Computer Simulation Studies Of Electrical Properties And Screening Method For Thermoelectric Materials

As an important role in the family of energy materials,thermoelectric?TE?materials can improve the efficiency of energy utilization due to the direct thermal-to-electrical energy conversion.The thermal energy,mainly coming from the daily waste heat,plays an important role in relieving the overuse of fossil resource and solving the global warming problems.The efficiency of thermoelectrics has been raised substantially because of the development and innovation of thermoelectric theory.Thermoelectric performance is governed by the figure of merit,ZT.The ZT of thermoelectric materials reported by experiments with good properties is usually more than 1.However,it is not enough for commercialized production?ZT>3.0?.To increase the efficiency of thermoelectrics,it needs further studies of the thermoelectric theory.Materials with high ZT performance are rare.This is because TE performance,characterized by ZT,is a contradicted property of matter?s,S andkusually couple together?.How to screen materials for thermoelectrics has been a tough problem because of the contradiction between transport quantities.Here,we developed a simple and effective electronic fitness function,EFF=sS²/(tN^2/3)(t^-1 is scattering rate and N is the volumetric density of states),based on first principles calculation and Boltzmann transport theory to efficiently screen materials for thermoelectrics.In addition,we predicted the crystal structure of Sn₂Se₃ and did detailed analysis of the electronic structure and the potential for thermoelectrics.The details are follows:1.The isoenergy surfaces of rutile phase TiO₂ and cubic phase KNbO₃show low dimensional characters among a series of n-type oxide compounds.They are interesting compounds to be investigated further as potential thermoelectrics.We investigated the electronic structures and transport properties of 16 n-type oxide compounds,including SrTiO₃,which is a known thermoelectric,based on the first principles calculation and transport theory.The isoenergy surfaces at the conduction band edges are simple screens that can be readily applied in screening for new oxide thermoelectrics.We found that rutile TiO₂ and cubic KNbO₃ were interesting compounds to be investigated further as thermoelectrics in addition to the known thermoelectric compound SrTiO₃.Our work provides a powerful roadmap for investigating potential n-type oxide thermoelectrics through the electronic structures and isoenergy surfaces.2.We developed and explored the simple and efficient Electronic Fitness Function?EFF?to screen potential materials for thermoelectric performance.We developed and explored the use of transport function,the electronic fitness function,EFF=sS²/(tN^2/3),that screens potential materials for thermoelectrics based on the first principles calculation and Boltzmann Transport Theory.Here,tis an inverse scattering rate and N is the volumetric density of states.The EFF can screen potential thermoelectric materials efficiently and effectively from the database of materials.The EFF measures the complexity of the electronic structures,which comes from the light-heavy band mixture,pocket degeneracy,pocket anisotropy and band convergence,to decouple S andsand to obtain large power factor?PF=sS²?.We compared our EFF peak values with the experimental maximum ZT at the same temperature.These two parameters showed positive relationship,which further proves that our method is effective.The EFF provides a simple but effective way for theoretical and experimental work to screen materials for thermoelectric performance.3.Several potential binary and ternary thermoelectric candidates are screened by EFF.We applied the EFF to 75 different binary and ternary thermoelectric and potential thermoelectric materials.We found an efficient screening using this transport function.The well known thermoelectric materials such as p-type rhombohedral GeTe and PbTe possess the large EFF values.Among all the compounds,the p-type cubic GeTe shows the largest EFF peak values at any investigated temperature range.Meanwhile,the EFF also predicted two promising alkali metal Zintl compounds,KSnSb for n-type and Na₂AuBi for p-and n-type,two promising half-Heuslers,IrNbSn and RhNbSn for p-type,and one full-Heusler,Li₂NaSb for n-type using for potential thermoelectric materials.These materials need to be further verified by experiments.Our method provides theoretical supports for experiments.4.We predicted the crystal structure of Sn₂Se₃ and investigated the structural stability,electronic structures and thermoelctric properties.The Sn₂Se₃ was predicted using CALYPSO code based on particle swarm optimization method and first principles calculation.The structural stability,electronic structures and transport properties were investigated in detail.We found a structure with the spacegroup P2₁/m based on Sn-Se ribbons with clear disproportionation of Sn into Sn?II?and Sn?IV?sites.Sn charge disproportionation is only marginally favored in this selenide.This leads to a semimetallic rather than semiconducting behavior.The Seebeck coefficient of this semimetallic phase is rather small which is not favorable for thermoelectric properties.Another metallic P-3m1 phase without the disproportionated Sn was also found,which the energy is only 0.54 meV/atom higher than the P2₁/m phase.The close competition of the P2₁/m and P-3m1 phase and the different valent Sn atoms in the P2₁/m phase may underlie the ready formation of an amorphous phase with low energy switching.