Thermoelectric Properties Of PbTe Under Hydrostatic Pressure And Uniaxial Stress

The thermoelectric conversion technology of heat and electricity can effectively use resources and relieve the problem of energy shortage and environmental pollution.The conversion efficiency of thermoelectric devices mainly depends on the ZT(thermoelectric figure of merit)values of the material.The classical thermoelectric material PbTe is a typical narrow band gap semiconductor with simple rocksalt structure and low lattice thermal conductivity,which has become one of the earliest thermoelectric materials studied and applied in practice.In this paper,the model of PbTe under hydrostatic pressure and uniaxial stress along the[100]?[110]and[111]directions are built.Based on the density functional theory and the semi-classical boltzmann transport equation,the electron and phonon transport properties of PbTe under different stress are analyzed.Our results show that PbTe exhibits a high ZT value of about 4 in the region of low carrier concentration with the increase of uniaxial stress along the[110]and[111]directions at 300K.Meanwhile,at a fixed carrier concentration of 1×1020cm-3,the p-type PbTe has a high ZT value above 3.0 at high temperature as the uniaxial stress along the[110]and[111]directions increases.The main work and conclusion of this paper are as follows:First,the band structure and phonon spectrum of PbTe under hydrostatic pressure and uniaxial stress along the[100]?[110]and[111]directions are calculated.With the increase of hydrostatic pressure,the band gap of PbTe first decreases and then increases,resulting in band inversion.Under the hydrostatic pressure,the longitudinal acoustic/transversal optical(LA/TO)has a decoupling phenomenon,leading to an increase in lattice thermal conductivity.When uniaxial stress of 1%?5%is applied along the[100]direction,the change of band shape and phonon behavior has slight effects on the transport properties.The uniaxial stress along the[110]and[111]directions are applied by changing the lattice angle of 61°?65°,where band splitting and band gap value increase slightly as the stress increases.Meanwhile,the acoustic branch exhibits obvious softening,which causes the lattice thermal conductivity of PbTe to drop to 0.1Wm-1K-1.Then,based on the obtained electron and phonon structures,the effects of hydrostatic pressure and uniaxial stress along the[100],[110]and[111]directions on the transport performance of PbTe are explored under different temperature and carrier concentration.For hydrostatic pressure within lOGPa,the ZT value of PbTe first decreases and then increases,and the value reaches minimum near 6GPa.The highest ZT value of n-type PbTe within lOGPa is about 0.3 at 300K.The p-type and n-type PbTe show high ZT values at carrier concentration of 1×1020cm-3,and the p-type has a ZT value above 2.5,which is larger than that of n-type.Under uniaxial stress applied along the[100]direction,the PbTe has a ZT value of about 0.4 at the low concentration region and 300K temperature.Besides,the p-type PbTe has better thermoelectric performance under high temperature at carrier concentration of 1×1020cm-3,for which the ZT value of over 3.0 is much higher than the value of 0.4 for n-type PbTe.With the increase of uniaxial stress applied along the[110]and[111]directions,a high ZT value about 4 could be obtained in the region of low carrier concentration at 300K.For fixed carrier concentration of 1×1020cm-3,the p-type PbTe has a ZT value above 3,showing better thermoelectric performance than n-type PbTe.This study demonstrates that application of hydrostatic pressure and uniaxial stress are effective ways to improve the thermoelectric performance of PbTe.