- 學位論文
運用分子動力學研究奈米多孔金之熱粗化機制與機械特性
Study on Thermal Coarsening Mechanism and Mechanical Properties of Nanoporous Gold Using Molecular Dynamics
摘要
奈米多孔金屬在多方領域都擁有不小的應用潛力，如隔熱、導熱、電化學、抗衝擊、能量吸收和感測器等。奈米多孔金屬受熱時原子會震盪加速，互相吸引而導致韌帶粗化和整體孔隙率下降，因此奈米多孔金屬在製備上常使用此熱粗化的方式來達到控制韌帶尺寸和孔隙率的目的。本文主要聚焦於升溫速率效應對奈米多孔金之熱粗化機制和其機械特性的影響。在奈米多孔金的模型建構上，首先使用耗散粒子動力學模擬旋節線分離現象以得到其基本的構型，並轉換回分子動力學模擬系統，於NPT系綜(等溫等壓)進行常溫能量釋放。在熱粗化的模擬上，先以六種不同的升溫速率從室溫個別加熱至溫度800 K，然後再以統一的冷卻速率降至常溫，觀察過程中韌帶的發展和其內部結構的變化。模擬結果顯示原子在粗化過程主要會往韌帶的質心方向流動，其流動方式也會受到鄰近韌帶的分布影響而變化。升溫速率存在著一個臨界值，低於此臨界值後粗化程度會減緩，升溫速率低於此臨界值的粗化程度則會繼續增加。先前的文獻已顯示韌帶尺寸越大則強度越低，但在熱粗化下後韌帶會成長，其強度卻反而變得越高。造成熱粗化前後的機械強度和韌帶尺寸的關聯性改變可能的原因是因為熱粗化也會使整體的孔隙率下降，當升溫速率越慢，孔隙率下降的越多，所以其拉伸和壓縮的強度因而增加。在承受衝擊負載下，低孔隙率的多孔金因為其密度較接近實心塊體而會出現衝擊波造成的裂紋，而高孔隙率則是隨著衝擊波而崩塌成實心結構。隨著衝擊波行進，崩塌的部分會持續增加，直到衝擊波完全被轉化為塑形能，這也暗示著高孔隙率受到衝擊後的應變率會明顯高於低孔隙率。對高孔隙率的多孔金進行短時間的熱粗化可以明顯提升其抗衝擊能力以及能量吸收能力。
並列摘要
Nanoporous metals have potential applications in various fields, such as thermal insulation, thermal conductivity, electrochemistry, impact resistance, energy absorption, and sensors. When the nanoporous metal is heated, the atoms will oscillate and accelerate, attracting each other, which leads to the coarsening of the ligament and the decrease of the overall porosity. Therefore, the thermal coarsening method is often used in the preparation of nanoporous metal to achieve the purpose of controlling the size and porosity of the ligament. . This paper mainly focuses on the effect of heating rate on the thermal coarsening mechanism and mechanical properties of nanoporous gold. In the model construction of nanoporous gold, dissipative particle dynamics are used to simulate the spinodal separation phenomenon to obtain its basic configuration, and the energy release at room temperature is carried out in the NPT ensemble (isothermal isobaric). In the simulation of thermal coarsening, the ligaments were individually heated from room temperature to a temperature of 800 K at six different heating rates, and then cooled to room temperature at the same cooling rate to observe the development of the ligament and the changes in its internal structure during the process. The simulation results show that atoms mainly flow towards the centroid of the ligament during the coarsening process, and the flow pattern is also affected by the distribution of adjacent ligaments. There is a critical value of the heating rate between 0.25 and 0.625 K/ps, and the coarsening degree will slow down when approaching this critical value, and the coarsening degree will continue to increase when the heating rate is lower than this critical value. Previous literature has shown that the larger the ligament size, the lower the strength, but after thermal coarsening the ligament grows and becomes stronger. The possible reason for the change in the correlation between mechanical strength and ligament size before and after thermal coarsening is that thermal coarsening also reduces the overall porosity. When the heating rate is slower, the porosity decreases more, so its tensile and The strength of the compression will be increased. Under shock loading, porous gold with low porosity will experience cracks caused by shock waves because its density is closer to that of a solid block, while high porosity will collapse into a solid structure with shock waves. As the shock wave travels, the collapsed part will continue to increase until the shock wave is completely converted into plastic energy, which also means that the strain rate of high porosity after shock is significantly higher than that of low porosity. Short-term thermal coarsening of porous gold with high porosity can significantly improve its impact resistance and energy absorption.
參考文獻
延伸閱讀
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