NEW YORK, Aug. 2 (UPI) -- An international team of scientists has devised a simple, easy way to make filters of nanotubes -- pipes less than a wavelength of visible light wide -- that could help generate high-octane gasoline and filter out germs.
The filters are made of carbon tubes. Each is only nanometers, or billionths of a meter, across. Research groups worldwide have pursued carbon nanotubes ceaselessly over the past decade because of their mechanical properties -- they are roughly 100 times stronger than steel -- as well as similarly remarkable thermal and electrical qualities.
IBM and NASA have investigated carbon nanotubes for circuitry, while Samsung, Dupont and NEC and working on incorporating them into flat panel displays.
A critical problem when it comes to making complex machines with carbon nanotubes is they are difficult to align together. They often clump up, much like piles of spaghetti.
"We are really interested in creating architectures with nanotubes and finding applications for them," materials scientist Pulickel Ajayan of Rensselaer Polytechnic Institute in Troy, N.Y., told United Press International.
Ajayan's group, along with Onkar Nath Srivastava's team at Banaras Hindu University in Varanasi, India, have invented a way to line up carbon nanotubes into porous sieves.
"People have been talking about using carbon nanotubes as membranes for bio and chemical separation, and in order for that to happen, you have to have an aligned film of them," said nanoscientist Charles Martin at the University of Florida at Gainesville, who did not participate in this research. "That is a very challenging thing. This work is very good," he told UPI.
The chief ingredient for the new method is liquid benzene, an organic chemical widely used in making plastics and synthetic fibers such as nylon and polyester. The benzene is mixed with ferrocene, an organic metal compound that catalyzes the nanotubes from benzene. The mixture is then sprayed, along with the chemically inert gas argon, into a quartz tube and then heated to 1650 degrees Fahrenheit (about 900 degrees Celsius).
The nanotubes then form on the inside of the quartz tube. The result is a hollow cylinder about 20 millimeters to 30 millimeters wide. The walls of this macrotube, as it is called, are about 100 to 500 microns thick and are made of tightly packed carbon nanotubes, each with its ends pointed toward the center of the macrotube.
The carbon macrotube then is removed from the quartz pipe via the careful use of acid. How densely packed the nanotubes are depends on the size of the nozzle used to spray the benzene into the quartz and the speed of the spray.
The cylindrical shape of the filter is critical, Martin said.
"You can pack a helluva lot more surface area in a given volume with a hollow cylinder than a sheet," he explained.
Increased surface area allows filtration to be performed more quickly, which can make or break an industrial process.
"Imagine you are trying to produce a pharmaceutical at 10 grams an hour, and you have a membrane that only produces a microgram per hour. It's obviously useless technology," Martin said.
Ajayan and colleagues have experimented with how well the macrotubes can filter. They found the macrotubes could filter out contaminants from drinking water, such as the fecal bacterium Esherichia coli -- which typically is 2,000 nanometers to 5,000 nanometers long and 400 nanometers to 600 nanometers wide -- as well as even tinier germs such as poliovirus, which is only 25 nanometers across.
The scientists report their findings in the upcoming September issue of the British journal Nature Materials, and are in the process of filing patents on their device.
Their work suggests the macrotubes could help produce high-octane gasoline by filtering out a number of heavy, undesirable hydrocarbons from crude oil in a single step. Hydrocarbons are carbon-based chemicals that store energy in their molecular bonds, and form the main ingredients of common fuels.
"Crude oil contains all kinds of hydrocarbons, each with three to almost 30 carbon atoms. Petroleum refining has the cracking process where you break down the higher hydrocarbons," Ajayan said. "An alternative way is to do a filtration and remove what you want in one step."
When it comes to conventional filters, such as those composed of activated carbon used to purify drinking water, "you have a totally random kind of pore arrangement, with a whole range of pore sizes. Here, with the macrotube, the arrangement and sizes can be very well-defined," Ajayan explained. "You want uniform pore sizes, because there are times when you want a very narrow window of pore sizes, so that what you want doesn't get through. That's a real problem with filters that membrane manufacturers work hard (to address)."
The group's filters also proved durable -- for instance, they are capable of working at 750 degrees F (400 degrees C), several times higher than a conventional polymer membrane filter's highest operating temperature. They also are easier to reuse via cleaning with heating and ultrasound. This makes them better than water filters made with cellulose that often are not reusable, because bacteria cling to such membranes and reduce filtration efficiency. Viral filters often are not reusable, either.
"There's a lot of work that remains if they want to make a practical device with these membranes," Martin said.
It is necessary to prove, for example, whether molecules are being filtered through the nanotubes, or through holes between the nanotubes, he explained.
"It would have been very cool to have unambiguous evidence the carbon nanotubes themselves were responsible for the filtration, and there's really no evidence for that," he said.
"Then again, if the membranes are selective, on one level it doesn't really matter where the selectivity is coming from," Martin concluded. "This is embryonic work, but it's very promising and very interesting."
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Charles Choi covers research and technology for UPI Science News. E-mail [email protected]
