材料組成與晶相結構對二氧化鋯基底固態電解質微結構與離子輸送影響之分子模擬解析

Effect of Material and Phase Compositions on the Micro-structure and Ionic Transport of Zirconia-based Solid Electrolyte: A Molecular Simulation Study

張楷焄(Kai-Shiun Chang)

指導教授 ： 童國倫

中原大學/工學院/化學工程研究所/博士(2011年)

https://doi.org/10.6840/cycu201100027

摘要

本研究以分子動態 (molecular dynamics, MD) 模擬技術解析二氧化鋯基底固態電解質之微結構與氧離子輸送機制，並討論混摻物種濃度、晶粒邊界 (grain boundary) 、混摻物材料組成與晶相結構等重要因素對固態電解質效能與穩定性影響。本文採用離子間作用力、徑向分佈函數、材料自由體積、與陽離子均方位移於原子尺度下分析材料結構；並以氧離子均方位移量、運動軌跡、位移量值分佈、擴散係數與離子導電度討論離子微觀輸送機制。由結果得知，由 Y2O3 混摻之 YSZ (yttria-stabilized zirconia) 最佳添加濃度約為 7 mol% ，係由 Y2O3 濃度提升而增加的氧離子空缺 (oxygen vacancy) 數與降低的空缺移動性相互競爭所決定。此外， YSZ 材料中存在的晶粒邊界明顯提高氧離子質傳阻力而降低氧離子移動性。此晶界影響對離子輸送行為影響隨溫度提升(高於1173 K 後)而漸減，係因高溫下離子含較高動能可越過晶界產生之能量障礙所致。並於複和混摻劑影響分析中發現，在 YSZ 材料中混摻 Sc2O3 (Sc-Y-SZ) 對長距離的離子運動有顯著提升，進而提高離子導電度。導電度的提升主因所添加的 Sc+3 具較小的離子半徑，可增加系統內自由體積，同時降低離子躍遷時能量障礙。但隨 Sc2O3 混摻濃度漸增，系統中陽離子 (Sc+3) 擴散行為亦趨，易形成結構形變 (structure deformation) 而降低材料穩定性。隨後，本文亦分析雙相結構 Sc-Y-SZ (dual-phase Sc-Y-SZ) 內氧離子輸送機制。由結果得知，雙相 (cubic/monoclinic) Sc-Y-SZ 中所引入的 monoclinic 結構提高晶粒內、晶粒間 (intra-/inter grain) 質傳阻力，而顯示出較單一 cubic 結構為低的離子導電度。同時，雙相結構 Sc-Y-SZ 模型可表現出真實材料並非完美結構的本質，進而獲得更為準確的定量結果。本論文採用 MD 模擬技術解析二氧化鋯基底固態電解質特性，結果驗證微觀理論方法於材料開發設計可行性與準確性。

關鍵字

晶界 ； 複合混摻物 ； 晶相結構 ； 分子動態模擬 ； 釔穩定態氧化鋯 ； 固態電解質

並列摘要

The molecular dynamics (MD) simulation was adopted to analyze the micro-structure characteristic and ionic transport performance of zirconia-based solid electrolyte in this work. Several key issues were investigated here, such as the dopant concentration, grain boundary resistance, complex dopant, and phase composition. The inter-ionic interaction, radial distribution function, free space, and cation mean-squared displacement (MSD) were adopted to discuss the solid electrolyte at an atom scale; and the oxygen ion MSD, travelling trajectory, displacement profile, diffusivity, and ionic conductivity were analyzed to study the ion transport mechanism. The optimized YSZ concentration locates around 7 mol%, which was attributed by the competition between the increased vacancy number and decreased vacancy mobility with the increase of Y2O3. In addition, the presence of grain boundary obviously lowered the oxygen ion mobility due to the apparent mass transport resistance. The grain boundary effect would be gradually eliminated at elevated temperature (beyond 1173 K) since the energy barrier was overcome by the improved ion kinetic energy. In the case of complex dopant, doing the Sc2O3 in YSZ (scandia-yttria-stabilized zirconia, Sc-Y-SZ) clearly raised the effective and continuous ion displacement, resulting in the released free space and migration barrier through the addition of Sc+3. However, the increase of Sc2O3 also lowered the structure durability because of the higher extent of cation diffusion (Sc+3). The excess of cation diffusion might lead to the deformation of cubic phase, lowering the ionic conductivity. In the fourth part, the oxygen ion diffusion mechanism in dual-phase (cubic/monoclinic phase) Sc-Y-SZ solid electrolytes was investigated. The inserted monoclinic phase enlarged in intra-/inter-grain resistance for oxygen movement and therefore lowered the ionic conductivity, especially in the high Sc2O3 system. The dual-phase Sc-Y-SZ reflected the imperfect nature of real material, which provided the more accurate ionic conductivity in comparison to that of cubic phase model. This thesis adopted MD technique to characterize the zirconia-based solid electrolyte, which validate the feasibility and accuracy of the theoretical work on the material development at a microscopic view.

並列關鍵字

phase structure ； YSZ ； grain boundary ； complex dopant ； solid electrolyte ； molecular dynamics simulation

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