固態氧化物燃料電池之電解質以其氧離子導電之特性在電化學元件中扮演著重要的角色，氧離子導體材料之結構具有較高的氧離子空缺濃度，以利於氧離子的傳送，氟化鈣型結構之立方相氧化鉍氧空缺濃度高達25%為氧離子導電率最高之固態氧離子導體(2.3 S/cm 於800℃)，而此缺陷結構也造成穩定性不佳，為了獲得較好的氧離子導電性質，必須將此缺陷氟化鈣型之立方結構穩定化，本研究以氧化鉍系固態電解質為主，探討其氟化鈣結構之穩定性，分為三個子題：(1)立方相與菱方相釔安定氧化鉍相之穩定性(2)添加氧化鈮之鉍鈣氧化物固態電解質及(3)以鉍金屬氧化合成氧化鉍。
添加氧化釔氧化鉍(Bi2O3-Y2O3)在氧化鉍系固態電解質中最具應用潛力，其中以25YSB(添加25%Y2O3之Bi2O3，Y0.5Bi1.5O3)格外受到重視，然而，在低於800℃操作溫度的環境下(600℃)，25YSB發生立方相至菱方相之相變化，導電率下降，立方相與菱方相兩種結構對於高溫操作條件所面臨的水氣氛，立方相較菱方相穩定，而根據氟化鈣型結構立方相之晶體結構，同樣有25%氧離子空缺濃度，卻因為熱處理，而使得氧離子空缺重新排列並集中於特定位置，造成 (111)面位移，形成菱方相，因而改變了特性。
基於立方相與菱方相之間的關係，改變氧離子空缺濃度或陽離子之大小可抑制菱方相之形成，以添加鹼土族氧化物之氧化鉍(Bi2O3-MO，M=Ca2+、Sr2+及Ba2+)而言，在常溫即可獲得菱方相，就缺陷化學之觀點，Bi3+被M2+取代時，使其晶格產生更多之氧空缺(>25%)，唯過高之缺陷濃度使其無法形成氟化鈣型結構之立方相而以菱方相穩定存在。此外，添加價數大於三之陽離子，例如Nb5+，除可降低缺陷濃度外，亦可將部分立方相穩定至室溫。本研究發現，添加6mol%氧化鈮於Bi1.4Ca0.3O2.4中，可獲得氟化鈣型結構之單一立方相(Bi1.4Ca0.3Nb0.06O2.55)，由於此成分具有27.56%之氧空缺濃度而使其導電率高於任何氧化鉍系固態電解質(1.97 S/cm 於800℃)，藉由高溫熱處理探討添加氧化鈮對其菱方相鉍鈣氧化物結構變化之影響，發現在600℃熱處理50小時，具有超晶格結構之第二立方相會逐漸析出，藉由擇區繞射，此超晶格同時具有立方結構以及菱方結構之對稱，除了氧空缺的集中之外，Ca2+與Nb5+也集中在(111)原子層，而此特殊之結構，經100小時之熱處理後，則形成另一菱方結構。
除了利用添加的方式探討氧化鉍系固態電解質之氟化鈣型結構之穩定方式外，本研究也利用金屬氧化之方式製備氟化鈣型結構之純氧化鉍。首先，將硝酸鉍溶解於乙二醇與水的混合溶液中作為電鍍液，藉由陽極化的氧化鋁模版(AAO)輔助之奈米結構化製程，直接以電鍍之方式製備鉍金屬奈米線後，再將試片於250~350℃氧化，即可獲得氟化鈣型結構之氧化鉍之奈米線，藉由穿透式電子顯微鏡探討其金屬/氧化物介面之晶體關係，發現氟化鈣型結構之純氧化鉍與菱方結構之鉍金屬皆具有相同原子組態之晶面，經快速氧化，氧離子迅速地擴散至鉍金屬中並形成離子鍵結，而此晶面於Bi3+排列組態不變的情況下，位移a/2則成為氟化鈣型結構。
以氟化鈣型結構之晶體而言，陰陽離子之半徑比必須介於0.732與1之間，但若於陰離子空缺存在的情況之下，則此比值並非絕對條件，本研究發現，藉由添加以及氧化的方式可合成新的氧化鉍系固態電解質或穩定純氧化鉍，而使得具有高濃度氧離子缺陷之氟化鈣結構得以在室溫穩定存在。

In this study, stabilization of CaF2-typed cubic phase of bismuth-based solid electrolyte is investigated. The analyses of crystal structure of cubic phase are carried out in the following sections: (1)effect of the crystal structure on the stability of (Y0.25Bi0.75)2O3 solid electrolytes (2)Nb2O5–doped Bi2O3-CaO solid electrolyte (3)preparation of Bi2O3 by oxidation of bismuth metal.
Highly conductive cubic (Y0.25Bi0.75)2O3 tends to transform to rhombohedral (Y0.25Bi0.75)2O3 when annealed at 600℃ for more than 200 hours. Although the rhombohedral phase of (Y0.25Bi0.75)2O3 was known to be the stable phase at temperatures <600°C, it was found that the annealed (Y0.25Bi0.75)2O3 was not thermodynamically stable in the water-containing environment. From XRD and TEM analyses, it was observed that the annealed (Y0.25Bi0.75)2O3 easily decomposed into monoclinic α-Bi2O3 and yttrium hydroxide at the temperature as low as 50°C. The monoclinic α-Bi2O3 further reacted with CO32- and formed Bi2O2CO3. Consequently, the annealed (Y0.25Bi0.75)2O3 degraded and became flaky powder. SEM micrographs of water-reacted (Y0.25Bi0.75)2O3 also show surface swelling and peeling. Such surface deterioration was caused by a large volume increase during the water reaction. Similar reaction was also observed when the annealed (Y0.25Bi0.75)2O3 was exposed in the humidified air at 300°C. As the temperature was raised to 500°C, little reaction was observed between water vapor and (Y0.25Bi0.75)2O3. The better stability of (Y0.25Bi0.75)2O3 at elevated temperature was observed.
A new δ-phase obtained in the bismuth–rich portion of Bi2O3-CaO-Nb2O5 system was investigated. Owing to the defect reactions in Bi-based oxide, substitutions of Bi3+ by Ca2+ and Nb5+ will produce and reduce the concentration of oxygen vacancy, respectively. Furthermore, Ca2+ and Nb5+ also play the opposite roles on the stability of δ-phase. In order to obtain a fluorite structured phase from rhombohedral Bi1.4Ca0.3O2.4, Nb2O5 is added and leads to phase transformations from rhombohedral to cubic. As a result, a single δ-phase (Bi1.4Ca0.3Nb0.06O2.55) containing 27.59% oxygen vacancy is successfully prepared by 6 mol% Nb2O5 doping. Structural characterizations are examined by the XRD, SEM, and TEM analyses. In addition, dynamic conductivity measurement also shows no transformation from room temperature to 800℃.
δ-Bi2O3 nanowires were successfully fabricated by thermal enhancement on electroplated Bi nanowires. Bi was first prepared using template-assisted electroplating. After thermal modification at 350℃ for 12h in air, Bi2O3 nanowires with high temperature phase (δ-phase) were obtained over a large area. High aspect ratio nanowire arrays were observed by SEM. To investigate the in-situ Bi to Bi2O3 phase transformation, the as-prepared nanowires were examined by high temperature X-ray diffraction (HTXRD) in air. Selected area electron diffraction (SAED) was used to investigate the morphology and structure of nanowires. A Bi/Bi2O3 core-shell structure was also examined by HRTEM after the Bi nanowires heated at 250℃ for 12h. The oxidation reaction of Bi occurrs at temperatures below the melting point of Bi (271.3℃). According to the analysis of HRTEM, the formation of high temperature δ-phase is caused by the coherent relationship between Bi and Bi2O3 nanowires.

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