Thermoelectric Properties Of Full Heusler Alloys Ca₂YZ（Y=Au,Hg;Z=As,Sb,Bi,Sn,Pb） And Doped In₂O₃

The problem of environmental pollution and energy shortage have caused the urgent need for the development of clean renewable energy.Thermoelectric devices can convert waste heat into useful electrical energy,while also converting electrical energy into heat for heating or cooling.In essence,thermoelectric coolers and generators are heat engines thermodynamically similar to conventional vapor power generation or heat pumping cycles,but they use electrons as the working fluid instead of gases or liquids.Thus,thermoelectric devices have the advantages of being solid-state devices,low maintenance costs and long service life,no pollution and no noise.Currently,the two main focuses in thermoelectrics research are the discovery of new materials with high thermoelectric efficiency and the design and optimization of thermoelectric generators.Our focus is on the search and optimization of thermoelectric applications to achieve efficient interconversion between thermal energy and electrical energy.In this paper,the first principle calculation method based on density functional theory and combined with the semi-classical Boltzmann theory,we study the electronic structure and thermoelectric properties of the full Heusler alloys Ca₂YZ（Y=Au,Hg;Z=As,Sb,Bi,Sn,Pb）and Sn doped In₂O₃(In_32-xSn_xO₄₈（x=0,1,2,3）).The main results are shown as following:（1）The electronic structure and transport properties of full Heusler alloy Ca₂YZ（Y=Au,Hg;Z=As,Sb,Bi,Sn,Pb）are investigated by the first principle method based on density functional theory and the semi-classical Boltzmann transport theory in consideration of the spin orbit coupling.Because of the electronic structure has a direct effect on the electronic transport properties,in order to get more accurate electronic structure,the electronic structures of these five substances were calculated using PBE,LDA and EV exchange correlation potentials with or without consideration of spin orbital coupling and MBJ（PBEGGA PBEGGASO,PBEGGASOMBJ,LDA,LDASO,LDASOMBJ,EVGGA,EVGGASO,EVGGASOMBJ）.It is found that the band gap calculated by whether PBEGGASOMBJ,LDASOMBJ or EVGGASOMBJ is similar.So we calculated the electronic structure and electronic transport property based on the method of PBEGGASOMBJ.There is a difference between the calculated band gap size and the calculated result of He et.al.,which may be they did not consider the impact of the spin orbit coupling.What’s more,their methods are based on the calculation of the electronic structure of PBEGGA,which artificially wide the band gap to the calculation of HSE（the band gap of HSE is calculated by VASP）.In order to prove the reliability of our calculation,we calculated the transport properties of Fe₂VAl in the same way.We compared the results with the experimental results and found that both the Seebeck coefficient and the conductivity are close to the experimental values,indicating that our method is reasonable.By comparing the transport properties of Ca₂YZ and Fe₂VAl,we found that the maximum power factor of Fe₂VAl is only one third of Ca₂YZ,when considering that the lattice thermal conductivity of Ca₂YZ is much smaller than that of Fe₂VAl,Ca₂YZ（Y=Au,Hg;Z=As,Sb,Bi,Sn,Pb）should be a very promising thermoelectric materials.（2）We study the effect of Sn substitution in two different sites In b and In d on the thermoelectric properties of In₂O₃ by using the primary principle and the semi-classical Boltzmann theory.By calculating the formation energy of two substitution bits,it is found that In b bit is easier to be replaced than In d,so we only consider the substitution of In b site.It is worth noting that the formation energy of In b replaced by Sn is negative when x=1,2,that is,one or two In b is replaced by Sn（x can take a value of 1,2,3）,but when x=3,the formation energy is positive.Considering the thermodynamic stability of materials,the electron structure and thermoelectric properties of one or two In b bits replaced by Sn are more significant.The atom doping concentrations of Sn in In₂O₃ are 3.12 at.%and 6.24 at.%,respectively,and by calculation with the results of In₂O₃ simulation under a rigid band,we found that the Seebeck coefficient is very small at higher doping concentrations,so we mainly studied the transport properties of one Sn doped.