臺灣博碩士論文加值系統

本篇以分子動力模擬的方法，分別研究鋰鹽LiBF4在diethyl carbonate(簡稱DEC)、propylene carbonate(簡稱PC)及其混合溶劑中的導電性。首先由鋰離子之均方位移求得其在溶液中之擴散係數，並以Nernst-Einstein公式估算其導電度。其次以自由鋰離子出現之機率修正導電度值，並由徑向分布函數圖計算鋰離子周圍之溶劑及陰離子總數。最後利用stress autocorrelation function對時間之作圖計算系統之黏度。
我們發現當DEC與PC以重量比1:1混合時，溶液之導電度隨鹽類濃度之遞增而先上升後下降，此與 Moumouzias所測得的實驗值相符。若把鹽類濃度固定而改變溶劑之混合比例，則發現導電度會隨著DEC比例之增加而下降。徑向分布函數圖顯示溶劑分子較陰離子接近鋰離子，即鋰鹽的正負離子被溶劑所隔離。此外，鋰離子周圍第一層的溶劑個數，並不隨鹽類濃度與溶劑之組成而改變，其值皆趨近於4，此結果與Soetens推測的四面體排列一致。
計算所得之鋰鹽溶液黏度值，皆比不含鋰鹽之純溶劑黏度值高;且當鋰鹽濃度增加時，黏度之計算值也隨之增高。但由Li+與週遭PC溶劑之距離隨時間變化的圖形得知，鋰附近第一與第二殼層之溶劑互換情形非常頻繁，而非如Soetens所述在整個模擬時程內第一殼層之四個溶劑分子均與鋰離子緊密結合，未曾變動。這可能是因為本文所用之鹽類濃度較高，使得溶劑層內分子易受其他正、負離子吸引所致。

Molecular dynamic simulation method has been used to study the conducting behavior of LiBF4 in diethyl carbonate(DEC)、propylene carbonate(PC) and their mixing solvent. The diffusion coefficient of lithium ion in the solution was computed firstly from its mean-square displacement. Utilizations were then made of the Nernst-Einstein equation to estimate the conductivity. The probability of finding free lithium ion was also calculated to revise the computed conductivity. The average numbers of solvent and anion around the lithium ion were also calculated from the plot of radial distribution function. Finally, the stress autocorrelation function vs. time was plotted to estimate the viscosity of systems.
The computed conductivity increases initially, and then decreases with the increase of the salt concentration for DEC/PC mixed system(mass ratio 1:1). The trend is consistent with that obtained by Moumouzias. At a constant salt concentration of 0.5m, the computed conductivity is found to decrease monotonically with the increase of DEC content. The plot of radial distribution function reveals that solvent molecules lie more closely to lithium ion than anions. That is, the positive and negative ions of the lithium salts are separated by solvent molecules.
Moreover, the number of molecules in the first solvation shell is close to 4, whatever the salt concentration and solvent composition are. The result is consistent with the tetrahedral arrangement conjectured by Soetens. But it is known from the time evolution of the distance between lithium ion and nearby PC molecules that the exchange of solvent molecules between the first and the second solvation shells of the lithium ion is frequent. Our result differs from that reported by Soetens where four solvent molecules are found to be bounded strongly with lithium ion during the whole simulation. The discrepancy is ascribed to the relatively high salt concentration used in the present work, which facilitate the attraction of solvent shell molecules by other positive and negative ions. The simulation also shows that the viscosity of salt solution is higher than that of pure solvent and that the viscosity increases further as more salts are added.

目錄
中文摘要 －－－－－－－－－－－－－－－－－－－－－－－－Ⅰ
英文摘要 －－－－－－－－－－－－－－－－－－－－－－－－Ⅱ
本文目錄 －－－－－－－－－－－－－－－－－－－－－－－－Ⅳ
圖目錄 －－－－－－－－－－－－－－－－－－－－－－－－－V
表目錄 －－－－－－－－－－－－－－－－－－－－－－－－ VII
第一章 緒論 －－－－－－－－－－－－－－－－－－－－－－ 1
第二章 分子動力－－－－－－－－－－－－－－－－－－－－－ 5
2-1分子動力模擬原理－－－－－－－－－－－－－－－－－－5
2.2分子動力模擬計算－－－－－－－－－－－－－－－－－－6
2.3 模擬方法 －－－－－－－－－－－－－－－－－－－－－10
2.4 力場 －－－－－－－－－－－－－－－－－－－－－－－11
2.5 模擬系統 －－－－－－－－－－－－－－－－－－－－－15
2.6 相關數據計算 －－－－－－－－－－－－－－－－－－－16
第三章 結果與討論 －－－－－－－－－－－－－－－－－－－ 20
3-1 擴散係數與導電度的關係 －－－－－－－－－－－－－－20
3-2 徑向分布函數的分析－－－－－－－－－－－－－－－－ 31
3-3 離子群聚的分析 －－－－－－－－－－－－－－－－－－47
3-4 溶液黏度的計算 －－－－－－－－－－－－－－－－－－52
3-5 陰陽離子與週遭溶劑之距離變遷分析－－－－－－－－－ 61
第四章 結論 －－－－－－－－－－－－－－－－－－－－－－64
參考文獻 －－－－－－－－－－－－－－－－－－－－－－－66

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