Computational Study of Structural and Energetic Properties of Ammonium Perchlorate at Interfaces

Abstract

We investigated the structural and energetic properties of the ammonium perchlorate (AP) crystal in bulk and at interfaces using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Thermal rotations of ammonium ions were observed in the bulk AP crystal at room temperature, in agreement with experimental findings. We investigated atomic density distributions near six crystallographic surfaces of the AP crystal. We observed significant thermal rotations of perchlorate ions near surfaces in contrast to their behavior in the bulk AP crystal. We also calculated the surface energies of these six surfaces of the AP crystal and found that the three surfaces (210), (010), and (001) have the lowest surface energies. We performed dynamic debonding and tensile deformation simulations to create (100), (010), and (001) surfaces. Clean (001) surfaces were formed with the debonding process, and a void bounded by (001) surfaces was formed during tensile deformations. However, clean (100) or (010) surfaces were not formed with either debonding or tensile deformation simulations even though the (010) surface has a lower surface energy estimated from MD simulations than the (001) surface. This is in agreement with the frequently observed (001) surface in the AP crystal. Our results suggest that the investigation of the surface energy and dynamic formation of surfaces with MD simulations can be a useful approach to predict the morphology of molecular crystals to complement accurate DFT calculations.

Reaction Network of Ammonium Perchlorate (AP) Decomposition: The Missing Piece from Atomic Simulations

“…The empirical potential approach (e.g., ReaxFF , ) has been widely used in investigating the initial decomposition pathways of energetic materials induced by thermal and mechanical stimulation. − Recently, MD simulations have been performed to reveal the atomic insights behind the mechanical and structural properties of AP. Yeh and Andzelm investigated the structural and energetic properties of the ammonium perchlorate (AP) crystal in bulk and at interfaces using molecular dynamics (MD) simulations. They observed significant changes in the atomic density distributions of the AP crystal and near surfaces compared to the corresponding distributions in the bulk phase.…”

Force Field Parameters for Ammonium Perchlorate Validated Using Surface Energies and Applied to Interactions with Polymer Binder

“…AP slabs used in QM calculations by Yeh and Andzelm 13 contain at least seven distinct layers of ammonium or perchlorate ions, which is significantly smaller than those used in MD simulations in this study. Three different types of simulation protocols were applied to see the effect of limited system sizes in QM calculations: (1) Atomic positions in two middle layers of the AP slab and lateral dimensions parallel to the interfaces were fixed at the bulk values while the positions in other layers were allowed to be optimized.…”

“…In this work, the (210), ( 001), ( 010), (100), (101), and (011) surfaces of AP were simulated using different force field parameter sets optimized to reproduce the lattice parameters of the bulk AP crystal to determine which agreed best with the corresponding surface energies estimated from QM calculations. 13 Using the best performing force field parameter set, simulations of the HTPB polymer binder in contact with the AP surfaces were performed. Strong interactions between the HTPB hydroxyl groups and the AP surface were observed to occur and could potentially corroborate previous suggestions that the interaction of the hydroxyl groups at the AP interface might explain differences in the AP−HTPB interfacial tension that are dependent on the AP surface.…”

“…As justified below in the Results and Discussion, out of the 70 parameter sets taken from Befort et al 12 parameter set #2 was chosen for AP−HTPB simulations since that parameter set resulted in 0-K surfaces energies in the closest agreement with those estimated from more accurate QM simulations based on DFT calculations. 13 Two sets of unit cell lattice parameters of AP were determined from NPT MD simulations of the bulk AP at 298 K using parameter sets #2 and #26. The temperature T was linearly increased from 0 to 10 K over a 5 ps duration with a 0.1 fs time step and then from 10 to 298 K with a time step of 1 fs for a 50 ps duration with the Langevin thermostat while the pressure P was maintained at zero target pressure.…”

Regulating Chemistry Composition on a Crystal Surface by Introducing a Cation Trapping Agent: A Novel Strategy to Tune Moisture Sensitivity of Crystals

“…From the AR-XPS analysis, we can know that the surface of ADN-1% MMT is covered with more −NO 2 groups. It can be concluded that −NO 2 groups have lower surface energy than NH 4 + ions, − so the ADN-1% MMT shows lower surface energy characteristics and narrower surface energy distributions (see PDI values in Figure d). The lower the surface energy, the better the flowability .…”