Preparation And Theoretical Study Of SnSe Single Crystal Thermoelectric Material

With the gradual improvement of industrial level,the problems of energy shortage and environmental pollution are becoming increasingly serious.It is of great significance to study clean and sustainable new energy technologies.The thermoelectric device based on thermoelectric material can realize the direct transformation from thermal energy to electric energy,can reuse the waste heat which has no economic value.Therefore,thermoelectric conversion technology is one of the important ways to solve the energy and environment problems,and thermoelectric devices are based on thermoelectric materials.Because of its very low thermal conductivity and high Seebeck coefficient,SnSe single crystal thermoelectric materials with layered structure have been paid more and more attention in recent decades,and have become a hot research topic in the field of thermoelectric materials.Although the single crystal SnSe thermoelectric materials have made a series of significant breakthroughs,the common preparation methods are demanding on the equipment at present,and the defects such as low conductivity and poor mechanical properties of the materials makes it still in the laboratory research stage.Commercial applications cannot be realized.In this paper,we try to prepare single crystal SnSe thermoelectric material by flux method,and study of the effects of preparation process,internal defects,stress and element doping on the crystallization quality,electrical transport characteristics,thermal stability and oxidation resistance of single crystal SnSe thermoelectric material by first principle and density functional theory,including.In order to reduce the fabrication cost of SnSe-based thermoelectric devices and realize the wide-range practical application of SnSe thermoelectric materials as soon as possible,the research focus of this thesis is to explore the better fabrication technology of SnSe-based thermoelectric devices.The main conclusions of this paper are as follows:1.The SnSe single crystal thermoelectric material with good crystallinity was successfully prepared by Sn-flux method,and the carrier concentration was fine-tuned by adjusting the solvent ratio of Sn.The density functional theory(DFT)calculation shows that the power factor can be significantly improved by adjusting the Seebeck coefficient and conductivity by adjusting the carrier concentration.The preparation of single crystal samples deviating from the chemical composition is an effective way to optimize the carier concentration because the Snse anti-site defects can reduce the Fermi energy level and enhance the carrier(hole)collection.The power factor of 7.6×10-5 W/m-K2 at 303K was obtained experimentally.2.By heating SnSe at 523K and 573K in air for 24 hours,the thermal stability and oxidation resistance of single crystal SnSe samples and the effect of heat treatment on the quality and electrical transmission characteristics of SnSe crystal were studied.The results show that SnSe crystal is not easy to be oxidized inside,but the sample shows slight oxidation to SnO2.However,even if the surface is oxidized,the SnSe crystal still shows stable thermoelectric properties.At the same time,heat treatment in air plays an important role in annealing of SnSe crystal.After annealing,the number of defects,such as dislocation in the crystal,decreases,which is beneficial to the increase of carrier mobility and conductivity.The density functional theory calculation(DFT)also proves that the defect density has a great influence on the conductivity and electron distribution.After one annealing,the maximum conductivity of the sample was increased from 3.75S/cm to 5.73S/cm,and the power factor was increased from 8.3×10-5 W/m-K2 to 1.33×10-4 W/m·K2.3.The effects of tensile strain and compressive strain on the electrical transmission characteristics of SnSe are studied by first-principles and Boltzmann transport theory.Under 4%compressive strain,the power factor of SnSe reaches its maximum in direction b and direction c,which are 7.7×10-4 W/m-K2 and 3.9×10-4 W/m·K2,respectively.Compared with the strain-free state,it has been expanded by 2.5 times and 2 times respectively.4.The Pb doped SnSe single crystals were prepared by Sn flux method,and the samples with different Pb contents were prepared according to the atomic ratios of Sn(1-x)+5PbxSe(x=0,0.15,0.3,0.6,0.9).The effect of Pb doping on the electrical transmission properties of SnSe single crystals was studied.The amount of Pb dopant can change the majority carrier of SnSe from hole to electron,and the earrier concentration also changes.The sample with the element ratio of Sn0.85+5Pb0.15Se is p-type,and the carrier concentration of undoped sample decreases to 3.3×1017cm-3.In contrast,most of the cariers in the samples of Sn0.1+5Pb0.9Se changed to n-type,andthe carrier concentration increased to 4.1×1017 cm-3.The maximum power factor of 1.2×10-4 W/m-K2 can be obtained at room temperature for the sample with the element ratio of Sn0.4+5Pb0.6Se,which means that Pb doping can be used to prepare the n-type SnSe thermoelectric material with excellent performance.