Christopher E. Shuck

Irradiation-Enhanced Reactivity of Multilayer Al/Ni Nanomaterials

We have investigated the effect of accelerated ion beam irradiation on the structure and reactivity of multilayer sputter deposited Al/Ni nanomaterials. Carbon and aluminum ion beams with different charge states and intensities were used to irradiate the multilayer materials. The conditions for the irradiation-assisted self-ignition of the reactive materials and corresponding ignition thresholds for the beam intensities were determined. We discovered that relatively short (40 min or less) ion irradiations enhance the reactivity of the Al/Ni nanomaterials, that is, significantly decrease the thermal ignition temperatures (Tig) and ignition delay times (τig). We also show that irradiation leads to atomic mixing at the Al/Ni interfaces with the formation of an amorphous interlayer, in addition to the nucleation of small (2-3 nm) Al3Ni crystals within the amorphous regions. The amorphous interlayer is thought to enhance the reactivity of the multilayer energetic nanomaterial by increasing the heat of the reaction and by speeding the intermixing of the Ni and the Al. The small Al3Ni crystals may also enhance reactivity by facilitating the growth of this Al-Ni intermetallic phase. In contrast, longer irradiations decrease reactivity with higher ignition temperatures and longer ignition delay times. Such changes are also associated with growth of the Al3Ni intermetallic and decreases in the heat of reaction. Drawing on this data set, we suggest that ion irradiation can be used to fine-tune the structure and reactivity of energetic nanomaterials.

Reactive Ni/Al Nanocomposites: Structural Characteristics and Activation Energy

Stochastically structured Ni/Al reactive nanocomposites (RNCs) were prepared using short-term high-energy ball milling. Several milling times were utilized to prepare RNCs with differing internal nanostructures. These internal structures were quantitatively and statistically analyzed by use of serial focused ion beam sectioning coupled with 3D reconstruction techniques. The reaction kinetics were analyzed using the electrothermal explosion technique for each milling condition. It is shown that the effective activation energy (Eef) ranges from 79 to 137 kJ/mol and is directly related to the surface area contact between the reactants. Essentially, the reaction kinetics can be accurately controlled through mechanical processing techniques. Finally, the nature of the reaction is considered; the mechanistic effect of the reactive and three diffusive activation energies on the effective activation energy is examined.

Kinetics of SHS reactions: A review

The current state of chemical kinetics for self-propagating high-temperature non-catalytic reactions has been reviewed for results over the past 50 years. Five different characterization techniques are primarily considered: differential thermal analysis (DTA), electrothermal explosion (ETE), electrothermography (ET), combustion velocity/temperature analyses (Merzhanov–Khaikin and Boddington–Laye approaches), and other advanced in-situ diagnostics, including time-resolved X-ray diffraction (TRXRD). Based on the summary of results thus far, recommendations are given for the future of SHS kinetic research.

Micro-heterogeneous regimes for gasless combustion of composite materials

ABSTRACT Reactive Ni/Al composite particles with different internal microstructures were fabricated by ball milling (BM). The propagation of gasless combustion waves through the compacted composite particle media was investigated using high-speed microscope video recording (HSMVR), with a resolution of 10 μm/pixel and 21.25 μs/frame. The microstructural combustion-wave characteristics, including hesitation time, propagation step size, instantaneous velocity, intraparticle reaction time, and average combustion-wave velocity, were studied as functions of measured internal microstructural parameters. The micro-heterogeneous relay-race combustion mechanism prevails across the investigated conditions. Decreasing the metal layer thicknesses in the composite particles leads to significant decrease in hesitation time, while only weakly affecting the instantaneous velocity. Characteristic times of hesitation and thermal relaxation defined two combustion front propagation regimes limited by interparticle heat transfer and by chemical reaction kinetics. Understanding the existence of these two discrete regimes allows us to effectively control the combustion parameters in this high-energy-density system.

TEM Analysis of Structural Transformation in Al/Ni Nanomaterials under High Energy Ion Irradiation

The paper addresses the effect of irradiation by accelerated ion beam on the structural transformation of Al/Ni multilayer nanomaterials studied by Transmission Electron Microscopy (TEM). The Al/Ni multi-layered nanomaterials are promising nanostructured energetic composite materials [1-2] that exhibit tunable ignition properties to a variety of external excitation methods including friction, shock waves, electrical sparks, and local heating. Because the ignition of such materials depends on their atomic structure and composition, irradiation may provide a novel approach for modification of the reactivity of the nanostructured energetic composite materials. Here we study the structural transformation in Al/Ni layers under irradiation. Magnetron sputtering and electron beam evaporation have been used to fabricate free-standing reactive multilayer nanostructured foils [3]. High energy carbon and aluminum ion beams with different charge states and intensities were used to irradiate the samples. The samples were analyzed by TEM using both high resolution TEM (HRTEM) and High Angle Annual Dark Field (HAADF) scanning TEM (STEM) modes at FEI Titan 80-300 electron microscope. The microscope was operated at 300 keV and equipped with an Oxford Inca EDX detector. A TEM cross-sectional sample that included the NiO-Ni interface was prepared from the top surface by Focus Ion Beam (FIB) using FEI Helios SEM/FIB dual beam equipment. It has been demonstrated that a significant enhancement of reactivity of Al/Ni materials after relatively short-term (40 min) high energy (20 MeV) irradiation by C ions (below the ignition threshold), is associated with structural transformations that lead to a decrease in the thermal selfignition temperature and ignition delay time. Indeed x-ray diffraction (Fig. 1) indicates that defect formation in the samples under irradiation leads to a decrease of the diffraction peak intensities of Al and Ni in irradiated materials as compared to the original foils. At the same time, the full width at half maximum (FWHM) of the Al and Ni peaks (not shown) exhibits different trends with irradiation time indicating that longer irradiation can facilitate the growth of Al crystallites while decreasing Ni crystallite size. This observation agrees well with TEM/STEM analysis (Figs. 1 and 2) that evidences the intermixing of Al and Ni at layer interfaces. Both high resolution TEM and electron diffraction indicate that formation of amorphous materials at the interfaces with small (2-3 nm) crystals of the Al3Ni intermetallic phase occur in the amorphous regions. It can be seen that the nuclei of Al3Ni crystals are distributed in the Al-rich phase close to every other Al/Ni interface for the 40 min irradiated foil (Fig. 2). It is interesting that these nuclei line-up perpendicular to the direction of the incident beam. Such structures confirm that the beam induces solid-state diffusion of Ni into the Al layer, where nucleation of Al3Ni phase takes place. Thus, the enhancement of multilayer energetic nanomaterial reactivity is shown to be associated with Paper No. 0292 583 doi:10.1017/S1431927615003712