Preparation And Performance Optimization Of Liquid-like Ag-based Thermoelectric Materials

At present,fossil energy such as oil,coal and natural gas are increasingly exhausted,and the burning of fossil energy has brought serious environmental pollution and energy waste,which has been perplexing people.The development of thermoelectric materials provides a new method to solve environmental pollution and energy waste.Thermoelectric materials can directly convert waste heat into electric energy,which has attracted more and more attention.On the one hand,thermoelectric materials can be prepared into wearable flexible thermoelectric devices,which work in the temperature range of 300-500 K;On the other hand,thermoelectric materials can be prepared into thermoelectric generators,which work in the temperature range of 500-700K and make effective use of the waste heat produced in the industrial and automotive fields.Therefore,compared with high temperature thermoelectric materials,the development of medium and low temperature thermoelectric materials has a wider application market.Among all kinds of thermoelectric materials,liquid-like thermoelectric materials are of great interest to researchers because of their inherent low lattice thermal conductivity,adjustable electrical properties and good thermoelectric properties.However,the thermoelectric properties of medium and low temperature liquid materials are still much lower than those of high temperature liquid materials.Moreover,most of the liquid materials in the medium and low temperature regions are Ag-based thermoelectric materials,so it is necessary to further optimize the thermoelectric properties of Ag-based materials.Therefore,this paper takes low-temperature Ag₂S and medium-temperature Ag Cu Te as the research object,deeply analyzes their thermoelectric properties,thermal stability and mechanical properties,and improves their properties in many aspects by means of doping,alloying and composite carbides.The specific research works are as follows:1.Synergistic optimization of thermal stability and thermoelectric properties of Ag₂S by alloying engineering.The phase transition temperature of Ag₂S can be changed by alloying different elements and compounds to avoid phase transition in the application temperature range and further improve the thermal stability of Ag₂S.The results show that only the endothermic peak of Cu-Se co-alloyed Ag₂S sample disappears,which indicates that Cu-Se co-alloying successfully restrains the phase transition of Ag₂S.Because the phase transition is suppressed,there is no sudden change in the properties of Ag_1.8Cu_0.2S_0.5Se_0.5samples in the temperature range of 300-500 K.In addition,positron annihilation experiments show that the Ag/Cu vacancy concentration in Ag_1.8Cu_0.2S_0.5Se_0.5 samples increases,and the carrier concentration increases from 10¹⁴ cm^-3 to 10¹⁹ cm^-3,which greatly improves its electrical properties.The secondary phase of Cu and a large number of free Ag/Cu ions in the sample increase the scattering of phonons,which greatly reduces the lattice thermal conductivity of Ag_1.8Cu_0.2S_0.5Se_0.5 in the whole temperature range.Finally,the Cu-Se co-alloying project improved the value of ZT for Ag₂S from 0.03 to 0.27,and significantly optimized its thermal stability.2.Effect of construction of multi-layer nanostructures on thermoelectric properties of Ag Cu Te.By doping Cu₂Se into Ag Cu Te matrix to construct multi-layer nanostructures,the scattering efficiency of phonons is maximized and the lattice thermal conductivity is greatly reduced.The results of high resolution transmission electron microscopy showed that additional phase boundaries,multiple moire fringes and regular nano-phases are formed in the samples doped with Cu₂Se.The lattice interference between Cu₂Se and Ag Cu Te produces moire fringes in the matrix,which leads to high density dislocations and strong stress field.These defects can strongly scatter medium and high frequency phonons.In addition,the nano-Cu₂Se phases of different sizes are uniformly distributed in the matrix,which also has a strong scattering effect on medium and high frequency phonons.The dissolution of a small amount of Cu₂Se at the temperature of 600-800K makes Se enter the Te site,which enhances the structural stability of Ag Cu Te.On the other hand,the dissolved Cu⁺ions participate in electrical transport,which optimizes the electrical properties.Finally,the thermal conductivity of Ag Cu Te-1%Cu₂Se samples reaches the lowest value of 0.45 Wm^-1K^-1 at 723 K,which is much lower than the average thermal conductivity of published Ag Cu Te-based thermoelectric materials.At the same time,the average ZT of Ag Cu Te-1%Cu₂Se samples is 1.13 between 523-723 K.This work provides a new strategy for further reducing the thermal conductivity of Ag Cu Te-based thermoelectric materials.3.Synergistic optimization of mechanical and thermoelectric properties of Ag Cu Te with composite nano-Si C.A series of Ag Cu Te/Si C composite thermoelectric materials were prepared by solid state reaction at high temperature.The test results of the properties of the samples showed that the mechanical strength and thermoelectric properties of Ag Cu Te were significantly improved after composite Si C.Nano-Si C not only refines the grains of Ag Cu Te,but also promotes the non-uniform nucleation of Ag₂Te secondary phase,which results in large-angle grain boundary defects in the matrix.High resolution transmission electron microscopy shows that nano-Si C is uniformly arranged on the grain boundary,and it is fastened tightly between two different grain boundaries like buttons on clothes.The button action of nano-Si C enhances the force between grain boundaries and significantly improves the mechanical strength of Ag Cu Te.In addition,nano-Si C produces a large number of edge dislocations and strong stress field on Ag Cu Te matrix.The strong stress field will increase the defect distortion energy of the sample,which provides enough energy for the Ag/Cu atoms in Ag Cu Te to escape from the lattice position,resulting in an increase in the number of Ag/Cu vacancies and a great improvement in electrical properties.On the other hand,there are a large number of edge dislocations,stress field changes and Ag/Cu vacancy defects in the matrix,which can significantly inhibit the transmission of thermal phonons and greatly reduce the lattice thermal conductivity of Ag Cu Te.Finally,due to the button action of Si C and grain refinement,the Vickers strength of Ag Cu Te/0.2%Si C sample increased by～20%.The ZT value of Ag Cu Te/0.2%Si C sample increased by～40%,and reached 1.32at 723K.