Enhanced Energetic Performances Based on Integration with the Al/PTFE Nanolaminates
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Integrating energetic materials on a chip has received great attention for its widely potential applications in the microscale energy consumption system, including electric initiation device. In this article, reactive Al/PTFE nanolaminates with periodic layer structure are prepared by magnetron sputtering, which consists of fuel Al, oxidant PTFE, and inert layer Al-F compound in a metastable system. The as-deposited Al/PTFE nanolaminates exhibit a significantly high energy output, and the onset temperature and the heat of reaction are 410 °C and 3034 J/g, respectively. Based on these properties, an integrated film bridge is designed and fabricated via integrating Al/PTFE nanolaminates with a Cu exploding foil, which exhibits enhanced energetic performances with more violent explosion phenomenon, larger quantities of ejected product, and higher plasma temperature in comparison with the Cu film bridge. The kinetic energy of flyers derived from the expansion of the Cu film bridge is also increased around 29.9% via integration with the Al/PTFE nanolaminates. Overall, the energetic performances can be improved substantially through a combination of the chemical reaction of Al/PTFE nanolaminates with the electric explosion of the Cu film bridge.
KeywordsAl/PTFE nanolaminates Nanostructured energetic materials Exploding foil initiator Electric initiation
Differential scanning calorimetry
Exploding foil initiator
Microelectronic and mechanical systems
Transmission electron microscopy
X-ray photoelectron spectroscopy
Over the last decade, investigations into nanostructured energetic materials have received worldwide concern and increasing research interest owing to their superior energetic performances, including low ignition temperature, rapid energy release, high energy density and tunable reactivity [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The chemical energy stored by these materials can be released upon electrical, optical, impact, or thermal actuation, which can be used for military purposes and civilian applications, such as initiation of secondary reactions , joining of materials , automotive air-bag propellants , and power supply . Many methods including the physical mixing of nanopowders, arrested reactive milling of dense nanocomposites, electrophoretic nanoenergetic coating, and periodical deposition of nanolaminates have been introduced to fabricate nanostructured energetic materials [15, 16, 17, 18, 19]. Among these methods, the fabrication of nanolaminates through alternately depositing two or more different films provides a fascinating structure for device integration with tunable energetic performances, because the number of layers and the thickness of monolayer are easily controlled, and consequently to tune their energetic performances.
Exploding foil initiator (EFI) is a type of electric generation pyrotechnic devices used for initiation of secondary reactions . After applying an electric pulse, instantly increasing current density causes the vaporization of metal film bridge and the generation of high pressure plasma. Then, the flyer on the film bridge is sheared and accelerated to impact the explosives. With the increasing requirements for electric ignition device miniaturization and low energy initiation, the integration of nanoenergetic layers with a metal film bridge based on microelectronic and mechanical system (MEMS) technology to achieve functional nanoenergetic-on-a-chip (NOC) constitutes a promising option for the development of EFI. The combination of the reaction heat of energetic materials with the traditional electrical joule of metal film bridge make it possible to improve the electric explosion performances of EFI with low energy initiation in a compact size.
Al/PTFE nanolaminate film is a promising candidate to be integrated with EFI based on the following reasons. First, the metal Al is a common material with a high energy density and energy release rate during oxidation. Meanwhile the fluorine content in PTFE is up to 76 wt.%, which can react with the metal Al to form AlF3 with a high theoretical energy release of 5571 J/g . Second, the potential gas release derived from the pyrolysis of PTFE film and the reaction product of oxycarbide in the atmospheric conditions can increase the pressure of generated plasma, which is beneficial for shearing and accelerating of the flyer . In this paper, an integrated film bridge was designed and fabricated via integrating the Al/PTFE nanolaminates with a Cu exploding film bridge. The structure and chemical composition of as-deposited Al/PTFE nanolaminates were studied by TEM and XPS analyses. The effects of the integrated Al/PTFE nanolaminates on the electric initiation performances were investigated through the electric explosion tests.
Deposition of the Al/PTFE Nanolaminates
Al/PTFE nanolaminates were prepared through alternately depositing Al layers and PTFE layers by direct current magnetron sputtering and radio frequency magnetron sputtering, respectively. The targets used for sputtering were pure aluminum foil (purity > 99.999%) and polytetrafluoroethylene foil (purity > 99.99%) with a size of 100 mm in diameter. A rotating substrate table was employed to realize multiple alternating depositions. The base pressure for film deposition was below 5 × 10− 4 Pa, and the argon gas was introduced as gas media. The deposition parameters were set as 1.1 Pa, 300 W for PTFE layers, and 0.45 Pa, 100 W for Al layers, to obtain an optimized film quality and stable deposition rate.
Preparation of the (Al/PTFE)n/Cu-Integrated EFI
Before deposition, the substrate was ultrasonic-cleaned sequentially by using acetone, alcohol, and deionized water for 10 min. Next, the cleaned substrate was blow-dried by argon gas and heat treated at 120 °C for 1 h for further drying. After drying, a 2-μm-thick Cu layer was deposited on the cleaned substrate by DC magnetron sputtering. Subsequently, the as-deposited Cu film was patterned through photolithography, and wet-etched by copper etching agent (CE – 100). The dimension of the patterned Cu film bridge was 600 μm × 600 μm. Then, ~ 2-μm-thick Al/PTFE nanolaminates were deposited on the top of the Cu film bridge and patterned with image reversal lift-off process. The stacking sequence for sputtering Al/PTFE nanolaminates was Al/PTFE/Al/PTFE/Al, and Al layer was left as the top layer. After that, two Cu contact pads patterned with mask were stacked on the both sides of the Al/PTFE nanolaminates for the connection to the voltage source. Finally, the finished sample was diced into individual units.
Characterization of the Al/PTFE Nanolaminates
The crystallinity and structural microscopic characterization of the Al/PTFE nanolaminates were performed using transmission electron microscopy (TEM). A ~ 1-nm-thick Al film was deposited on the PTFE layer to determine the chemical compositions of the interface between Al layer and PTFE layer by X-ray photoelectron spectroscopy (XPS). The PTFE nanolaminates were scrapped from the substrate and transferred in an alumina crucible for the analysis of energy release by differential scanning calorimetry (DSC). The sample mass for each test was ~ 10 mg, and the tests were carried out from 25 to 800 °C at a heating rate of 10 °C/min in flowing argon.
Electric Explosion Test of the Film Bridge
The electric explosion properties of the samples were tested by an electric explosion measurement system, which is similar to the previous report for Cu/Al/CuO film bridge . The electric explosion temperature characteristics were determined by an electric explosion temperature diagnosis mode based on the “double-line atomic emission spectroscopy of a copper element” [24, 25]. The electric explosion phenomena were recorded synchronously by a high-speed camera with 20,000 frames per second. The acceleration process of flyers was obtained through photonic Doppler velocimetry (PDV) to investigate the ability to drive flyers.
Results and Discussion
Characterization of the Al/PTFE Nanolaminates
Electric Initiation Performances of the (Al/PTFE)n/Cu Film Bridges
In summary, reactive Al/PTFE nanolaminates with periodic layer structure were successfully fabricated by magnetron sputtering. The Al/PTFE nanolaminates was composed of PTFE layers (amorphous), Al layers (polycrystalline), and inert layers (Al-F compound) in a metastable system, which could provide a high energy output of 3034 J/g. Through MEMS technology, the Al/PTFE nanolaminates were integrated with a Cu exploding foil to construct an integrated film bridge. The chemical reaction of Al/PTFE nanolaminates is well consistent with the electric explosion of the Cu film bridge. The electric explosion temperature and the energy output of the integrated film bridge are also increased evidently. Overall, the initiation performances of the Cu film bridge can be improved obviously through integration with the Al/PTFE nanolaminates.
This work was supported by the military support project fund (No. JPPT-125-5-161), the basic research project fund of the central university (2672018ZYGX2018J023), and The China Postdoctoral Science Fund (2018M633344).
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article.
ZYX gave the idea and designed and performed the experiment, data processing, and manuscript drafting. YYC, AMT, WY, and WL assisted in the measurement and data analysis. JHC made corrections to the manuscript. ZXH, ZWL, and LYR supervised the whole work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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