(701d) Mechanism for Rapid Mechanical Activation of Gasless High-Density Energetic Mixtures

The ignition mechanism and ability to control the initiation and combustion processes in advanced heterogeneous high-density energetic mixtures (HDEMs) is of particular interest because of a variety of potential applications, including chemical energy storage and micro-scale energetic systems, materials synthesis, as well as propellants in rocket, air-breathing and other energetic formulations. In this work, HDEMs of metal-based compositions are in specific focus. The ignition (or so-called thermal explosion, TE) characteristics of such systems have been extensively investigated during the last ten years both in inert and reactive atmospheres. In gasless heterogeneous systems, the ignition temperature is typically related to specific features of the system phase diagram. For example, the eutectic melting point (or m.p. of the least refractory reagent), at which a significant increase in reactant contact area occurs, typically determines the system ignition temperature. This conclusion presents challenges to control the ignition characteristics for such gasless HDEMs.

The influence of rapid high-energy ball milling on the ignition characteristics of a gasless heterogeneous reactive system, i.e. Ni-Al, has been investigated. It was shown that 5-15 minutes of such mechanical treatment leads to a significant decrease of ignition temperature, which appears to be well below the eutectic in the considered binary system. Thus the thermal explosion process is defined by solid-state reactions. Several hypotheses have been suggested to explain this effect, i.e. mechanical activation (MA) of chemical reaction. One is related to a substantial increase of contact area between reactive particles and fresh inter-phase boundaries formed in inert atmosphere ball milling. A second considers a substantial decrease in effective activation energy of interaction between the reactants. A third takes into account the formation of non-equilibrium solid solutions. Finally, theories suggest that defects and strain imparted on the crystalline structure are the driving force.

In this work a set of advanced analytical tools, such as High-Speed Thermal Imaging, Time-Resolved X-ray Diffraction (TRXRD), and High-Temperature Eletcro-Thermal Analysis was used in order to address the question about the contribution of the aforementioned hypotheses on the apparent mechanism of MA in such systems.