Explosion Study of Nitromethane Confined in Carbon Nanotube Nanocontainer via Reactive Molecular Dynamics Jeong Hyeon Lee, †,⊥ Jin Chul Kim, †,⊥ Woo Cheol Jeon, †,⊥ Soo Gyeong Cho,* ,‡ and Sang Kyu Kwak* ,† † School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡ Agency for Defense Development (ADD), P. O. Box 35, Yuseong, Daejeon 305-600, Republic of Korea * S Supporting Information ABSTRACT: Explosion dynamics of confined nitromethane (NM) fluid has been investigated by using nonequilibrium reactive molecular dynamics. For the confinement, NM was encapsulated into a nanocontainer, which is the capped (20, 20) armchair carbon nanotube (CNT). After thermal energy was injected into confined NM at various densities, the nanobomb consisting of NM and CNT was fully decomposed including bursting phenomena. We found that the time for explosion was reduced as density and initial temperature increased. While NM was being decomposed into intermediates, defects of Stone-Wales type (5-7 carbon atoms ring) or high-order rings were randomly formed at the cap and side wall of CNT. Subsequently, the intermediates functionalized carbon atoms at the defects, from which nanoholes were evolved. The CNT burst when the size of nanohole became about 8 Å. Further, we demonstrated that defective CNT with vacancy exploded faster because carbon atoms at defect sites played a seed role to make nanoholes. This theoretical study, which is related to nanoscale explosion, provides a new insight into confined NM system to apply for a small-size target. 1. INTRODUCTION Nitromethane (NM), which is the simplest type of nitro compound in high explosive energetic material (HE), has been used for years in explosion-related applications. Many researchers have conducted experimental and theoretical studies on, to name a few, optical 1 and thermal decom- positions 2-8 as well as phase transitions including melting 9,10 and solidification 11 (or crystallization). Recently, specific interest was begun on the behavior of NM in confinement environment by focusing on the decomposition activity. For instances, Liu et al. 7 conducted ab initio molecular dynamics (AIMD) to show fast decomposition of NM between functionalized graphene sheets. Smeu et al. 12 showed the stabilizations of several HEs (e.g., FOX-7, RDX, HMX, etc.) encapsulated in carbon nanotube (CNT) and graphene bilayer by density functional theory (DFT) calculations. Especially, via MD, NM confined in CNT was found to undergo special intermolecular arrangement 13,14 and to have low activation energy for reaction. 15,16 In a way, the idea of nanobomb, which is composed of nanocontainer and enclosed HEs, has been already shown, 17 but it was not concretely realized even in in silico studies. Under the encapsulation, HE is expected to be intact from outside by nanocontainer, which prohibits the change of chemical properties of confined molecules, at normal conditions. However, when in use, the effect of explosion would be enhanced by the built-up pressure (i.e., by decomposition of HE) inside before the burst of nanobomb. In order to model the conceptual nanobomb, we define a nanocontainer, which is a small container and can encapsulate a few tens to hundreds of molecules. A promising nanocontainer, CNT, is a good candidate because of its excellent thermal and mechanical properties. 18 In particular, it can endure internally developed pressures of 30-100 GPa because of high axial tensile strength in intrinsic structural stability. 19 Furthermore, CNT with cap is expected to transport encapsulated materials safely by making isolated conditions. 20 In this study, therefore, we conceptually constructed a nanobomb with NM and CNT as the explosive and nanocontainer, respectively. In general, thermal decompositions of HE materials follow very complex reaction mechanisms, which are very difficult to trace at harsh condition of high pressure and temperature under confined environment. Also, there have been few data of confined NM on fundamental knowledge of decomposition phenomena including detailed interaction mechanism and information on various intermediates and products. In order to capture the desired information, temporal and methodo- logical limits of generic MD and DFT must be lifted. To do so, reactive force field (denoted as ReaxFF) developed by van Duin et al. 21 was considered for this study since it can handle reactive dynamics of atoms via the bond order information describing Received: November 22, 2016 Revised: March 7, 2017 Published: March 8, 2017 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.6b11757 J. Phys. Chem. C XXXX, XXX, XXX-XXX