Thermal and Impact Reaction Initiation in Ni/Al Heterogeneous Reactive Systems Robert V. Reeves,* ,† Alexander S. Mukasyan, ‡ and Steven F. Son † School of Mechanical Engineering, Purdue UniVersity, Zucrow Laboratories, 500 Allison Road, West Lafayette, Indiana 47907, and Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, 210 Stinson-Remick Hall, Notre Dame, Indiana 46556 ReceiVed: May 21, 2010; ReVised Manuscript ReceiVed: July 12, 2010 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 self- ignited 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. I. Introduction Gasless reactive systems potentially have different applica- tions, including chemical energy storage, microscale energetic devices, materials synthesis, and as additives to rocket propel- lants. To fully exploit such a system though, a fundamental understanding of the ignition mechanisms is necessary. It is well- known that these systems are ignitable through external heating, internal Joule heating, and mechanical means, such as high strain rate impact. While many efforts have been made for investigat- ing the ignition of heterogeneous reactive mixtures (a suggested review was written by Barzykin 1 ), such a fundamental under- standing of the phenomenon is not complete. The behavior of exothermic systems during external preheat- ing often can be described through thermal explosion (TE) theory, developed by Semenov, 2 Todes, 3 Rice and co-workers, 4,5 and Frank-Kamenetskii 6 and primarily focused on gas-phase reactions. Merzhanov and colleagues created the basis for such initiation in condensed homogeneous systems. 7,8 These classical theories introduced several terms which require definition to continue discussion. First, one has to consider reactions initiated in static and dynamic conditions. In the former case, the reactive media are typically immersed in a volume (e.g., furnace) with constant high temperature, and under certain conditions, rapid reaction starts. In the latter case, the ambient temperature of the furnace changes with time. In both of these situations, the only driving mechanism for rapid reaction initiation is the heat released through the exothermic reaction; therefore, a strong dependence of reaction rate on temperature is a required condition for rapid reaction initiation. In Arrhenius-type kinetics, this condition translates to large activation energy (E a ) of chemical reaction. This concept reveals that when the rate of heat release exceeds that for heat losses, sufficient conditions are met for the rapid reaction, or TE, to proceed. The concept of volume self-ignition or TE versus local ignition phenomena also needs to be delineated. When the characteristic thermal relaxation time of the reactive media is much greater than the chemical reaction time, the TE phenom- enon, that is, the simultaneous reaction initiation in the bulk of the whole sample, prevails. In local ignition, the reaction initiates at a discrete location of the sample which meets the definition of the system’s ignition criteria. The reaction may then self- propagate in the form of a combustion wave through the sample volume. The propagation mode (steady-state, oscillatory, etc.) and combustion front velocity are dependent on the forward transport of heat and mass, mainly through thermal conduction and mass diffusion. 9 Finally, so-called adiabatic thermal explo- sion may occur when experimental conditions allow TE with negligible heat losses. These classical theories of reaction initiation suggest that changes to experimental conditions, like heat loss rate or system configuration, may significantly affect TE characteristics. There- fore, it is not physically sufficient to introduce parameters such as a self-ignition temperature for the TE regime or an ignition temperature for local ignition. Both parameters will vary as experimental conditions are changed. In general, the TE process is characterized by three main parameters, (i) induction time, (ii) heating of the system below the TE threshold, and (iii) the TE threshold, which is defined by characteristic times of heat * To whom correspondence should be addressed. Phone: 765-494-0072. Fax: 765-494-0530. E-mail: rvreeves@purdue.edu. † Purdue University. ‡ University of Notre Dame. J. Phys. Chem. C 2010, 114, 14772–14780 14772 10.1021/jp104686z  2010 American Chemical Society Published on Web 08/16/2010