Researchers from the University of Texas at Dallas, US, and CSIRO Textile and Fibre Technology, Australia, have dry-spun multiwalled carbon nanotubes into twisted yarns that are both strong and good electrical conductors. The fibres could have applications in areas such as structural composites, protective clothing, artificial muscles, electronic textiles, heat pipes and supercapacitors.

"Our yarns are strong, tough, extremely flexible; they are knot, creep, chemical and radiation resistant; electrically and thermally conducting; sewable; weavable; and can be used from near absolute-zero to ultra-high temperatures," said Ray Baughman of the University of Texas at Dallas NanoTech Institute.

To make the yarns, the scientists adapted the traditional textile spinning techniques that have been around since at least the late Stone Age. They drew 10 nm-diameter multiwalled carbon nanotubes from a "forest" of similar length tubes deposited on a substrate, applying a twist at the same time.

The technique builds on a previous dry-spinning method developed in China but produces yarns that the scientists say are 1000 times stronger.

"The trick is that we use yarn twist and the resulting nanoscale friction to provide inter-nanotube mechanical coupling leading to yarn strength, rather than weak van der Waals interactions or a polymer binder," said Baughman. "The absence of this polymer is important for maximizing properties for multifunctional applications, such as for a yarn used for structural purposes that also functions as an artificial muscle, supercapacitor, heat pipe or fuel cell."

The team drew the yarns by hand while they were twisted with a motor at about 2000 rpm. This limited the length of the yarn to about 1 m because of "the arm length of the person doing the drawing". But the researchers say the spinning process is amenable to automation, which would enable the production of continuous yarns.

"We see no barrier to commercially practicing our spinning process - and modifications of it that are described in our pending patent application," said Baughman. "We are working with CSIRO to upscale the process."

To adjust yarn diameter, the team altered the width of the forest sidewall that they used to generate an initial wedge-shaped ribbon. Using forest sidewall widths from less than 150 microns to around 3 mm gave yarn diameters of between 1 and 10 microns.

The team typically applied a twist of around 80,000 turns per metre. This compares to about 1000 turns per metre for a highly twisted conventional textile yarn with a diameter 80 times larger.

The researchers also made two-ply yarns by overtwisting a singles yarn and allowing it to untwist until it reached a torque-balanced state. Then they made four-ply yarns by repeating the procedure with a two-ply yarn, this time twisting in the opposite direction.

Despite the good conductivity of individual nanotubes and pure nanotube yarns, composite fibres containing nanotubes and insulating polymers generally have low conductivities. But Baughman and colleagues found that the "intertube mechanical coupling" brought about by twisting helped maintain the electrical conductivity of the yarn after the infiltration of polyvinyl alcohol (PVA).

Introducing PVA decreased the yarn's electrical conductivity by around 30%, resulting in nanotube/PVA composite yarns with an electrical conductivity more than 150 times that of coagulation-spun nanotube composite fibres containing PVA.

"Although not yet quite as tough as the Kevlar used for antiballistic vests, our nanotube-based yarns are tougher than graphite fibre," added Baughman. "Moreover, our multiwalled nanotube yarns have advantages over Kevlar in terms of thermal stability, resistance to creep and resistance to ultraviolet-induced degradation."

The yarns also showed extremely large Poisson's ratios: 4.2 compared to the typical value for a solid of around 0.3. As a result, applying a strain to the yarns produces a considerable densification. The team say this might be used for tuning the absorption and permeability of the yarns.

The researchers reported their work in Science.

About the author
Liz Kalaugher is editor of nanotechweb.org.