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Electronic Properties of Carbon Fiber Open Field to 'Smart' Structures

Imagine an airplane that is smart enough to take energy from the sun and power its own aisle lights or let you know when it develops a dangerous crack in its wing. Or a solar car that soaks in enough sunlight to avoid those long lines at the gas station.

That's what scientists hope for the future using a carbon-fiber material recently discovered to have electronic properties.

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The material, a combination of hair-thin carbon fibers glued together and then built up in layers, has been used for decades to build airplanes and other structures. But researchers recently discovered it also can conduct electricity, making it an unusual combination of both a building material and an electronic device.

"With this material, the electronics vanish into the structure itself. The whole piece is smart, and no electrical interconnection is needed," says Deborah Chung, professor of mechanical and aerospace engineering at the University of Buffalo in New York. Chung discovered the electronic properties of the carbon fiber and presented her findings at the International Symposium on Smart Structures and Materials in San Diego in early March.

The material opens up the possibility of smart structures that could sense and behave intelligently and of new kinds of electronics that are less costly to make and save space, Chung adds. Electronics could be embedded in the panels of cars or the wings of airplanes, much as nerves are embedded in the human skin.

The discovery also lays the foundation for a new technology called structural electronics, Chung says. That technology could make possible aircraft components that are huge energy-storage devices, solar cars whose body panels can store energy, and even specialized computers and electronic devices without traditional silicon chips.

'THIS is an important finding and an interesting innovation," says Victor Li, professor of civil engineering at the University of Michigan, Ann Arbor. He is familiar with Chung's work. "It implies combining electronics and structural functions, which has not been done before."

It also gives engineers a way to monitor more closely the structures they build. "If an airplane wing develops cracks you can't always see them," says ceramic engineer Patrick Nicholson of McMaster University in Hamilton, Canada. "Until now, there has been no way of monitoring the behavior of materials in flight."

David Tunnicliffe, a physicist at the functional materials department of British Aerospace in Bristol, England, agrees. "This may be useful to detect damage, and thus in the future may help reduce maintenance costs of airplanes by pointing out to inspection engineers where to look for a problem."

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The carbon fiber is both a metal and a semiconductor, so it does not need electrical wires and contacts. A semiconductor is a material that can conduct electricity efficiently. The silicon used for computer and other electronics is a semiconductor. Although commercial applications for the carbon semiconductor are promising, they are a way off. Chung says she doesn't know its performance versus silicon, and she is just beginning to build devices using the carbon.

Still, the carbon-fiber material is stronger than silicon. And it can be used throughout the entire body of a product rather than in a concentrated area such as the main box of a computer. That means built-up heat from operation dissipates more easily, and the product lasts longer.

The material is made by laying down grids of carbon fibers that are about 1/10th the diameter of a human hair, and then binding them with epoxy.

The advantage of having a metal and semiconductor in one material is that the metal provides a system of built-in electrical contacts to which a meter can be connected for measurements.

Currently, optical or electronic sensors for detecting strain and deformation are embedded in carbon fiber materials used to make aircraft parts, a process that can actually weaken the structure, Chung says.

The carbon semiconductor may act like a series of tiny thermometers. That means it can detect potentially dangerous ice on an airplane wing, Tunnicliffe says.