Crystallization kinetics of pentaerythritol tetranitrate (PETN) thin films on various materials

Abstract

The nature of the crystal growth process in explosives can dramatically impact their safety and performance. Thin films are a commonly used tool for investigating the fundamental crystallization behavior in many systems, and the interfacial physics between a crystallizing species and the substrate can significantly affect many factors associated with the crystal growth process. In this work, atmospheric vapor deposition was used to prepare thin films of supercooled PETN droplets on the surfaces of copper, gold, and silicon. The gold and copper surfaces were prepared on silicon substrates using ultra-high vacuum vapor deposition. Crystallization of the supercooled PETN droplets was induced by scanning the surface of each substrate type with an atomic force microscope upon which crystallization occurred in the form of compact, circularly shaped dendrites reminiscent of spherulites. All three substrates were characterized relative to their respective surface roughness and surface energy. The surface energy was assessed from contact angle measurements with three liquids: water, ethylene glycol, and dodecane. High-resolution, time-lapse images of the crystal growth process were acquired under high magnification on each material surface, and the crystal growth rates were measured based on the radial distance of “ring” features that appeared in the crystal growth patterns after inducing changes in the substrate temperature. The growth rates were used to determine the Arrhenius activation energy of crystal growth on each surface, and the results indicate that there is a significant correlation between the surface energy and the crystallization behavior of the supercooled PETN droplets.