Investigation On The Structure And Properties Of M?M=Cu,Ag?₂X?X=S,Se,Te? Thermoelectric Compounds

The thermoelectric technology can realize the mutual conversion between thermal energy and electric energy,possessing the advantages of no transmission parts and high reliability.Especially,it plays an irreplaceable role in micro-refrigerators and space power sources.In order to improve the energy conversion efficiency of thermoelectric devices,it is necessary to develop high-performance thermoelectric materials.With the characteristics of‘phonon-liquid,electron-crystal',liquid-like materials has become an important category of high performance thermoelectric materials due to its low lattice thermal conductivity and adjustable electrical properties.The copper chalcogenide Cu₂X?X=S,Se,Te?is a kind of typical liquid-like thermoelectric materials.In the post decade,the research on Cu₂X thermoelectrics has mainly focused on the optimization of preparation process,nano-engineering and anion-site solid solution for tuning thermoelectric transport.However,there is a lack of research on doping or solid solution in the cationic site.In this paper,M₂X?M=Cu,Ag;X=S,Se,Te?-based compounds were studied,and their crystal structure,phase composition and thermoelectric transport properties were characterized.Particularly,the emphasis has been put on the discussion on the screening of doping cations and the influence of cation doping on the thermoelectric properties of Cu₂Se and Cu₂S.New quaternary compounds Cu_2-2xAg_2xSe_1-xS_x were discorvered by dual-site solid solution and their thermoelectric performance have been optimized.1.Li element was screened out according to the difference of ionic radius,and Cu_2-xLi_xSe and Cu_2-xLi_xS compounds were prepared by solid reaction process.The doping limit was less than 2.5 at%.However,Li-doping has a significant effect on the thermoelectric properties even if in a small amount.Li-doping leads to an increase in the thermal conductivity of cubic phase of copper chalcogenide,and simultaneously optimizes the carrier concentration effectively without changing the band structure.The thermoelectric figure of merit zT for Cu_1.99Li_0.01S reached 0.84 at 900 K,which is 133%higher than that of Cu₂S matrix.2.The Cu and Se sites in Cu₂Se were simultaneously substituted by Ag and S,respectively.When x is less than 0.2,all Cu_2-2xAg_2xSe_1-xS_x samples at room temperature consists of two phases.These two phases take the structures of Cu₂Se and CuAgSe?room temperature phase for both?,respectively,while the composition is different from that of Cu₂Se and CuAgSe.As x value increases,the ratio of the two phases changes irregularly accompanied with changing the composition of the two phases,while their structural symmetries keep unchanged.The anion sites of the two phases in all the samples have the same composition,while the cationic components changes but without obvious regularity.At high temperature all samples transfered to the cubic phase,as the same structure with copper selenide.3.The thermoelectric properties of Cu_2-2xAg_2xSe_1-xS_x?x=0.01⁰.2?samples were characterized.The lattice thermal conductivity of the solid solution is greatly reduced as compared with the matrix,because of the enhanced phonon scattering by S and Ag subsititution in the Se and Cu sites,respectively.The mass field fluctuations,as well as the stress field fluctuations are considered contributing to the enhanced phonon scattering.The single parabolic band model?SPB?calculations shows that the Ag and S substitution bring little changes of band structure.The thermoelectric figure of merit zT for Cu_1.8Ag_0.2Se_0.9S_0.1 at 900 K reached 1.6,which is 33%higher than that of the copper selenide matrix.4.Two new compounds,Cu_1.4Ag_0.6Se_0.7S_0.3 and CuAgSe_0.5S_0.5,were synthesized by melt-annealing-SPS process.The obtained compounds are single phase with uniform distribution of constituent elements in the whole scale from nanometer to micrometer.Both of them are stable in different heat treatment processes.The Cu_1.4Ag_0.6Se_0.7S_0.3takes the crystal structure of orthorhombic as CuAgSe at room temperature,and CuAgSe_0.5S_0.5 takes the same structural symmetry as Ag₂Se at room temperature.The thermal conductivity of the two compounds is extremely low as compared with those of most reported copper chalcogenides and their solid solutions.However,because of the low electrical conductivity,the thermoelectric performance of the obtained two new compounds is not efficiently good with the maximum zT value of about 0.25.5.Introducing a small amount of cation vacancies in Cu_1.4Ag_0.6Se_0.7S_0.3 does not affect the crystal structure of the compound,while it can effectively optimize the electrical properties and reduce the lattice thermal conductivity.The electrical conductivity of copper-deficient Cu_1.39Ag_0.6Se_0.7S_0.3 and silver-deficient Cu_1.4Ag_0.59Se_0.7S_0.3 samples are two orders of magnitude higher than that of the matrix,and the power factor is up to 35 times than that of the matrix.Benefited from the significantly improved conductivity and very low thermal conductivity,the thermoelectric figure of merit zT for Cu_1.39Ag_0.6Se_0.7S_0.3 was enhanced to 1.7 at 750 K,which is comparable to the best of liquid-like thermoelectric materials.6.Inspired by the research work of Cu_2-2xAg_2xSe_1-xS_x compounds,Cu_2-2xAg_2xSe_1-yTe_y compounds were synthesized by solid reaction process,and its phase composition and thermoelectric properties were investigated.Three new quaternary compounds were discovered.Especially,the zT value of the n-type thermoelectric compound Cu_0.6Ag_1.4Se_0.7Te_0.3 reaches 0.7 as the maximum and larger than 0.6 in a wide temperature range from 500 K to 800 K.This new compound would fill a gap in n-type Cu-based liquid-like thermoelectric materials,and accelerate the development of Cu-based liquid-like thermoelectric device technology.