Control of Large-Scale Single-Phase Ni Silicide Formation from Reactive Multilayers

The Ni/Si reactive multilayer (RML) is full of application potentials including reactive joining, igniters, and power sources. Here, the Ni/Si RMLs with atomic ratios of Ni and Si (2:1, 1:1, and 1:2) are studied, where the bilayer thickness and the total thickness are controlled to be 53 nm and 2 μm, respectively. After laser ignition, a rapid explosive reaction is observed where there are two self-propagation wavefronts and their velocities reach 7.20 and 11.93 m s⁻¹. The uniform, small grains, and single-phase Ni silicides are obtained and controlled by tuning growth parameters. From the understanding of the Gibbs free energy diagram, the first wavefront comes from the solid-state amorphization reaction between the Ni and a-Si. The formation of the final Ni silicide causes the second wavefront. The entire reaction is described by a transient heat transfer model. It proposes a key parameter called heat loss rate, which explains the importance of RML thickness and the thermal diffusivity of the substrate. It allows to accurately control the reaction rate, which masters the final silicide phase. Thus, the oxidation of samples downgrades the reliability as the self-propagation velocity of the two wavefronts dropped to 1.34 and 1.91 m s⁻¹ in samples after 200 h of oxidation.