The High-Temperature Structure and Conductivity of Oxygen Ion Conductor La2Mo2O9

• Main Authors: Tseng , Jang-Chumg, 曾讓忠
• Other Authors: 蔡大翔
• Format: Others
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/24724127944020409448

碩士 === 國立臺灣科技大學 === 化學工程系 === 90 === This thesis investigates the relations among the compositions, ionic conductivities, and crystal structures of oxygen ion conductor La2Mo2O9. The high-temperature form of La2Mo2O9 has a structure similar to that of b-SnWO4 crystal, space group P213, which is different from the conventional oxygen ion conductors. The undoped La2Mo2O9 exhibits an ionic conductivity of 0.06 S cm-1 at 800°C, which is higher than that of yttrium-stabilized ZrO2. It shows a first-order phase transition near 580°C. Its ionic conductivity increases drastically after the phase change. We synthesized the doped and undoped La2Mo2O9, using the solid state reaction method. X-ray diffraction patterns of quenched La2Mo2O9 powders indicate the high temperature form gradually converts to the low temperature form at room temperature. This fact seems to point out that the phase transition is reversible. The phase transition of La2Mo2O9 is inspected by the differential thermal analysis. La2Mo2O9 doped with 10 mol% Bi, Gd, Sm, or Yb (at La site) exhibits no thermal event up to 800°C, meanwhile, samples doped with 10 mole% Ca, Nd, Ce show marked endothermic peaks at 577, 570, 566°C, individually. The undoped La2Mo2O9 displays an endothermic peak at 564°C. The sintered density of a powder compact, which is uniaxially cold-pressed, reaches 95.9% relative density at a temperature of 930°C. Measurements on the grain size of sintered bodies indicate that doping 10mol% Bi and Gd increases the grain size, and doping 10mol% Ca has an insignificant effect on the grain growth. Microstructure analysis of 10mol% Ca sample indicates grain boundary precipitates, and its density is lower than other doped samples. Much higher resolution on X-ray diffraction was achieved, using the synchrotron radiation source. The reflection patterns under various temperatures point out that La2Mo2O9 of low temperature form exhibits superlattice reflections, owing to its ordered oxygen vacancies. The low temperature form demonstrates more complex reflection patterns before (110), 2q=17.4°, than that of the high temperature form. Seven weak reflections that vanish after the phase transition are preliminarily identified as the superlattice reflections. The ionic conductivity, between 300-800°C, was measured by AC impedance spectroscopy. We found that doping 10mol% Bi, Gd, and Ca can enhance the ionic conductivity at 800°C. Among the specimens of the 10mol% Bi, Gd, Ca doped La2Mo2O9 and undoped La2Mo2O9, the calcium doped specimen possess the highest ionic conductivity. The Arrhenius plot of ionic conductivity versus reciprocal temperature shows that the high temperature activation energy of undoped La2Mo2O9 is 8.9 kcal/mol, and the low temperature activation energy is 24.9 kcal/mol. The activation energy of 10mol% Bi is 29.8 kcal/mol. The high temperature activation energy of 10mol% Gd is 8.6 kcal/mol, and the low temperature activation energy is 34.9 kcal/mol. The high temperature activation energy of 10mol% Ca is 11.9 kcal/mol and the low temperature activation energy is 25.3 kcal/mol. The temperature dependence of ionic conductivity is generally higher in the low temperature region.