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Reactive Foils
Turn off the ovens. Put away the blow torches. Save the Velcro for clothing accessories. In 1994 Dr. Tim Weihs (a professor in JHU’s Materials Science and Engineering Department) and Dr. Troy Barbee of Lawrence Livermore National Laboratory made a groundbreaking discovery that held the promise of changing the way many materials are held together. Components that are joined by soldering or brazing in ovens or with blowtorches can be damaged by heat exposure, and the heating process also introduces oxygen, compromising the strength of the joint. Recent advances in ceramic armor materials have seen limited use because they have proven so difficult to attach to metal. Weihs and Barbee’s invention, a thin sheet of foil made up of nanoscale layers of alternating materials, makes it possible to create ultra-strong bonds without overheating components and without the presence of oxygen.
Weihs and Barbee patented their invention in 1996, and set to the lengthy task of making these foils ready to manufacture. In this effort they are joined by Professor Omar Knio, who is spearheading the computational modeling effort that will eventually lead to optimizing the foil’s design.
Each layer in a multilayer foil is from 1 to 100 nanometers thick, alternating between a light element such as aluminum and a transition metal such as nickel. The layers (about 1,000 of them) are deposited by magnetron sputtering to create a foil sheet about 10 microns thick (for reference, a human hair is about 60 microns in diameter). Because nickel would rather be bonded to aluminum than to itself, according to Weihs, once a reaction is started, say, with a spark or a match, a self-propagating exothermic reaction begins and speeds through the foil at about 5m/s. The temperature goes from 25C to 1600C in about 10 milliseconds as the reaction front flashes through the foil. When sandwiched between two components and two sheets of solder and ignited, the foil melts the solder, joining the materials together. Only the surface layer of the material being joined is exposed to the heat during the process, and this is the beauty of the invention. Because the entire component is not subjected to an external heat source during bonding, the kinds of materials, solders, and brazes that are used in manufacturing processes can expand dramatically. Appli-cations include soldering of temperature-sensitive microelectronics and semiconductors, hermetic sealing, and metal-ceramic joining.
The trick is finding the right thickness and composition for a given joining application. The velocities, heats, and temperatures of the reactions can be controlled by varying the thicknesses of the alternating layers. A foil with very thin layers, about 20-50 atoms thick, will get very hot very fast, melting a lot of braze or solder and creating an extremely strong joint. But if the individual layers in the foil are too thin, oscillations in the temperature front can quench the reaction. Prof. Knio has been numerically modeling the heat transfer taking place in the exothermic reaction, in order to optimize the melting of solders while at the same time minimizing the heating of components. By combining numerical predictions with experimental measurements of mechanical properties, Prof. Knio plans to develop software that can be used to determine the combination of foils, brazes, solders, and geometries that optimizes the joint shear strength and interfacial fracture resistance.
Weihs and Knio have set up a company called Reactive NanoTechnologies and are working to commercialize the reactive foil technology. The patents are rolling in. This spring, they patented a method to manufacture the foils and two foil structures.