Finite size effects on aluminum/Teflon reaction channels under combustive environment: A Rice–Ramsperger–Kassel–Marcus and transition state theory study of fluorination

Intermolecular Energy Transfer Dynamics at a Hot-Spot Interface in RDX Crystals

The phonon mediated vibrational up-pumping mechanisms assume an intact lattice and climbing of a vibrational ladder using strongly correlated multiphonon dynamics under equilibrium or near-equilibrium conditions. Important dynamic processes far from-equilibrium in regions of large temperature gradient after the onset of decomposition reactions in energetic solids are relatively unknown. In this work, we present a classical molecular dynamics (MD) simulation-based study of such processes using a nonreactive and a reactive potential to study a fully reacted and unreacted zone in RDX (1,3,5-trinitro-1,3,5-triazocyclohexane) crystal under nonequilibrium conditions. The energy transfer rate is evaluated as a function of temperature difference between the reacted and unreacted regions, and for different widths and cross-sectional area of unreacted RDX layers. Vibrational up-pumping processes probed using velocity autocorrelation functions indicate that the mechanisms at high-temperature interfaces are quite different from the standard phonon-based models proposed in current literature. In particular, the up-pumping of high-frequency vibrations are seen in the presence of small molecule collisions at the hot-spot interface with strong contributions from bending modes. It also explains some major difference in the order of decomposition of C-N and N-N bonds as seen in recent literature on initiation chemistry.

Activating Aluminum Reactivity with Fluoropolymer Coatings for Improved Energetic Composite Combustion

Aluminum (Al) particles are passivated by an aluminum oxide (Al2O3) shell. Energetic blends of nanometer-sized Al particles with liquid perfluorocarbon-based oxidizers such as perfluoropolyethers (PFPE) excite surface exothermic reaction between fluorine and the Al2O3 shell. The surface reaction promotes Al particle reactivity. Many Al-fueled composites use solid oxidizers that induce no Al2O3 surface exothermicity, such as molybdenum trioxide (MoO3) or copper oxide (CuO). This study investigates a perfluorinated polymer additive, PFPE, incorporated to activate Al reactivity in Al-CuO and Al-MoO3. Flame speeds, differential scanning calorimetry (DSC), and quadrupole mass spectrometry (QMS) were performed for varying percentages of PFPE blended with Al/MoO3 or Al/CuO to examine reaction kinetics and combustion performance. X-ray photoelectron spectroscopy (XPS) was performed to identify product species. Results show that the performance of the thermite-PFPE blends is highly dependent on the bond dissociation energy of the metal oxide. Fluorine-Al-based surface reaction with MoO3 produces an increase in reactivity, whereas the blends with CuO show a decline when the PFPE concentration is increased. These results provide new evidence that optimizing Al combustion can be achieved through activating exothermic Al surface reactions.

Determination of the spatial temperature distribution from combustion products: a diagnostic study

Temperature measurements within the highly complex reaction field of energetic materials are complicated but existing technology enables point source measurements that identify a maximum temperature at a single location. This study presents a method to extend point source measurements to thermally map the spatial distribution of temperature over a large field of interest. The method couples point source temperature measurements from a multi-wavelength pyrometer with irradiance measurements from an infrared camera to produce a highly discretized thermal map that includes the reaction and surrounding field. This technique enables analysis of temperature gradients within the field of interest and an understanding of energy propagation beyond the point of reaction. Point source measurements of maximum temperature are within 10% of reported values. The method was illustrated for the aluminum and polytetrafluoroethylene reaction and the thermal distribution of temperature produced 30,720 temperature measurements over a field of interest corresponding to 3.5 cm × 8 cm.