Preparation,Electrical Properties And Mechanism Of Sr₂Nb₂O₇ And Ca₁₂Al₁₄O₃₃-based Oxygen Ion Conductor Materials

Solid oxide fuel cell（SOFC）,as a high efficiency and clean green power generation device,has great potential in the green market.In order to achieve the development needs of commercialization,the research and development of intermediate and low temperature SOFC（IT/LT-SOFC）has become an inevitable trend.Electrolyte is the core component of SOFC.With the decrease of operating temperature,electrolyte materials with high ionic conductivity and structural stability at low and intermediate temperature face new challenges.The purpose of this work is to explore new high-performance oxygen ion conductors and improve the conductivity of electrolyte materials.The details are as follows:（1）Study on electrical properties and transport mechanism of layered perovskite Sr₂Nb₂O₇.Sr₂Nb₂O₇ with space group of Cmc2₁ was synthesized by solid phase method.Through the conductivity measurements under various of oxygen partial pressure and the electromotive force measurement of an oxygen concentration cells,it was confirmed that the material was dominated by oxygen ion conduction,with a small part of electron conduction.The conductivity measurements of the samples prepared under pure O₂ atmosphere,Ar atmosphere and 5%H₂+95%N₂ atmospheres revealed that the oxide ion conduction in Sr₂Nb₂O₇ originated from the slight reduction of Nb⁵⁺ions during the high-temperature synthesis process.And with the increase of reduction degree,the material changes from oxygen ion conductor to electron conductor.The bond-valence-based method shown that the oxygen ions are most likely transport via two-dimensional paths in the perovskite slabs.（2）Defect engineering and ionic conduction of layered perovskite Sr₂Nb₂O₇materials.Three methods were used to create oxygen defects,such as donor-,acceptor-and equivalent doping of different ions at Sr and Nb sites for Sr₂Nb₂O₇materials;The raw material Nb₂O₅ was replaced by Nb O₂ to prepare Sr₂Nb₂O₇materials;The topological reduction for Sr₂Nb₂O₇ material with different molar ratio of solid reducing agent Ca H₂.The conductivity measurements shown that the three methods could not improve the conductivity of Sr₂Nb₂O₇.（3）Conductivity study of the composite of Na₂W₂O₇ and Sr₂Nb₂O₇ materials.The composites with different mass ratios（denoted as x W-y N）were calcined at1000℃,it was found that for the composite with the ratio of Na₂W₂O₇/Sr₂Nb₂O₇higher than 5:5,there would be an abrupt increase in conductivity at 600℃,reaching10^-2S/cm.This method can improve the conductivity,but XRD measurements shown that the Na₂W₂O₇ would react with Sr₂Nb₂O₇ and yield new main phase peaks of Sr WO₄ and Na Nb O₃.DSC measurement on the 6W-4N composite revealed that there were three exothermic peaks at 600℃,which may related with the impurity of Na₂WO₄.（4）Cations doping and electrical properties of mayenite Ca₁₂Al₁₄O₃₃.Two methods of aerodynamic levitation laser heating method and solid phase reaction were used to synthesis materials of doping equivalent In³⁺at Al site and donor Y³⁺at Ca site.The results shown that the aerodynamic levitation laser heating method can successfully prepare Ca₁₂Al_13.9In_0.1O₃₃ materials which cannot be synthesized by solid phase method.However,this method cannot expand the solution limit of Y³⁺doped at Ca site.In addition,Ca₁₂Al_14-xCu_xO_33-λ（0≤x≤0.4）materials were synthesized by solid phase reaction method.However,none of the above three methods improved the materials conductivities.