Energetic Al/Ni Superlattice Enables High‑Speed Micro‑Plasma Generation with Enhanced Flyer Velocities

Abstract

We report the fabrication and characterization of an energetic Al/Ni superlattice, deposited by high‑vacuum magnetron sputtering onto Al₂O₃ substrates. The resulting bilayer thickness of ~25 nm (≈15 nm Al + 10 nm Ni) forms a well‑ordered periodic structure, as confirmed by transmission electron microscopy (TEM). When subjected to a 0.22 µF capacitor charged between 2.9 kV and 4.1 kV, the superlattice exhibits a pronounced electrical‑exploding response that generates a micro‑plasma within a few hundred nanoseconds. The plasma output, quantified by flyer‑velocity measurements, surpasses that of conventional 500 nm Al/Ni multilayers, reaching >3 km s⁻¹. These findings demonstrate that the Al/Ni superlattice can serve as a highly efficient micro‑plasma generator, enabling improved initiator performance with lower energy input.

Energetic Al/Ni Superlattice Enables High‑Speed Micro‑Plasma Generation with Enhanced Flyer Velocities

Introduction

Reactive multilayer foils (RMFs) store chemical energy in alternating nanometric layers that react explosively when triggered by an external stimulus. Their reaction velocity and peak temperature are strongly governed by the composition and layer geometry. Al/Ni RMFs are particularly attractive due to their high reaction heat (~330 cal g⁻¹), excellent manufacturability, and low cost. Extensive studies have established that reducing the bilayer thickness increases the fuel/oxidizer interfacial area and shortens diffusion distances, thereby accelerating the reaction and raising the peak temperature. However, when the bilayer thickness falls below ~20 nm, excessive intermixing can diminish performance.

When the bilayer spacing is driven into the sub‑nanometer regime, an energetic Al/Ni superlattice is obtained. Such a structure offers ultrashort reactant separations and a large intermixing region, leading to unique reaction kinetics that differ from conventional RMFs. Previous work has characterized these superlattices using differential scanning calorimetry, TEM, and time‑resolved X‑ray micro‑diffraction, revealing the absence of metastable phases due to the limited diffusion distance.

Despite the wealth of combustion‑characterization studies, the electrical response and plasma generation of Al/Ni superlattices under high‑voltage stimulation remain underexplored. In this study, we deposit an Al/Ni superlattice on Al₂O₃ substrates, pattern it into a bowtie bridge, and evaluate its electrical and plasma behavior when driven by a high‑pulse capacitor.

Experimental Methods

Energetic Al/Ni superlattice samples were fabricated by alternately depositing Al and Ni layers onto Al₂O₃ substrates from high‑purity (99.99 wt %) targets. The base pressure of the sputtering chamber was 5 × 10⁻⁵ Pa, and argon was introduced at 0.8 Pa. Both Al and Ni were sputtered at 90 W, yielding deposition rates of ≈15 nm min⁻¹ and ≈10 nm min⁻¹, respectively. This produced a bilayer thickness of ~25 nm (≈15 nm Al + 10 nm Ni) and a total film thickness of ~4 µm, maintaining an overall 1:1 atomic ratio (3:2 thickness ratio). For comparison, conventional 500 nm Al/Ni RMFs were also deposited under identical conditions. A 20 nm copper layer was sputtered on top to improve electrical contact to the ceramic plug.

The micro‑plasma generator was fabricated using standard MEMS processing. A 0.5‑mm thick 4‑in. Al₂O₃ wafer was cleaned sequentially with acetone, isopropanol, and deionized water in an ultrasonic bath, then baked at 100 °C for 30 min. After oxygen plasma cleaning, the energetic superlattice was deposited. A positive photoresist (AZ5214E) was spin‑coated at 5000 rpm for 60 s and pre‑baked at 100 °C for 90 s. UV exposure (16 mJ cm⁻²) followed by development in NaOH created the bowtie bridge pattern. The structure was etched in an aluminum etchant (Al Etchant Type A, Transene) at 30 °C to a depth of ~4 µm. After dicing, residual photoresist was removed with acetone, and the chip was mounted onto a ceramic plug to complete the generator.

The cross‑sectional structure was examined by TEM. The micro‑plasma generator was then energized with a 0.22 µF capacitor charged to 2.9–4.1 kV. Current–voltage waveforms were captured with a Rogowski coil and high‑voltage probe, recorded on an oscilloscope. Simultaneously, plasma evolution was recorded with an ultra‑high‑speed camera (SIM, SIL3001‑00‑H06) using a 10 ns exposure and 20–50 ns inter‑frame interval, while light intensity was monitored by a photodiode. Timing synchronization among the pulse generator, camera, and oscilloscope was achieved with a digital delay generator (DG535). Additionally, the flyer‑velocity was measured with photonic Doppler velocimetry (PDV) by driving a 30 µm Kapton flyer with the bowtie bridge.

Results and Discussion

Figure 3a shows a bright‑field TEM image of the energetic Al/Ni superlattice. The periodic bilayer stack is clearly resolved, with ≈15 nm Al and ≈10 nm Ni layers. Selected‑area electron diffraction confirms a well‑defined polycrystalline structure for both phases. In contrast, Figure 3d illustrates the thicker 500 nm bilayer RMFs, where the layer contrast is noticeably less distinct due to increased intermixing.

Figure 4a presents the evolution of voltage, current, light intensity, and energy for a 3.5 kV charged capacitor. The initial Joule heating raises the bridge temperature, causing a gradual voltage rise. When the temperature crosses a threshold, the Al/Ni layers vaporize and ionize, creating a low‑resistance plasma path that spikes the current and abruptly drops the voltage. This explosive transition is accompanied by a sharp burst of light, indicating plasma formation.

Figure 4b captures the plasma expansion in real time. The initial Joule‑heating phase lasts ~168 ns. At ~218 ns, a sudden voltage surge coincides with intense light emission across the bowtie bridge, marking the onset of evaporation. The peak voltage at ~258 ns signals the explosion and plasma generation, followed by rapid expansion that can produce a shock wave. The absence of discrete combustion products suggests a uniform explosion driven by the plasma.

By integrating the voltage‑current curves for charging voltages between 2.9 kV and 4.1 kV, we extracted the delay time (T_b) and critical explosion energy (E_c). As shown in Figure 5a, T_b decreases with higher charging voltage, while E_c increases. This trend indicates that a higher electrical energy input shortens the ignition delay and raises the energy required for a complete explosion. The more homogeneous current distribution at 4.1 kV explains the reduced delay and increased critical energy.

Flyer‑velocity measurements reveal a clear dependence on the charging voltage. As depicted in Figure 6a, the maximum velocity rises from ~2.3 km s⁻¹ at 2.9 kV to >3.0 km s⁻¹ at 4.1 kV. These velocities are markedly higher than those obtained from 500 nm bilayer RMFs, confirming the superior energy density of the superlattice. The Gurney energy model, which relates flyer velocity to the energy per unit mass delivered by the plasma, predicts that a higher exploding current density should yield a higher velocity. However, the experimental data show the opposite trend for the conventional RMFs, implying that the chemical reaction of the Al/Ni superlattice—enhanced by plasma formation—contributes additional energy beyond the purely electrical input.

Conclusions

We have demonstrated that an Al/Ni superlattice with a 25 nm bilayer thickness, fabricated by magnetron sputtering on Al₂O₃, can function as a highly efficient micro‑plasma generator. Under a 0.22 µF capacitor charged to 4.1 kV, the device exhibits a rapid electrical‑exploding response that produces plasma within <300 ns, generating flyer velocities exceeding 3 km s⁻¹. These velocities surpass those of 500 nm Al/Ni RMFs, confirming that the chemical reaction of the superlattice contributes significantly to the plasma expansion. The results suggest that energetic Al/Ni superlattices are promising candidates for micro‑ and nano‑plasma initiators, offering improved energy transduction and system reliability for advanced applications.

Abbreviations

RMFs:: Reactive multilayer foils
TEM:: Transmission electron microscopy

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