Ignition mechanism in nanocomposites thermites

IGNITION MECHANISM IN NANOCOMPOSITES THERMITES by Rayon Williams Nanocomposite thermites (n-thermites) have been actively investigated for a wide range of potential applications including propellants, explosives, and pyrotechnics. There have been several recent efforts aimed at understanding ignition mechanisms of nanocomposite reactive materials. Although significant progress has been made, ignition mechanisms remain elusive. At the same time, a robust ignition model is required to incorporate these materials in practical energetic formulations. A challenge of this effort is to describe the mechanisms of ignition of n-thermites prepared by Arrested Reactive Milling (ARM) with different stimuli, including heat, spark and impact and also develop a multi-step kinetic model describing different processes affecting ignition. The role of thermally initiated heterogeneous exothermic reactions is evaluated and the effect of decomposition of oxidizer and respective oxygen gas release on ignition is described. N-thermite powders are prepared by ARM and evaluated using thermal analysis, electron microscopy and other analytical techniques. Experimental studies of ignition of n-thermites stimulated by heating, electric spark and impact are conducted with the goal of developing a reaction model capable of describing different experimental data sets. State of the art thermo-analytical equipment and advanced isoconversion methods are used to describe stability and redox reaction mechanisms in the prepared samples. Multiple reaction steps are identified and described quantitatively. Thin layers of the prepared powders coated onto an electrically heated Ni-Cr filament are ignited at heating rates between 200-17000 K/s in a miniature vacuum chamber. Ignition is monitored based on both photodiode and pressure transducer signals recorded simultaneously. For spark-induced ignition, powder layers of different thickness are placed in a grounded brass holder. A needle-like electrode is placed above the powder and sparks with different energies are produced. Real time measurements of current and optical signatures produced by the ignited sample at different wavelengths are taken. The results are processed to determine the spark energy, minimum ignition energy, ignition delay, and other parameters. Shock ignition of nanocomposite 8Al∙MoO3 thermite particles are independently carried out at the University of Illinois Urbana Champaign. An individual particle is targeted by a miniature, laser-driven flyer plate accelerated to a speed in the range of 0.5-2 km/s. Ignition delays observed in both shock and spark ignition experiments for the same material are close to each other and vary in the range of 120 – 200 ns. A reaction mechanism including multiple oxidation steps starting with the Cabrera-Mott (CM) reaction followed by direct oxidative growth of and phase changes in different alumina polymorphs is validated for a stoichiometric 2Al∙3CuO nanocomposite powder prepared by ARM. The reaction kinetics describing these reaction steps are shown to remain credible for the ARM-prepared reactive composites with different scales of mixing, interface morphologies, and component ratios, as long as the components remained Al and CuO. This work presents a further validation and development of this multistep model to describe reaction in another ARM-prepared thermite system, 8Al∙MoO3. IGNITION MECHANISM IN NANOCOMPOSITES THERMITES