Control of self-propagating reactions and phases in Ni/Si reactive multilayers†

Reactive multilayers (RMLs) are alternating nanoscale exothermic materials that release stored energy upon localized energy ignition. We report in this paper the examination of the effects of the number of bilayers, roughness, and substrate on self-propagating reactions and product phases in equiatomic Ni/Si RMLs. Ni/Si RMLs, either freestanding or on FR4 substrates, with bilayer thicknesses of 50 nm, 65 nm, and 170 nm, were fabricated by magnetron sputtering at −20 °C. Infrared laser ignition and high-speed camera observations revealed two-step self-propagation wavefronts. With a fixed bilayer thickness, increasing the number of bilayers reduced radiative heat loss and enhanced self-propagation velocity. Surface undulations on FR4 substrates caused rough freestanding RMLs to have slower self-propagation velocities than flat freestanding RMLs. RMLs with a 50 nm bilayer and total thickness of 3.34 μm enabled successful self-propagating reactions on FR4 substrates. The product phase of the explosive silicidation reaction depended on the bilayer thickness and overall composition. In ultra-thin bilayer, a thickness of 50 nm favored single-phase NiSi₂ formation, while 65 nm favored θ-Ni₂Si. In thicker bilayers, a thickness of 170 nm favored single-phase NiSi, the equilibrium phase corresponding to the overall composition.