Modeling and quantitative nanocalorimetric analysis to assess interdiffusion in a NiAl bilayer

A computational model is developed to describe the evolution of the temperature field in a nanocalorimeter that comprises inert material layers on which a nanoscale NiAl bilayer has been deposited. The model incorporates a reduced continuum description of mixing and heat release in the NiAl bilayer, and of the energy equation in the inert layers. Due to the small thicknesses of individual layers that are several orders of magnitude smaller than the corresponding length, a simplified, transient, homogeneous representation of the temperature field can be adopted. The resulting lumped model is valid over short enough timescales, which are nonetheless sufficiently large to capture the formation reaction. By using experimental observations of the evolution of the temperature on the surface of the nanocalorimeter, the model is used to estimate the transient heat release rate. Assuming an Arrhenius model for the mixing between Ni and Al, the estimated heat release rate is used to determine the Arrhenius pre-exponent and activation energy of the atomic diffusivity. Computed results indicate that the present approach provides a promising means of characterizing atomic diffusion rates. Limitations arise, however, due to the low amplitudes of the heat release term at low temperature, and also due to phase-change effects, which make the heat release rate unobservable in the neighborhood of the melting temperature of individual constituents. For the present system, reliable estimates are extracted for temperatures ranging from about 600 K to the Al melting temperature.