A或B位置金屬離子取代鈰酸鑭燒綠石(La,M)2(Ce,N)2O7 (M=Yb, N=V)於物理及電化學性質之影響

Abstract

本篇論文以溶膠-凝膠法(sol-gel method)分別合成鐿離子取代在A位以及釩離子取代在B位之La2Ce2O7化合物，並探討金屬離子摻雜對材料物理及電化學性質之影響。 對於(La1-xYbx)2Ce2O7系列，在X光繞射圖譜中發現，於0 ≤ x ≤ 1，可以合成出單一純相，其組成也透過感應耦合電漿原子發射光譜儀及能量散射光譜儀得分析獲得證實。經由結構精算可以發現隨著Yb取代量增加，結構單位晶格是逐漸下降的。從熱程控還原反應實驗結果得知樣品在400-750oC會與氫氣反應，且至900oC仍能維持結構穩定性。本材料之碇材緻密化最佳燒結條件為1600oC之下持溫1小時，其相對密度可達95%以上，足以供後續導電度量測使用。使用交流阻抗分析譜量測離子導電度，發現在x=0.1時，離子導電度有顯著的提升，在600oC至900oC之下導電度為3.5×10-3 - 6.9×10-2 S/cm 。當x>0.1時，離子導電度會開始下降，這是由於燒綠石晶體結構的轉變，此結果可由X射線光電子能譜儀的分析結果證實。而此材料的導電度隨著氧氣分壓的增加而提升，是屬於P型半導體的導電性質。最後由四點探針進行電子導電度的測試，整個系列化合物都有著較低的電子導電度，並且在氫氣及氧氣底下沒有太大的差異，於600oC-900oC電子導電度約為10-5 – 10-3 S/cm。綜合以上性質，鐿離子取代La2Ce2O7之化合物相當有潛力作為固態氧化物燃料電池之材料。 對於La2(Ce1-yVy)2O7系列，在0 ≤ y ≤ 0.2可以合成出純相化合物，並透過元素組成的分析作證實。晶格精算結果發現材料的晶格常數隨著釩離子取代量增加而下降。從熱程控還原反應實驗得知樣品在400-750oC會開始與氫氣反應，並且至900oC尚能維持結構穩定性。供電性測試使用而壓製成的碇材的最佳燒結條件為1500oC持溫一小時，其相對密度可以達到95%以上。此系列化合物離子導電度隨著摻雜比例增加而上升，於y=0.2時有最大值，於600oC-900oC，離子導電度為3.7×10-3 - 10-1 S/cm，其導電度略高於YSZ而相近於(La0.9Yb0.1)2Ce2O7. 此部分之材料仍待更完整的研究，例如X射線光電子能譜儀分析以及電子導電度測試。

In this thesis, series of (La1-xMx)2(Ce1-yNy)2O7 (M=Yb or N=V) were prepared by sol-gel method to study the influence of metal cation to their physical and electrochemical properties. For (La1-xYbx)2Ce2O7 series, single-phase were observed in XRD with a substitution range of 0 ≤ x ≤ 1 and their compositions were confirmed by ICP-AES/SEM-EDS. The refined cell constants decreased as the amount of substituted-Yb increased. According to the result from TPR, these materials react with hydrogen in the temperature of 400-750oC and their structures remain stable up to 900oC. Dense ceramic pellets for the materials were obtained through the optimal sintering condition, at 1600°C for 1 h under air, and the relative density of the pellets reached 95%. Total ionic conductivity increases significantly with x = 0.1 of substituted-Yb content. For (La0.9Yb0.1)2Ce2O7, the measurement of ionic conductivity via electrochemical impedance spectroscopy showed that the conductivities were in a range of 3.5×10-3 – 6.9×10-2 S/cm at 600-900oC. When x>0.1, the drop trend in total ionic conductivity with increasing Yb contents were attributed to the crystal phase transformation of pyrochlores, which could be speculated by XPS analysis. The total ionic conductivity rises as oxygen partial pressure increased, indicative of a p-type semiconductor. The electronic conductivity of (La1-xYbx)2Ce2O7 measured via four probe method were in a range of ~10-5 – 10-3 S/cm at 600-900oC, and exhibited no significant difference between the results measurement under O2 and H2. The Yb-substituted La2Ce2O7 exhibits high total conductivity and low electronic conductivity compare to that of electrolyte materials Y0.16Zr0.84O2 (YSZ), which is a potential electrolyte material for solid oxide fuel cell. For La2(Ce1-yVy)2O7 series, pure phase were observed in a range of 0 ≤ y ≤ 0.2 and their compositions were analyzed by ICP-AES/SEM-EDS. The refined cell constants decreased as V contents increased. From the TPR result, these materials started to react with hydrogen in the temperature range of 400-750oC and they exhibited structural stability up to 900oC. The dense ceramic pellets were sintered at 1500oC for 1h under air, and the corresponding relative density is over 95%. Total ionic conductivity increases with the substituted-V content. La2(Ce0.8V0.2)2O7 exhibited high total ionic conductivity of in the range of 3.7×10-3 - 10-1 S/cm at 600-900oC, which is higher than the total ionic conductivity of YSZ and similar to (La0.9Yb0.1)2Ce2O7.