Theoretical Study On Thermoelectric Properties Of RbSnX₃（F,Cl） And Doped Mg₂Si/FeGa₃ Compounds

At present,about 2/3 of the world’s energy is wasted as heat and released into the atmosphere in various forms,including oil and gas as raw materials of chemical industry,industrial waste incineration and automobile exhaust emissions.Devices based on thermoelectric materials have attracted wide attention because of their ability to convert heat energy and electric energy into clean electric energy resources.Thermoelectric materials are safe,reliable,long life,easy to control,no pollution to the environment and other advantages,but most of the discovered thermoelectric materials conversion efficiency is low,so they have not been widely used and developed.It is of great research significance to discover the materials with high thermoelectric conversion efficiency or to improve the thermoelectric materials with low performance.In this paper,the structure of the material is optimized based on the first-principles density functional theory.The relationship between the carrier type and concentration and the thermoelectric optimal value at different temperatures is obtained by means of the deformation potential theory and the semi-classical Boltzmann transport theory.The thermoelectric optimal value（bp-ZT）is calculated by combining the intrinsic carrier concentration and considering the bipolar effect.It provides research ideas for experimental design and preparation of thermoelectric materials.The main contents are as follows:1.Built and optimized the architecture of Rb Sn X₃（X=F,Cl）.The mechanical stability of the compound was verified by calculating the elastic coefficient matrix.The lattice thermal conductivity of Rb Sn F₃ is calculated by Slack method.The results show that the thermal conductivity of the lattice is consistent with those reported in the literature.Based on the first principles and the high precision energy band calculated at dense k points（18×18×18）,the semi-classical Boltzmann equation is solved,and the electron transport coefficient（S,σ,κ_e）of Rb Sn X₃ is obtained.Then the density of states is used to calculate the intrinsic carrier concentration,and further based on bipolar effect to calculate the S,σandκ_e,won the 300-900 K temperature range thermal power optimal value of bp-ZT values.The results showed that the maximum bp-ZT of Rb Sn F₃ and Rb Sn Cl₃ appeared at 900 K,with 3.17 and 1.66 respectively,corresponding to the thermoelectric conversion efficiencyηof 29.26%and 15.49%.At the same time,it is found that the bipolar effect of these two compounds is not obvious.Therefore,Rb Sn F₃and Rb Sn Cl₃,especially the former,can be used as potential thermoelectric materials.2.The thermoelectric properties of Ca-substituted Mg₂Si compounds（Mg₇Si₄Ca and Mg₄Si₄Ca₄）and their intrinsic systems were studied.The geometric structures of the three compounds were optimized and their kinetic stability was verified by phonon spectra.The lattice thermal conductivity of Mg₂Si was calculated according to Debye-Callaway method,and the results were in agreement with the experimental values.Furthermore,the lattice thermal conductivity of the doped structure was calculated using the same scheme.It is found that doping results in obvious decrease of lattice thermal conductivity,which is conducive to the improvement of thermoelectric properties.Based on the calculation of the two carrier transport coefficients based on the energy band of each compound,the transport coefficients including the bipolar effect are calculated by combining with the intrinsic carrier concentration.The results show that the bp-ZT of Mg₂Si is in agreement with the experimental values in the temperature range of 323-823 K,which confirms the reliability of the calculation scheme constructed by us.The maximum value of bp-ZT of Mg₇Si₄Ca and Mg₄Si₄Ca₄ is 700 K and 500 K,respectively,which are 1.35 and 5.70,significantly higher than the 0.61 of Mg₂Si at the same temperature.Therefore,Ca doping can effectively improve the thermoelectric properties of Mg₂Si.Comparing the two doping systems,it can be found that the effect of heavy doping is more obvious than that of low concentration doping.By analyzing the effect of doping on lattice thermal conductivity and electron transport coefficient,it can be found that the decrease of lattice thermal conductivity caused by doping plays a major role in the increase of bp-ZT.In the temperature range considered,the bipolar effect of Mg₂Si and its doped compounds is not obvious below 500 K,but the bipolar effect gradually becomes obvious with the increase of temperature,especially in the heavily doped system.3.The calculation scheme established in the last chapter is further extended to Fe Ga₃and the thermoelectric properties of Fe doped Cu atoms are studied.Different from Mg₂Si system,both Fe Ga₃ and its doped compound Ga₈Fe₄Cu₄ have obvious anisotropy,so we calculated the thermoelectric properties in different directions respectively.Comparing the calculated results of lattice thermal conductivity with the experimental values,it is found that Debye-Callaway method also gives satisfactory results for Fe Ga₃.In the temperature range of 300-900 K,the bp-ZT values of Fe Ga₃ are in good agreement with the experimental values,which further confirms the reliability of our calculation scheme.The maximum values of bp-ZT of Ga₈Fe₄Cu₄ along x and z directions at 300 K are 0.62and 2.02,respectively,which are significantly higher than that of Fe Ga₃(3.16×10^-3),indicating that Cu-doped Fe Ga₃ can effectively improve the thermoelectric properties of the materials.The bipolar effect of Ga₈Fe₄Cu₄ was found to be obvious at all high temperatures except 300 K.Similar to Mg₂Si,the main reason that Cu doping improves the thermoelectric properties is that it effectively reduces the lattice thermal conductivity of Ga₈Fe₄Cu₄.Unlike the large bp-ZT of Fe Ga₃,which occurs at high temperatures of 900K,the bp-ZT of Ga₈Fe₄Cu₄ is lower at 400 K and thus is more suitable for thermoelectric applications in lower temperature environments.