Scientists have converted a gentle flow of gas directly into an electric current by flowing common gases like oxygen, nitrogen, or argon over a special kind of semiconductor. Although physicists say the technique has a way to go before it can be applied, they envision its being used for developing tiny measuring devices or to make tiny generators.
Last year in Science, Ajay K. Sood and Shankar Ghosh of the Indian Institute of Science in Bangalore and colleagues reported inducing an electrical signal by flowing water over single-walled nanotubes (Science, 14 February 2003). They were curious whether similar currents could be generated when a gas was used.
In this latest experiment, the physicists released pure compressed gas over a 3-millimeter-long semiconductor set on an incline and onto carbon nanotubes. The semiconductors the scientists used were "doped," meaning they'd been impregnated with certain impurities, such as boron or phosphorus, which would enhance their conductive properties. Sometimes the team released gas traveling a leisurely average walking speed of a few kilometers per hour; other times, they let the gas flow 100 times faster. Each time, they got a measurable electrical signal; when metals such as platinum replaced the semiconductor strip, the effect disappeared, they report in the 17 August online Physical Review Letters.
Creating an electrical current with a gas is possible in part because when gas strikes an inclined surface, it produces a pressure gradient, similar to the effect that keeps airplanes in the air. This pressure gradient in turn produces a temperature differential on the surface, which prompts production of a tiny electrical current.
"The device is potentially very useful," says Narayanan Menon, an experimental physicist at the University of Massachusetts, Amherst, who says he's surprised that no one has done this experiment before. He could envision the approach's being used to create new flow sensors or electric generators without moving parts.