Optimization Of Thermoelectric Performance Of SrTiO₃ Ceramics By Configurational Entropy Design

Thermoelectric materials are new energy materials that use Seebeck effect and Peltier effect to realize direct conversion of heat energy and electric energy.They are safe,stable,environmental,pollution-free and have a wide application prospect in energy conservation and emission reduction,waste heat recovery and utilization.Oxide thermoelectric materials have advantages incomparable to alloy thermoelectric materials,such as good chemical and thermal stability at high temperature,simple preparation process,low cost,non-toxic and so on.Especially significant high thermal stability can make thermoelectric devices to maintain a large temperature difference to obtain high conversion efficiency.Among the oxide materials,n-type SrTiO3 material has attracted the attention of researchers due to its high symmetry and high Seebeck coefficient,while intrinsic SrTiO3 is a wide-gap(3.2 eV)insulator with low carrer concentration(～1015 cm-3)and it also has high thermal conductivity due to strong TiO ionic bonding.Therefore,many studies focus on reducing thermal conductivity while optimizing electrical transport performance.As far as it goes,the main strategies are doping,in situ vacancy,nanostructure and others to optimize the thermoelectric properties of SrTiO3 materials.However,the thermoelectric properties of SrTiO3 materials are still poor compared with the matching p-type materials,which requires further optimization.Entropy engineering is a new strategy originating from high entropy alloys,which is used to cooperatively optimize the electrical and thermal transport properties of materials.Therefore,this paper aims to optimize the electrical and thermal transport performance of SrTiO3 through entropy engineering strategy.On the one hand,by increasing the type of elements and increasing entropy,the single-phase structure of the material is stabilized and the phase-boundary electron scattering in the polyphase material is weakened,so as to improve the electrical transport performance of the material.On the other hand,the anharmonic vibration and lattice distortion induced by multiple elements at the same lattice site will effectively enhance the multiphonon scattering to optimize the thermal properties,so as to finally realize the collaborative optimization of thermoelectric properties.The research results of this paper are as follows:1.The low entropy of(Sr1-4xLaxNdxDyxYx)TiO3(x=0.005,0.01,0.015,0.02,0.025,0.03)ceramics were prepared by solid phase method.Due to the introduction of configurational entropy term,the total Gibbs free energy is reduced by 2.5%～10%;which proves that configurational entropy also plays a certain role in the stability of phase structure in the low entropy range,which also is conducive to the optimization of electrical performance.The higher power factor(-700 μW K-2 m-1)is obtained at 973 K for the samples of x=0.02,0.025,0.03 due to the significant increase of carrier concentration.Point defects and porous structures effectively enhance phonon scattering and reduce thermal conductivity.Finally,the zT of x=0.02,0.025 and 0.03 samples reach 0.2 at 973 K.It is found that the optimal configurational entropy of the system ranges from 3.24 to 4.43 J mol-1 K-1.In addition,the materials maintain excellent mechanical properties,and the microhardness is about 6～7 GPa.2.Based on the optimal ratio of La and Nb doped Lao.1Sr0.9Ti0.9Nb0.1O3 reported in the literature,the medium entropy Sr0.9-xBaxLa0.1Ti0.9Nb0.1O3-δ(x=0.1,0.2,0.3,0.4,0.5)ceramics were designed and prepared.It is found that the introduction of configurational entropy term reduces the total Gibbs free energy by about half.This proves that the entropy plays an important role in driving the stability of phase structure.The difference of oxygen vacancy leads to the higher carrier concentration of x=0.2 and x=0.3 samples,so the maximum power factor PF=602.1 μW K-2 m-1 of x=0.3 samples is obtained at 923 K.In addition,the point defect scattering enhancement caused by large mass field and stress field fluctuations effectively reduces the lattice thermal conductivity.Finally,the x=0.2 sample obtains the maximum zT=0.15 at 973 K.Considering the electrical and thermal properties,the optimal configurational entropy range of this work is 9～10 J mol-1 K-1.At the same time,all samples maintain the excellent mechanical properties and the Vickers hardness is 7～11 GPa.3.High entropy Ca0.25Ba0.25Sr0.25Y0.25Ti0.9Nb0.1O3,Ca0.25Ba0.25Sr0.25Nd0.25Ti0.9Nb0.1O3 and Ca0.25Ba0.25Sr0.25La0.25Ti0.9Nb0.1O3 ceramics were designed and prepared.Single-phase Ca0.25Ba0.25Sr0.25La0.25Ti0.9Nb0.1O3 ceramic was successfully synthesized.Single-phase Ca0.25Ba0.25Sr0.25La0.25Ti0.9Nb0.1O3 ceramic compared to other components maintain a relatively high electrical conductivity,which makes the power factor to get the maximum 442μW K-2 m-1 at 873 K.At the same time,the phonon scattering is enhanced due to the anharmonic vibration induced by multiple elements and lattice distortion,which effectively reduces the lattice thermal conductivity in high entropy ceramics.The minimum lattice thermal conductivity is 1.4 W m-1 K-1 for Ca0.25Ba0.25Sr0.25La0.25Ti0.9Nb0.1O3 ceramic at 873 K.Finally,the maximum zT=0.17 is obtained for Ca0.25Ba0.25Sr0.25La0.25Ti0.9Nb0.1O3 sample at 873 K.