Preparation Of Lanthanum Silicate Oxyapatite Electrolyte Materials By Combustion Process

Apatite-type La_9.33（SiO₄）₆O₂is a new solid state electrolytes with greationic conductivity and stability at intermediate temperature. It is of greatsignificance to implementation of commercial application of La_9.33（SiO₄）₆O₂,through adopting appropriate preparation technology to lower synthesistemperature and improve ionic conductivity at lower temperature.In this thesis, the urea-nitrates combustion was introduced to prepare theLa_9.33（SiO₄）₆O₂electrolyte at lower temperature, and the gelation time ofprecursor was shortened by utilizing adequate solvent system. Theas-prepared samples were investigated by DCS-TGA, XRD, SEM and laserparticle test method. The results suggested that the amorphous electrolytepowders, calcined at a temperature as low as800℃for12h, transformed intowell-crystallized apatite-type La_9.33（SiO₄）₆O₂powders with an average size of80nm.Parallel experiments and the thermodynamics analysis were applied tostudy the role of nitric acid during the combustion process. The resultsindicated that nitric acid was not only the solvent of rare-earth oxide but alsothe supplier of ammonium nitrate. The most well-crystallized pureLa_9.33（SiO₄）₆O₂powders could be prepared corresponding to the optimalmolar ratio （R=10） of nitric acid/La2O3.Batch experiment revealed the density of La_9.33（SiO₄）₆O₂sintered bodywas influenced by the pretreatment, forming pressure, sintering temperatureand holding time. The results showed that introduced attrition pretreatmentand fitting forming pressure of225MPa could eliminate inter-agglomerateand lower sinter point, it was benefit to improve density of the samples efficiently. Dense ceramic with a relative density of96%was obtained bysintering the green compact of these La_9.33（SiO₄）₆O₂nanopowders at1400℃for3h with a heating rate of2℃/min. Electrochemical impedancespectroscopy （EIS） was utilized to study the electrical property of the sinterbody. The results demonstrated that La_9.33（SiO₄）₆O₂ceramics possessed wellresistors reversibility and stability. The relationship between sample’sconductivity and temperature were in accord with empirical formula ofArrhenius. The sintered body shows high ionic conductivity (4.38×10^-3S·cm^-1at700℃).The experiments of Sr-and Nd-doped LSO were performed to study theinfluence of dopants on the properties of LSO. The results proclaimed thatthere were little effects of doping on the crystal structure and feature of LSO.However, the transport properties of oxygen ion could be improved efficientlyby proper amounts of Sr and Nd dopants. The ionic conductivities ofLa_9.33M_x（SiO₄）₆O_2+δ（M=Sr、Nd） reached a maxium of δ_Sr=7.248×10^-3S cm^-1and δ_Nd=1.782×10^-2S cm^-1at500℃, corresponding to the optimal dopingcontent of x=0.3. The preliminary study on the conduction mechanism ofdoped electrolyte was carried out, this thesis thought that apatite-typeLa_9.33M_x（SiO₄）₆O_2+δvia interstitial oxygen ion conduction mechanism, and theinterstitial oxygen ion made a more effect on the ionic conductivity thancation vacancy. The oxygen ion conductivity was improved because of Sr andNd doping increased the amounts of interstitial oxygen ion.