近年來有許多實驗發現將儲氫材料硼氫化鋰(LiBH4)儲存於奈米孔洞材料中可大幅度地提升熱力學性質、加快反應速率與改善可逆性質。在本文中，我們分別利用蒙地卡羅法與基因演算法得到反應物奈米化硼氫化鋰原子團(LiBH4)n與生成物硼化鋰原子團(LiB)n(n=2~12)的結構。其他生成物鋰(Lin)、硼(Bn)與閉式硼烷鋰化合物(Li2BnHn)的結構則由文獻中取得。我們利用密度泛函理論計算反應物與其可能生成物之總能量，且發現奈米化硼氫化鋰原子團(LiBH4)n之穩定性與其塊材相比並無太大差異，而其他生成物卻因為奈米化後穩定性下降。此一現象顯示奈米化後的硼氫化鋰(LiBH4)n需要更高的反應溫度。我們也研究奈米化硼氫化鋰(LiBH4)n之反應路徑，結果顯示反應路徑由塊材狀態的LiBH4 → LiH + B + 3/2H2 轉變為(LiBH4)n → (LiB)n + 2nH2。另外我們也發現較大的硼氫化鋰原子團(LiBH4)n在釋放氫氣過程中越有可能產生中間產物閉式硼烷鋰化合物(Li2BnHn)。我們深入搜尋硼氫化鋰(LiBH4)m(m=2、3、4與6)之可能中間產物以了解奈米化硼氫化鋰(LiBH4)m的釋放氫氣過程。結果顯示，不同大小的奈米化硼氫化鋰(LiBH4)m具有不同的氫氣釋放特性。壓力在1bar之下，當m=3與6時，在氫氣釋放初期，大量氫氣會快速地釋放在較小的溫度區間。而當m=2與4時，在氫氣釋放初期，氫氣釋出量與溫度呈現線性關係。然而我們的研究結果與實驗上的結果並不一致，原因是由於實驗上所使用的奈米孔洞材料對於奈米化硼氫化鋰(LiBH4)n具有促進其熱力學性質的效果，而在我們的計算我們並沒有考慮奈米孔洞的影響。

Several experimental studies have found improved thermodynamic, kinetic, and reversibility properties when LiBH4 is confined in nano-porous materials. In this study, we used Monte Carlo method and genetic algorithm to search the conformations of reactant (LiBH4)n and one of products (LiB)n. Other products, Lin, Bn, and Li2BnHn, were obtained from literatures. We used the density functional theory to calculate the total energies of reactants and all possible products, and found that the stability of LiBH4 nanoclusters have only little difference compared with that of bulk-LiBH4. However, for the products, the stabilities decrease with reducing sizes. This phenomenon indicates nano-LiBH4 would have higher reaction temperatures. We also investigated the reaction paths of nano-LiBH4, and the results show that the reaction path change from bulk-phase reaction (LiBH4 → LiH + B + 3/2H2) to ((LiBH4)n → (LiB)n + 2nH2) after reducing size. We also found intermediate compound Li2BnHn might appear dehydrogenation. The possible intermediate compounds of (LiBH4)m(m=2,3,4,6) were further included in the calculations to understand the hydrogenation releasing processes. Our results show that different (LiBH4)m have different hydrogen release properties. When m=3 and 6, under 1 bar, both of them release hydrogen rapidly in small temperature interval near their starting points of hydrogen release. When m=2 and 4, we found the amount of releasing hydrogen has a linear relation with temperature near their starting points. However, our results do not replicate experimental results due to the exclusion of the substrate effects i.e. to the nano-porous materials used in experiments which improve the thermodynamic properties of nano-LiBH4.

中文摘要 i
Abstract ii
目錄 iii
圖次 iv
表次 v
I. 介紹 1
II. 理論與方法 8
II.1 熱力學系統介紹 8
II.2 結構搜尋 9
II.3 密度泛函理論 14
II.4 振動正交模態 24
II.5 計算細節 26
III. 結果與討論 27
III.1 硼氫化鋰LiBH4塊材系統 27
III.2 奈米原子團之結構與能量 29
III.3 奈米化硼氫化鋰LiBH4之反應路徑 34
III.4 奈米化硼氫化鋰LiBH4釋放氫氣過程 35
IV. 結論 41
參考文獻 42

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