Microstructure Of Cu₂S Thermoelectric Materials And Its Relationship With Properties

Thermoelectric conversion technology can convert the waste heat dissipation existed in nature and human activities into useful electricity.This technology has a wide range of important applications in the industrial waste heat power generation,automotive waste heat power generation,solar energy utilization,thermoelectric cooling and special energy fields.The natural coupling between S long-range ordering and Cu short-range ordering in Cu₂S ensures electrical transport and greatly reduces thermal conductivity.It is a very promising high-performance thermoelectric material.The stoichiometric Cu₂S compound has been reported to possess two phase structures at room temperatures,namely low-chalcocite?L-chalcocite?and djurleite.The phonon conduction is related to the anharmonicity of the lattice vibration.The more complex the crystal structure is,the greater the degree of anharmonicity of the lattice vibration is,and the larger the scattering of the grating wave is,so the smaller the mean free path of the phonon,the lower the thermal conductivity.However,this still cannot explain that this ceramic material has a low thermal conductivity comparable to that of amorphous glass at room temperature,and it is apparent that there are special microstructures in the material that strongly scatter phonons.Meanwhile,it can be seen that the thermal conductivity continuously and drastically drops to the extreme low values when approaching the first phase transition.Such reduced thermal conduction whose value is largely lower than that of superionic state suggests dramatic structural evolution occurred during heating.Transmission electron microscopy,with its high-resolution image observation and structural analysis capabilities,and in-situ characterization techniques that can perform real-time structural evolution under variable temperature conditions,has an unique advantage in the characteristics of fine microstructure characterization and temperature-induced structural evolution of copper-based thermoelectric materials.Thus in the present study,we performed in a transmission electron microscope?TEM?to investigate the phases,microstructure,phase transition and structural evolution of Cu₂S,and discusses the relationship between the microstructure and properties of the materials.For the study of unusually low thermal conductivity in Cu₂S at room temperature,we start from the room temperature phase structure and microstructure characteristics.Firstly,using the mother-subgroup relationship and group theory correlation principle between high-temperature hexagonal phase space group and two room temperature phases,we calculated two kinds of transformation matrices between room temperature phases and hexagonal phase space.Electron diffraction theory was used to analyze the low-temperature phase structures.The results show that there are many phase structures in the Cu₂S system,and these phases coexist in the system,forming an approximately continuous S-atom sublattice.However,due to multiphase coexistence,rich phase interfaces will be produced.Meanwhile,there are a large number of defect structures,such as dislocation,twinning nanostructures,and moire stripes.This special microstructure is consistent with the idea that the thermal transport properties and the electrical transport properties are coordinated and controlled.The continuous sublattice of the S atom provides a good channel for the transport of charge,while structural fluctuations at different scales of Cu ions strongly scatter phonons,which make the thermoelectric performance of Cu₂S system at room temperature better than that of general semiconductor materials.On the basis of Cu₂S low temperature phase structure characterization,in-depth in-situ TEM investigation by either heating or electron beam irradiation has been performed to examine the structural evolution of high performance thermoelectric Cu₂S with temperatures.Upon raising the temperature,the L-chalcocite structure could be transformed to the djurleite structure suggesting a disorder-order process of Cu vacancies.Then,the djurleite phase may either change the structural orientation or inversely transform back to the L-chalcocite phase,which could be associated to the slow heat transportin this thermoelectric material with ultra-low thermal conductivity.Sometimes,an intermediate phase would be formed during heating.Thus,there exist varieties of ordered configurations for Cu atoms in Cu₂S which are extremely sensitive to the temperature.The multiple phase transitions and drastic structural oscillation along with formation and movement of phase boundaries must have enhanced the phonon scattering,hence should be responsible for the observed sharp drop of lattice thermal conductivity at elevating temperature.