Exothermic Self-Sustained Waves with Amorphous Nickel

The synthesis of amorphous Ni (a-Ni) using a liquid-phase chemical reduction approach is reported. Detailed structural analysis indicates that this method allows for efficient fabrication of high surface area (210 m(2)/g) amorphous Ni nanopowder with low impurity content. We investigated the self propagating exothermic waves associated with crystallization of Ni from the amorphous precursor. Time-resolved X-ray diffraction indicates that amorphous nickel crystallizes in the temperature range 445-480 K. High-speed infrared imaging reveals that local preheating of compressed a-Ni nanopowder triggers a self-sustaining crystallization wave that propagates with velocity similar to 0.3 mm/s. The maximum temperature of crystallization wave depends on the sample density and can be as high as 600 K. The Kissinger approach is used to determine the apparent activation energy (55.4 +/- 4 kJ/mol) of crystallization. The self-diffusion activation energy of Ni atoms in a-Ni is similar to 60 kJ/mol, determined through molecular dynamics (MD) simulations. This agreement of experimentally derived and theoretically calculated activation energies allows us to conclude that self-diffusion of Ni atoms is the rate-limiting stage for crystallization. Furthermore, utilization of amorphous metal as a reactant significantly increases the rate of solid-state reactions. For example, in reactive intermetallic forming systems, such as Ni + Al, the self-sustaining reaction propagation velocity with a-Ni is twice higher than with crystalline Ni of the same morphology. Additionally, using a-Ni increases the maximum reaction temperature in the Ni + Al system by 300 K.