Observation of Oscillating, Spin-Like Propagation in Reactive Multi-Layer Foils Using the Dynamic Transmission Electron Microscope

Self-propagating high temperature synthesis (SHS) in reactive multilayer foils (RMLFs) has been systematically studied in situ and ex situ. RMLFs are layered materials comprised of two constituents with a high enthalpy of mixing. The two constituents are deposited in an alternating fashion. The 10s--100s nanometer-thick layers produce short diffusion distances to enhance mixing. When initiated by an external heat source, the foils react in a self-propagating fashion driven by exothermic mixing. The propagation characteristics, namely velocity and maximum temperature, depend on the chemistries involved as well as the foil architecture. The Al/Ni 3:1 system was chosen because of its potential application in microelectronics and its lower reaction temperature. The foils were grown by magnetron sputtering with bilayers measuring 25 or 27nm and a final thickness of 125 and 189nm.;In situ and ex situ experiments have yielded significant cumulative trends about RMLF behavior. Ex situ experiments rely on reaction quenching and post mortem examination with XRD to reveal intermetallic phase evolution. Quenching can introduce intermediate phases not necessarily native to the original process. In situ optical observation yield temperature and velocity information, but not necessarily phase information. In situ x-ray microdiffraction has been applied to study phase evolution but samples a large portion of the foil. The dynamic TEM (DTEM) has the spatial and temporal resolution to study these reactions in situ to better our understanding of the reaction process, which tends to be rather uncontrollable and occurs at very high temperatures. Using SHS of RMLFs as a novel method for intermetallic formation will be benefited by a more thorough understanding of the thermodynamics and kinetics involved, especially for heat-sensitive application.;The dynamic transmission electron microscope (DTEM) has been a unique instrument allowing for in situ examination of RMLFs during the propagating reaction. The DTEM utilized two lasers, one to initiate a reaction in the foil by providing the heat stimulus and the second to cause photoemission of 2 billion electrons in a 15ns pulse. The pulse length determines the temporal resolution of the DTME, which in this case is a 15ns snap-shot of the reaction propagation. The spin-like propagation observed in time-resolved images and the variable microstructure which suggests oscillations were an unexpected finding in the Al:Ni system where the relatively large heat of mixing produces steady-state, fast propagation.;The DTEM shows the reaction propagates in a transverse fashion, as in a spiral away from the laser ingestion source. While the original grain sizes were on the order of the bilayer size, the final grain sizes range from ∼50nm to 3 microns. The bands produced by the spiraling propagation show areas of fine and course grains in an alternating fashion along the band length. Additionally, although Al3Ni is a line compound, electron diffraction reveals the presence of two phases, Al3Ni and Al3Ni 2 with elemental Al. The heating and cooling rates are both significantly high and non-uniform to cause two phases and the large grain size distribution. The results give experimental credence to previous numerical modeling.