Preparation and properties of doped La2Mo2O9 oxide as electrolyte materials for solid oxide fuel cell

• Main Author: 謝孟儒
• Other Authors: 蔡大翔
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/37826388308358646609

碩士 === 國立臺灣科技大學 === 材料科技研究所 === 92 === The current study investigates the relations among composition, ion conductivity, thermal expansion, and structure of A-site substituted La2Mo2O9. La2Mo2O9 is a recent-discovered oxygen ion conductor by Lacorre, its 800C conductivity is near 0.08 S cm-1, higher than yttria-stabilized zirconia. La2Mo2O9 exhibits a phase transition near 560C, its high temperature structure resembles -SnWO4, space group P213. After phase transition, the ion conductivity and the thermal expansion jump to higher values. This investigation searches the best composition of Lanthanide rare-earth or other elements substituted La2Mo2O9. The definition of best composition is based on the following criteria; effective depression of phase transition, high ion conductivity, and thermal expansion matching with that of yttria-stabilized zirconia. We have prepared substituted La2Mo2O9 through sintering at 930C, since the uniaxially-pressed specimens are sintered to the highest relative densities at this temperature. We found that 10mol% Sm2O3, Gd2O3, Dy2O3, Er2O3, Yb2O3 substituted La2Mo2O9 exhibit no thermal events during scanning the temperature range of 30-800C using differential thermal analysis, while 10mol% CeO2 and Nd2O3 do. X-ray diffraction analysis using synchrotron analysis indicates that low-temperature phase of La2Mo2O9 possesses ordered structure of superstructure owing to the oxygen vacancies, but the X-ray diffraction patterns of (La1.8Gd0.2)Mo2O9 before and after the probable phase transition temperature are the same. The diffraction work shows that Gd and several rare-earths are capable of depressing the phase transition. Measurements of thermal expansion coefficients using thermal mechanical analysis indicate that the compositions showing phase transitions also exhibit obvious jumps in thermal expansion at their phase transition temperatures. With respect to the matching between the substituted La2Mo2O9 and zirconia in thermal expansion coefficient, the composition of (La1.8Dy0.2)Mo2O9 possessing a thermal expansion coefficient of 10.3310-6C and that of (La1.8Er0.2)Mo2O9 10.6110-6C are near that of yttria-stabilized zirconia. Both of (La1.8Dy0.2)Mo2O9 and (La1.8Er0.2)Mo2O9 do not exhibit phase transition. Among all the 10 mol% substituted La2Mo2O9, the 700C ion conductivities of (La1.8Dy0.2)Mo2O9 and (La1.8Er0.2)Mo2O9 are the highest, 0.26 Scm-1 approximately. In addition, the 500C ion conductivity of (La1.8Gd0.2)Mo2O9 is higher than the rest of 10 mol% rare-earth substituted La2Mo2O9. From the relation between ion conductivity and ion concentration, we find that doping of 1 mol% Dy and Er can effectively depress the phase transition influence on ion conductivity, doping of 5 mol% Dy and Er can eliminate the phase transition influences. On the other hand, the composition of 10mol% Gd appears to show a second order phase transition in the Arrhenius plot of ion conductivity. For those compositions displaying phase transitions, hysteresis in ion conductivity is also observed. In summary, we have found that the Dy and Er-substituted La2Mo2O9 possess the capability of effective depressing phase transition, higher ion conductivities, and lower thermal expansion coefficients. This paper proposes the La2Mo2O9 and the Yttria-Stabilized Zirconia of the complex electrolyte. We find that ion conductivity of Yttria-Stabilized Zirconia films lower than disks, but when the operating temperature rises, the influence not too great of ion conductivity on complex electrolyte, and the one that can improve the La2Mo2O9 is resisted the shortcoming not good of the reducing atmosphere.. They are potential candidates in the La2Mo2O9 family to be used as the electrolyte of intermediate temperature SOFC.