How Thermal Aging Affects Ignition and Combustion Properties of Reactive Al/CuO Nanolaminates: A Joint Theoretical/Experimental Study

Abstract

The paper reports a joint experimental/theoretical study on the aging of reactive Al/CuO nanolaminates, investigating both structural modifications and combustion properties of aged systems. We first show theoretically that the long-term storage (over several decades) in ambient temperature marginally affects nanolaminates structural properties with an increase in an interfacial layer of only 0.3 nm after 30 years. Then, we observe that the first thermal aging step occurs after 14 days at 200 °C, which corresponds to the replacement of the natural Al/CuO interfaces by a proper ~11 nm thick amorphous alumina. We show that this aging step does impact the nanolaminates structure, leading, for thin bilayer thicknesses, to a substantial loss of the energetic reservoir: considering a stoichiometric Al/CuO stack, the heat of reaction can be reduced by 6–40% depending on the bilayer thickness ranging from 150 nm (40%) to 1 µm (6%). The impact of such thermal aging (14 days at 200 °C) and interfacial modification on the initiation and combustion properties have been evaluated experimentally and theoretically. Varying Al to CuO ratio of nanolaminates from 1 to 3, we show that ignition time of aged systems does not increase over 10% at initiation power densities superior to 15 W·mm−2. In contrast, burn rate can be greatly impacted depending on the bilayer thickness: annealing a stoichiometric nanolaminates with a bilayer thickness of 300 nm at 200 °C for 14 days lowers its burn rate by ~25%, whereas annealing a fuel rich nanolaminates with the same bilayer thickness under the same thermal conditions leads to a burn rate decrease of 20%. When bilayer thickness is greater than 500 nm, the burn rate is not really affected by the thermal aging. Finally, this paper also proposes a time–temperature diagram to perform accelerated thermal aging.

Unexpected enhanced reactivity of aluminized nanothermites by accelerated aging

“…Obviously, caution must be taken, due to many assumptions associated with the isoconversional methods, but even more from the subtle low temperature aging mechanisms that do not leave an exploitable thermal signature in the DSC traces. Therefore, thermokinetic study must be combined with a precise evaluation of morphological and structural changes at the nanoscale in the nanothemites upon heating as it was recently proposed for nanolaminate systems [36,38]. Such an approach would make it possible to identify the main reactional steps of low temperature, as recorded in the DSC but without any a priori knowledge of their fundaments.…”

“…(1 α), limited to independent reactions occurring successively after each other, within specific temperature ranges. However, the thermite reactions, which combines chemical oxido-reduction reactions with mass transport through barrier layers [35][36][37][38] that have dynamical changes in their structures over temperature cannot rigorously be reduced to a single Arrhenius law activation. Thus, we opted for the use of a differential isoconversional method based on the Friedmann method.…”

Modeling the self-propagation reaction in heterogeneous and dense media: Application to Al/CuO thermite

“…Beyond the fact that huge discrepancies do exist in diffusion coefficient values in the literature [56][57][58], defining diffusion pathways and associated parameters for both Al and oxygen species of thermite steady state combustion is a vast issue, out of the reach of current understanding. Therefore, it is generally accepted to fit effective coefficients through specific set of measurements, whatever their nature [50,59,60]. We assume diffusion of species to follow a classical Arrhenius law as given in Equation 13 which defines how an Arrhenius law is implemented for species diffusion.…”

Multifunctional Reactive Nanocomposites via Direct Ink Writing

“…With the batch sizes employed, the total time to produce 60 grams of particles is ≈17 h. Once milled, these particles are expected to retain their reactivity for decades at room temperature, as the microstructure is not so fine that diffusive mixing at room temperature will substantively affect the stored chemical energy. [ 19,20 ]…”

“…With the batch sizes employed, the total time to produce 60 grams of particles is ≈17 h. Once milled, these particles are expected to retain their reactivity for decades at room temperature, as the microstructure is not so fine that diffusive mixing at room temperature will substantively affect the stored chemical energy. [19,20] In a printed modality, the reactivity of the constituent particles is only part of the equation-the other components comprising the ink will also affect a printed structure's reactivity and properties. Producing an ink incorporating a wide range of particle sizes and morphologies has been demonstrated with sol-gel based inks.…”