Session H7: Reactive Materials I

Reactive Nanolaminates as Model Materials for Controlling Initiation Thresholds under Shock and Electrical Loading

Over the last ten years a group of researchers at Johns Hopkins University have demonstrated the ability to control and predict the initiation and energy of exothermic reactions that self-propagate in foils with nanoscale layering. These exothermic reactions can be ignited with mechanical, electrical, optical or thermal pulses of energy and provide model materials for systemically varying and predicting initiation thresholds. This presentation will describe our efforts to quantify and predict how the initiation and propagation of these reactions depend on the nanoscale spacing of the reactants and their heats of reaction for mechanical and electrical loadings. Studies of mechanical deformation will also be presented. The free-standing foils or sheets are fabricated using vapor or mechanical processing methods and range in total thickness from 10$\mu $m to 1000$\mu $m. The individual layers within the foils range in thickness from 10nm to 10,000nm. Rod and plate geometries can also be fabricated. A common chemistry for formation reactions includes Ni and Al while a typical chemistry for a reduction/oxidation reaction would include Al and CuO$_{x}$. The reactants and their spacing are chosen to enable exothermic reactions that self-propagate at velocities ranging from 0.1 to 10m/s with maximum temperatures above 1000\r{ }C. Using mechanical impact tests and electrical discharge experiments we have measured thresholds for initiating reactions in these foils, and we have shown that the thresholds increase significantly with reactant spacing and with pre-mixing between the reactants. These measurements are compared with numerical predictions and show strong agreement. The mechanisms controlling the initiation of the reactions will be reviewed and the metastable phases that appear within the self-propagating reactions will be identified using in situ XRD experiments. Lastly, the strength of these materials will be characterized as a function of reactant spacing using standard tension tests.   

Investigation of Formulations Containing Perfluoro-Coated Oxide-Free Nano-Aluminum

Plastic Bonded Explosive (PBX) samples were formulated incorporating oxygen-free nano-aluminum (nano-Al) that is passivated with a fluorinated carboxylic acid. This project investigated explosive formulations containing this coated nano-Al (C14/nanoAl). Small-Scale Shock Reactivity and Internal Blast Tests (SSBT) determined the contribution of the C14/nanoAl in the early stages of the reaction. These results show that the volume of the dent of samples containing C14/nanoAl is twice as great as the dent volume of standard formulations, suggesting faster aluminum combustion than conventional Al formulations.   

Preparation and Shock Reactivity Analysis of Novel Perfluoroalkyl-Coated Aluminum Nanocomposites

The barrier to realization of the energy potential of metals is the comparatively slow rate of oxidation. The rate of oxidation is governed by diffusion of the fuel and oxidizer species. For Al, the Al$_2$O$_3$ surface oxide further slows this process. Replacement of this layer with organic molecules containing oxidizer species should result in a material that reacts fast enough to enable the energy release to contribute to explosives detonation. Passivation of unpassivated, oxide-free Al nanoparticles using C$_{13}$F$_{27}$CO$_{2}$H and C$_{15}$F$_{29}$CO$_{2}$H forming self assembled monolayers (SAMs) is reported with materials containing as much as 33 \% Al. The fast reaction capability of the SAM-passivated material was investigated using time-resolved emission measurements of laser ablation experiments. Laser ablation can transfer momentum to a surface, since the ablated material applies a pulse of high pressure to the surface underneath it as it expands. Time-resolved emission results of the SAM-passivated materials were compared with oxide passivated Al nanoparticles coated with the same acids, C$_{13}$F$_{27}$CO$_{2}$H and C$_{15}$F$_{29}$CO$_{2}$H.   

Phase Stability of Epsilon and Gamma HNIW (CL-20) at High-Pressure and Temperature

Hexanitrohexaazaisowurtzitane (CL-20) is one of the few ingredients developed since World War II to be considered for transition to military use. Five polymorphs have been identified for CL-20 by FTIR measurements ($\alpha $, $\beta $, $\gamma $, $\varepsilon $, and $\zeta )$. As CL-20 is transitioned into munitions it will become necessary to predict its response under conditions of detonation, for performance evaluation. Such predictive modeling requires a phase diagram and basic thermodynamic properties of the various phases at high pressure and temperature. Theoretical calculations have been performed for a variety of explosive ingredients including CL-20, but it was noted that no experimental measurements existed for comparison with the theoretical bulk modulus calculated for CL-20. Therefore, the phase stabilities of epsilon and gamma CL-20 at static high-pressure and temperature were investigated using synchrotron angle-dispersive x-ray diffraction experiments. The samples were compressed and heated using diamond anvil cells (DAC). Pressures and temperatures achieved were around 5GPa and 175$^{o}$C, respectively. No phase change (from the starting epsilon phase) was observed under hydrostatic compression up to 6.3 GPa at ambient temperature. Under ambient pressure the epsilon phase was determined to be stable to a temperature of 120$^{o}$C. When heating above 125$^{o}$C the gamma phase appeared and it remained stable until thermal decomposition occurred above 150$^{o}$C. The gamma phase exhibits a phase change upon compression at both ambient temperature and 140$^{o}$C. Pressure -- volume data for the epsilon and gamma phase at ambient temperature and the epsilon phase at 75$^{o}$C were fit to the Birch-Murnaghan formalism to obtain isothermal equations of state.