Thermoelectric Properties And Transport Mechanism Of Perovskite-type Oxides And Layered Oxyantimonides

Thermoelectric(TE)materials are able to directly convert heat to electric power,and in reverse,use electricity to regulate temperature.TE devices have long been used in space exploration and semiconductor refrigeration,now offering a potential solution for waste heat recovery.The efficient TE materials require the high electrical conductivity,large magnitudes of Seebeck coefficient and low thermal conductivity.The three parameters are interdependent and hard to be optimized simultaneously.There are several strategies to improve the electrical transport properties,such as carrier concentration modulation and band structure engineering.However,manipulating lattice thermal conductivity is relatively difficult.Introducing new scattering mechanism is so far the only effective way to reduce the lattice thermal conductivity,often with compromising the electrical conductivity.Therefore,the materials with intrinsically low thermal conductivity are good choices for achieving high TE performanceFrom this point of view,strontium barium niobate(SBN)was selected at the beginning of my research.The material possesses the intrinsically low lattice thermal conductivity because of its complex structure and shows good electrical conductivity after oxygen reduction.The TE properties of SBN were investigated by the experiments and calculations.The results indicate that the electrical transport behavior follows the Anderson localized theory.The polarized regions as one of the disorder sources also play an important role on lowering the lattice thermal conductivity.The strong anisotropic TE properties of SBN are induced by oxygen vacancy.The thermoelectric figure of merit(Z7)of SBN increases to 0.35 after manipulating the components and micro-structure,which is almost as twice as that of the matrix material.However,the value is still far below the expectation,which is ascribed to two reasons.The phonon scattering caused by the boundaries of polarized regions weakens at high temperatures,leading to the increase of lattice thermal conductivity.On the other hand,the electron localization impedes electrical conduction,which deteriorates the TE performance of SBN.Electron localization has a negative effect on the TE performance and occurs in many materials besides SBN.It is meaningful to figure out the mechanism of the localization phenomenon.Thus,the localization mechanisms of three perovskite thermoelectric oxides(SBN,CaMnxNb1-xO3 and SrTi1-xNbxO3-?)were investigated in terms of the space and energy distribution of electronic states by using first principles calculations.The results indicate that the origin of electron localization of SBN is ascribed to its complex structure.The 4d states of the Nb atoms are distributed at different energy levels,leading to the electron localization near conduction band minimum.The carriers of CaMnxNb1-xO3 are localized because of the energy difference of Mn 3d states and Nb 4d states.The localization mechanism of SrTi1-xNbxO3-? is different.Both Nb doping and oxygen vacancy have an effect on electron localization.Besides,it was found that introducing Sr vacancy is an effective way to achieve delocalization.According to the results,it is hard to delocalize the charge carriers of SBN because the localization origin is intrinsic.Then ZrCuSiAs-type oxyantimonides with a simple structure were selected to avoid electron localization.The materials also possess intrinsically low lattice thermal conductivity because of the two-dimensional layered structure.The TE properties of LnTSbO(Ln= La-Gd and T= Zn,Mn)were investigated by the first-principles calculations and semi-classical Boltzmann theory.The conduction bands show a multiple valley structure near the band bottom.The TE properties are determined by the energy difference between the valleys.The values of energy difference of LnZnSbO linearly increase with the ionic radius of Ln.The thermoelectric properties are thus effectively manipulated by varying the lanthanides.As a result,LnZnSbO(Ln= Ce-Nd)shows a better TE performance.The calculations of n-doped NdZnSbO show that the doped F atoms act as electron donors and provide charge carriers for NdZnSbO.By contrast,p-type doping(Sr,Ba,Ag,Cu)not only provides holes as charge carriers,but also influence the structure near the top of valence band.The band degeneracy is lifted at X point,and the band dispersion becomes weak at Z point.Generally,heavy doping is preferable for n-type NdZnSbO,and slight doping is suitable for p-type NdZnSbO.