Robert Veeh Reeves

Thermal explosion in Al-Ni system: influence of mechanical activation.

The influence of short-term (5-15 min) highly energetic ball milling on the ignition characteristics of a gasless heterogeneous Ni-Al reactive system has been investigated. By using Al-Ni clad particles (30-40 microm diameter Al spheres coated by a 3-3.5 microm layer of Ni, that corresponds to a 1:1 Ni/Al atomic ratio), it was shown that such mechanical treatment leads to a significant decrease in the self-ignition temperature of the system. For example, after 15 min of ball milling, the ignition temperature appears to be approximately 600 K, well below the eutectic (913 K) in the considered binary system, which is the ignition temperature for the initial clad particles. Thus, it was demonstrated that the thermal explosion process for mechanically treated reactive media can be solely defined by solid-state reactions. Additionally, thermal analysis measurements revealed that mechanical activation results in a substantial decrease in the effective activation energy (from 84 to 28 kcal/mol) of interaction between Al and Ni. This effect, that is, mechanical activation of chemical reaction, is connected to a substantial increase of contact area between reactive particles and fresh interphase boundaries formed in an inert atmosphere during ball milling. It is also important that by varying the time of mechanical activation one can precisely control the ignition temperature in high-density energetic heterogeneous systems.

Reaction instabilities in Co/Al nanolaminates due to chemical kinetics variation over micron-scales

The reaction front dynamics of Co/Al reactive nanolaminates were studied as a function of the initial temperature of the unreacted material. Sample geometries that exhibit stable reaction fronts as well as geometries that present “spinning” reaction front instabilities were investigated at initial temperatures ranging from room temperature to 200 °C. It was found that reactions in samples with small reactant periodicities (<66.4 nm) were stable at all temperatures, reaction in large periodicity samples (≥100 nm) were unstable at all temperatures, and reactions in samples with intermediate periodicities transitioned from unstable behavior to stable behavior with increasing initial temperature. The results suggest that behaviors typical of two types of reaction kinetics are present in unstable reaction fronts: slow, diffusion-limited kinetics in the regions between transverse reaction bands, and a faster mechanism at the leading edge of the transverse bands.

The dynamics of Al/Pt reactive multilayer ignition via pulsed-laser irradiation

Reactive multilayers consisting of alternating layers of Al and Pt were irradiated by single laser pulses ranging from 100 μs to 100 ms in duration, resulting in the initiation of rapid, self-propagating reactions. The threshold intensities for ignition vary with the focused laser beam diameter, bilayer thickness, and pulse length and are affected by solid state reactions and conduction of heat away from the irradiated regions. High-speed photography was used to observe ignition dynamics during irradiation and elucidate the effects of heat transfer into a multilayer foil. For an increasing laser pulse length, the ignition process transitioned from a more uniform to a less uniform temperature profile within the laser-heated zone. A more uniform temperature profile is attributed to rapid heating rates and heat localization for shorter laser pulses, and a less uniform temperature profile is due to slower heating of reactants and conduction during irradiation by longer laser pulses. Finite element simulations of laser heating using measured threshold intensities indicate that micron-scale ignition of Al/Pt occurs at low temperatures, below the melting point of both reactants.

Effects of oxidation on reaction front instabilities and average propagation speed in Ni/Ti multilayer foils

Vapor-deposited, equiatomic Ni/Ti multilayer foils exhibit low-speed, self-propagating formation reactions that are characterized by a spin-like reaction front instability. In addition to the intermetallic reaction between Ni and Ti, reactions performed in air can also exhibit a discrete combustion wave associated with the oxidation of Ti. In general, the oxidation wave trails the complex intermetallic reaction front. Multilayers that have a large reactant layer periodicity (≥200 nm) exhibit a decrease in net reaction speed as air pressure is reduced. Oxidation has a much smaller effect on the net propagation speed of multilayers with small layer periodicity (<100 nm). The net propagation speed of the multilayers is increased when air is present, due to the added energy release of Ti oxidation. High-speed optical microscopy shows that the increased front speed is associated with an increased nucleation rate of the reaction bands that typify the spinning reaction instability of the Ni/Ti system.

MICROSTRUCTURAL EFFECTS ON IGNITION SENSITIVITY IN NI/AL SYSTEMS SUBJECTED TO HIGH STRAIN RATE IMPACTS

The effect of microstructural refinement on the sensitivity of the Ni/Al (1:1 mol%) system to ignition via high strain rate impacts is investigated. The tested microstructures include compacts of irregularly convoluted lamellar structures with nanometric features created through high-energy ball milling (HEBM) of micron size Ni/Al powders and compacts of nanometric Ni and Al powders. The test materials were subjected to high strain rate impacts through Asay shear experiments powered by a light gas gun. Muzzle velocities up to 1.1 km/s were used. It was found that the nanometric powder exhibited a greater sensitivity to ignition via impact than the HEBM material, despite greater thermal sensitivity of the HEBM. A previously unseen fast reaction mode where the reaction front traveled at the speed of the input stress wave was also observed in the nanometric mixtures at high muzzle energies. This fast mode is considered to be a mechanically induced thermal explosion mode dependent on the magnitude of the traveling stress wave, rather than a self-propagating detonation, since its propagation rate decreases rapidly across the sample. A similar mode is not exhibited by HEBM samples, although local, nonpropagating reaction zones shear bands formed during the impact event are observed.

COMPARISON OF MECHANICAL AND THERMAL IGNITION CHARACTERISTICS FOR REACTIVITY ENHANCED Ni/Al POWDERS

Recently, efforts have been made to understand and tailor the ignition of gasless reactive systems. It is known Arrested Reactive Milling (ARM) can enhance reactivity through extensive plastic deformation, thorough mixing of materials, and introduction of crystal defects in the material. Alternatively, reducing the particle size of the constituent materials to nano‐scales also enhances reactivity. However, the effect of these reactivity enhancing processes on the ignition mechanisms is not well understood. The ignition parameters of the Ni‐Al system is studied by comparing those for mixtures of micron size Ni/Al powders, nano‐scale Ni‐Al powders, as well as Ni‐Al powders after ARM. Differential thermal analysis (DTA), as well as electro‐thermal explosion (ETE) methods, were used to study thermal ignition properties. Ignition from mechanical stimulus was studied by impacting samples with a projectile from a gas gun. DTA and ETE showed the ARM materials reacted at temperatures below the eutectic point (913 ...

ULTRAFAST X‐RAY PHASE CONTRAST IMAGING OF A GASLESS REACTIVE SYSTEM USING 3RD GENERATION SYNCHROTRON RADIATION

We report anultrafast x‐ray phase‐contrast imaging study of a gasless composite reactive (Si‐coated W wire) system undergoing high heating rates (104–2.5×105 K/s). Construction of an imaging system utilizing a high‐speed CMOS camera and the third‐generation synchrotron at the Advanced Photon Source of Argonne National Laboratory allows for imaging of microstructural changes of the reactive system over previously unstudied time frames and length scales. Imaging was performed at speeds up to 36,000 frames per second with 10 μm spatial resolution. Using Computer‐Assisted Electrothermography (CAE), the heating rate of the gasless reactive system W‐Si is controlled and its kinetics is measured. A physical description of the changes undergone by the system during melting and reaction are captured by the high‐speed imaging system and correlated to the recorded CAE data. The initial Si melt, as well as the initial reaction, is seen to be non‐uniform along the wire. A secondary reaction, undetected by CAE data, is...