以分子動力學及奈米壓痕分析一維奈米結構材料機械性質

On the Mechanical Properties of 1-D Nanostructured Materials Using Molecular Dynamics Simulation and Nanoindentation Testing

徐語晨(Yu-Chen Hsu)

指導教授 ： 陳文華 ； 鄭仙志

國立清華大學/工學院/動力機械工程學系/博士(2010年)

摘要

自奈米碳管發現以來，由於其優異材料性質，引起對於各式奈米結構材料廣泛的研究。近年來，隨著電子元件尺寸的微小化，一維奈米結構如奈米碳管、奈米線及奈米桿等，皆被認為可作為電子元件或互連結構應用於微奈米電子及機電之裝置。然而，奈米結構材料雖具相當潛力於各種工程應用上，但相關機械與熱性質仍未被完整且確實地瞭解。特別對於其相關影響因子，如尺寸、晶格結構，乃至缺陷等，對於材料性質之影響，均須予以深入探討。
近來，由於數值計算方法的發展，分子動力學已被廣泛應用於奈米材料機械性質之探討。本論文旨在建立一精確、有效的分子動力學分析模型以探討奈米結構材料之機械性質。首先將針對不同結構之單/多層奈米碳管，包括鋸齒型、扶手椅型及混合型，對於其基本機械性質進行估算。其中層間凡得瓦力對於奈米碳管機械性質之影響為本研究主要重點項目之一。接下來更將研究延伸至多層奈米碳管層間凡得瓦力之效應，並對於其層間剪切作用力及強度進行討論。此外，由於製造技術之限制，奈米碳管常被發現於製程中有缺陷產生。因此，本論文希望藉由分子動力學模擬，有系統地探討缺陷效應對於奈米碳管機械性質及破壞行為之影響。主要討論之影響因子包括缺陷數量、型式、位置及分布。本研究中更針對局部應力分布及裂縫成長路徑之關聯性進行探究。
除了奈米碳管，金屬奈米線也是另一種被廣泛應用之奈米結構材料。因此，本論文藉由分子動力學模擬及奈米壓印試驗，估算三種不同金屬奈米線(金、銀及鈷)機械性質。在此同時對於長度與截面積、晶格結構、缺陷及晶界改變等效應進行分析。進一步經由拉伸試驗模擬，更可得到金、銀奈米線之極限強度及頸縮結構。此外，根據奈米壓印試驗所求得之鈷奈米線彈性模數，更可與分子動力學所估算之數值相互比對並驗證模型之正確性。
最後，本論文將建立一多材料之分子動力學分析模型，以深入瞭解自行組裝層(self-assembly monolayer, SAM)之塗覆對於金基板與樹酯材料間介面接著及金接點之熱壓合接合之影響。本論文中，將針對三種具有不同官能基之烷基硫醇(SH(CH2)nX, X=CH3, OH, NH2)進行討論。首先經由軸向拉伸模擬，探討各式自行組裝層之彈性性質。接下來，在自行組裝層對於介面接著影響的討論中，除了烷基硫醇鏈長及官能基之影響，亦對於其於相異晶格結構之金基板表面上之接著行為進行比較。本論文之研究結果與文獻中部份計算數值及實驗數據相互比較，結果均相當脗合。
本論文之成果，不僅對於奈米結構材料基本物性及相關機械行為，可有較全面的瞭解，對於異質材料間之黏著力亦可準確評估，並為未來進行奈米力學行為研究及奈米結構材料之工業應用，建立堅實基礎。

關鍵字

奈米碳管 ； 金屬奈米線 ； 機械性質 ； 破壞行為 ； 介面接著 ； 分子動力學分析

並列摘要

Ever since the exciting discovery of carbon nanotubes (CNTs), there has been a huge growth in research in material science on finding novel nanostructured materials with advanced material properties. Recently, due to the shrink of feature size in IC technology, nanostructured materials, especially one-dimensional (1-D) nanostructures such as CNTs, nanowires and nanorods, have been considered for use in nanoscale electronic or electromechanical devices as active electronic components or interconnects. Despite of their potential, as claimed, for various engineering applications, the thermal-mechanical properties of nanostructured materials remain not fully determined or clear, not mentioning the effects of the relevant influence factors, such as size, crystal structure and defect.
Recent progress in computational methods based on molecular dynamics (MD) methods has allowed the characterizations of the mechanical properties of nanomaterials. The study aims at developing an accurate and effective MD simulation model to explore the thermal-mechanical characteristics of nanostructured materials. The study starts from the evaluation of the fundamental mechanical properties of various single/multi-walled carbon nanotubes (S/MWCNTs), including zig-zag, armchair and hybrid types. The study first focuses on the exploration of the effect of the weak inlayer van der Waals (vdW) atomistic interactions on the mechanical properties of S/MWCNTs. The influence of the axial orientation mismatch between the inner and outer layers of MWCNTs on the associated mechanical properties are also addressed, followed by the investigation of the behaviors of the interlayer shear force/strength of MWCNTs. The effectiveness of the MD simulation is demonstrated through the comparison with the theoretical/experimental data available in literature. Besides, due to the limitation of fabrication technologies nowadays, atomistic defects are often perceived in carbon nanotubes (CNTs) during the manufacturing process. Thus, the second goal of the study is to perform a systematic investigation of the effects of atomistic defects on the nanomechanical properties and fracture behaviors of single-walled CNTs (SWCNTs) using MD simulation. Key parameters and factors under investigation include the number, type (namely the vacancy and Stone-Wales defects), location and distribution of defects. The correlation between local stress distribution and fracture evolution is also discussed. To demonstrate the feasibility of the proposed MD model, the present results are compared with the theoretical/experimental data available in literature.
The third goal of the study aims to estimate the elastic properties of three different metal nanowires, namely made of gold (Au), silver (Ag) and cobalt (Co), through MD simulations and nanoindentation testing. The investigation also addresses the effects of the length and cross-sectional area of the nanowires, crystal structure, presumed defect and the variation of grain boundary of the metal crystal on the mechanical properties. Furthermore, tensile test simulation for both the Au (gold) and Ag nanowires is carried out, where the ultimate strength and the necking structure are also evaluated. Verification of the MD simulation model in terms of elastic modulus is made using nanoindentation experiment, and the literature theoretical and experimental data.
Finally, the last goal of the study is to establish a multi-material MD simulation model to look into the insight of the effects of self-assembly monolayer (SAM) coating on the interfacial adhesion of an Au-epoxy system and on the bondability of the thermocompression-bonded Au-Au joints. Three different types of functionalized alkanethiol SAMs (SH(CH2)nX, X=CH3, OH, NH2) chemisorbed onto Au substrates, are considered in the investigation. The investigation first explores the elastic properties of these SAMs through uniaxial tensile simulation, followed by exploring the effects of the SAMs on the adhesion behaviors of the Au-epoxy system and the Au-Au system, and those of chain lengths and tail groups of the n-alkanethiolates on the adhesion strength. The study also reports a comparative analysis of the effects of the crystal orientation of Au on the associated interfacial behaviors. The calculated results are partly compared with the published experimental data, and also with each other to identify the optimal SAM candidate in terms of adhesion strength for the Au-epoxy system.
The achievements made in this study can not only provide a more thorough and clear understanding of the basic mechanical properties and behaviors of the nanostructured materials and the adhesion behaviors at the Au-Au and Au-epoxy bi-material interfaces, but also give a solid foundation for future research on the nanomechanics and industrial application of the nanostructured materials.

並列關鍵字

Carbon Nanotubs ； Metal Nanowires ； Mechanical Properties ； Fracture Behaviors ； Interfacial Adhesion ； Molecular Dynamics Analysis

參考文獻

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