Thermodynamics and kinetics of phase formation in magnetron-sputtered Ni/Al multilayer thin films with nanoscale morphology
Aluminide thin film multilayer systems are materials far from equilibrium that are highly reactive enabling self-propagating high-temperature syntheses (SHS)  where a high amount of heat is released. Using magnetron sputter-deposition many hundreds of individual nanometer thick layers can be stacked by alternating pure Al with one or more other metals that are highly miscible in Al, e.g., Ni, Ru, Fe, Pd, Pt, or Ag [1-3]. The reaction mechanism toward equilibrium is controlled by a pronounced atomic diffusion asymmetry across the layer interfaces and it is characterized by large negative heat of formation. For example, in the case of the formation of the intermetallic NiAl and RuAl B2-structured phases the specific heat of formation is -59 and -62 kJ mol-1, respectively. This large amount of heat released during the solid state reaction is able to melt solder layers and thus considered relevant as micron-sized heat source or energy carriers for reactive joining in microelectromechanical systems.
The reaction itself is not only determined by the choice of reactant metals and the relative stoichiometry - thus by the thermodynamics of the system, but also by the type of imprinted morphology, as well as post-processing factors such as thermal and stress history, nature of the substrate, and ignition method. Here we will present thermodynamic and kinetic data associated to the reaction in Ni/Al sputtered multilayer thin films by differential scanning calorimetry (DSC) as a function of several of the abovementioned factors. For this reason, DSC will be employed for slow heating rates and Flash DSC for heating rates from 10^4 to 10^6 K s-1 similar to the heating rates observed in SHS reactions occurring in a semi-solid liquid state. We will discuss the effects of the layer periodicity, overall composition, sputtering conditions, and thermal post-processing treatments to the reaction mechanisms. Furthermore, we will compare the results with those obtained by annealing Al/Ni alternate foils after repeated cold rolling  and Ni-Al nanopowders after high pressure torsion .
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