John Wen1,Anqi Wang1
University of Waterloo1
Metastable intermolecular composites (MICs) have shown promising characteristics in storing and supplying thermal energy to produce heat and power especially for propulsion and miniature systems. To improve their ignition and flame propagation properties, core-shell and other assembled structures are commonly fabricated as aerogels, thin films, multi-layers and powders with enhanced interfacial compatibility between reactive components. We developed wet-chemistry synthesis routes to fabricate a variety of spherical and colloid core-shell structured aluminum-based MICs including Al/CuO, Al/Fe3O4 and Al/NiO. Electron microscopies, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to examine the geometry and microstructure of core-shell products. A good coverage of metal oxide nanoparticles has been achieved on aluminum micro- and nano-sized particles, as the crystallization of metal oxide or complex nanoparticles under controlled chemical environment was the key to homogeneously fabricated core-shell structures. The onset temperature, energy release and activation energy corresponding to the major exothermic events were found to be distinct yet comparable to physically mixed powders of aluminum and oxides nanoparticles due to the shortened diffusion distance between aluminum and oxidizer nanoparticles, evidenced by the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results.<br/>Combustion of these core-shells, patterned as thin-films or loaded in a combustion chamber as loose powder, exhibited improved ignition and flame propagation characteristics. High-speed microscopic imaging and infrared thermography were utilized to reveal the mass and thermal transport phenomena near the flame front of a burning film composed of energetic particles. While the reactive sintering mechanism seems valid for both physically mixed and core-shell samples, the thickness of the flame front and its propagation speed demonstrated the competitive roles of the reactivity, sintering and heat conduction. The formation of standalone core-shell reactive structures reduced the role of reactive sintering of aluminum nanoparticles and generated much finer hotspots along the propagation of flame front. Generally the global burning rates at different fuel/oxide equivalence ratios are determined by the reactivity and thermal transport. Near the stoichiometric condition, although reactivity is high, low thermal conductivity limits the thermal feedback, leading to non-uniform burning propagation on the thin film. On the other hand, at fuel-rich conditions, despite uniform burning propagation, the global burning rate is limited by low reactivity. During the combustion tests with loose powders, a two-stage combustion behavior has been observed as laser induced ignition was achieved first on the surface exposed to air before the combustion propagation swept through the entire sample volume. Such two-stage combustion behavior, being more commonly observed for physically mixed MICs, is less evident in core-shell structured MICs. The formation of individually reactive core-shell particles allows for enhanced condensed-phase reactions within a smaller zone which accelerates the propagation of combustion once the bulk reaction is initiated. While the ignition delay and burning rate of these powders are not necessarily corelated to the reactivation kinetics found from the DSC measurements, thermodynamic properties and energy transport paths within the packed sample play a more significant role in the combustion chamber tests.