Observation of Transient Phase Formation in Reactive Multilayer Foils using Conventional and Dynamic Transmission Electron Microscopy (DTEM)

Abstract

Free-standing, polycrystalline Al/Ni multilayer thin films, or reactive multilayer foils (RMLF), react to form intermetallic compounds in a rapid, self-propagating reaction. The exothermic reaction front heats up to temperatures above 1120 K [1] and travels at a high velocity of ~10m/s in air. Since RMLFs produce extremely high heat over a small surface area, they are used in application to fuse dissimilar materials such as metal and ceramic, exposing only contacting surfaces to destabilizing heat.In this study, we perform a comparative study of transient intermetallic phase formation within the reaction front region using both optical and electron microscopy. Five bilayers of Ni and Al are repeated for an electron transparent 125 nm thick sample. STEM and TEM micrographs in Fig. 1 show the specimen geometry which appears to have insignificant interphase premixing from the deposition process. The formation reaction is initiated by a Nd:YLF ultraviolet laser inside the vacuum of the microscope. Optical time-resolved imaging is done to observe reaction front velocities and the evolution of a steady-state propagation of intermetallic formation. The specimen generates light during this exothermic formation and is detected every 2 µs for direct observation of this dynamic process. In Fig. 2, the reaction initiates at the position of the laser spot and gradually radiates outward until the ends of the jagged-shaped sample have been illuminated. The intensity can be related to temperature of the foil and the passing of the reaction front. It appears that the reaction front travels at a greater velocity than that mentioned above due to the vacuum pressure and sample dimensions. Analysis of the detected light levels indicates a non-steady state propagation of the reaction front. The reaction products are investigated with postmortem STEM and TEM studies to characterize any changes in the reaction front behavior.The metastable phase formation is also studied with the in-situ methods of the DTEM. The DTEM maintains the high resolution of conventional TEM but is also capable of fast (ns) time dependence studies. The instrument at Lawrence Livermore National Laboratory is modeled after the DTEM made by . It uses high-speed ultraviolet laser pulses to induce electron photoemission, creating a fast, pulsed incident electron beam. Because the dynamic evolution of phases are contained within the RMLF reaction front region alone, a single timeresolved image or diffraction pattern of this significant area gives significant information on transient states, rate of formation and also fundamental mesoscale kinetics. Such data will develop a complete picture of intermetallic phase formation in RMLFs down to the atomic and nanosecond scale. When this is captured at high time and spatial resolution, the self propagating reaction fronts can be related to the foil parameters, improving modeling [3] currently based upon diffusion and energy.

DC Photoelectron Gun Parameters for Ultrafast Electron Microscopy

“…Single-shot dynamic transmission electron microscopes (DTEMs) driven by nanosecond laser pulses (Bostanjoglo & Leidtke, 1980; Bostanjoglo et al, 2000; Dömer & Bostanjoglo, 2003; LaGrange et al, 2006 a ; Armstrong et al, 2007 a ) have thus far achieved 1–10 nm·ns (>1000 nm·ps) space-time resolution (LaGrange et al, 2006 a ; Armstrong et al, 2007 a ). This has already allowed the investigation of the a → b phase transition in Ti (LaGrange et al, 2006 a , 2006 b , 2007; Armstrong et al, 2007 a ) and the dynamics of reactive multilayer foils (Kim et al, 2006, 2007; Armstrong et al, 2007 a ). The space-time resolution of these operational DTEMs, however, is limited by both the drive laser pulse duration and the use (and hence retro-fit) of conventional electron microscope columns, which were not designed to handle the propagation of high charge density electron pulses.…”