Microstructural Characteristics Of Cu₂X(X=S,Se) And Their Correlation To The Thermoelectric Properties

With the development of human society and economy,the energy crisis and environmental problems are increasingly serious.It is imperative to find pollution-free sustainable sources and energy conversion technologies.Thermoelectric materials can realize the direct conversion between heat and electricity,showing promising potentials for applications in the waste heat power generation and solid-state refrigeration.However,the low energy conversion efficiency of available thermoelectric devices compared to the conventional heat engines constrains their application in large scales.Cu₂X(X=S,Se)binary compounds and their solid solutions exhibit ultralow lattice thermal conductivity and moderate-or narrow-gap owing to their complex crystal structures,microstructures and electronic structures,which lead to excellent thermoelectric properties with the figure-of-merit z T value up to 2.0 or above.However,the extreme complexity of both crystal and electronic structures makes it hard to comprehend the structure-property correlations in Cu₂X,which thus hinders the consequent material design aiming to improve the thermoelectric performance.In this dissertion,in-depth structural characterization was carried out on the Cu₂X(X=S,Se)binary compound and solid solution materials by mainly using transmission electron microscopy.The chemical composition,solid solution structures and electronic structures in nanometer and atomic scales along with in-situ structural evolution at increasing temperature have been analyzed.On the one hand,the effects of solid solution of S and Se were investigated on the regulation of structure,microstructure and thermoelectric properties.On the other hand,both common and diverse microstructural features were comprehensively analyzed among Cu₂X(X=S,Se)binary materials and their solid solutions to which the properties and stability were correlated.Among the Cu₂S_xSe_1-x solid solutions,Cu₂S_0.5Se_0.5equimolar solid solution with the highest entropy and Cu₂S_0.08Se_0.92 solid solution with the best thermoelectric performance were selected as the representative samples for the thorough structural and microstructural analysis.The atom-resolved aberration corrected scanning transmission microscopy revealed the unique intergrowth structures of hexagonal and cubic phases at nano-and even unit cell-scales in the Cu₂S_0.5Se_0.5 solid solution.High-density phase interfaces were formed upon high-frequency oscillations of different structures in the material.The relaxation structure model of the phase interfaces has been established.In addition,twins were observed in the cubic phase,which also illustrate platelets at several unit cell-scales.Subsequently,EDS compositional analysis at atomic scales has been performed in these characteristic microstructures.It is found that the specific intergrowth is correlated to the concentration undulation of Se-S solution elements at the nano-and unit cell-scales.The atomic ratio of Se/S is always higher than 1 in the hexagonal phase and lower than 1 in the cubic phase,suggestive of high sensitivity of structures to the composition.The unique dual-phase intergrowth and twin structures lead to the formation of high-density of interfaces,which combine with the chemical disorder or increased entropy and compositional oscillation,eventually to strongly scatter phonons at different wavelengths and lower the lattice thermal conductivity.The lattice thermal conductivity of the Cu₂S_0.5Se_0.5 solid solution(K_L=0.22 Wm^-1K^-1)is the lowest in the Cu₂X(X=S,Se,Te)system.However,the trapping states at the interfaces may have negative impacts on the electrical transport properties,resulting in poor electrical performance.Improvement of electrical property is expected to be achieved by,e.g.introducing Cu vacancies,which deserves further research in the future.After resolving the room-temperature phase structures,in-situ TEM analysis was performed to explore the structural evolution in Cu₂S_0.5Se_0.5 samples upon changing the temperature.Anomalous phase transition has been observed,which is consistent to the in-situ XRD measurement,i.e.,the continuous transformation from hexagonal to cubic structure without a critical transition temperature.In-situ TEM indicated that the abnormal structural transition was due to the variation in configurations of intergrowth in local micro-regions in the material,which led to diversity in local stress field,hence the consecutive changes in structural transition temperatures among different micro-regions.Along with the phase transition,the phase interfaces gradually shortened and disappeared,which corresponds to the abnormal phenomena with respect to the macroscopic thermoelectric properties at increasing temperatures,i.e.,unexpectedly,thermal conductivity increased and electrical conductivity decreased.Thus,this further verifies that the high-density phase interfaces can greatly enhance phonon scattering but hinder electrical transport.The dual-phase intergrowth was not observed in Cu₂S_0.08Se_0.92 solid solution.However,abundant microstructures were formed in the materials,including the specific mosaic structures,i.e.,the grains are single crystal-like but consisting of nano-subgrains with tiny orientation differences.In addition,nanocrystals,multiscale twin structures and different superperiodic structures were observed in Cu₂S_0.08Se_0.92 solid solution.These microstructural features also generated abundant interfaces in the material,which can effectively scatter phonons.As a result,the lattice thermal conductivity of Cu₂S_0.08Se_0.92 solid solution(K_L=0.26 Wm^-1K^-1)was reduced by 52%compared to that of Cu₂Se(K_L=0.55 Wm^-1K^-1).Although the lattice thermal conductivity of Cu₂S_0.08Se_0.92is not the lowest among the Cu₂X solid solutions,it possesses the highest thermoelectric figure of merit(1000K,z T=2.0)among the Cu₂S_xSe_1-x solid solutions,which could be ascribed to its excellent electrical properties resulted from nearly optimum carrier concentration.Therefore,the design of the solid solution thermoelectric materials requires global consideration of both thermal and electrical properties,which should be balanced and optimized to reach as high as possible thermoelectric performance.An interesting and common structural feature has been observed in Cu₂X(X=S,Se)binary and solid solutions,i.e.high-density of lamellar defects with similar atomic structure in all of Cu₂S,Cu₂S_0.5Se_0.5,Cu₂S_0.08Se_0.92 and Cu₂Se.Structurally,the lamellar defects locate on the close packed planes of S/Se anion sublattice and contain chalcogen double layers but lack of Cu element by two atomic layers compared to the Cu₂Se fcc structure.EELS examinations indicate that the copper element in the defects unexpectedly exhibits characteristic of an atom hence weakened bonding with sulfur/selenium element,while comparatively strong Se(S)-Se(S)paired interactions are established in chalcogen double layers.Formation of these specific lamellar defects and complex oxidation states of elements there involved should have resulted from the inherent nature of Cu-S/Se bonding by taking account of small difference in electronegativity between the cations and anions.The atomic Cu in the lamellar defects is considered to be closely correlated to the copper precipitation phenomenon under external electric fields in Cu₂X(X=S,Se)materials.Thus,the as-revealed structural features provides an important perspective for the study of the copper precipitation problem along with structure-property relationships in Cu₂X(X=S,Se)materials.