Elucidating the dominant mechanisms in burn rate increase of thermite nanolaminates incorporating nanoparticle inclusions

Abstract

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes. A detailed theoretical analysis of the local temperature and thermal gradients at the vicinity of these two additives shows that highly conductive inclusions do not accelerate the combustion front while poor conductive inclusions result in the distortion of the flame front (corrugation), and therefore produce high thermal gradients (up to 1010 K.m−1) at the inclusion/host material interface. This results in an overall slowing down of the combustion front. These theoretical findings contradict the experimental observations in which a net increase of the flame front velocity was found when Au and SiO2 particles are added into the thermite. This leads to the conclusion that the faster burn rate observed experimentally cannot be fully associated with thermal effects only, but rather on chemical (catalytic) and/or mechanical mechanisms: formation of highly-stressed zones around the inclusion promoting the reactant mixing. One additional experiment in which physical SiO2 particles were replaced by voids (filled with Ar during experiment) to cancel the potential mechanical effects while preserving the thermal inhomogeneity in the thermite structure confirms the hypothesis that instead of pure thermal conduction, it is the mechanical mechanisms that dominate the propagation velocity in our specific Al/CuO multilayered films.

Influence of process parameters on energetic properties of sputter-deposited Al/CuO reactive multilayers

“…An interesting approach to create a high density and a high interfacial surface area composite in thermites is by creating laminate structures through Physical Vapor Deposition (PVD). Indeed, thermite nanolaminates offer a highly controllable architecture, and have been incorporated into a variety of micro-pyrotechnic devices commonly used in micro-electromechanical systems (MEMS) [21,[32][33][34][35][36][37]. From a material point of view, Al/CuO nanolaminates have drawn particular attention due to their low ignition threshold, high reactivity (burn rate up to 100 m/s) and gas generation ability.…”

“…Incorporating a Cu nanolayer at the interface of the Al/CuO multilayers has been reported to have enabled the reduction of the onset reaction temperature and a better reactivity. Adding additives such as gold nanoparticles [36,48] or micropores accelerate the combustion rate under certain conditions. From a technical perspective, the emergence and development of Al/CuO nanolaminates have benefited from the versatile magnetron sputtering technique, which enables a reliable high-quality deposition as well as well-adhered metallic and oxide thin-films.…”

Effect of Process Parameters on the Properties of Direct Written Gas-Generating Reactive Layers

“…The mean linear burn rate was recorded using a high-speed camera Photon SA3 at a speed of 75 000 frames per second, with a 128 × 32 image resolution: a printed line of each material was deposited in a polycarbonate mold (trench of 30 mm long, 1 mm wide, and 1 mm deep) 44 and ignited at one end with a pyroMEMS. 14 The density of the reactive material inside the mold was evaluated to be at 0.08 g/ cm 3 , ∼0.24% TMD.…”

“…The presence of bubbles or voids can typically create instabilities: as the reaction front encounters a void, the heat will be transported by convection whereas in dense media the main transport mechanism will be conduction. Hence, inhomogeneities will lead to velocities variation at the microscale as described in 44 . In addition, the presence of bubbles, if they are big, can lead to pressurization inside the material which may induce cracking and even disintegration of the film.…”

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.…”