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 initiation in the Ni-Al heterogeneous gasless system due to thermal and mechanical stimuli was investigated. Reactive systems with different microstructures, including micro-and nanoscaled powder mixtures, as well as composite particles formed during short-term (15 min) high-energy ball milling (HEBM) of Al/Ni clad particles were examined. Thermal and mechanical responses were tested by differential thermal analysis and shear impact testing, respectively. It was shown that nanomixtures and HEBM samples thermally selfignited at temperatures (T ig ) well below eutectics for Ni-Al (T eut ) 913 K), while the ignition temperature for conventional microscale mixtures is at least T eut . Moreover, T ig for HEBM samples is typically lower than that for nanomixtures. For the HEBM system, the apparent activation energy (E HEBM ) 28 ( 2 kcal/mol) appeared to be half of the nanosystem's measured value (E nano ) 55 ( 5 kcal/mol). Oppositely, it was shown that nanomixtures were mechanically ignitable through shear impacts of the investigated energy range, while HEBM samples were not. Thus, the HEBM samples were comparatively more sensitive to thermal initiation, while the nanomixtures were more sensitive to mechanical initiation. It is believed that the different microstructures contribute to this phenomenon; HEBM material has larger interfacial areas between active materials, which reduces its activation energy and increases thermal sensitivity. The nanomaterials consist of small, hard particles which allow for increased contact stresses during impact and increasing mechanical sensitivity.
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.
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