Investigation of Al/CuO multilayered thermite ignition

Abstract

The ignition of Al/CuO multilayered material is studied experimentally to explore the effect of heating surface area, layering and film thickness on the ignition characteristics and reaction performances. After the description of the micro-initiator devices and ignition conditions, we show that the heating surface area has to be properly calibrated to optimize the nanothermite ignition performances. We demonstrated experimentally that a heating surface area of 0.25 mm 2 is sufficient to ignite a multilayered thermite film of 1.6 mm wide by a few cm long, with a success rate of 100%. A new analytical and phenomenological ignition model based on atomic diffusion across layers and thermal exchange is also proposed. This model considers that CuO first decomposes into Cu 2 O, the oxygen diffusing across Cu 2 O and Al 2 O 3 layers before reaching the Al layer where it reacts to form Al 2 O 3 . The theoretical results in terms of ignition response times confirm the experimental observation. The increase of the heating surface area leads to an increase of the ignition response time and ignition power threshold (go/no go condition). We also evidence that for any heating surface area, the ignition time rapidly decreases when the electrical power density increases until an asymptotic value, named minimum response ignition time, which is a characteristic of the multilayered thermite itself. At the stoichiometric ratio (Al thickness is half of CuO one), the minimum ignition response time can be easily tuned from 59 µs to 418 ms by tuning the heating surface area. The minimum ignition response time increases when the bilayer thickness increases. Not only this work gives a set of micro-initiator design rules to obtain the best ignition conditions and reaction performances but it also details a reliable and robust MEMS process to fabricate igniters and it brings new understanding of phenomena governing the ignition process of Al/CuO multilayers.

Engineering of Al/CuO Reactive Multilayer Thin Films for Tunable Initiation and Actuation

“…The propagation of the combustion front is therefore recorded using both the high speed camera (a SA3 photron with max 120 000 fps) and by measuring the travel time between the Figure 8. Ignition times as a function of the ignition power density and comparison with simulations (details in [38]). Curves and points correspond to five 300 nm thick 1 : 1 Al/CuO bilayers deposited on a thin Ti filament evaporated on Kapton (left) and Pyrex (right).…”

“…As an illustration, Figure 8 provides the ignition time vs the ignition power for a thin reactive film made of five 300 nm thick 1 : 1 Al/CuO bilayers, deposited on a thin Ti filament evaporated on either a Kapton membrane or directly onto the Pyrex substrate. The simulation results (solid line) are detailed in [38]. Below a certain electrical power, no ignition occurs, regardless the duration of application of the power.…”

“…Our team has developped a simple hot-wire initiator consisting of a Ti thin film resistance on a Pyrex substrate or Kapton film from the DuPont Corporation. The fabrication process summarized in Figure 7, as well as the ignition experimental set up and procedures were already published in [38].…”

“…For any heating surface areas, the ignition time rapidly decreases when the ignition power density increases until an asymptotic value, defining the minimum ignition response time, which is a characteristic of the multilayered material itself. In [38], authors investigated both experimentally and theoretically, the main parameters influencing the minimun ignition response time such as the heating surface area, the multilayers characteristics (bilayer thickness, film thickness and stoichiometry). For a given electrical power, they showed that the minimum ignition response time is highly affected by the substrate thermal conductivity, the heating surface area, defined through the resistance design, and nanothermite film layering.…”

“…As summary, in addition to the stoichiometry and layers' thickness [38], both the hot point surface area and substrate thermal conductivity, have a great impact on the ignition threshold. They have to be carefully chosen to fullfil the application requirements.…”