Electrophoretic deposition and mechanistic studies of nano-Al/CuO thermites

Abstract

Electrophoretic deposition was used to deposit thin films (∼10–200 μm) of nano-aluminum/copper oxide thermites, with a density of 29% the theoretical maximum. The reaction propagation velocity was examined using fine-patterned electrodes (0.25 × 20 mm), and the optimum velocity was found to correspond to a fuel-rich equivalence ratio of 1.7. This value did not correlate with the calculated maximum in gas production or temperature, and it is suggested that it is a result of enhanced condensed-phase transport, which is speculated to increase for fuel-rich conditions. A ∼25% drop in propagation velocity occurred above an equivalence ratio of 2.0, where Al2O3 is predicted to undergo a phase change from liquid to solid. This is expected to hinder the kinetics by decreasing the mobility of condensed-phase reacting species. The effect of film thickness on propagation velocity was investigated, using the optimum equivalence ratio. The velocity was seen to exhibit a two-plateau behavior, with one plateau between 13 and 50 μm film thickness, and the other above ∼120 μm. The latter had nearly an order of magnitude faster velocity than the former, 36 m/s vs. 4 m/s, respectively. For film thicknesses in the 50-120 μm range, a linear transitional regime was observed. Images from the combustion studies showed an increase in forward-transported particles as the film thickness increased, along with more turbulent behavior of the flame. It was suggested that the two-plateau behavior indicated a shift in the energy transport mechanism. While nanocomposite thermites have been traditionally thought to exhibit convective energy transport, we find in this work that particle advection may also be important. The velocity of particles ejected through a thin slit mounted above a thermite strip was measured, and was found to be even faster (∼2-3×) than the flame propagation velocity. The morphology of captured particles was examined with an electron microscope, and indicated that reactive sintering had occurred. A non-dimensional number (A) was used to relate the rate of gas pressurization (1/τp) to the rate of gas escape by Fickian diffusion (D/L2), A = L2/(D*τp). For small values of A, gases rapidly escape and do not accumulate within the thermite films. Thus, the resultant energy transport is relatively slow. For large values of A, gases are entirely trapped, thus activating enhanced energy transport via oscillating pressure buildup and unloading of the material. This analysis is suitable for thermites which can produce sufficient gas to raise the local pressure above some critical value to overcome material adhesion strength, inducing pressure-driven unloading and resulting in enhanced energy transport. We suggest that further improvements in nanocomposite thermites can be made by examining the coupling of multiple length scales. Not only is nano-scale mixing important, in this work we found that a second length scale (∼120 μm) was necessary to fully activate a pressure buildup/unloading mechanism, which significantly enhances the reactivity.