Microstructure-reactivity Relationship for Gasless High-energy Density Materials

Reactive nanocomposites (RNCs), comprised of stochastically layered metals, were fabricated using short-term high-energy ball milling (HEBM). By varying the milling conditions, the internal nanostructure of the RNCs can be controlled. Using a combi- nation of two approaches (synchrotron-based X-ray nanotomography and focused-ion- beam sectioning), the 3D-structure of these materials were quantitatively analyzed. HEBM was initially applied to the Ta/C system to generate a variety of different composite structures. This system was chosen because it is a true solid flame system; the adiabatic combustion temperature is below any phase transition or eutectic tem- peratures. The ignition temperature was 1243 ± 15 K and the maximum temperature observed was 2487 ± 50 K. It was shown that this reaction proceeds solely due to solid state mechanisms.

This approach was then applied to the Ni/Al system. The reactivity, including ig- nition and combustion parameters, for different RNCs was analyzed using high-speed infrared imaging. The direct relationships between the 3D structural characteristics and reactivity parameters were determined. It was shown that all of these features are primarily controlled by the Al layer thickness. This is most likely due to the reaction mechanism, where the controlling step is Ni diffusion into the Al matrix.To better understand the driving force behind the observed ignition and combustion parameters, 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 aremol 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 was examined. A comparison with existing theoretical models allows us to conclude that, for specially designed RNCs, the re- action can be initiated and self-propagates solely due to solid-state mechanisms, i.e., in the solid flame mode. It was directly demonstrated that, by understanding the fundamental quantitative relationship between the structure and properties of RNCs, unprecedented control over the reaction can be achieved.