Self-propagating reactions in Al/Ni nanostructured multilayer foils are examined both experimentally and computationally to determine the impact of variations in reactant spacing on reaction properties. Heats of reaction and reaction velocities have been characterized as a function of average bilayer spacing for sputter-deposited, single-bilayer foils (having a uniform bilayer spacing) and for dual-bilayer foils (having two different bilayer spacings that are labeled thick and thin). In the latter case, the spatial distribution of the thick and thin bilayers is found to have a significant effect on reaction velocity, with coarse distributions leading to much higher reaction velocities than fine distributions. Numerical simulations of reaction velocity match experimental data well for most spatial distributions, with the exception of very coarse distributions or distributions containing very small bilayer spacings. A simple model based on thermal diffusivities and reaction velocities is proposed to predict when the spatial distribution of thick and thin bilayers becomes coarse enough to affect reaction velocity. This combination of experiment and simulation will allow for more effective design and prediction of reaction velocities in both sputter-deposited and mechanically processed reactive materials with variable reactant spacings.

Abstract:Phase transformations and transient morphologies are examined as exothermic formation reactions self--propagate across Al/Ni nanolaminate films. The rapid evolution of these phases and sub--µm morphological features require nanoscale temporal and spatial resolution that is not available with traditional in situ electron microscopy. This work uses Dynamic transmission electron microscopy (DTEM) to identify intermetallic products and phase morphologies as exothermic formation reactions self--propagate in nanolaminate films grown with 3:2, 2:3, and 1:1 Al/Ni atomic ratios. Single--shot, diffraction patterns with 15 ns temporal resolution reveal that the NiAl intermetallic forms within 10 nanoseconds of the reaction front's arrival in all three types of films and is the only intermetallic phase to form as the reactions self--propagate and quench very rapidly. Concurrently, time resolved imaging reveals a transient cellular morphology in the Al--rich and Ni--rich foils, but not in the equiatomic films.The cellular features in the Al--rich and Ni--rich films are attributed to a cooling trajectory through a two--phase field of liquid + NiAl.

The time scale associated with shock-induced detonation is a key property of energetic materials that remains poorly understood. Herein, we test aspects of one potential mechanism, the phonon up-pumping mechanism, where shock compression excites lattice phonon modes, transferring energy to intramolecular vibrations leading to chemical bond cleavage and reaction. Using ultrafast infrared pump−probe spectroscopy on pentaerythritol tetranitrate (PETN), we reveal sub-picosecond vibrational energy transfer (VET) from the photoexcited band at 1660 cm −1 into every other infrared-active mode in the probed frequency range 800−1800 cm −1 . Energy transfer processes remain incomplete at 150 ps. Computational predictions from density functional theory are used in tandem to elucidate VET pathways in PETN.

Pentaerythritol tetranitrate (PETN) is a common secondary explosive and has been used extensively to study shock initiation and energy propagation in energetic materials. We report 2D IR measurements of PETN thin films that resolve vibrational energy transfer and relaxation mechanisms. Ultrafast anisotropy measurements reveal a sub-500 fs reorientation of transition dipoles in thin films of vapor-deposited PETN that is absent in solution measurements, consistent with intermolecular energy transfer. The anisotropy is frequency dependent, suggesting spectrally heterogeneous vibrational relaxation. Cross peaks are observed in 2D IR spectra that resolve a specific energy transfer pathway with a 2 ps time scale. Transition dipole coupling calculations of the nitrate ester groups in the crystal lattice predict that the intermolecular couplings are as large or larger than the intramolecular couplings. The calculations match well with the experimental frequencies and the anisotropy, leading us to conclude that the observed cross peak is measuring energy transfer between two eigenstates that are extended over multiple PETN molecules. Measurements of the transition dipole strength indicate that these vibrational modes are coherently delocalized over at least 15-30 molecules. We discuss the implications of vibrational relaxation between coherently delocalized eigenstates for mechanisms relevant to explosives.

Abstract