ChBE Seminar Series: Timothy Weihs
Tuesday, October 19, 2010
11:00 a.m.-12:15 p.m.
Room 2110, Chemical and Nuclear Engineering Bldg.
Professor Michael Zachariah
Rapid Formation Reactions in Nanolayered Foils: Scientific Studies and Commercial Applications
Department of Materials Science and Engineering
Johns Hopkins University
Over the last 15 years we have investigated exothermic formation reactions that self-propagate in multilayer foils where the nanoscale layers alternate between materials with negative heats of mixing. These exothermic reactions can reach temperatures as high as 3000°C and can travel at velocities greater than 30 m/s. One common example of such a foil includes alternating layers of Ni and Al. With a pulse of electrical current at one end of the foil, ohmic heating provides sufficient thermal energy to initiate atomic diffusion between the Ni and Al layers and the formation of Ni-Al compounds, which then leads to energy release and propagation of the reaction away from the point of ignition. Using a combination of ignition experiments, velocity and temperature measurements, continuum modeling and molecular dynamic simulations we have identified many of the physical parameters that control ignition and the propagation of the reactions. We have also used in situ X-ray diffraction and TEM experiments to identify the phase transformations that appear in these foils under very rapid (~106 K/sec) heating conditions.
In addition to these scientific studies, we have also developed commercial applications for these reactive nanomaterials. In the most common example, the foils act as local heat sources that melt solder layers and bond two components without thermal damage. The lack of global heating has proven particularly beneficial when bonding temperature sensitive devices such as CPUs and LEDs. This presentation begins with a demonstration and a review of commercial applications, but focuses on results from the scientific studies.