Synthesis,Structure And Electrical Properties Of LaSrGa₃O₇-based Melilite Materials

With the scarcity of traditional energy sources,the development of new energy has become an inevitable trend of development.Clean,noiseless,efficient and compressible fuel cell technology is one of the most effective technologies for solving environmental pollution caused by fossil fuels^[1].Solid oxide fuel cells?SOFCs?consist of three main components:electrolyte,cathode and anode.Among them,the conductivity and thickness of electrolyte play a key role on determing the operating temperature of SOFCs.Oxide ion conductivity over 10^-2 S/cm for the electrolytes is necessary for the practical application in SOFCs.This paper aims to screen out oxygen ion conductors in the ABGa₃O₇ melilite structure system by introducing interstitial oxygen or oxygen vacancy defects through not equivalent substitution,and design and synthesize new oxide ion conductor.These results would provide new thought for designing and prepare oxide ion conductors.The specific findings are list as follows:For the La³⁺and Sr²⁺ions in LaSrGa₃O₇,the cations with similar effective ionic radius similar to that of La³⁺and Sr²⁺ions were selected to substitute for La/Sr,such as Bi³⁺,Y³⁺and Pb²⁺to dope the La³⁺ions or Sr²⁺ions.A series of LaBixSr1-x Ga3O7+0.5x?0?x?0.4?,La_0.5Y_0.5SrGa₃O₇ and La_1+xPb_1-xGa₃O_7+0.5x?0?x?0.2?materials were synthesized by a conventional solid state reaction method.Rietveld refinement and energy dispersive spectrum?EDS?analysis confirmed that the Bi³⁺ions occupied La/Sr sites and introduced interstitial oxygen ions into the structure.The formation energy of this interstitial defect calculated by the density functional theory?DFT?is about 2.21 eV,which is consistent with the high solid solubility of Bi³⁺in LaSrGa₃O₇ obtained from experimental data.The substitution of Bi³⁺ions for Sr²⁺ions can significantly enhance the oxygen ion conductivity.For example,for a composition of x=0.4 in LaBixSr1-xGa3O7+0.5x?0?x?0.4?,the electrical conductivity is 1.62?10^-2 Scm^-1 at 800°C,making these materials promising for practical applications in SOFC and oxygen separation membranes.At the same time,it is also proved that Pb²⁺ions and Y³⁺ions also enter the lattice of LaSrGa₃O₇ and occupy the La/Sr position.For the Ga³⁺ions in LaSrGa₃O₇,the cations with similar effective ionic radius to Ga³⁺ions,such as Fe³⁺?Co²⁺?Ni²⁺?Nb⁵⁺?Sn⁴⁺and Mo⁶⁺ions were selected for doping.A series of LaSrM₃O₇ melilite structure materials were also synthesized using traditional solid state reaction methods.XRD data and Rietveld refinement analysis verify that Fe³⁺ions and Co²⁺ions are more readily for doping into Ga³⁺ions than other ions,try to change the La/Sr ratio in the La/Sr position to increase the oxygen ion conductivity of the material.This article conducts electrical performance testings and analysises of this series of materials.For the Fe³⁺and Co²⁺doped materials,it was found that the ionic conducting properties were not apparently,while electronic conduction was introduceds.