Pure Nanotubes by the Kilo

An improved process for making large amounts of pure metallic carbon nanotubes could hold the key to overhauling the electrical power grid with more efficient transmission lines.

Researchers at Rice University plan to generate a large quantity of this material by the end of summer. They'll use these nanotubes to make long and highly conductive fibers that could be woven into more efficient electrical transmission lines.

 Amped up: This jumble of carbon nanotubes has tripled in volume after two runs through a growth process called amplification.

There are a few different classes of carbon nanotube, each with slightly different properties and different potential uses. Unfortunately, existing production methods result in a mixture of different nanotubes, with varying dimensions and wildly different electrical properties. Purely semiconducting nanotubes, useful for future integrated circuits, are in the mix with metallic nanotubes that could be used to make highly conductive wires. So nanotubes have to be separated by type, a slow and expensive process, says Andrew Barron, professor of chemistry and materials science at Rice.

"There is a subset of nanotubes that are the best conducting materials to be found, that don't lose any energy to heat," says Barron.

Barron is part of a group at Rice that wants to make something very large from these nanotubes: miles and miles of highly conductive electrical transmission lines for a more efficient energy grid, which will be important as the use of renewable energy grows. This was one vision of the late Rice professor Richard Smalley, who won the Nobel Prize in Chemistry for his codiscovery of fullerenes, a new type of carbon structure. The Rice researchers have made long, pure carbon nanotube fibers, but since they have been working from impure samples, these fibers are not as conductive as they could be.

Barron and his colleagues have now improved on a method for making pure nanotubes that they first developed in 2006. Called "amplification," it should eventually allow them to turn a nanogram of pure carbon nanotubes into a gram, then a kilogram, then a ton. They start by separating a small amount of pure metallic nanotubes from a mixture, and then attach a catalyst to the tip of each tube. They then put the nanotubes into a pressurized, temperature-controlled chamber and feed in a mixture of gases. Under these conditions, the nanotubes double in size, growing from the catalyst at the tip. The existing nanotube acts as a template that dictates the diameter, structure, and properties of the extra length of the nanotube. The nanotubes are then cut and the process is repeated.

Barron and colleagues first demonstrated amplification a few years ago, but it wasn't very efficient. In a paper published online in June in the journal Nano Letters, they described a combination of the right catalysts and growth conditions that would ensure that every single nanotube would be amplified. Previously they'd assumed these conditions should be identical to the ones used to make the starting batch of nanotubes, but it didn't work very well. Barron says they have now found the conditions to make amplification work.

The Rice researchers are using the amplification process to accumulate enough pure metallic nanotubes to make a fiber of the type that would be used to make an electrical transmission line. They've made long, conductive nanotube fibers in the past using a spinning process also developed at Rice, but they've had to use impure nanotubes to make any great length of the material.

Aaron Franklin, a researcher at IBM's Watson Research Center, says the new study probably doesn't "reveal the golden ticket for achieving high volumes of metallic-only tubes." The amplification process is still not producing very large quantities of the material, Franklin notes. 

While the Rice group continues to work on amplification, other researchers are exploring alternative ways of making pure nanotubes in quantity. Mark Hersam, a professor of chemistry at Northwestern University, developed what is now one of the most commonly used separation methods. He founded a company called NanoIntegris to sell pure nanotubes. He says ramping up production "is now essentially an industrial optimization exercise."

By Katherine Bourzac
From Technology Review


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