Numerical Calculations and Measurements of Energy Transduction in Electrically Exploded Ni/Al Laminates
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Numerical Calculations and Measurements of Energy Transduction in Electrically Exploded Ni/Al Laminates Christopher J. Morris1, Paul R. Wilkins2, Chadd M. May2, and Nicholas W. Piekiel1 1
U.S. Army Research Laboratory, 2800 Powder Mill, Rd, Adelphi, MD, 20783, USA
2
Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
ABSTRACT The electrical heating of Ni/Al laminate foils allows interrogation of phenomena at heating rates as high as 10^12 K/s. In the 2011 Fall MRS meeting, we reported on emission spectra from rapidly heated Ni/Al laminates resolved temporally over 350 ns, which provided qualitative evidence of rapid and exothermic vapor phase mixing of Ni and Al in these experiments which we term electrical explosions. These results were significant, because thermal diffusion processes normally limit Ni/Al reactions to much slower energy release rates, potentially limiting their applications. Here we present further evidence of exothermic Ni/Al mixing, quantified by experimental velocity measurements of encapsulation material and interpreted by numerical calculations of energy partitioning into different processes. These calculations agreed well with experiments from different Al, Cu, and Ni samples, sputter-deposited and lithographically patterned into bow-tie bridge structures. Velocity measurements of up to 5 km/s for 11.5 µm thick parylene encapsulation layers were accurately predicted using a single, empirical fitting parameter which depended on the electrical circuit used. The calculations also agreed with encapsulation layers accelerated by electrically exploded Ni/Al laminates as long as an additional 1.2 kJ/g of energy was included in the model. This value is precisely the enthalpy of mixing between Ni and Al, and therefore quantifies the transduction of energy into encapsulation layer kinetic energy. INTRODUCTION The study of electrically exploding wires or foils has been ongoing for a number of decades [1], with applications in energetic material initiation, rail gun accelerators [2], x-ray generation [3], fast opening switches for pulsed power [4], bonding of ceramic materials [5], and the evaluation of material properties at high temperatures and pressures [6, 7]. We have focused on foils composed of Ni/Al laminates, heated at 1011–1012 K/s over the course of 1–100 ns. Both the measurement of encapsulation layer velocities [8] and the emission spectra of conductor materials [9-11] have provided qualitative evidence of exothermic Ni and Al mixing over these short timescales. In this paper we provide further evidence by validating an empirical model using measurements from Cu and Al bridges, and showing that the same model required an additional 1.2 J/mg of input energy in order to accurately predict encapsulation layer velocity from Ni/Al laminate bridges. This energy quantity was precisely the energy expected to be released upon mixing Ni and Al [12], strongly suggesting that this mixing contributed directly to the increase in kinetic energy.
EXPERIMENT We fabricated
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