Putting it simply: a new way to make nanotubes is found

Oct 17, 2003

A lot of scientific discovery can be attributed to serendipity. A recent happy accident occurred on a nano-sized scale when researchers at Kettering University discovered a different method for producing nanotubes.

A lot of scientific discovery can be attributed to serendipity. A recent happy accident occurred on a nano-sized scale when researchers at Kettering University discovered a different method for producing nanotubes.

Nanotubes are tiny (ranging in size from 0.8 nanometers to 300 nanometers a nanometer equals one billionth of a meter) structures capable of great things. Imagine a rolled tube of graphite. Since graphite has very strong planar carbon bonds, nanotubes have very high tensile strength. They are also very light and have lots of surface area for something so small. In addition to their physical characteristics, nanotubes can also function as either conductors or semiconductors in electronic circuitry.

Their biggest attractions are their combination of surface area and unique electronic properties. Nanotubes can have thermal conductivity better than diamonds, electro-conductivity better than copper and can withstand very high temperatures because they are the strongest fibers known. This makes them the perfect material for a variety of applications from medicine to microprocessing to space exploration.

Until recently, there were four methods of producing nanotubes. Then, on the way to looking for something else, three students and two professors at Kettering stumbled across a different method of nanotube production.

"It's actually a simpler way of doing it than had previously been done," said Bahram Roughani, associate professor of Applied Physics. Established methods include arc discharge, laser ablation or pulsed laser vaporization (PLV), chemical vapor deposition and gas phase processes such as high-pressure carbon monoxide (HiPCO).

Roughani and David Parker, professor of Applied Physics and director of Applied Optics, started the research project with students John Henry, of Flint, Mich.; Paul Thomas, of Tyler, Texas; and Nadia Van Huffel, of Grand Rapids, Mich.

The Kettering team's approach used a laser, but they did it without special environments such as vacuum systems, high pressure or extreme temperatures. "We made our nanotubes in atmospheric pressure, at room temperature and we started with different materials," said Roughani, referring to the silicon carbide wafers they exposed to the laser and use as a solid substrate for the nanotubes. Most other methods use graphite as a starting point.

"We break up the silicon carbide disks and radiate them with a laser," said Parker, "this burns away the silicon leaving behind the carbon nanotubes." The students were originally looking for a way to reduce the occurrence of micropipes (voids) in silicon carbide as a project in Parker's Applied Physics class. Roughani suggested the project based on previous work he had done with Wright Patterson Air Force Base.

"We were looking for crystalline structures," Roughani explained, "we thought they would be the most interesting result." What they found instead were "haystacks." "I thought they (the students) had burned the heck out of the sample and had damaged it somehow," he said, "then I took another look and decided to do a little more work on the sample."

To verify what they thought they saw, Roughani subjected the sample to Raman Spectroscopy to determine which kind of carbon they had. It was definitely nanotubes. Raman Spectroscopy is an interaction between laser light and a material that shows the frequency at which the material vibrates (in other words, it's energy). Every material has it's own rate of vibration that acts as a fingerprint.

Carbon nanotubes, specifically multi-wall carbon nanotubes, resemble chicken fence, a six sided hexagonal structure. "If you fold it over to a seamless cylinder you have a carbon nanotube. This material can be like copper or silicon. We get different results based on the conditions of the fold and where the hexagons link together," said Roughani.

"When we're doing the "folding" we don't have control of where it folds, but we do have control over the conditions occurring during the folding process so that we know what the potential is," he said.

After verifying their discovery of a new method of nanotube production, the Kettering researchers began concentrating on perfecting their technique. Nanotubes are difficult to produce in large quantities. Most nanotube research is focused on increasing the yield.

"If they could be produced in volume, one could create composite materials because they are extremely light if you look at the ratio of strength to mass of this material compared to steel, they are hundreds of times higher, so you can make things that are much, much lighter and much, much stronger," said Roughani. "If they could be made in quantity they would make cables stronger than any cable currently available," Parker added.

"The students last term were successful in getting only three samples with nanotubes, the process isn't really controlled yet. This term we are trying to refine the process. We've made ten or so shots so far and we were successful so we have made progress, but we still have a long way to go," Parker said. Students Patrick Herbst, of Sterling Heights, and Daniel M. Neill, of Rochester Hills, worked with Parker during the second round.

But when they get there, the potential applications are the stuff of science fiction. "Beam me up Scotty" may become a reality if nanotubes can be used as the building blocks for a ladder to the space station. "It's science fiction now, but people are dreaming about it," said Roughani. A Sept. 23 article in the "New York Times" reported on a conference at the Los Alamos National Laboratory exploring the feasibility of a space ladder 60,000 miles high. This "space elevator" would carry cargo and personnel to and from the Space Station, and could lower the cost of putting a satellite into space from $10,000 a pound to $100.

In the realm of not-so-fictional science, nanotubes are being used in electronics, alternative energy research and medical applications. A South Korean company is currently making high resolution display screens using nanotubes. "They have to be stacked one by one, so the technology is very expensive but the resolution is incredible," said Roughani.

The electronics/microprocessing industry is also poised to make the leap to tiny technology. "It appears to me that the size of integrated circuits now has just about reached it's tiny limit," said Parker, "another thing people dream about is making integrated circuits using nanotubes, enabling them to be made even smaller. With the present technology, what they've done over time is simply made integrated circuits based on processing techniques that use shorter and shorter wavelengths of radiation. They can't make the wavelengths any shorter. To go any smaller they will have to go to nanotube technology," he said.

The full measure of this tiny technology is how big it can be in surface area that is. Fuel cells need to store large amounts of hydrogen. To store hydrogen for a fuel cell requires large amounts of surface area, and because they are hollow, carbon nanotubes have large amounts of external and internal surface area just right for storing hydrogen.

Nanotubes are also being considered for medical applications, most specifically in the internal delivery of drugs and drug therapies to specific locations in the human body.

It's no wonder that nanotube research is one of the most highly funded areas of technological research by the U.S. government. It has tremendous potential for the future, with market applications possibly exceeding $230 million within five years.

Written by Dawn Hibbard