A molecular dynamics study of the role of pressure on the response of reactive materials to thermal initiation

Abstract

To elucidate the mechanisms of energy release in a reacting nickel/aluminum bilayer, we simulate the exothermic alloying reactions using both microcanonical and isoenthalpic-isobaric molecular dynamics simulations and an embedded-atom method type potential. The mechanism of the mixing consists of a sequence of steps in which mixing and reaction first occurs at the interface; the resulting heat generated from the mixing then melts the Al layer; subsequent mixing leads to further heat generation after which the Ni layer melts. The mixing continues until the alloying reactions are completed. The results indicate that pressure has a significant influence on the rates of atomic mixing and alloying reactions. Local pressures and temperatures within the individual layers at the time of melting are calculated, and these results are compared with the pressure-dependent melting curves determined for pure Al and pure Ni using this interaction potential.

Amorphous Ni/Al nanoscale laminates as high-energy intermolecular reactive composites

“…The chemical reactions of the equilibratedsamples are then modeled withisobaric isoenthalpic MD simulations (NPH) at initial temperature T 0 and atmospheric pressure; this allows the system to heat up and change its volume due to the chemical reactions and has been used before to study reactions in crystalline Ni/Al. 12 The time step used to integrate the equations of motion during chemical reactionis 0.5 femtoseconds with a velocity Verlet algorithm. The barostat relaxation constant is set at 1 ps.…”

“…Recent experiments have captured the dynamical nature of the phase transformations using time-resolved synchrotron x-ray micro-diffraction6 and nanosecond insitu TEM5 in Ni/Al multi-layer reactive foils. Both MD 8,9,10,11,12 and experimental investigations6 ,13,14 revealed melting of the crystalline solids during the reaction of Ni/Al nanolaminates and Ni/Al nano-particleswhile evidence for solid-state reactions was observed in mechanically activated Ni/Al. 13 The process of melting absorbs a fraction of the energy generated during the chemical reactions (due to the heat of fusion) and consequentlyleads to a local drop in temperature degrading performance.…”

“…13 The process of melting absorbs a fraction of the energy generated during the chemical reactions (due to the heat of fusion) and consequentlyleads to a local drop in temperature degrading performance. 9,10,12 In this Communication we explore the viability ofamorphous materials for intermolecular reactive composites(aIRCs) which, with melting a 2 nd order phase transition with no heat of fusion,we hypothesize will outperform their crystalline counterparts. We study the reaction mechanism in a model system consisting of alternating layers of amorphous Ni and amorphous Al and find that indeed these composites are more energetic than their crystalline counterparts and explain the increase in performance in terms of fundamental properties of the amorhous system.…”

Interdiffusion of Ni-Al multilayers: A continuum and molecular dynamics study

“…Earlier research includes the study of intermixing behavior for the interface of Fe/Co/Ni-Al thin film systems during deposition [21][22][23]. It has been used successfully to investigate the melting and crystallization in the Al 50 Ni 50 system [24], the melting and alloying of Ni-Al nanolaminates under shock loading [25][26][27] and the role of pressure and misfit strain on the response of Ni-Al multilayers to thermal initiation [28][29][30][31]. Of direct relevance to this work, MD work by Baras and Politano [32] revealed the sequence of phase formation within a Ni-Al bilayer system during exothermic reactions, and work by Cherukara, Vishnu and Strachan [33] elucidated the effect of surfaces and voids on the rate of intermixing.…”

A perspective on modeling the multiscale response of energetic materials

“…10 In that study, we initially attempted to use microcanonical (NVE) MD simulations to understand mechanistic details of thermal initiation of the nanolaminate at 1200 K, 1 atm. The system was fairly small (~63000 atoms), having periodic cell dimensions of ~100 x 90 x 83 Å. Equilibration simulations, performed within the isothermal-isobaric (NPT) ensemble to relax the system to the desired thermodynamic state, were terminated before full equilibration of the system, due to onset of chemical reaction at the interface.…”