Characteristics of the Energetic Igniters Through Integrating Al/NiO Nanolaminates on Cr Film Bridge
© Yan et al. 2015
Received: 21 November 2015
Accepted: 14 December 2015
Published: 30 December 2015
The energetic igniters through integrating Al/NiO nanolaminates on Cr film bridges have been investigated in this study. The microstructures demonstrate well-defined geometry and sharp interfaces. The depth profiles of the X-ray photoelectron spectroscopy of Al/NiO nanolaminates annealed at 550 °C with a bilayer thickness of 250 nm show that the interdiffusion between the Al layer and NiO layer has happened and the annealing temperature cannot provide enough energy to make the diffusion process much more complete. The electrical explosion characteristics employing a capacitor discharge firing set at the optimized charging voltage of 40 V show that the flame duration time is about 700 μs, and an excellent explosion performance is obtained for (Al/NiO)n/Cr igniters with a bilayer thickness of 1000 nm.
The energetic igniters have attracted much attention in recent years because of lower ignition energy, faster ignition time, and smaller dimensions in volume compared with conventional hot-wire devices, especially for doped polycrystalline silicon [1–5], platinum , titanium , chromium , and tantalum nitride bridges . Nevertheless, this kind of energetic igniters has some disadvantages such as relatively low transient ignition temperature and output energy. A variety of energetic nanolaminates consist of alternating nanoscale layers of metal or metal oxide such as Al/Ni [10–13], B/Ti [14, 15], Al/CuO [16–20], and Al/MoOx , which can provide large negative reaction heats. The reaction heat and burning rate are mainly determined by the ingredient and different bilayer thicknesses with different contact surface areas or volume ratios between alternate layers. Integrating nanolaminates on a film ignition bridge has proved to be an effective way to improve the performance, where a self-propagating exothermic reaction can be initiated in these nanolaminates with generated thermal plasma when the applied current passes through a film bridge. The integrated structure combines the advantages of the film bridge and reactive multilayer films, which can optimize the ignition performance with low electrical energy consumption, fast energy release rate, and a large amount of reaction heat.
Chromium can be a promising energetic material for its good physical properties including high stability, good reliability, and large temperature coefficient of resistance (TCR). Among all of the nanolaminates, it is found in the literature  that the released reaction heat in Al/NiO nanolaminates is about 3440 J/g and relatively much higher than that of others. In this study, an energetic igniter by integrating Al/NiO nanolaminates on a Cr film bridge is fabricated and characterized. The elemental diffusion process and solid state reaction mechanism in nanolaminates are systematically investigated, and the electrical explosion properties of (Al/NiO)n/Cr igniters are also studied.
Cr films are deposited onto alumina substrates (10 mm × 5 mm × 0.5 mm) by direct current (DC) magnetron sputtering. Before deposition, the substrates are cleaned with acetone, alcohol, and de-ionized water in an ultrasonic bath for 10 min, which subsequently are dried by nitrogen gas and placed in the oven for 1.5 h. The distance between the chromium target (99.99 % purity) and alumina substrate is 70 mm to obtain a homogeneous deposited film. When the base pressure is pumped down to 5 × 10−4 Pa, argon gas is firstly introduced into the chamber as work gas and then the Cr film is deposited for 1.5 h with a sputtering pressure and power of 0.9 Pa and 128 W, respectively. The spin-coated photoresist (AZ9260) on the Cr film is patterned using photolithography with the designed mask. Subsequently, the exposed Cr film is directly wet-etched in the corrosive liquid and then placed in the oven at 150 °C for 5 min.
Al/NiO nanolaminates are alternately deposited from Al (99.99 % purity) and Ni (99.95 % purity) targets of diameter 50 mm through a shield with an optimized shape by DC magnetron sputtering and DC reactive magnetron sputtering at room temperature, respectively. The distance between the target and substrate is 60 mm with the sputtering power at 100 W for the Al and Ni targets. After each NiO monolayer is obtained with argon partial pressure at 1.5 Pa and oxygen partial pressure at 0.25 Pa, the argon gas and oxygen gas in the chamber are pumped out completely to prevent the oxidation of deposited metal aluminum with argon partial pressure at 1.5 Pa. The deposition rates of Al and NiO monolayers are calculated by dividing the monolayer thickness by the deposition time. The total thickness of Al/NiO nanolaminates is 3 μm, and the thickness ratio of the Al monolayer and NiO monolayer is maintained at 1:1.5 in order to react completely between the Al and NiO films. The resistivity value of the (Al/NiO)n/Cr igniter is about 2 Ω. The nanolaminates with bilayer thicknesses of 250 and 1000 nm are fabricated in this paper with the NiO monolayer on the bottom and the Al monolayer on the top.
The cross-sectional and top-view morphologies are determined using a scanning electronic microscopy (SEM, JEOL-7500F). The elemental distribution through the thickness of several layers during the diffusion process in the nanolaminates after post-deposition annealed at 550 °C in flow argon gas is confirmed using X-ray photoelectron spectroscopy (XPS) depth profiling with the interval time of 5 min (Axis Ultra DLD, Kratos Analytical Ltd). The electrical explosion parameters including ignition voltage, ignition current, and ignition delay time are achieved using a capacitor voltage discharging firing set, and the literatures [23–25] describe the principle of open-air electrical explosion testing of the igniter; the explosion process is observed using a high-speed camera (HS4540MX12) with 20,000 frames per second. Component analysis of the corresponding products after electrical explosion experiment is identified using XPS.
Results and Discussion
The (Al/NiO)n/Cr igniters are fabricated by integrating Al/NiO nanolaminates on the Cr film bridge. The microstructure of Al/NiO nanolaminates with the bilayer thickness of 1000 nm is densely packed and with well-defined interfaces. The XPS depth profiles of Al/NiO nanolaminates annealed at 550 °C with bilayer thickness of 250 nm show that the diffusion between the Al layer and NiO layer has happened and the annealing temperature cannot provide enough energy to make the diffusion process much more complete. The electrical explosion test and XPS spectra exhibit that the (Al/NiO)n/Cr igniter with bilayer thickness of 1000 nm has more advantages over that of 250 nm. The (Al/NiO)n/Cr igniter is supposed to have a variety of potential applications in both military and civilian areas, and it should be noted that it allows batch fabrication and a high level of integration by standard microfabrication techniques.
This work is mainly supported by military pre-research fund (9140A12040412DZ02138).
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