Thin, Fiery Foil
Soldiers in a remote desert must make emergency equipment
repairs. They have no welding torch and no 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 flashy new form of welding could be put into practice soon, using highly reactive metallic films--no thicker than a strand of human hair--co-invented by Timothy Weihs, an assistant professor of materials science and engineering. These foils are made by microscopic layering of two elements in alternating rows, each only 50 to 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 9-volt battery is enough to trigger a molecular marriage, causing the foil to erupt in a self-propagating reaction described as exothermic because it gives off heat. It 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, Weihs says, raising the foil's temperature 2,900 degrees Fahrenheit over a square-foot area within a millisecond. Because the heat erupts and ends so quickly, it can bond objects at their surfaces without damaging the rest of the material. The process has another great advantage: because it requires no oxygen, it can be used under water or in airless outer space.
Weihs is now refining the fabrication of these foils and exploring military, industrial and medical applications. He says the foils could be used to set off a secondary reaction that requires a strong burst of heat, such as activation of an adhesive material. They might be used to burn off cancer cells without harming healthy ones nearby. The foils might also be used in military flares to divert heat-seeking missiles. The multilayer films will probably find their widest use, however, as a bonding agent.
Using exothermic reactions to join materials is hardly new. In the thermite process, patented in 1908, aluminum and iron oxide powders were ignited to join the ends of steel railroad ties. But the powders have drawbacks because the atoms in one powder must travel greater distances to mix with an opposite atom in a neighboring powder. Thus, more heat is needed to trigger a reaction in powders than in foils. In addition, the air that is present between the powders can weaken the final joint.
Multilayer foils are superior because they place the reactive atoms much closer to one another, giving scientists greater control over the way they join and release heat.
"This is a way of taking those thermite reactions to the next level," says Weihs. "We can engineer the foils. We can control the rate of reaction. Essentially, we can engineer the materials to produce the precise heat-release rate and final temperature that we want."
Making these foils, however, is not easy. Weihs uses devices called magnetron sputtering guns inside a vacuum chamber to "knock" atoms from aluminum and nickel targets and deposit them on a substrate. By switching between the two sputter guns, Weihs is able to build up alternating layers, each perhaps 50 to 100 atoms thick. A thin layer of a third barrier material can also be added to keep the volatile atoms from mixing quite so quickly.
Weihs co-developed these reactive foils in 1994 as a postdoctoral student at Lawrence Livermore National Laboratory in California. He and co-inventor Troy Barbee Jr., a senior research scientist at Lawrence Livermore, were awarded a U.S. patent for the foils in July 1996.
The foils will not immediately replace traditional welding or heating tools because of the high cost of producing the films. But Weihs is optimistic. "In most cases, it's going to be more expensive to use the foils now, but there are a number of applications where the additional cost is justified," he says.
"What's needed is to have companies make these foils in large quantities. Then the price will come down."
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