Enhancement Of The Thermoelectric Properties Of Cu₂Se Liquid-Like Materials By Alloying Ag₂S And Compositing Ti₃C₂T_X

Developing new energy materials is an important way to alleviate the current energy crisis.Thermoelectric materials can realize the direct conversion of heat and electricity,which is a new energy material with great application potential.Cu₂Se has attracted extensive attention from thermoelectric researchers as a phonon-liquid electron-crystal（PLEC）material.In the medium-and high-temperature range,Cu ions in the material show liquid-like transport characteristics with high electrical transport performance,and disordered free Cu ions can lead to strong phonon scattering,thus low thermal conducitivity.Therefore,Cu₂Se based materials are known as the most promising thermoelectric materials.Although Cu₂Se has a low thermal conductivity(about 0.7 Wm^-1K^-1),there is still a certain gap compared with other high-performance thermoelectric materials SnSe,AgCuTe,BiCuSeO(0.4-0.0.5 Wm^-1K^-1).It has been proven that strategies such as elemental doping,optimizing carrier concentration,and building nanostructures can effectively improve the thermoelectric properties of Cu₂Se-based materials.In this paper,alloying Ag₂S and compositing Ti₃C₂T_x are used to realize a variety of optimization strategies for Cu₂Se,resulting in a substantial reduction in thermal conductivity and good thermoelectric performance.The specific research works are as follows:1.The inluence of thermoelectric properties of Cu₂Se by alloying Ag₂S:In this thesis,the alloying of Ag₂S powder with Cu₂Se matrix was achieved by high-energy ball milling and spark plasma sintering.The diffraction peak of AgCuSe was found in the X-ray diffraction（XRD）of the alloyed samples,indicating that the Ag and S elements diffused during the preparation and reacted with the Cu₂Se matrix.Synchrotron powder diffraction（SPD）and thermal absorption peak test showed that the phase transition temperature of Cu₂Se shifted to low temperature with the alloying concentration,which proved that some Ag and S entered the lattice position.In addition,the aggregation of Ag and S elements can be clearly observed in the energy dispersive spectrometer（EDS）element mapping.The AgCuSe nanosecondary phase can produce strong scattering of low-and mid-frequency phonons,resulting in a significant decrease in the thermal conductivity of the samples.The thermal conductivity of the 2%Ag₂S alloyed sample at 820 K is reduced to 0.46 Wm^-1K^-1,which is 23%lower than that of the unalloyed sample.In addition,the diffusion and substitution of S and Ag can effectively inhibit the formation of Cu vacancies and suppress the excessive carrier concentration in the unalloyed sample.The electrical conductivity of the 2%Ag₂S alloyed sample decreas to 1.0×10²⁰ cm^-3.The decrease of the thermal conductivity and the optimization of the carrier concentration result in a ZT value of 1.5 at 820 K for the Ag₂S alloying concentration of 2%,which is 50%higher than the ZT value of 1.0 for the unalloyed sample.2.The optimization of the thermoelectric properties of Cu₂Se by compositing Ti₃C₂T_x:It has been studied that there is a large mismatch in the phonon density of states between C and Cu₂Se,and the recombination of low-dimensional carbon-based materials in Cu₂Se can play a significant role in optimizing the thermal conductivity.In this thesis,an innovative method is proposed to combine the 2D MXene material Ti₃C₂T_x with Cu₂Se.The Ti₃C₂T_x nanosheets can be found that attached to the surface and interlayer positions of the Cu₂Se matrix in the scanning electron microscope（SEM）.A large number of Cu₂Se/Ti₃C₂T_x hetero-interfaces were generated in the Cu₂Se matrix as can be seen in the transmission electron microscope（TEM）images.The high density of dislocations and nanosheet stacking can be clearly observed by high-resolution transmission electron microscopy（HRTEM）.These different scales of defects and impurity phases can enhance the scattering of phonons at different frequencies,resulting in a large reduction in the lattice thermal conductivity over the whole range of testing temperatures.The total thermal conductivity of the Cu₂Se/0.4 wt%Ti₃C₂T_x sample at 795 K is 0.27 Wm^-1K^-1,a decrease of 55%compared to 0.6 Wm^-1K^-1 of the uncomposited sample.In this work,the thermal conductivity has been optimized more substantially in the medium-and high-temperature range compared with other works of composite carbon nanotubes,carbon nanodots,graphene,etc.In addition,the Hall test results at room temperature show that there is a certain energy filtering effect when the Ti₃C₂T_x composite concentration is small.However,when the composite concentration is 0.4 wt%,Ti₃C₂T_x can be used as a carrier reservoir to provide a carrier transfer channel for Cu⁺,resulting in an increase in the carrier concentration.Finally,although the electrical properties show slight decrease,the low thermal conductivity leads to a maximum ZT value of 2.11 at 795 K for the Cu₂Se/0.4wt%Ti₃C₂T_x sample,which is 109%higher than that of the uncomposited sample.