| Summary: | 碩士 === 淡江大學 === 化學學系碩士班 === 100 === In this study, two series of oxides and their carbonate composites with formula as R2(Zr,Ti)2O7 (R = Y, Gd, Sm, Nd) were prepared. Dense R2Zr2O7 and R2(ZrTi)O7 oxides were sintered at 1600oC and 1500oC, respectively. Dense composites were prepared by immersing porous oxides into 1:1 molar ratio of Li2CO3 and Na2CO3. When cation radius ratio (rR/r(Zr,Ti), 6-coordination number used for the Zr4+ and Ti4+ ions) is ≥ 1.48, samples have pyrochlore phase; on the other hand, they have fluorite phase. Electrical conductivities of the R2Zr2O7 and R2(ZrTi)O7 are in the reange of 10–4 and 10–5 S‧cm–1, respectively that is related to the radius of the R cation and no apparent relation with the pyrochlore and fluorite phase transformation is observed. Activation energy (Ea) of these oxides is in the range of 0.52–1.30 eV that decreases with increasing of the unit cell a-axis. For the same R cation, Ea of the R2Zr2O is higher than that of the R2(ZrTi)O7. For the composites, at temperature lower than the melting point of the carbonates, 495oC, conductivity is higher than that of the oxides for about 2–3 orders. On the other hand, at higher temperatures, conductivity closes to 10–1 S‧cm–1 with Ea in the range of 0.05–0.21eV. Ionic transference numbers (ti) of the R2(ZrTi)O7 is 100%. Nevertheless, for the R2(ZrTi)O7 series, ti is 100%, 90–91% and 81–83% at 500oC, 600oC and 700oC, respectively. Ti atoms are in the mixed-valence state with a valence of 3.4+. Increasing temperature, electronic conduction increases. At 490oC, after 11 d, conductivity of the Dy2(ZrTi)O7–composite is 4.65(5) x 10–2 S‧cm–1 and levels off. All of the composites studied in this research are potential electrolytes for using in the SOFC at temperatures slightly lower than 500 oC.
|
|---|
