce lone pair electrons to increase the anharmonicity of lattice vibration,resulting in a significant decrease in the lattice thermal conductivity of the material,which is lower than the lattice thermal conductivity of In element-substituted Fe site samples without lone pair electrons.The Cu Sn_0.08Fe_0.92S₂ sample reaches a maximum z T value of 0.19 at 673 K,which is twice that of the undoped CuFeS₂ sample.