Session C24: Focus Session: Materials in Extremes: Chemistry under Extreme Conditions

Meso-scale Simulations and Instrumented Experiments in Metastable Intermolecular Composites

Impact initiation of reactions in various aluminum-based intermolecular composites in the form of powder mixture compacts and cold-rolled laminates are being investigated using instrumented gas-gun impact experiments under conditions of uniaxial-strain and uniaxial-stress loading. Time-resolved stress and particle velocity measurements as well as high-speed imaging are used for monitoring the deformation and reaction states to obtain evidence of reaction based on changes in compressibility and shock-velocity, as well as via direct light emission. Meso-scale numerical simulations with CTH multimaterial hydrocode are also performed on actual (imported) micrographs. The simulations allow qualitative and quantitative probing of the local configurational changes and their effects on impact-initiated reaction mechanisms, following validation of macroscopic properties by correlations with experiments. The heterogeneous nature of wave-propagation through reactants of dissimilar elastic and plastic properties and morphological characteristics, produce effects that give rise to turbulent flow, vortex formation, and dispersion of reactants across large distances. Understanding of these processes as a function of mathematically represented constituent configuration and state of stress/strain is essential for designing energetic/reactive materials systems with tunable energy release characteristics.   

Polymerization in Substituted Acetylenes: A Comparison between Static, Medium-Strain Rate, and Shock Compression Studies.

Fast timescale of reactions occurring during shock compression create significant diagnostics challenges to fully quantify the mechanisms involved. Static compression provides a complementary route to investigate the equilibrium phase space and metastable intermediates during high pressure chemistry. Intermediate strain rate compression (0.001/s or higher) with time-resolved probes is a novel way to extract reaction kinetics and underlying pathways. In this study, we present our results from high pressure in situ synchrotron x-ray diffraction (XRD) and infrared (IR) spectroscopy studies on substituted acetylenes: tert-butyl acetylene [TBA: (CH3)3-C$\equiv $CH] and ethynyl trimethylsilane [ETMS: (CH3)3-SiC$\equiv $CH]. We observed that the onset pressure of chemical reactions at room temperature (C$\equiv $C $\to $ C$=$C polymerization) in these compounds was typically higher in static compression (TBA: 11 GPa and ETMS: 26 GPa) when compared to shock input pressures (TBA: 6.1 GPa and ETMS: 6.6 GPa). Expectedly, thermal effects during heating drive the threshold pressure were close to shock conditions as observed during the high temperature measurements. Under compression at medium strain rate (1 GPa/s or higher), a clear progression of the chemical reaction was observed via time-resolved XRD patterns obtained at 0.5s intervals. It is noted that the reaction products were visually observed to be glassy and recovered to ambient conditions, remaining stable with no degradation.   

Cellular Structure and Oscillating Behavior of PBX Detonations

Efforts are aimed on bridging experimental and theoretical studies of localizations/instabilities manifested in detonation reaction zone (DRZ) at micro-, meso-, and macro-scale. In molecular level, the theoretical/computational studies of detonation (RDX, HMX) show: reaction localizations onset/growth is caused by kinetic nonequilibrium stimulated by different levels of activation barriers/reaction energies at bonds dissociation processes (C-NH2, C-NO2, C$=$C). At micro- and meso-scale levels, leading role of kinetic nonequilibrium in reaction localizations onset was established in experiments with single beta-HMX crystals-in-binder subjected to 20 GPa-shock and PBX detonation. Reaction localizations and further ejecta formation were spatially resolved by 96-channel optical analyzer at simultaneous recording reaction light and stress field around crystal. Spatially-resolved measurements reveal fundamental role of shear-strain in triggering initiation chemistry. At macro-scale level, formation of the cell-structures and oscillating detonation regimes revealed in HMX- and RDX-based PBXs at wide variation of grain-sizes, wt. {\%} filler/binder, residual micro-voids and binder nature. Emphasizes placed on effect of DRZ-induced radiation upon oscillating regimes of detonation front motion.   

Ultrafast shock induced chemistry in hydrogen peroxide

Although strong compression waves have been used to study the equilibrium high pressure and temperature properties of materials for more than half a century, the study of ultrafast strain rate dependent material transformations, while promising, is only beginning to be fully explored. Shock waves can change the thermodynamic state of a material over a picosecond time scale, i.e. faster than the time scale of quasi-equilibrium reaction kinetics for many reactive systems. This fundamental property of shock compression suggests the possibility of selecting reaction paths via modulation of applied compression waves on a time scale that is faster than the time scale of reaction kinetics. Here we present experiments and thermochemical and molecular dynamics simulations on a model system, hydrogen peroxide, which demonstrate that the applied strain rate can be used alongside the pressure and temperature to control reactivity in bulk matter, thus enabling the exploration of otherwise inaccessible chemical reaction paths.   

A possible crystal defect mediated mechanism governing energy release in solid organic secondary explosives

Work has been ongoing in our group for several years to produce a global chemistry model of thermal ignition for the solid organic secondary explosive octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) valid over the entire temperature range of energetic response from thermal ignition to detonation. We have made considerable progress recently, resulting in the first broadly accurate model of this type for HMX. We have also recently provided the first theory of the phenomenon of melt acceleration in the thermal decomposition which indicates a universal mechanism applicable to this entire class of materials. The success of these models derives from the kinetic rate equations used, which are based upon rates activated by energies of vaporization and sublimation. The equations can be reduced to dimensionless form, yielding melt accelerated rates of thermal decomposition, ignition and detonation which are functions of two rate constants, one proportional to the liquid activity and another that can be interpreted as the simultaneous occupation of two defect states of the crystal. In this reduced form, data from a number of secondary explosives may be superposed on common curves. In this talk we explore the possibility that the underlying mechanism responsible for this behavior is linked to the equilibrium population of a crystal defect described by a vacancy in contact with local disorder.   

Fragmentation of explosively driven Al-W granular composite rings

Al oxidation has a chemical potential nearly 5 times that of traditional high explosives, however, the oxidation rate scales with the Al particle size. To oxidize on a time scale of $\sim$1ms, Al particle size needs to be on the order of 20microns. Continuum theory and experiments of homogeneous materials show that fragments generated under typical loading conditions have much larger sizes (order 1-10mm). Using a highly heterogeneous material with constituents that have drastically different shock impedances (such as Al and W) provides additional mesoscale mechanisms that allow for further pulverization of the material into smaller fragments. Explosively driven expanding ring experiments were conducted on Al-W granular composite rings and recovered fragments showed a significant reduction in the fragment size compared to a homogeneous sample. Examination of the fragments under SEM showed a propensity for fragments to be composed of a cluster of Al and W particles with little plastic deformation in the interior Al. Hydrocode simulations were conducted to gain an insight into this clustering behavior. Understanding of the mesoscale mechanisms may be used to generate mesostructures that could tailor the size of generated fragments based on the loading conditions.   

Dynamic Behaviors of Two PBX Explosives under Ramp Wave Loading

By means of the magnetic force produced by pulsed power generator CQ-1.5 and CQ-4, two PBX explosives are dynamically characterized under ramp wave loadings from several GPa to 10 GPa in experiments and calculations. The experimental and calculated results show that the PBX explosives exhibit visco-elastic or elastic effects, and the Mie-Gr\"{u}neisen EOS can't well reflect the dynamic nature of PBX-1 and PBX-2 explosives at lower pressure of below 1 GPa. And it can describe their dynamic behaviors well above 1GPa. In this paper, the SG constitutive model is also used to describe this property of PBX9501, which shows good agreement with the experimental results and those of calculated from visco-elastic model by Baer.   

Novel energetic materials for quantum optical initiation

The development of new photoactive materials, which optically initiate through quantum controlled photochemical dynamics, would provide a transformational advancement in the laser-based ignition of energetic materials. Ideal materials should have low initiation thresholds for specific optical pathways while simultaneously having high initiation thresholds for all other conventional stimuli. Optical control can only be effective in newly designed materials that are synthesized to take advantage of such control; consequently, quantum control of optical initiation requires a thorough understanding of the excited state molecular dynamics that leads to photochemical decomposition. To date, our efforts have focused on making new materials with energetic optical chromophores and validation of their non-linear optical response properties through experiment and simulation.   

Molecular dynamics simulation of spinning detonation in energetic AB material

Spinning detonation-wave structure is observed in molecular dynamics simulation of a solid energetic material (EM) confined in the round tube with smooth walls. The EM is represented by a modified AB model with adjustable barrier height for exothermic reaction AB$+$B $\rightarrow$ A$+$BB, which allows us to study the evolution of detonation-wave structure produced by instabilities of the planar detonation front as a function of physico-chemical properties of the EM material, including its thermochemistry and reactive equation of state. The planar detonation wave in a tube of relatively small radius evlolves into an unstable pulsating detonation through the development of longitudinal perturbations, which can later lead to a collapse of the detonation wave. However, as the tube radius is increased, the detonation wave structure is stabilized by a development of a single-headed spinning detonation having an unusual four-wave configuration. Further increase of the tube radius results in a multi-headed detonation structure with turbulent-like distributions of pressure and other physical variables at the front, similar to that observed in gases.   

Quantum mechanical simulations of condensed-phase decomposition dynamics in molten RDX

A reaction model for condensed-phase decomposition of RDX under pressures up to several GPa is needed to support mesoscale simulations of the energetic material's sensitivity to thermal and shock loading. A prerequisite to developing such a model is the identification of the chemical pathways that control the rate of the initial dissociation and the subsequent decomposition of the dissociation products. We use quantum mechanics based molecular dynamics simulations to follow the decomposition dynamics under high-pressure conditions and to identify the reaction mechanisms. This presentation will describe current applications to liquid-phase decomposition of molten RDX.   

Formation of 2D Graphene-like Structures in Reacting Carbon-Rich Energetic Materials

The late stages of extreme reactivity in carbon-rich energetic materials such as 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) are characterized by the slow evolution of carbon to different phases. Slow growth from clusters to graphite and even nano-diamonds have been noted experimentally in detonating TATB. We conducted long-time scale, constant volume-temperature molecular dynamics simulations on pre-shocked TATB crystals for over 3 nanoseconds. Using the reactive force-field Reaxff, and at conditions of temperatures of 2500 and 3000 K, and a pressure of 16 $\sim$ 20 GPa, we discover the formation of 2D graphene-like structures of predominantly carbon, with very low heterogeneity of oxygen and nitrogen at the edges. While these simulations have enabled us to track the reactivity of TATB well into the formation of several stable gas products, such as H$_{2}$O, N$_{2}$, and CO$_{2}$, the formation of graphene-like structures and its slow evolution into final graphite and diamond like structures may finally explain the very low reactivity of TATB, as evidenced in its large reaction zone.   

A Generalized Reduced Model of Uniform and Self-Propagating Reactions in Reactive Nanolaminates

Reactive nanolaminates are comprised of alternating layers of materials that react exothermically. Self-propagating reaction fronts, traveling at speeds that can exceed 10m/s, can be initiated in these materials using an external heat source. The wide range of length and time scales involved in such reactions presents a typical modeling challenge due to the inherent interplay of the different scales in the underlying dynamics and the eventual end-product. In this presentation, we will discuss the development of a reduced reaction model for Ni/Al nanolaminates. The model incorporates a generalized, anisotropic description of thermal transport that also accounts for the dependence of thermal conductivity on composition and temperature. A generalized description of intermixing is also developed, that incorporates information derived from disparate experimental observations, and molecular dynamics (MD) computations. Using insights gained from MD computations, intermixing is described using a simplified, temperature-dependent composite diffusivity relation that enables us to reproduce measurements of low-temperature ignition, homogeneous reactions at intermediate temperatures, as well as the dependence of reaction fronts on micro-structural parameters.   

Ultrafast Vibrational Spectroscopy of Shock Compressed and Flash-Heated Single Molecular Layers

We report the shock compression and flash-heating of single molecular layers on metallic substrates probed with an ultrafast nonlinear coherent vibrational spectroscopy, vibrational sum frequency generation (SFG). Laser-driven shock compression and flash-heating resulted in pressures of a few GPa and temperatures greater than 500 K, respectively. Due to shock velocities of a few nm/ps, single molecular layers allowed picosecond time resolution of shock loading. Monolayers further allowed the measurement of heat transport from the monolayer-metal anchor point to the monolayer's terminus. SFG spectroscopy was utilized due to its sufficient monolayer sensitivity. Shock loading dynamics were analyzed with the help of static high pressure measurements in a diamond anvil cell, and flash-heating results were compared with simulations.\footnote{Y. Zhang, \textit{et al.}, Phys. Chem. Chem. Phys. 12, 4435-4445 (2010).}$^,$\footnote{P. Manikandan, \textit{et al.}, J. Phys. Chem. C 115, 9622--9628 (2011).}