Theoretical Study On The Thermoelectric Properties Of BXGa(X=Be,Mg,Ca),BeMF₃(M=Al,Y) And AgMF₃(M=Zn,Cd)

In today's society,the demand for non-renewable resources such as oil and coal is increasing year by year with the continuous progress of world industrialization and global technology.The large-scale use of non-renewable resources has caused my country's energy shortage and serious environmental pollution.So looking for clean and sustainable energy is the key to solving these problems.Many of the substances emitted during the combustion process of some non-renewable resources are discharged in the form of heat energy and have not been effectively used.If this part of heat energy can be used,it is very important to alleviate environmental pollution and energy shortages.For this purpose,thermoelectric materials,as a kind of functional materials that can directly convert thermal energy into electrical energy,have attracted wide attention from scientists.Thermoelectric materials have the advantages of non-polluting,easy to control,and long service life,but there are not many materials with high conversion efficiency,so they have not been widely used.Therefore,it is of great significance to find thermoelectric materials with high conversion efficiency and to promote and develop them.Because the conversion efficiency of thermoelectric materials is determined by the dimensionless quality factor ZT value at a certain temperature,it is a very meaningful research topic to find thermoelectric materials with high ZT value.Based on first-principles-based density functional theory(DFT),this paper uses theoretical calculation methods to study the thermoelectric properties of semiconductor materials,and provides theoretical guidance for the design of experiments and preparation of thermoelectric materials.The research work of theoretical calculation is as follows,1.The calculation of the phonon spectrum confirmed the dynamic stability of the compound Half-Heusler(HH)BXGa(X=Be,Mg,and Ca).Based on the obtained structure,VASP software is used to calculate the electronic structure properties of the BXGa compound.Analysis of the results shows that they are direct band gap semiconductors with a narrow band gap.Their relaxation time(?)and carrier mobility(?)are calculated using deformation potential theory,and the results show that the?and?of electrons are significantly higher than those of holes.The lattice thermal conductivity is calculated using the Slack model.Based on the first-principles calculation of the energy band structure diagram,the Boltzmann equation is solved by Boltz Tra P2 software to obtain the transport coefficient.Finally calculate the ZT value of these materials.The results show that n-type BCa Ga has a ZT peak of 7.38 at a temperature of 700 K and a carrier concentration of 1.14×10¹⁹cm^-3,which is a very promising candidate for thermoelectric materials.2.The thermal conductivity is calculated based on the Debye-Callaway model,and the thermoelectric transfer characteristics of the fluoride perovskite Be MF₃(M=Al,Y)at different temperatures are studied.The calculated results are close to the relevant experimental and theoretical data reported by previous authors.The effective mass m^*_dosof holes is greater than that of electrons.For?and?,the electrons are larger than the holes in the same system at the same temperature.When the temperature is1300 K,the maximum value of ZT of n-type Be YF₃can reach 4.76;when the temperature is 800 K,the maximum value of ZT of p-type Be Al F₃is 1.52.When the cold end temperature is 300 K,the maximum thermoelectric conversion efficiency of n-type Be YF₃can reach 48%.Therefore,Be MF₃(M=Al,Y)is expected to become a functional thermoelectric material with excellent thermoelectric properties.3.The thermoelectric properties of Ag MF₃(M=Zn,Cd)were studied.Analysis of the calculation results of the electronic structure shows that they are all semiconductors with indirect band gaps.Based on the calculation of lattice thermal conductivity and transport coefficient,the relationship between ZT value and temperature and carrier concentration is established.The results show that at a temperature of 1200 K,the ZT value of p-type Ag Cd F₃reaches 5.09 at a carrier concentration of 2.1×10²⁰cm^-3;the ZT value of p-type Ag Zn F₃at a carrier concentration of 9.3×10²⁰cm^-3reached 2.56.It shows that Ag MF₃(M=Zn,Cd)is a potential high-temperature thermoelectric material.