It is an amazing discovery that CNTs can be directly drawn out in a dry state from so-called super-aligned CNT (SACNT) arrays. Up until now, there have been two methods to make these drawn-out CNTs into a yarn: twisting CNTs by down-sizing an industrial twisting process and shrinking CNTs using some solvents, but the two separate methods have some disadvantages for making uniform and high-strength yarns.

Recently, researchers at Tsinghua University have developed a simple and continuous method that combines the twisting and shrinking processes to make high-strength CNT yarns. In this method, a yarn freshly spun from a SACNT array is first twisted and then passed through a solvent for shrinking. The two processes deliver a yarn that consists of densely packed CNTs and has a tensile strength of more than 1 GPa. The tensile strength depends on the diameter and the twisting angle of the yarn, and peaks at a yarn diameter around 10 µm. Different kinds of solvents, such as water, ethanol and acetone, have used to shrink the twisted yarns, with acetone showing the best shrinking effect.

Constant width

The team uses a laser scanning method to parallel etch its SACNT array, which ensures that the CNT yarn is produced with a uniform diameter. By this method, CNT yarns with a wide diameter range can be obtained: the thinnest is about 4 µm and the thickest can reach up to 40 µm in diameter.

The CNT yarns are conductive (Rs ~ 1.1 × 10–5 Ω m) and much stronger and lighter than thin metal wires. These advantages make CNT yarns superior materials for use as high-strength and ultra-thin conducting wires with a diameter less than 10 µm.

Full results are available in the journal Nanotechnology.