Low-Temperature Exothermic Reactions in Al/CuO Nanothermites Producing Copper Nanodots and Accelerating Combustion

Abstract

Several Al/CuO nanocomposite reactive materials, prepared recently by arrested reactive milling, were found to exhibit a distinct low-temperature exothermic peak around 600 K seen by differential scanning calorimetry. This work is an experimental study aimed to establish whether this low-temperature reaction affects combustion and why it is observed in some but not all materials with identical compositions. The peak is only observed when aluminum is initially separately milled in acetonitrile. Electron microscopy showed resulting composite powders to be porous, unlike fully dense powders obtained without premilling aluminum. The as-prepared and partially reacted powders recovered after heating just past the peak to 650 K were examined using high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. Additionally, X-ray diffraction measurements were performed for all materials. Finally, the reactive powders were ignited using plasma and shock generated by an electrostatic discharge. Results show that the low-temperature exothermic peak is associated with a redox reaction pathway involving release of oxygen by CuO that is not in direct contact with Al and freemolecular transport of that oxygen to the nearby surface of Al. This reaction pathway inhibits the formation of Al/Cu intermetallic phases. Instead, nanometer-scale metallic Cu particles are formed due to CuO reduction. It is also found that this reaction pathway accelerates ignition of reactive powders, which in turn leads to a higher burn rate of aerosolized powder.

Atomic Scale Insights into the First Reaction Stages Prior to Al/CuO Nanothermite Ignition: Influence of Porosity

“…As a direct consequence of the early enrichment of the alumina layer with Al, molecules of oxygen trapped in the pore are adsorbed and chemically incorporated into the alumina layer at 600 K, explaining the small exotherm at 600 K found on differential scanning calorimetric (DSC) traces of Al/CuO dense composites containing nanopores. Noteworthy the pore O 2 pressure drop may trigger the decomposition of the adjacent CuO surface, thus supporting the presence of copper-rich nanodots (local reduction of CuO) observed in the partially reacted composites with voids at the interfaces . Not only does this study bring a consistent understanding of the Al/CuO pre-ignition mechanisms in porous versus fully dense composites, but it also explains the decrease in the ignition temperature and increased reactivity experimentally observed for layered or milled Al/CuO composites containing nanopores.…”

“…Accordingly, at 600 K, reoxidation is observed (loss of Al II and Al I ), due to chemisorption of the oxygens initially present in the form of O 2 molecules in the pore. This can explain the small exotherm at 600 K found on DSC traces of Al/CuO dense composites containing nanopores . Importantly, the adsorption and reaction of the 13 O 2 molecules into the alumina layer at 700 K (12 O at 600 K and the remaining 14 O at 700 K) provoke a drop in the oxygen partial pressure in the pore, which further prompts the CuO reduction into Cu 2 O.…”

“…That is why various preparation methods have been employed to combine the aluminum fuel and oxidizer together in close proximity, including sputter deposition, − arrested milling methods, electrosprays, sol–gel processing, and self-assembly. − Interestingly, in Al/CuO fully dense materials such as composites prepared by arrested reactive milling and reactive multilayers, the exothermic redox reaction is restricted to condensed phase reactions at the CuO–Al interface, where an amorphous product Al 2 O 3 layer grows − and the ignition temperature is reduced compared to other types of thermites to ∼850 K. , Recent studies also showed that the ignition temperature can be further decreased by adding micro- or nanovoids within the interfacial layer, which was unexpected. Mursalat et al observed that modifying the milling process parameters, i.e., adding a pre-milling step for Al powders using acetonitrile as a process control agent, produces nanovoids in the thermite, resulting in lowering the ignition temperature down to ∼650 K , and accelerating the reaction. Wu et al also showed that incorporation of microsized pores into Al/CuO reactive multilayers decreases the ignition threshold around the pores and provokes the acceleration of their burn rate (×18% for 20% void loading).…”

A Benchmark Study of Burning Rate of Selected Thermites through an Original Gasless Theoretical Model

“…While nanosized particles were found to outperform their micron counterparts, the performance enhancements were below expectation, and, thus, the exact mechanisms by which these augmentations are accomplished remains debated [28][29][30][31][32][33][34][35]. In theory, mixed nanoparticles possess higher interfacial contact and a decrease in mass-transport lengths resulting in much wider reaction fronts.…”

Pushing the Limits of Energy Performance in Micron-Sized Thermite: Core–Shell Assembled Liquid Metal-Modified Al@Fe2O3 Thermites

“…Attempts to facilitate the diffusion process and improve the reactivity of the thermite system could be broadly classified as altering the mesoscale architecture/assembly of fuel and oxidizer moieties (improved mixing) or altering the performance/properties of the fuel (modified the fuel) (Figure ). The former method focuses on the meticulous architecture design of the thermite system, realizing a variety of structural forms, for instance multilayered, ,− 3D macroporous, − and core–shell − structures. Among these shapes, the core–shell structure has received extensive attention because of its unique properties, including highly specific surface area, controllable pore size, uniform distribution, excellent exothermicity as well as compatibility with MEMS systems. ,, The latter method focuses on accelerating the participation of Al in the reaction, such as reducing the fuel size, − multimetal fuels, , annealing and quenching aluminum, ,, and decomposing the alumina layer. , Among these methods of modifying fuels, reducing the aluminum size could bring many advantages, including fast reaction rate and low initial redox reaction temperature .…”