Variable Laser Ignition Pathways in Al/Pt Reactive Multilayers across 10 Decades of Pulse Duration

Abstract

Pulsed laser irradiation is used to investigate the local initiation of rapid, self-propagating formation reactions in Al/ Pt multilayers. The single pulse direct laser ignition of these 1.6 μm thick freestanding foils was characterized over 10 decades of pulse duration (10 ms to 150 fs). Finite element, reactive heat transport modeling of the near-threshold conditions has identified three distinct ignition pathways. For milli-to microsecond pulses, ignition occurs following sufficient absorption of laser energy to enable diffusion of Al and Pt between layers such that the heat released from the corresponding exothermic reaction overcomes conductive losses outside the laser-irradiated zone. When pulse duration is decreased into the nanosecond regime, heat is concentrated near the surface such that the Al locally melts, and a portion of the top-most bilayers react initially. The favorable kinetics and additional heat enable ignition. Further reducing pulse duration to hundreds of femtoseconds leads to a third ignition pathway. While much of the energy from these pulses is lost to ablation, the remaining heat beneath the crater can be sufficiently concentrated to drive a transverse self-propagating reaction, wherein the heat released from mixing at each interface occurs under kinetic conditions capable of igniting the subsequent layer.

Stability Criteria for Self-Propagating Reaction Waves in Co/Al Multilayers

“…34 For context, it is known from nanosecond laser irradiation studies that even in the presence of tens of microns of material locally heated beyond the thermodynamic ignition condition, constraining reactant diffusion to the solid state will quench the reaction by longitudinal conduction. 43 These constraints on propagation can be seen in the stair-step propagation typical of mode II spin instabilities. Mode I propagation, the criterion suggests, occurs when h fus,Co > Q net > h fus,Al and the physical significance of this regime will be discussed below.…”

“…However, unlike with substrate quenching, the energy lost to forward conduction remains within the thermodynamic system of the multilayer and an event such as two intersecting spin bands can nucleate forward propagation . For context, it is known from nanosecond laser irradiation studies that even in the presence of tens of microns of material locally heated beyond the thermodynamic ignition condition, constraining reactant diffusion to the solid state will quench the reaction by longitudinal conduction . These constraints on propagation can be seen in the stair-step propagation typical of mode II spin instabilities.…”

“…When the leading edge of the reaction front cannot melt Co, mixing occurs via solid/liquid dissolution. Due to the difference in nearest neighbor sites of a two-dimensional Co surface compared to a three-dimensional bulk Co melt, there is actually a small increase in the maximal diffusion coefficient, D 0 , in the mode I enthalpy regime (i.e., from 1.38 × 10 –7 to 2.22 × 10 –7 m 2 /s) . Alone, this should reduce the ratio between forward heat conduction and reaction front propagation velocity, which would promote wave stability.…”

“…Due to the difference in nearest neighbor sites of a two-dimensional Co surface compared to a three-dimensional bulk Co melt, 48 there is actually a small increase in the maximal diffusion coefficient, D 0 , in the mode I enthalpy regime (i.e., from 1.38 × 10 −7 to 2.22 × 10 −7 m 2 /s). 43 Alone, this should reduce the ratio between forward heat conduction and reaction front propagation velocity, which would promote wave stability. The increased diffusivity of solid/liquid dissolution in the Co/ Al system has been isolated experimentally during previous studies that removed heat from the flame in thin 150 nm total thickness Co/Al via radiation.…”