Paper TF+EM+SE+NS-ThM5
Effects of Nanometer Scale Periodicity on the Self-Propagating Reaction Behaviors of Sputter-Deposited Multilayers

Thursday, November 1, 2012, 9:20 am, Room 10

Nanometer-scale, vapor-deposited multilayers are an ideal class of materials for systematic, detailed investigations of reactive properties. Created in a pristine vacuum environment by sputter deposition, these high-purity materials have well-defined reactant layer thicknesses between 1 and 1000 nm, minimal void density and intimate contact between layers. If designed appropriately, these energetic materials can be ignited at a single point and exhibit a subsequent, high-temperature, self-propagating formation reaction. The nanometer-scale periodicity set through design tailors the effective diffusion length of the subsequent self-propagating reaction.

With this presentation, we describe effects of the nanometer-scale, multilayer periodicity on i) the reactivity of multilayers in different surrounding gaseous environments and ii) the reaction front morphology as viewed in the plane of the multilayer. We show that nickel/titanium and titanium/boron multilayers are affected by the surrounding gaseous environment, and describe how the magnitude of average propagation speed depends on multilayer periodicity. Fine multilayer designs are characterized by fast reaction waves, and there is no difference in average propagation speed when reacted in air (atm. pressure) versus vacuum (1 mTorr). Coarse multilayer designs are generally slower and are affected by secondary oxidation reactions when conducted in air. These thick multilayer designs are affected by the pressure of the surrounding gaseous environment with enhanced propagation speeds owing to the highly exothermic reaction of Ti with O. Regarding the effects of nanometer-scale multilayer periodicity on reaction front morphology, we show that reactive multilayers often have a smooth reaction front when layer periodicity is small. However, multilayers having larger periodicity (and hence larger effective diffusion lengths) exhibit reaction front instabilities and complex reaction front morphologies.

In this talk, we also stress how the propensity to oxidize and the propensity to form reaction front instabilities (as affected through nanometer-scale design) impact final properties of the multilayers for applications such as localized joining.

Sandia is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.