Imaging of Thermal Propagation in Combustion of Al/CuO Core-Shell Nanothermite Film

Abstract

Nanoenergetics have been a topic of great discussion over the past few decades due to their extraordinary combustive properties. In particular, heavy investment has been put into researching nanothermites due to the low cost of their components and high energy release of the products. However, much is still unknown about the kinetics and thermodynamics of nanothermites on both the spatial and temporal micro-scale. Here, we report the design and implementation of a high-speed thermographic facility ("pyrometry setup") as well as the investigation of the combustion mechanism and flame propagation of Al/CuO nanothermite films at such scales using an alternate high-speed thermography (3.75 µm/px, 2.5 ms resolution) and videography (2.8 µm/px, 50 µs resolution) system. Sub-millimeter films of both Physically-Mixed (PM) and Core-Shell (CS) Al/CuO nanothermite were fabricated and subsequently ignited by hot wire, and the combustion event was recorded onto footage. This work demonstrates the differences between the combustion of PM and CS films with respect to temperature, flame propagation, and particle ejection. Discussions have been made on the so-called "reactive sintering" mechanism — in which reacting particles amalgamate before burning — and its effects on micro-scale combustion behavior. It has been shown that the self-contained nature of the CS film promotes spatio-thermometric uniformity within the pre-combusted regions of the film (uniform temperature banding), whereas the PM film exhibits irregularities within those regions as well as notable "hot spots" along the flame front. Additionally, the flame propagation from the CS film has been recorded to be faster than that of the PM film by a factor of 1.4 (~10.80 vs. ~7.44 cm/s, respectively), which may bear a relationship with the particle ejection seen in both samples. In the CS film, it is revealed to be more akin to a particle "exhaust" rather than an "ejection" of aggregated particles as displayed by the PM film (presumably caused by the hot spots), resulting in a hotter post-combustion region of space. This suggests that for CS films, the energy from the reaction remains relatively local to the reaction zone rather than being carried away by distantly ejected particles. Temperature measurements using an emissivity correction value of 0.15 indicate that the maximum temperature along the flame front for the PM film is ~3400 K and the distance ("conduction distance") between that and the unburned film (room temperature) is ~645 µm, giving an average spatial temperature gradient of 4.8 K/µm . For the CS film, the maximum temperature and conduction distance were found to be ~2500 K and ~600 µm, respectively, giving an average spatial temperature gradient of 3.7 K/µm.