Characterization of self-propagating formation reactions in Ni/Zr multilayered foils using reaction heats, velocities, and temperature-time profiles

We report on intermetallic formation reactions in vapor-deposited multilayered foils of Ni/Zr with 70 nm bilayers and overall atomic ratios of Ni:Zr, 2 Ni:Zr, and 7 Ni:2 Zr. The sequence of alloy phase formation and the stored energy is evaluated at slow heating rates (similar to 1 K/s) using differential scanning calorimetry traces to 725 degrees C. All three chemistries initially form a Ni-Zr amorphous phase which crystallizes first to the intermetallic NiZr. The heat of reaction to the final phase is 34-36 kJ/mol atom for all chemistries. Intermetallic formation reactions are also studied at rapid heating rates (greater than 10(5) K/s) in high temperature, self-propagating reactions which can be ignited in these foils by an electric spark. We find that reaction velocities and maximum reaction temperatures (T-max) are largely independent of foil chemistry at 0.6 +/- 0.1 m/s and 1220 +/- 50 K, respectively, and that the measured T-max is more than 200 K lower than predicted adiabatic temperatures (T-ad). The difference between T-max and T-ad is explained by the prediction that transformation to the final intermetallic phases occurs after T-max and results in the release of 20%-30% of the total heat of reaction and a delay in rapid cooling. (C) 2011 American Institute of Physics. [doi:10.1063/1.3527925]