| Self-propagating reactions were studied in multilayer foils. These sputter-deposited microlaminates consist of alternating layers of two materials which mix and react exothermically. The heat generated by the reaction propagates the reaction through the foil. A thorough understanding of the thermodynamics and kinetics of these reactions, and the sequence of intermediate phase formation, is vital for engineering these foils for applications such as joining.; Here, two reactive systems were investigated: nickel/aluminum (formation reaction) and copper oxide/aluminum (reduction-oxidation reaction). Reaction paths and kinetics were studied with differential scanning calorimetry, Auger depth-profiling, x-ray photoelectron spectroscopy, and energy-filtered transmission electron microscopy.; In the Ni/Al foils, the metastable phase Al9Ni2 formed as the first phase in a series of Ni/Al multilayer foils, but it did not form in foils with a small bilayer period (12.5 nm) where the stable phase Al3Ni formed first. The heat of formation and Gibbs free energy for Al9Ni2 were both calculated to be −28 kJ/mole·atom, and the average activation energy for Al9Ni2's formation was calculated to be 1.58 eV. A nucleation model based on thermodynamics and diffusive intermixing is proposed to explain why Al9Ni2 forms before Al3Ni in most cases, but not in foils with small bilayers.; CuOx/Al multilayers were successfully sputter-deposited. These thermite reactions self-propagate at 1 m/s and the heat released is −3.9 kJ/g. The CuOx deposited in the foils is non-stoichiometric Cu 4O3 (paramelaconite); the heat of formation and Gibbs free energy for Cu4O3 were calculated to be −453 kJ/mole and −279 kJ/mole, respectively. When these foils react in a differential thermal analyzer, the paramelaconite decomposes into CuO and Cu2O. The reaction then proceeds via two exotherms. First, an interfacial layer of Al2O3 grows to coalescence; second, this layer thickens and the final reaction products are Cu, Al2O3, and Cu 2O. A Modified Coffey Model was applied to help explain the calorimetry results. The first exotherm was assumed to be controlled by the two-dimensional, interface-limited growth of the Al2O3 layer, while the second exotherm was assumed to be controlled by one-dimensional growth of the Al2O3 and the reduction of Cu2O. |
