Nanotube cathode breakthrough may make portable X-rays possible

Carbon nanotube-based cathodes already are making a commercial splash in handheld X-ray devices used by industries to sort alloys and identify materials. Now a university-industry research team in North Carolina reports that it successfully produced a nanotube X-ray cathode for medical applications.

They say the X-ray tubes, which unlike conventional cathodes need little heat to function, will outlast today’s products and allow manufacturers to design portable X-ray equipment. The tubes could be on the market as early as 2004, according to Otto Zhou, an associate professor of physics and materials sciences at the University of North Carolina at Chapel Hill (UNC) and chairman of Applied Nanotechnologies Inc. (ANI).

UNC and ANI researchers collaborated on the project. Their findings appear in the July 8 issue of Applied Physics Letters.

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Zhou’s team found that a small amount of single-wall carbon nanotube film applied to a cathode produced currents high enough to obtain images with the clarity of standard X-rays. To prove the cathode’s effectiveness, they placed Polaroid films behind a hand and a fish to get X-rays images of their bones.

Other researchers and companies have produced nanotube-based cathodes. Zhou’s achievement was boosting current intensity to the level needed for medical applications.

“You need it to be intense in a short time,” Zhou said. X-rays, which can pass through many forms of matter, are used to examine inner structures such as bone. But prolonged exposure can damage genetic material.

Conventional X-rays work by heating a metal filament, the cathode, to more than 1,000 degrees Celsius. The cathode emits electrons, which then hit an anode to produce X-rays. In the 1990s, scientists discovered carbon nanotubes emit large quantities of electrons through their narrow tips when exposed to low electric forces. Since then, numerous industrial and academic researchers have incorporated nanotubes into field emission displays, and more recently cathodes, to make smaller and more energy-efficient devices.

“The design hasn’t changed over 100 years,” Zhou said of X-ray technologies. “All you need is an electron source and a target.”

Zhou predicts use of nanotubes will lead to longer lasting X-ray tubes, a need that inspired the project two years ago. As director of the North Carolina Center for Nanoscale Materials, Zhou often relied on X-ray diffraction techniques to study materials. He learned firsthand how the intense heat needed to produce X-rays took its toll on cathode tubes, which can cost about $6,000 and up to $29,000 or higher.

“We had to buy one every year,” he said. “One day our tube broke down and we discussed: ‘Why not make a carbon nanotube tube?'”

Shan Bai, chief executive of ANI, said the company is “in deep discussions” with several X-ray cathode manufacturers to use their technology. Bai and Zhou would not name their potential partners, but said the ability to extend the lifetime of cathodes and possibly miniaturize them for portable devices were attractive features. Ambulance personnel could use portable X-ray equipment to assess injuries at an accident scene, for instance.

The market potential is significant. The cathode-ray market has been pegged as high as $60 billion. X-ray cathodes would be a segment of that market, which includes blockbusters such as flat panel displays.

Two-year-old ANI is not the only company exploring the use of nanotubes as electron emitters. The Korean electronics company Samsung demonstrated a nanotube-based field emission prototype as early as 1999, and Samsung and Ise Electronics Corp. of Japan unveiled prototype displays this May. NanoLab, a startup in Massachusetts, is working with General Electric Co. and NASA on carbon nanotube cathodes, said David Carnahan, NanoLab’s president and co-founder.

Applied Nanotech Inc.’s nanotube X-ray cathode already appears in a handheld X-ray spectrometer made by Oxford Instruments PLC. The company introduced its Horizon 600 spectrometer in 2001 for analyzing inorganic material. Applied Nanotech is a subsidiary of SI Diamond Technology Inc. Applied Nanotech and Applied Nanotechnologies are not related.

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