Research On Thermal Transport And Thermoelectric Properties Of New Semiconductor Materials

The growing demand on energy over the past few decades makes the shortage of resource increasingly severe,however,traditional fossil fuels are neither ecological nor durable,so the task of finding sustainable and environmental friendliness energy is becoming more and more urgent.In recent years,thermoelectric materials have received extensive attention and research as a class of sustainable and ecologically friendly material which provide direct heat-electricity conversion.In this paper,based on the first-principles calculations,we studied the thermal transport and thermoelectric properties of a series of new semiconductor materials such as 2D transition metal chalcogenides MX₂and M₂X₃(M=Ni,Pd;X=S,Se and Te),2D ?-? semiconductors MX(M=Ge,Sn;X=S,Se),?B-? group compounds Zn(Cd)Se,Zn(Cd)Te and alkali antimonide compounds M₃Sb(M=K,Rb and Cs)by solving the Boltzmann transport equation.For 2D transition metal chalcogenides,we divide them into two groups according to the lattice structure,one is the hexagonal H-MX₂(M=Ni,Pd;X=S,Se and Te),the other is the orthorhombic O-M₂X₃(M=Ni,Pd;X=S,Se and Te).For the isotropic H-MX₂,due to the small phonon group velocity and short phonon lifetime,H-NiSe₂and H-PdTe₂have the lowest lattice thermal conductivity,for the anisotropic O-M₂X₃,the overall lattice thermal conductivity of the material along the b-axis is smaller than that along the a-axis.Finally,comparing these 2D transition metal chalcogenides,we found two materials with the smallest lattice thermal conductivity,namely H-NiSe₂ and H-PdTe₂,at a temperature of 300K,the lattice thermal conductivity are 1.08 Wm^-1K^-1 and1.30 Wm^-1K^-1of H-NiSe₂ and H-PdTe₂,respectively.As we all know,low lattice thermal conductivity is a prerequisite for good thermoelectric performance,at a temperature of700K,for n-type doped H-NiSe₂and H-PdTe₂,the calculated maximum ZT values are0.61 and 1.08,respectively.2D ?-? semiconductors MX(M=Ge,Sn;X=S,Se)can be obtained by mechanical stripping from few-layer MXs which has already been experimentally synthesized,based on the first-principles calculations,we conducted a comprehensive study of the thermal transport properties of these materials.The calculated lattice thermal conductivity is anisotropic,and the lattice thermal conductivity along the a-axis is slightly larger than that of b-axis,which can be attributed to the larger phonon group velocity of a-axis,besides,the optical modes of Sn Se made a great contribution to the lattice thermal conductivity as a result of the most optical modes are located in the low frequency range.In addition,the lattice thermal conductivity of GeX is generally larger than that of SnX,which can be also attributed to the larger phonon group velocity.Our research reveals the thermal transport characteristics of these 2D materials and points out the difference in lattice thermal conductivity of GeX and SnX.?B-? materials Zn(Cd)Se and Zn(Cd)Te are well known for their excellent photoelectric performance,however,their thermoelectric(TE)properties are usually ignored,and there are few reports on this aspect.By taking advantage of first-principles calculations,we executed a series of thermal transport and thermoelectric properties investigations on these materials.Our results show that Zn Se and Cd Se belong to the hexagonal crystal system,containing 4 atoms in the primitive cell,Zn Te and Cd Te belong to the cubic crystal systems and there are 8 atoms in the primitive cell.The calculation results shows that Cd Se has the lowest lattice thermal conductivity among the four materials,at a temperature of 300K,the thermal conductivity along the a-axis and c-axis are 3.85 Wm^-1K^-1and 4.70 Wm^-1K^-1,respectively,besides,at a temperature of 1200K we obtained a pretty large power factor,²,of 4.39×10^-3Wm^-1K^-1,based on the low lattice thermal conductivity and large power factor,we got the ideal figure of merit,the ZT values along the a-axis and c-axis are 1.8 and 1.6,respectively,which indicates that Cd Se is not only an excellent optical material,but also a potential thermoelectric material.Then we studied the thermal transport properties of the alkali antimonide compounds M₃Sb(M=K,Rb and Cs).The conventional calculation methods are invalid in these severely anharmonic materials.We have investigated the role of the quartic anharmonicity in lattice dynamics and the thermal transport properties of these materials by using the self-consistent phonon theory combined with compressive sensing techniques.We found that the alkali-metal atoms in K₃Sb,Rb₃Sb and Cs₃Sb have a strong quartic anharmonicity and play a crucial role in the lattice dynamic stability.According to the Boltzmann transport equation,the calculated lattice thermal conductivity shows a very small value at a temperature of 300K,which is 1.26?0.63and 0.52 Wm^-1K^-1for K₃Sb,Rb₃Sb and Cs₃Sb,respectively.The calculated lattice thermal conductivity of these compounds is smaller than many famous thermoelectric materials,indicating that these alkali antimonide compounds are potential thermoelectric materials.