單壁奈米碳管儲氫性能之分子動力學模擬

Abstract

氫氣燃料已為現代不可缺少的能源，而碳管目前為儲氫材料中的佼佼者，過去於儲氫理論模擬以巨正則蒙地卡羅(GCMC)為主。所以本文有別於過去使用巨正則蒙地卡羅，而改以古典分子動力學的方法考慮古典統計力學中正則系綜的系統，於固定粒子數、固定體積及溫度的情況下，假設氫氣吸附機制為物理吸附，並利用Lennard -Jones Potential(12-6) 來代表氫和氫、氫和碳、不同碳管間碳和碳的位勢能，Lennard-Jones Potential(8-4)代表碳管內部碳分子無振動時碳管間的交互作用位勢能，採用Tersoff potential來表示碳管內碳原子的交互作用，模擬單根、三根碳管束分別於77˚K、300˚K隨著壓力變化其體積吸附量、絕對吸附重量百分比及額外吸附重量百分比。另外在第四章也考慮影響碳管儲氫的其它參數，於不同管徑、不同溫度對儲氫量的影響，並以定性的結果觀察大管徑碳管束的變形，了解到於大管徑碳管束中現象之不可忽略。最後提出不同的模擬晶胞，使模擬的物理問題能擴展到較接近真實材料情況，使利用分子動力學方法模擬出來的結果能更接近應用，及以此提供管間隙的不同影響儲氫定性的情形。

In this paper, the molecular dynamics (MD) is used to simulate the hydrogen storage in single-walled carbon nanotubes (SWNTs). In according to the result of experiment we assume the hydrogen storage in SWNTs is physical absorption. We select Lennard-Jones potential to represent the intermolecular force acting between hydrogen (H2) molecules. The interactions between carbons of nanotube are modeled with Tersoff potential. The Verlet leapfrog algorithm is used to calculate the trajectories of atoms. The rescaling technique is utilized to keep temperature of system. In this thesis, the absolute adsorption weight, volumetric adsorption, and excess gravimetric storage capacity in one SWNT and three SWNTs are simulated at 77˚K and 300˚K. In the procedure, the hydrogen molecules are absorpted to interior and exterior of SWNT and the space between SWNTs. It is found that absolute absorption is decreasing with increasing temperature and increasing with pressure. In addition, the other parameters, including the diameter of carbon nanotube, the space between SWNT, play a small role in the hydrogen storage of SWNT. Finally, we observe that deformation of bigger carbon nanotube deformed could not only affect the hydrogen storage in carbon nanotubes but also provide a direction to simulate array of infinite carbon nanotubes in further studies.