Preparation And Performance Modulation Of Strontium Niobate-based Thermoelectric Materials

Thermoelectric materials are semiconductor materials that can convert thermal energy into electrical energy or vice versa.Oxide thermoelectric materials exhibit high chemical stability and thermal stability,while being cost-effective and easily scalable in terms of synthesis.The numerous advantages of oxide thermoelectric materials have attracted significant attention.However,the relatively low thermoelectric efficiency of oxide materials hinders their practical applications,making it a crucial research topic to explore effective approaches for enhancing their performance.Strontium niobate（Sr Nb₂O₆）possesses a slightly distorted perovskite-like structure,where the niobium atoms exhibit complex mixed valence states between+4 and+5.As a result,such compounds demonstrate excellent electrical transport properties and rich magnetism.In our study,we selected Sr Nb₂O₆as the matrix material and employed doping modifications and the introduction of nanoscale microstructures to tailor its thermoelectric performance.The main research findings are summarized as follows:（1）A series of Sr_1-xLa_xNb₂O₆（x=0.1,0.2,0.3,0.4）oxide thermoelectric materials were synthesized by heavy lanthanum（La）doping into strontium niobate（Sr Nb₂O₆）.The influence of lanthanum content on the crystal structure,electronic properties,valence states of constituent elements,and thermoelectric performance was investigated.Due to the substitution of trivalent La³⁺ions for divalent Sr²⁺ions,the crystal lattice of Sr_1-xLa_xNb₂O₆（0<x≤0.4）underwent a transition from the monoclinic system（P21/c）of Sr Nb₂O₆to the orthorhombic system（Pnma）.This structural transformation,accompanied by an increase in negative charge density,suggests intriguing electrical transport properties.Upon reduction annealing in a reductive atmosphere,Sr_1-xLa_xNb₂O_6-δexhibited a significant amount of oxygen vacancy defects,leading to lattice distortion and enhanced phonon scattering,thereby resulting in low thermal conductivity.The power factor gradually decreased monotonically with increasing lanthanum content,and the Sr_0.6La_0.4Nb₂O_6-δsample achieved the maximum ZT value of 0.13 at 1073 K.（2）A series of composite oxide thermoelectric materials,Sr_0.7Ba_0.3Nb₂O₆/xwt%Ti（x=5,10,15,20）,was synthesized by incorporating nanoscale Ti into the Sr_0.7Ba_0.3Nb₂O₆base material and introducing a sintering aid,Bi₂O₃.The investigation revealed that during the sintering process,the nanoscale Ti powder underwent oxidation,transforming into titanium dioxide（Ti O₂）.This oxidation reaction resulted in the generation of additional oxygen vacancies within the lattice of the base material,which contributed to the enhancement of carrier concentration and the reduction of electrical resistivity.Moreover,the addition of titanium facilitated the formation of nanosized bismuth-rich regions.The Bi₂O₃located at the grain boundaries established electrical connections between the Sr_0.7Ba_0.3Nb₂O₆and Ti O₂particles,thereby effectively reducing carrier scattering at the grain boundaries and enhancing carrier mobility.Ultimately,the Sr_0.7Ba_0.3Nb₂O₆/5wt%Ti sample reached the highest ZT value of 0.30 at 1063K,which is approximately a 50%improvement compared to the Sr_0.7Ba_0.3Nb₂O₆matrix.