Ignition and self-propagating reactions in Al/Pt multilayers of varied design

• Published in: Journal of applied physics Vol. 124; no. 9
• Main Authors: Adams, D. P., Reeves, R. V., Abere, M. J., Sobczak, C., Yarrington, C. D., Rodriguez, M. A., Kotula, P. G.
• Format: Journal Article
• Language: English
• Published: Melville American Institute of Physics 07.09.2018
• Subjects: Activation energy; Applied physics; Constraining; Diffusion coefficient; Diffusion rate; Heat of reaction; Ignition; Mass transport; Mathematical models; Melting points; Multilayers; Platinum; Self propagation; Solid state; Stoichiometry; Thermal diffusion; Thermodynamic properties; Videography
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.5026293

The different rate-limiting processes underlying ignition and self-propagating reactions in Al/Pt multilayers are examined through experiments and analytical modeling. Freestanding, ∼1.6 μm-thick Al/Pt multilayers of varied stoichiometries and nanometer-scale layer thicknesses ignite at temperatures below the melting point of both reactants (and eutectics) demonstrating that initiation occurs via solid-state mixing. Equimolar multilayers exhibit the lowest ignition temperatures when comparing structures having a specific bilayer thickness. An activation energy of 76.6 kJ/mol at. associated with solid state mass transport is determined from the model analysis of ignition. High speed videography shows that equimolar Al/Pt multilayers undergo the most rapid self-sustained reactions with wavefront speeds as large as 73 m/s. Al- and Pt-rich multilayers react at reduced rates (as low as 0.3 m/s), consistent with reduced heat of reaction and lower adiabatic temperatures. An analytical model that accounts for key thermodynamic properties, preliminary mixing along interfaces, thermal transport, and mass diffusion is used to predict the wavefront speed dependencies on bilayer thickness. Good fits to experimental data provide estimates for activation energy (51 kJ/mol at.) associated with mass transport subject to high heating rates and thermal diffusion coefficient of premixed interfacial volumes (2.8 × 10−6 m2/s). Pt dissolution into molten Al is identified as a rate-limiting step underlying high temperature propagating reactions in Al/Pt multilayers.