Tikalon Blog by Dev Gualtieri

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Carbon Thread

January 21, 2013

Carbon seems to be the most researched element of the past few years, supplanting silicon which brought us into a new electronic age. Graphene , one of the many allotropic forms of carbon , is being researched for electronic applications. I wrote about the amazingly short interval between the discovery of graphene and the award of a Nobel Prize for its discovery in a previous article (2010 Nobel Prize in Physics, October 6, 2010).

Other allotropes of carbon, carbon nanotubes (CNTs), are being researched for both electronic and structural applications. The regular arrangement of carbon atoms in nanotubes, combined with the 346 kJ /mole bond energy of the carbon-carbon bond , leads to a high strength material. The Young's modulus of single-walled CNTs is about 1000 GPa ; and their tensile strengths are close to fifty GPa.[1]

Carbon nanotubes have desirable properties beyond their strength. They have low density , high electrical conductivity , and high thermal conductivity . Nature, however, requires a little work from scientists before CNTs can replace the structural materials we presently use. The practical problem is that CNTs are individually very small, so you need to assemble many of them together to get a macroscopic fiber or ribbon from which composites can be made.

In an article at the end of last year (Strong Carbon Nanotube Composites, November 12, 2012 ), I wrote about research in the dry-drawing of carbon nanotubes into electrically-conductive sheets. These sheets were formed into composites of high electrical conductivity and tensile strengths of 1.8 GPa, improved to 3.8 GPa by stretching.[2-5] The best such material had a Young's modulus as high as 293 GPa, a tensile strength of 3.8 GPa, a thermal conductivity of 41 W-m⁻¹-K⁻¹, and an electrical conductivity of 1230 S-cm⁻¹.[5]

A method of producing threads of carbon nanotubes has just been published in Science by an international team from the Departments of Chemical and Biomolecular Engineering , Chemistry , Electrical and Computer Engineering , Physics and Astronomy , the Applied Physics Program , and The Smalley Institute for Nanoscale Science and Technology at Rice University (Houston, Texas ), Technion-Israel Institute of Technology and the Russell Berrie Nanotechnology Institute (Haifa, Israel ), Teijin Aramid (Arnhem, Netherlands ), and the Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio ).[6-7]

Says Matteo Pasquali , research team leader and a professor of chemical and biomolecular engineering and chemistry at Rice, "It looks like black cotton thread but behaves like both metal wires and strong carbon fibers."[6] These carbon nanotube threads have about the same thermal conductivity as, and an order of magnitude greater electrical conductivity than, the best graphite fibers . Graphite fibers are brittle , but the CNT threads are as flexible as textile thread. The electrical conductivity is like that of copper , gold and aluminum .[6]

Light fixture suspended by carbon nanotube threads

Conductive and strong.

This fifty gram light fixture is suspended by carbon nanotube threads that also provide the electrical current to power the LED lamp .

(Rice University Image, used with permission.)

A wet spinning process is used to create the CNT threads. The key to the process was the discovery that CNTs are highly soluble in chlorosulfonic acid .[6] A concentrated solution of CNTs allows the same type of wet spinning process used for other fibers. The wet spinning process produces CNT fibers with good alignment of the fibers and a high fiber packing , as shown in the scanning electron micrograph , below.[6]

Scanning electron micrograph of a CNT thread

scanning electron micrograph of a carbon nanotube thread.

This cross section of a fiber produced by the Rice University-Teijin Aramid process shows few voids within the fiber.

(Rice University Image, used with permission.)

The wet-spun CNT threads are about 10 μm in diameter, and they are built from a parallel alignment of tens of millions of CNTs. Strength and alignment are highly correlated , so a close alignment of the CNTs is important.[6]

Rice University teamed with a manufacturer of commercial aramid fiber , Teijin Aramid (Arnhem, Netherlands), to improve the wet spinning process. Teijin Aramid's involvement began in 2010, when they provided funding and access to their own scientists. Additional funding was provided by the Air Force Office of Scientific Research, Technion's Russell Berrie Nanotechnology Institute, the US Department of Defense , and the Welch Foundation .[6]

Wet-spun carbon nanotube thread on a spool

Wet-spun carbon nanotube thread on a spool.

(Still from a YouTube video.)[8]

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