Session D7: CH.3 Chemistry: Energetic Material Initiation

Simulations of the chemistry of shocked energetic materials on the nanosecond timescale

The initiation of chemistry by a shock wave occurs through complex interactions between chemical and mechanical mechanisms. The first few nanoseconds after shock arrival is a crucial time period, where the initiation of exothermic chemical reactions leads to the growth of local hot spots. We are simulating these mechanisms in two ways. First, we are performing atomistic simulations of shocked TATB using a modified version of the ReaxFF force field. We have found that modification of ReaxFF is necessary in order to accurately model charge transfer and ionization under sustained high density conditions. We have simulated overdriven detonation waves in TATB using the multi-scale shock technique (MSST). In our simulations of overdriven shocked TATB, we find that large-scale graphitic structures encompassing thousands of atoms form after 1 ns. A second simulation technique uses continuum mechanics with an anisotropic crystal-level plasticity model for shocked HMX. We use a thermochemical approach to model the equation of state and chemistry of the shocked energetic material. In the simulations a pore is compressed by a shock wave, resulting in material deformation and chemistry. We find that simulations with crystal-level plasticity predict chemistry that is localized in fluid regions. The results of the crystal plasticity model will be compared with a more traditional treatment of plasticity in HMX via shock viscosity.   

Toward quantum controlled initiation of energetic materials

Successful quantum controlled initiation requires understanding the photochemical reactions that occur when time dependent electric fields interact with energetic materials. Steering the outcome of a chemical reaction with light requires optimally shaped ultrafast laser pulses to guide energy flow along desired reaction coordinate pathways. The ability to measure the complex photo-chemical dynamic molecular vibrations is key to not only understanding but controlling the photodecomposition mechanisms of energetic materials. We have successfully built a Femtosecond Stimulated Raman Spectroscopy (FSRS) system that works in tandem with our existing 400 nm broadband shaped pump (actinic pump) and 400-700 nm Transient Absorption (TA) probe experiment. This gives us a unique capability of photo-exciting energetic materials with an actinic shaped broadband femtosecond pump pulse and measuring the resulting dynamics simultaneously using FSRS and TA. The measured evolution of the TA and, more importantly, the vibrational spectrum during the photodecomposition transformation provides key structural data on the reaction mechanisms. We have tested our new capability on both energetic and non-energetic materials and have observed vibrational dynamic changes suggesting possible decomposition mechanisms.   

Study of the laser-induced decomposition of energetic materials at static high-pressure by time-resolved absorption spectroscopy

A detailed description of the reaction rates and mechanisms occurring in shock-induced decomposition of condensed explosives is very important to improve the predictive capabilities of shock-to-detonation transition models. However, direct measurements of such experimental data are difficult to perform during detonation experiments. By coupling pulsed laser ignition of an explosive in a diamond anvil cell (DAC) with time-resolved streak camera recording of transmitted light, it is possible to make direct observations of deflagration phenomena at detonation pressure. We have developed an experimental set-up that allows combustion front propagation rates and time-resolved absorption spectroscopy measurements. The decomposition reactions are initiated using a nanosecond YAG laser and their kinetics is followed by time-resolved absorption spectroscopy. The results obtained for two explosives, nitromethane (NM) and HMX are presented in this paper. For NM, a change in reactivity is clearly seen around 25 GPa. Below this pressure, the reaction products are essentially carbon residues whereas at higher pressure, a transient absorption feature is first observed and is followed by the formation of a white amorphous product. For HMX, the evolution of the absorption as a function of time indicates a multi-step reaction mechanism which is found to depend on both the initial pressure and the laser fluence.   

Ultrafast laser diagnostics for studies of shock initiation in energetic materials

Ultrafast laser diagnostics have opened new pathways for investigation of shock physics and initiation of energetic materials. Recent work (Bolme LANL/Armstrong LLNL) has demonstrated that short laser pulses can be utilized for direct laser drive and coupled with imaging, spectroscopic, and interferometric tools for studies of dynamic shock loading on picosecond time scales. At Sandia, we are developing diagnostic platforms which extend this earlier work including: (1) Ultrafast Shock Interferometry (USI) (Armstrong LLNL) for tabletop measurement of Hugoniot/Equation-of-state data and characterization of shock structure in heterogeneous materials with micron spatial resolution; (2) coherent Raman diagnostics, including Coherent anti-Stokes Raman spectroscopy (CARS) and stimulated Raman scattering (SRS) for measurement of temperature and dynamic changes in chemical bonding; and (3) femtosecond transient absorption spectroscopy, which can monitor shock-induced shifts in electronic structure, which have been proposed to drive rapid chemical changes behind the shock front. We are pursuing a path where each of these tools is being developed independently and then combined for the study of shock physics studies in thin films of energetic materials. At the APS/SCCM, we will describe the details of our measurement systems, as well as recent progress toward new laser-diagnostic data on inert/explosive thin-film samples.   

Toward a Role of Light Absorption in Initiation Chemistry of Shocked HMX single Crystals and Crystalline High Explosives

Question which mechanism is driving radiation-induced reactions, thermal or athermal becomes a subject of conflicting discussions. Major challenge of this work is to identify at micro- (sub-granular), meso- (grain level) and macro-scale roles of these two mechanisms in triggering initiation chemistry in HMX-based HEs. Four acceptor-patterns were tested at 20 GPa input pressure: single HMX crystal-in-water, HMX/water-slurry, PBX(HMX/HTPB) {\&} inert PBX-simulant (HMX-particles replaced by crystalline sucrose). Scenario of reaction onset-localizations-dissipation was spatially resolved using Multi-Channel Optical Analyzer MCOA-UC (96 channels, 100um-spatial accuracy, 0.2ns-timeresolution, 450-850 nm-spectral range) through real-time panoramic recording emitted reaction light and shock field in standard optic monitor. Experiments reveal a dual nature of initiation chemistry: athermal and thermal. Single-crystal tests disclose origination of photo-induced reactions downstream of emitting reaction spot due to intensified radiation absorption in surface micro-defects. Polycrystalline samples reveal cyclic reproducibility of radiation-induced thermal precursors in which radiation absorption causes thermal expansion/phase-changes of HMX-grains resulting in oscillating detonation.