Study On The Thermoelectric Properties And Mechanical Properties Of IB₂-VIA Compounds With Disorder Structures

In response to the global energy crisis and the debilitating impact of fossil fuels on the environment,thermoelectric（TE）materials have attracted worldwide attention for their ability to collect and convert industrial waste heat into useful electricity.IB₂-VIA-group thermoelectric materials with the liquid-like property have extremely low thermal conductivity and high thermoelectric figure of merit,which regard as a promising thermoelectric material.Furthermore,the Multi-scale disorder is one of the primary reasons for its excellent thermoelectric performance.In this dissertation,the disorder structural characterization and thermoelectric performance of IB₂-VIA-group thermoelectric materials are systematically investigated.Especially for high disorder conditions of multi-principle solid solution and phase transition,the relationship between the disorder and thermoelectric performance was investigated,and the method to optimize TE performance by disorder parameters was deeply analyzed.The main achievements as follow:1.By introducing configuration entropy to describe the disorder in multi-principle solid solution,the relationship between the configuration entropy and thermoelectric performance was fully discussed.It found that the TE properties are significantly optimized and improved by increasing the material’s configuration entropy.Moreover,High throughput calculations and predictions have been successfully performed for the discovery of high configuration entropy TE materials.Finally,the high configuration entropy TE materials Cu₂S_1/3Se_1/3Te_1/3 was synthesized,and have a maximum zT value of 2.0 at 1000K,50%higher than that in matrix compounds.2.Through considering the absorption heat of the phase transition,the thermal transport equation during the phase transition has been modified.We show that the measurement on thermal diffusivity can also be affected by material’s phase transition with the degree determined by how fast of the phase transition.Thus,the true value of thermal conductivity during phase transition should be corrected by removing the contributions from enhanced heat capacity and lowered thermal diffusivity,which is well demonstrated in four selected kinds of phase transition materials（Cu₂Se,Cu₂S,Ag₂S,and Ag₂Se）in experiment.Moreover,it found that the thermal conductivity can be significantly lowered by the critical scatterings of phonons and electrons during material’s second-order phase transitions.3.By introducing order parameters to describe the phase transition,we discussed the influence of the structure fluctuation and soft mode on the electric transport property,and built the relationships between resistivity and seebeck coefficient during the phase transition.It founds that the structure fluctuation was increasing with the b parameter decreasing,and lead to a large seebeck coefficient during the phase transition.But the phonon energy of the soft mode was decreasing with the b parameter decreasing,and lead to a small resistivity during the phase transition,which is well demonstrated in Cu₂S_xSe_1-x（x=0,0.04,0.08,0.12）in experiments.As the S doping,the b parameter was increasing,seebeck coefficient was decreasing as resistivity increasing.Combining the decreasing of the thermal conductivity,the Cu₂S_0.08Se_0.92.92 have a maximum zT value of 1.4 during the phase transition.4.It founds that theα-Ag₂S is fundamentally different from other semiconductors and ceramics with the good ductility.By performing the density functional theory calculation to simulate the slipping process,it shows that the S atom are always bonded to the part of these six Ag atoms during the slipping and prevent the whole materials cleaving and cracking.Moreover,the fluent change in bonding between Ag and S leads to a low energy barrier during the slipping process,and finally results on the good ductility ofα-Ag₂S.