Fiery foils produce new sealing process.

Soldiers in a remote desert must make emergency equipment repairs. They have no welding torch or access to an industrial furnace. Instead. a soldier slips a metallic film. thin as aluminum foil, between the broken parts and touches a match to the edge. The foil ignites in a brief, but powerful, blast of heat, creating a sturdy seal between the critical pieces.

This new form of welding could be put into practice soon, using highly reactive metallic films no thicker than a strand of human hair, says Timothy Weihs, assistant professor of materials science and engineering, Johns Hopkins University, Baltimore, Md. The foils are made by microscopic layering of two elements in alternating rows, each 50-100 atoms deep. Weihs deposits rows of atoms -- aluminum and nickel, for example -- that have a strong desire to bond with one another. A common match flame or a spark from a nine-volt battery is enough to trigger a molecular marriage, causing the foil to erupt in a self-propagating exothermic reaction. (An exothermic reaction is one that gives off heat. The process is self-propagating because the heat released by atomic bonding triggers the same reaction in adjacent atoms, causing the entire sheet of foil to ignite.)

This process occurs very quickly, raising the foil's temperature 2,900[degrees]F over a square-foot area within...