Energetic materials – the overarching class of propellants, pyrotechnics, and explosives – have been studied by thousands of researchers over the past couple of millennia, however, the community only has a loose understanding of how these materials react at the relevant length scale (nm - µm) of the reaction. Our group at the University of California, Riverside specifically focuses on “nanothermites” that are prepared with nanoscale metal/metal oxide (e.g. Al/CuO) particles since they have a higher energy density than traditional monomolecular explosives and are more reactive than micron-sized thermite powders because of the reduced diffusion distance between reactants. Unfortunately, the measured performance of nanothermites is significantly lower than expected for using nanomaterials and the presence of micron-sized products suggests that one inefficiency lies in the loss of nanostructure during the reaction – a process referred to as “reactive sintering.” It had been shown in other studies that particles can reactively sinter, but these studies were done with a small number of particles and it was unclear if such a phenomenon could happen on the time scale (µs) of the reaction in a real-world application.
Haiyang recently developed a way to 3D-print high mass loading nanothermite propellants and I was working on techniques to image ultrafast reactions at the particle scale, so in an act of curiosity, we decided to test a printed Al/CuO composite. Unlike most printed propellants we tested to that point (Al/PVDF) which burned steadily and had no visible products other than soot, we saw large glowing particles reacting and growing against the surface of the glass of this stochastically-propagating reaction. It was immediately clear that we had witnessed a reactive sintering event on the length (µm) and time scale (µs) for the first time in a scenario that could represent in-operando solid propellant combustion. The ~20µm particles that formed had actually melted into the surface of the (now broken) glass slide and, since we now had a unique opportunity to witness a reaction take place and retain the products, Haiyang challenged us to mark the spot where we were looking and find the actual products in the SEM. For context, trying to find a 0.5x0.5 mm square over a 22x22mm piece of broken glass is a lot like trying to find a needle in a haystack, but with some practice, we had figured out a system to label and find the post-reaction products very consistently.
Figure 2: (left) Still image from high-speed microscopy video showing reactively-sintered particles formed during the combustion of high-loading Al/CuO/MethoCel/PVDF. (right) Scanning electron microscopy image showing one-for-one particle matching with the high-speed microscopy video and corresponding element maps demonstrating post-reaction forensic capabilities.
In tandem with the results we could get from the reaction products, we could also extract temperatures of the composite from the calibrated high-speed camera to perform thermal analysis on the temperature gradient in the reacting front and in a single particle. Between the thermal analysis and particle-tracking in the videos, we had seen that the macro-scale propagation rate (~5 cm/s) of the composite was actually limited by the heat transfer from sintered/cooling particles and that the stochastic advances (~50 cm/s) we saw were actually driven by the reaction/sintering event.
All of this work culminated in our article, “In-operando high-speed microscopy and thermometry of a reaction propagation and sintering in a nanocomposite,” with two main conclusions: (1) reactive sintering happens on the length and time scale of large scale propellant reaction and (2) propagation velocity of printed nanothermite composites is largely controlled by heat released from cooling particles. More importantly, we are excited to show how a high-speed microscopy/videography apparatus can be used to characterize reactions on small length- and time-scales and demonstrate how these results reveal and/or confirm physics that had previously been based exclusively on reaction products.
Original journal article:
In-operando high-speed microscopy and thermometry of a reaction propagation and sintering in a nanocomposite by H. Wang*, D.J. Kline*, & M.R. Zachariah, Nature Communications 10, 3032 (2019).