Rice's breakthrough in carbon nanotubes

Mon, 06/03/2013 - 9:49am
M. Simon, Technical Contributor

Rice University has been making considerable progress in devising methods to manufacture carbon nanotubes (CNT) for use as electrical conductors. The breakthrough came in 2003.

Rice’s breakthrough stems from the fact that the researchers are using a wet method to produce the CNT thread. While almost every other research group uses a dry method to create bulk CNTs — usually chemical vapor deposition — Rice uses a “wet spinning” method that it pioneered back in 2003. Wet spinning is basically what it sounds like: Clumps of nanotubes are dissolved in a bath of chlorosulfonic acid, and then squirted through small holes to create long strands of CNTs. These long strands — which can be hundreds of meters in length — are then wound onto a big spool to dry out.
It just so happens that Rice’s method of producing CNT thread is very similar to how aramid fibers, such as Kevlar, are produced. Teijin, an Israeli company that produces a material similar to Kevlar called Twaron, supported Rice University with this research. While it hasn’t yet been confirmed, there is a strong chance that Rice and Teijin will be able to use existing equipment and expertise to mass-produce this CNT yarn — and that’s a very exciting prospect indeed.
All told, these long nanotube threads have comparable electrical conductivity to copper, gold, and aluminium alloy — but its thermal conductivity is best-in-class and its tensile strength is four times that of aluminium alloy, or more than 20 times that of copper. Because copper is so weak, copper wires must be fairly thick to withstand manufacturing processes. At its most basic, thin CNT thread could be used in the place of these thick copper wires, resulting in svelter data cables, and perhaps a minimization of some consumer electronics devices too.

This article from 2011 gives more details on the manufacturing methods.

The process of making nano cables begins with a lump of double-walled nanotubes that have been treated to remove impurities. The researchers add sulfuric acid to the nanotubes so they can spread them into a thin film. They then grasp the edge of the film with tweezers to start making a fiber, and pull with a steady force to yield a long cable—similar to how wool yarn is made by pulling and twisting fleece. They rinse the acid from the cable and expose it to iodine vapor at high temperatures. The iodine penetrates into the nanotubes within the cable and increases the cable’s conductivity without compromising its mechanical properties. And the Rice group has shown that conductivity isn’t affected when the cables are knotted together to make greater lengths.

This Rice paper discusses using CNT for undersea cables where its corrosion resistance will come in handy.

I'm looking forward to the day when CNTs replace copper for grid interties. That might reduce wire losses by a factor of two or more. Reducing total grid losses by as much as one or two percent. Not much you say? True. But with electrical energy making up a part of everything manufactured it could have very wide spread ripple effects. If nothing else, by making longer distance grid transmission economical, the need for new backup generating capacity will be reduced. That will also enhance grid stability.

Problems? There are still more than a few that need to be worked out. Like the best way to terminate such wires.

In parallel with efforts to improve bulk conductivity, integration of CNT materials into cabling architectures will require development in electrical contacting. Several methods for contacting bulk CNT materials to metals are demonstrated, including mechanical crimping and ultrasonic bonding, along with a method for reducing contact resistance by tailoring the CNT-metal interface via electroless plating.

Obviously that problem is not insurmountable. I expect to see these cables first on aircraft and rockets where every ounce counts and dollars are not too critical. Then autos followed by general use. The future is looking brighter every day.

M. Simon's e-mail can be found on the sidebar at Space-Time Productions.

Engineering is the art of making what you want from what you can get at a profit.


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