The Electrothermal Transport Properties Of Copper-and Silver-based Ⅵ Binary Compounds At High Pressure And High Temperature

Thermoelectric material is a kind of green energy material.Improving the thermoelectric performance and finding the new thermoelectric material system are still hot topics in current research.High pressure can realize the lattice shrinking,and then adjust the band structure,so as to effectively regulate the thermoelectric properties,which makes in situ thermoelectric property measurement technology at high pressure attracted much attention.In the existing in situ thermoelectric properties measurement technology at high pressure,the in situ thermoelectric properties measurement at temperature and pressure is still limited by technology.It is of great significance to study the regulation and influence mechanism of the synergistic effect of temperature and pressure on thermoelectric properties to find high performance thermoelectric materials.Therefore,it is extremely critical and challenging to achieve in situ the measurement of electrical/thermal transport signals under extreme conditions of high temperature and high pressure,and to break through the in situ measurement technology of thermoelectric properties at high temperature and high pressure.Copper/silver groupⅥbinary compound M（M=Cu/Ag）₂X（X=S,Se,Te）is known as phonon-liquid electron-crystal thermoelectrics,which has excellent thermoelectric properties.Cu₂Se and Ag₂S have multiple structural phase transitions under high temperature or high pressure.Cu₂Se and Ag₂S change from semiconductor to"fast ion conductor"after the temperature-induced phase transition,and the conduction mechanism changes from hole/electron conduction to hole/electron and ion conduction.At present,most of the research in situ thermoelectric properties at high pressure focuses on conventional semiconductor materials（hole/electron conduction）,and there is still a lack of the research in situ ionic conduction mechanism at high pressure.Breakthrough in situ thermoelectric property measurement technology at high temperature and high pressure,which is expected to explore the regulation mechanism of high pressure on the ion conduction of high temperature phase Cu₂Se and Ag₂S,and to obtain the essential difference of high pressure modulation between carrier conduction and ion conduction.Finally,the thermoelectric properties of Cu₂Se and Ag₂S are regulated by the synergistic effect of temperature and pressure,and high performance thermoelectric materials are developed.In this paper,the in situ measurement method of electrothermal transport properties under extreme conditions of high temperature and high pressure is studied.Cu₂Se and Ag₂S are taken as the research object,and the variation rule of thermoelectric properties under the synergic action of temperature and pressure is studied by using the developed in situ measurement method at high temperature and high pressure combined with first-principles calculation.The innovative research results are as follows:1.Based on the high pressure generation device of six-sided top hydraulic press,the experimental technology of in situ electrothermal transport property measurement at high temperature and high pressure is developed.Through the lead design of the test loop of unique high pressure sample cavity assembly,the introduction of independent signal lines was realized,which successfully solved the problem that the lead was easy to break in the complex stress field under high pressure and mutual interference of test signals,thus improving the accuracy of the test results and the success rate of the experiment.This measurement method is used to collect data of experimental points at a stable temperature and pressure environment,and realize the measurement of electrical/thermal transport signal.2.Using the in situ measurement method at high temperature and high pressure,it is found that the reversible temperature transition point T_c of Cu₂Se has a negative pressure effect under high pressure.Under high temperature and high pressure,Cu₂Se shows the characteristics of heavily doped semiconductor before and after the phase transition;the sudden increase of resistivity（ρ）and Seebeck coefficient（S）at T_c is due to the phonon-carrier scattering caused by the temperature transition,and the pressure reduces the energy required for the temperature transition.The monoclinicα-Cu₂Se existed stably in the range of 328–353 K and 0.8–3.0 GPa,and itsρdecreased and S increased with the increase of pressure.This anomaly results in an optimal power factor PF_max of 4.15×10^-4 W m^-1 K^-2 forα-Cu₂Se at 333 K and3.0 GPa,which increases by about 76%compared with that at 0.8 GPa.The decrease ofρand the abnormal increase of S result from the increase of band gap and effective mass caused by high pressure.However,the high pressure limits the ion migration,resulting in increased simultaneously inρand S of the cubicβ-Cu₂Se（fast ion conductor）.The optimal PF_max ofβ-Cu₂Se is 7.75×10^-4 W m^-1 K^-2（750 K,2.4 GPa）by HPHT regulation,which is 26%higher than that of the same temperature of 0.8GPa.The high pressure has a reverse regulation effect on the conductivity ofα-Cu₂Se andβ-Cu₂Se due to their different conduction mechanisms.Under high pressure,the band gap ofα-Cu₂Se increases and effective mass increases S,and the decrease ofρmay be due to the increase of mobility;the lattice shrinking lead to ion transport channel is blocked,which inhibits the movement of Cu⁺inβ-Cu₂Se and weakens the electrical conductivity.While optimizing the thermoelectric performance at high temperature and high pressure,the thermodynamic stability of the system is improved.3.Using the in situ measurement method at high temperature and high pressure,it is found that the reversible temperature transition point T_c of Ag₂S has positive pressure effect under high pressure.Under high temperature and high pressure,Ag₂S shows the characteristics of semiconductor before and after the phase transition.The resistivity（ρ）and Seebeck coefficient absolute value（|S|）decrease sharply at T_c,which is result of temperature phase transition,caused atomic rearrangement and Ag⁺disorderly,and pressure increases the enthalpy difference between the two kinds of Ag₂S structure.Monoclinic phaseα-Ag₂S existed stably in the range of 320–450 K,1.5–4.6 GPa,With the increase of temperature and pressure,theα-Ag₂S band gap decreases,and lead to theρgreatly reduced,and the|S|also decreases.Finally,at 447 K and 3.3 GPa,the optimal thermoelectric value ZT_max is 0.127,which is about 2.4 times of ZT_max at0.8 GPa.However,high pressure limits the ion migration.High pressure on theβ-Ag₂S（fast ion conductor）ions and electrons of strength is different,and leads to theρand|S|increase first and then decrease and then increase again.By HPHT regulation,the optimal ZT_max ofβ-Ag₂S was 0.18（574 K,2.2 GPa）.High pressure can reversely regulate the conductivity ofα-Ag₂S andβ-Ag₂S due to their different conduction mechanisms.The effect of Ag-S atoms is enhanced under high pressure,resulting in the decrease of band gap and the enhancement of electrical conductivity ofα-Ag₂S.However,lattice shrinking hinder the ion transport channel,which inhibits the Ag⁺movement inβ-Ag₂S and weakens the electrical conductivity,and improves the thermodynamic stability of the system while optimizing the thermoelectric performance at high temperature and high pressure.