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futurity_sci_tech 00138.txt

#How to make carbon thread without clumps Made into fibers single-walled carbon nanotubesâ line up like a fistful of raw spaghetti noodles thanks to a new process. The tricky bit according to Rice university chemist Angel Martã is keeping the densely packed nanotubes apart before they re drawn together into a fiber. Left to their own devices carbon nanotubes form clumps that are perfectly wrong for turning into the kind of strong conductive fibers needed for projects ranging from nanoscale electronics to macro-scale power grids. Earlier research at Rice by chemist and chemical engineer Matteo Pasquali a coauthor of the new paper used an acid dissolution process to keep the nanotubes separated until they could be spun into fibers. Now Martã Pasquali and their colleagues are producing eatfibers with the same mechanical process but they re starting with a different kind of feedstock. atteo s group used chlorosulfonic acid to protonate the surface of the nanotubesmartã says. hat would give them a positively charged surface so they would repel each other in solution. The technique we use is exactly the opposite. process revealed last year by Martã and lead authors Chengmin Jiang a graduate student and Avishek Saha a Rice alumnus starts with negatively charging carbon nanotubes by infusing them with potassium a metal and turning them into a kind of salt known as a polyelectrolyte. They then employ cage-like crown ethers to capture the potassium ions that would otherwise dampen the nanotubes ability to repel one another. Put enough nanotubes into such a solution and they re caught between the repellant forces and an inability to move in a crowded environment Martã says. They re forced to align a defining property of liquid crystals and this makes them more manageable. The tubes are forced ultimately together into fibers when they are extruded through the tip of a needle. At that point the strong Van der waals force takes over and tightly binds the nanotubes together says Martã an assistant professor of chemistry and bioengineering and of materials science and nanoengineering. But to make macroscopic materials Martã s team needed to pack many more nanotubes into the solution than in previous experiments. s you start increasing the concentration the number of nanotubes in the liquid crystalline phase becomes more abundant than those in the isotropic (disordered) phase and that s exactly what we neededmartã says. The researchers discovered that 40 milligrams of nanotubes per milliliter gave them a thick gel after mixing at high speed and filtering out whatever large clumps remained. t s like a centrifuge together with a rotary drummartã says of the mixing gear. t produces unconventional forces in the solution. eeding this dense nanotube gel through a narrow needle-like opening produced continuous fiber on the Pasquali lab s equipment. The strength and stiffness of the neat fibers also approached that of the fibers previously produced with Pasquali s acid-based process. e didn t make any modifications to his system and it worked perfectlymartã says. The hair-width fibers can be woven into thicker cables and the team is investigating ways to improve their electrical properties through doping the nanotubes with iodide. he research is basically analogous to what Matteo doesmartã says. e used his tools but gave the process a spin with a different preparation so now we re the first to make neat fibers of pure carbon nanotube electrolytes. That s very cool. asquali says that the spinning system worked with little need for adaptation because the setup is sealed. he nanotube electrolyte solution could be protected from oxygen and water which would have caused precipitation of the nanotubeshe says. t turns out that this is not a showstopper because we want the nanotubes to precipitate and stick to each other as soon as they exit the sealed system through the needle. The process was not hard to control adapt and scale up once we figured out the basic science. he Welch Foundation supported the research which appears in ACS Nano T

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