Reactive foils are nanoscale metallic multilayers comprising two or more elements with a large negative heat of mixing. Applying a small spark can cause mixing between the layers; the mixing process itself generates additional heat, resulting in a highly exothermic reaction front that propagates across the foil. The temperature can rise 1000 ºC or more in a few milliseconds, corresponding to a heating rate in excess of one million degrees per second. But although the temperature can be quite high, the total heat released is small because the foil is thin. This makes reactive multilayers useful as local heat sources for a variety of applications, including welding or soldering thermally-sensitive materials. The reactive foil technology has been commercialized by Prof. Tim Weihs, who co-founded Reactive NanoTechnologies.

The rapid heating makes the reactions themselves scientifically interesting, but studying them is a considerable challenge. The reaction front is only about 100 μm wide and propagates at ~1 m/s, so it passes a fixed point on the foil in less than 100 μs. In order to study the phase transformations occuring in the reaction front, we (in collaboration with Sol Gruner at Cornell) developed a time-resolved x-ray microdiffraction experiment using intense synchrotron radiation along with an extremely fast position-sensitive x-ray detector. By collecting several diffraction patterns as the reaction front passes the x-ray beam, we can determine the order in which phases form and watch the reaction sequence in detail. These in situ experiments are showing that previous conceptions of the phase formation sequence (based on either slower heating rates or quenching experiments) are incorrect.

The experimental setup for the initial proof of concept experiments (conducted at the Cornell High Energy Synchrotron Source, or CHESS) is show at left. The synchrotron x-ray beam is focused to a small (~50 μm spot) by a quartz capillary. The reactive foil is mounted in a holder which allows the reaction to be triggered remotely. As the reaction propagates across the foil, the light emitted is sensed by a photodetector coupled to a fiber optic; this signal is used to trigger the pixel array detector (PAD). The PAD, developed by Sol Gruner's group at Cornell, is ideal for these experiments because it allows several diffraction patterns to be recorded in succession with microsecond resolution. This is important because, unlike pump-probe expeiments which achieve high time resolution by repeatedly exciting reversible transformations, the self-propagating reactions studied here are irreversible and so all of the data must be captured in a single event.

Recently, we have moved these experiments to the Advanced Photon Source at Argonne National Laboratory, where we use Kirkpatrcik-Baez mirrors to focus a "pink" undulator beam. This provides about the same number of photons (roughly 10¹³ ph/s) into an even smaller spot.