Preparation And Performance Of Apatite Type Lanthanum Silicate Electrolyte Materials Doped In Silicon Site

With excellent ionic conductivity and lower activation energy, hexagonal apatite-typeLa9.33(SiO4)6O2（LSO for short）, a new kind of solid state electrolyte materials, areconsidered potential electrolyte candidates for solid oxide fuel cells at medium and lowtemperature. A series of researches indicate that, it is an effective approach to enhance theoxygen ion conductivity of LSO by adopting appropriate preparation technology anddoping modification.In this thesis, the La9.33Si6-xMxO26+0.5x（LSMO for short）conductor was obtained bythe urea-nitrates combustion. Compared with the traditional preparation methods, thegelatin time and the synthesis temperature of powder were effectively reduced by thecombustion. Aluminum, titanium and vanadium with different valence states were selectedas doping elements of Si site.The phase, element and crystal structure of as-prepared electrolyte powders wereinvestigated by the measures, such as XRD, XPS, EDS and FT-IR. The results suggestedthat the amorphous powders transformed into well-crystallized apatite-type LSMO at acalcination temperature as low as800℃for12h. The analyses also showed that theelements Al, Ti, V doped into the crystal lattice of LSO, which did not affect its apatitetype crystal structure.Combined with the microstructure analysis of the sintered LSMO, the relation curvesbetween the relative density and the sintering temperature showed that the best sinteringtemperature of LSMO doped Al, Ti, V were respectively to be1500℃,1500℃and1450℃.The electrochemical property of the sintered LSMO was measured by theelectrochemical impedance spectroscopy. The results demonstrated that the relation curvesbetween sample’s conductivity and testing temperature were in accordance with theempirical formula of Arrhenius. The oxygen ionic conductivity of LSMO with the samedoping amount and same testing temperature showed that the electrochemical performanceof LSMO doped V5+was superior to LSMO doped Al3+. In view of the LSMO doped V5+,the optimal doping amount is1.0, and the conductivity of La9.33Si5VO26.5could reach to1.46×10-2S·cm-1at test temperature of800℃. However, LSMO doped Ti4+showed lower conductivity, which indicated that equivalent ion was not beneficial to improve theconductivity. When the doping amount was too high, the conductivity of the LSMO dopedabove three kinds of ions all appeared different degrees of decrease.Based on the interstitial oxygen conduction mechanism, the two kinds of point defectmodels which are “interstitial oxygen” and “defects composite center” were establishedand researched. Through analyzing the relationship among the conductivity, the dopingelement of different valence state and the dosage of the doping element, the enhancementmechanisms of interstitial oxygen defect concentration and space conductance were alsoput forward.