Connecting agglomeration and burn rate in a thermite reaction: Role of oxidizer morphology

Agglomeration (sintering) of aluminum particles has been commonly observed in solid propellants and pyrotechnics and it is believed to play a key role on the combustion performance of these materials. One of the most commonly studied nanothermites – Al/CuO – has also been well documented to exhibit this agglomeration phenomenon. In this work, high-speed in-operando microscopy and pyrometry were used to observe agglomeration in an Al/CuO reaction with different oxidizer particle size and shape (microparticles, microwires and nanoparticles). We found that the use of a microwire oxidizer resulted in propagation velocities and agglomerate sizes more similar to nanoparticles rather than microparticles. Replacing CuO microparticles (5 µm) with similarly-sized CuO microwires (equivalent diameter: 3 µm) was shown to dramatically elevate the burn rate by ~27 × (2 m/s vs 55 m/s) and increased the flame temperature from 2550 K to 3200 K, resulting in ~30 × higher heat flux (energy release rate). Adding 10 wt.% polymer into the above three thermite systems slowed down the burn rates from ~m/s to ~cm/s, which allowed our microscopic system to probe the details of the agglomeration process. The agglomeration size was found to be reduced from ~40 µm to ~13 µm when replacing CuO microparticles with microwires in the 3D printed Al/CuO composites, similar to that of CuO nanoparticles case. Our results point to a “pocket” model to explain these trends. In a reactive composite, CuO microwires should create an Al “pocket” closer in size to that when using CuO NPs and considerably smaller that with CuO microparticles. The reduced pocket size should also enhance burn velocity. These results also suggest the “pocket” offers a pathway to think about architectures to minimize the sintering size which is known to lead to two-phase losses in propellants.