探討奈米複合材料中奈米碳管之力量傳遞

Abstract

奈米複合材料承受由基材傳遞至奈米碳管的應力，奈米碳管之力量傳遞效率於奈米複合材料的力學反應中扮演重要的角色，因為奈米碳管之力量傳遞效率會影響奈米材料的效能。對於多壁奈米碳管而言，不僅奈米碳管中外層碳管能承擔力量而且奈米碳管中內層碳管亦能承擔力量，因此奈米碳管中內層碳管所能承擔力量的能量會影響奈米複合材料的整體效能。本研究目的為利用剪力遲滯模型與多尺度模擬方法，對奈米複合材料受力時，力量由基材傳遞至奈米碳管，探討奈米碳管的力量傳遞效率與應力分佈，本研究中單壁奈米碳管和多壁奈米碳管二者的力量傳遞均納入探討。 一個包含奈米碳管和基材的圓柱形代表性的體積元素應用於剪力遲滯模型分析，由剪力遲滯模型分析描述奈米碳管的力量傳遞效率與軸應力分佈特性。為了探討原子級的奈米碳管界面層作用力，研究雙層奈米碳管於奈米複合材料中界面層作用力對內層碳管應力傳遞的影響，由於奈米碳管是奈米尺寸但基材於複合材料中視為巨大的材料，因此利用多尺度模擬方法探討雙層奈米碳管的力量傳遞效率。多尺度模擬方法包含兩個步驟，首先、利用分子動力學模擬雙層奈米碳管原子間的界面層作用力，以一個等效圓柱連續體、並嵌入彈簧元素，用此彈簧元素來模擬分子動力學所獲得的原子間的界面層作用力，利用此等效圓柱連續體來模擬雙層奈米碳管。此等效圓柱連續體考慮兩種不同的雙層奈米碳管界面層作用力，分別為凡得瓦爾力與人造的共價鍵。其次、此等效連續體於奈米碳管複合材料微觀力學模型中當作輔助的工具用以計算力量傳遞效率。本文分析奈米碳管層數、原子級的界面層作用力、及奈米碳管的長度與直徑比對力量傳遞效率的影響，結果顯示奈米複合材料於相同的奈米碳管體積比的情況下，單層奈米碳管比多壁奈米碳管呈現更佳的力量傳遞效率。這是因為多壁奈米碳管、隨著奈米碳管的層數增加力量承載能量降低，且結果更進一步顯示，即使於多壁奈米碳管層間建立共價鍵，亦無法因此而提升多壁奈米碳管力量傳遞效率的不足。

In CNT nanocomposites, the efficiency of the load transfer from the matrix to the carbon nanotubes (CNTs) plays an important role in the mechanical response, since it can influence the efficacy of the nano-reinforcements. In multi-walled carbon nanotubes (MWCNTs), not only the outer, but also the inner graphene layers may be responsible for supporting the load. The loading capacity of the inner layers may therefore also influence the overall performance of the nanocomposites. This study aims to investigate the load transfer efficiency and stress distribution in CNT nanocomposites from the surrounding matrix to the CNTs, using shear lag model analysis and multi-scale simulation methods. Both single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) were considered in the investigation. A cylindrical representative volume element (RVE) containing the CNTs and matrix phases was employed in the shear lag analysis; using this, a characterization was made of the axial stress distribution and the load transfer efficiency in the CNTs. To investigate the atomistic interactions between the adjacent layers, the effect of the interlayer interactions on the stress transfer efficiency in the interior layers of DWCNTs (in the nanocomposites) was considered. CNTs have nanoscale dimensions, but in composites the matrix is often regarded as a bulk material; the load transfer efficiency within DWCNTs was therefore investigated using a multi-scale simulation scheme. The multiscale simulation consisted of two steps. First, the atomistic interactions between the adjacent graphite layers in DWCNTs were characterized using a molecular dynamic (MD) simulation, from which a cylindrical equivalent continuum solid of DWCNTs with embedded spring elements was proposed to describe the interactions between neighboring graphene layers. Two kinds of interatomistic properties—van der Waals (vdW) interactions, and artificially built covalent bonds—were considered in the equivalent DWNT solid. The equivalent solid was subsequently implemented as reinforcement in a micromechanical model of CNT nanocomposites, to allow an evaluation of the load transfer efficiency to be made. The effects of the number of layers, the atomistic interactions between the graphite layers, and the aspect ratio of the CNTs on the load transfer efficiency were investigated. The results indicated that SWCNTs exhibited a superior load transfer efficiency compared with MWCNTs, for the same CNT volume fraction in the nanocomposites. This was attributed to the fact that in MWCNTs, the load carrying capabilities reduce as the number of graphitic layers increases. Moreover, it was revealed that the inferior load transfer efficiency properties of the MWCNTs were not improved, even when covalent bonds were constructed between the graphite layers.