The role of two-stage phase formation for the solid-state runaway reaction in Al/Ni reactive multilayers

While extensively studied for heating rates below 1.7K/s and above 1000K/s, the solid-state phase transformations in Al/Ni reactive multilayers have not been examined at intermediate heating rates between 100K/s and 1000K/s. Combined nanocalorimetry and time-resolved synchrotron x-ray diffraction studies are utilized to address this range of heating rates for multilayers with an overall composition of 10at. % Ni and a bilayer thickness of 220nm. It was found that a two-stage phase formation of Al3Ni proceeds up to a heating rate of 1000K/s. The two growth stages occur in the solid-state and are kinetically separated. The activation energy of the first growth stage is determined to be 137kJ/mol, which agrees well with the literature data at low heating rates. At 1000K/s, a transition to a runaway reaction is observed. Unusual for metallic multilayers, the reaction proceeds completely in the solid-state which is also known as "solid flame." Using nanocalorimetry, a critical input power density for ignition of 5.8 x 10(4) W/cm(3) was determined. The rapid succession of the two Al3Ni formation stages was identified as the underlying mechanism for the self-sustaining reaction.