Reactive MD simulations: From nanoscale electrochemistry to detonation initiation
Alejandro Strachan, Nicolas Onofrio, Mathew Cherukara, and Mitchel Wood
This presentation will discuss recent developments in reactive MD simulations to describe electrochemical processes and exemplify its use with two applications where chemical reactions are driven by different external stimuli.
Nanoscale resistance-switching cells that operate via the electrochemical formation and disruption of metallic filaments that bridge two electrodes are among the most promising devices for post-CMOS electronics. Despite their importance, the mechanisms that govern their remarkable properties are not fully understood, limiting our ability to assess the ultimate performance and scalability of this technology. We present the first atomistic simulations of the operation of conductive bridging cells using reactive MD with a charge equilibration method extended to describe electrochemical reactions. The simulations predict the ultrafast switching observed in these devices, with timescales ranging from hundreds of picoseconds to a few nanoseconds for devices consisting of Cu active electrodes and amorphous silica dielectrics and dimensions corresponding to their scaling limit. We find that single-atom-chain bridges often form during device operation but they are metastable with lifetimes below a nanosecond. The formation of stable filaments involves the aggregation of ions into small metallic clusters followed by a progressive chemical reduction as they become connected to the cathode. Contrary to observations in larger cells, the nanoscale conductive bridges often lack crystalline order. An atomic-level mechanistic understanding of the switching process provides new guidelines for materials optimization for such applications and the quantitative predictions over an ensemble of devices provide insight into their ultimate scaling and performance.
Detonation initiation of RDX under dynamical loading. Defects play a central role in the initiation of chemical reactions in high energy density materials under dynamical loading by spatially localizing the shock energy. Despite many decades of research the details of how the energy in the shockwave leads to self-sustained chemical reactions remain obscure. We used large-scale reactive MD simulations to identify the critical pore size that results in sustained chemistry for the nitramine RDX (C3H6N6O6) under shock loading with a piston velocity of 2 km/s. The mechanical shock causes pore collapse and the initial chemical reactions occur within picoseconds of this event and under local non-equilibrium conditions. Following these initial reactions the chemical front grows most rapidly into the collapsed pore (i.e. in the upstream direction) which we attribute to the heat lensing effect of the pore and the increased sensitivity of the amorphous material. Above the critical pore size we see the formation of final, exothermic, products that leads to significant local heating which accelerates the chemical reactions. These simulations provide atomic insight into the coupling of dynamical mechanical loads and chemical reactions that occur under extreme conditions of temperature and pressure and away from local equilibrium.