探討粉體類奈米複合材料之機械性質

Abstract

本研究旨在提出連續微觀力學模型，來描述以奈米尺寸的二氧化矽(Silica)球體為內含物，聚亞醯胺(Polyimide)奈米複合材料的機械性質。為了建立穩定且適合的二氧化矽/具聚亞醯胺奈米複合材料，藉由分子動力學(Molecular dynamic)模擬，依序完成NVT系綜及NPT系綜分析。模擬過程中檢查位勢能的變化，當能量在平均值附近變動持續一段時間，則系統達到平衡，且此分子結構達到了能量最低的狀態。藉由分子動力學模擬，可以分別計算出奈米內含物及週遭基材 (Matrix) 間的非鍵結間距(Non-bonded gap)，及非鍵結能量(non-bonded energy)。假設單位化後的非鍵結能量 (非鍵結能量除上內含物的表面積)，與球形內含物及其週遭基材互相作用之間，存在相互關係。接著發展包含球形內含物、基材以及等效介面層的三相連續微觀力學模型，以描述奈米複合材料的行為。在微觀力學模型中，等效介面層的尺寸假設為非鍵結間距，且藉由線性彈簧模型，可將單位化非鍵結能量對應為等效介面層的勁度。另一方面，由分子動力學模擬，分別給予分子結構三個方向微小的位移場，可直接計算出楊氏模數。此結果將與為微觀力學模型的結果比較。 除了分子模擬之外，將利用拉伸以及破壞實驗研究球形內含物的奈米複合材料的機械性質。考慮20nm、200nm及1m三種不同的球形內含物，以瞭解內含物尺寸對於奈米複合材料之機械性質的影響。最後，實驗結果將與分子模擬以及連續微觀力學模型計算的結果，相互對照及比較。

This research aims to propose a continuum-based micromechanical model to characterize the mechanical properties of the polyimide nanocomposites with nano-scale silica spherical inclusions. In order to build an equilibrated molecular structure suitable for silica/polyimide nanocomposites, the NVT and NPT ensembles were performed sequentially in the molecular dynamic (MD) simulation. During the simulation, the total energy variation was examined, and when the quantity fluctuating around a certain mean value lasted for a while, the system was considered in equilibrium and the equilibrated molecular structure with minimized energy was accomplished. Based on the MD simulation, the non-bonded gap as well as the non-bonded energy between the nano-sized inclusion and the surrounding matrix was evaluated, respectively. It was postulated that the normalized non-bonded energy (the non-bonded energy divided by surface area of the inclusion) is correlated with the intensity of the interfacial interaction between the spherical inclusion and surrounding matrix. Subsequently, a three phase continuum-based micromechanical model including spherical inclusion, matrix and effective interface was developed to describe the behaviors of the nanocomposites. It is noted that in the micromechanical model, the dimension of the effective interface was assumed equal to the non-bonded gap and the corresponding stiffness of the effective interface was calculated from the normalized non-bonded energy with the assistance of a linear spring model. On the other hand, by applying tiny displacement fields in the three different directions of the molecular structures, the Young』s modulus of the nanocomposites can be directly evaluated from the MD simulation. The Young』s modulus obtained from the MD simulations will be compared to those derived from the micromechanical model.In addition to the molecular simulation, the tensile and fracture tests will be carried out to investigate the mechanical properties of the nanocomposites with spherical inclusions. Basically, three different sizes of the inclusion, 20 nm, 200 nm and 1 μm, will be taken in account to understand the influence of the inclusion sizes on the mechanical properties of the nanocomposites. Finally, experimental results will be interrelated to those analytical results calculated using the molecular simulation and the continuum-based micromechanical model.