金屬叢集的融化現象; Melting scenario in metallic clusters

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题名:	金屬叢集的融化現象;Melting scenario in metallic clusters
作者:	羅謹賢;Jin-shian Luo
贡献者:	物理研究所
关键词:	合金金屬叢集;金屬叢集;融化;bimetallic cluster;melting;velocity autocorrelation function;metallic cluster
日期:	2007-12-24
上传时间:	2009-09-22 10:59:17 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	我們使用定溫Brownian-type 分子動力學來分析研究(合金)金屬叢集的溫度效應。比熱CV與δ(Lindemann-like parameter) 這兩個普遍在巨觀系統中運用來描述固—液相變的物理量，當運用在金屬叢集這樣的小系統中，對融化溫度(Tmelt) 的預測始終不一致。而這背後的機制現在已能被深入探討。而為了能更深入了解融化現象，我們加入了速度自相關函數(velocity autocorrelation function) 以及它的傅立業轉換(即power spectrum)，進而發現此物理量與CV主峰所預測的融化溫度範圍一致。本論文中選取了兩個合金叢集系統Ag1Cu13、Au1Cu13並與一純金屬叢集系統Cu14做深入的比較分析。由這三個不同的金屬叢集，我們可以歸納出不同相對位置的溫度效應以及合金系統中異質原子所造成的差異性。由分析這些動態資訊，我們觀察到：在低溫區間(T? Tmelt) 有原子的遷移行為(migrational relocation)，以及在中溫區間(T< Tmelt) 與分子間交換行為(permutation) 的相互疊加，而在高溫區間(T? Tmelt) 原子的行為會呈現liquid-like或甚至gas-like。 The isothermal Brownian-type molecular dynamics simulation has been applied to study the melting behavior of bimetallic clusters. It was found that the specific heat and Lindermann-like parameter customarily used in bulk system to describe solid-liquid transition show incongruity in the prediction of melting temperature, T_{melt}. The underlying mechanisms that lead to the incompatibility of melting temperatures deduced from these two quantities were analyzed further. To gain insight into the melting behavior, we calculated in addition the velocity autocorrelation function and its Fourier transform, the power spectrum, to extract separately the melting temperature. It appears that the melting temperature inferred from the latter quantities is closer to that deduced from the principal peak position of specific heat. Two bimetallic clusters, Ag₁Cu₁₃ and Au₁Cu₁₃, were selected for thorough investigation. For the cluster morphology, we carried out a quantitative comparison of Ag₁Cu₁₃, Au₁Cu₁₃ and Cu₁₄ to locate the atomic distribution and contrasted between a bimetallic cluster and a pure cluster to learn the thermal reaction of atoms, in particular the impurity atom in the bimetallic cluster. On analyzing the dynamical data, we observed at a lower temperature (T≪T_{melt}) migrational relocation of atoms whose dynamics was superimposed by permutations between atoms at an intermediate temperature (T<T_{melt}), and at a higher temperature (T≈T_{melt}), the atoms behave liquid- or even gas-like.
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