Carbon nanotubes

Carbon nanotubes (CNTs) are an allotrope of carbon, they can be visualised as graphene sheets rolled to form seamless cylinders with diameters ranging from ~0.4 to several nanometres and length-to-diameter ratios of up to 132,000,000:1. If a single sheet is used, one obtains a single-walled nanotube (SWNT), while several nested SWNTs give a multi-walled nanotube (MWNT), a double-walled nanotube (DWNT) being a special case of these. These all-carbon macromolecules are known to have superb mechanical (e.g. Young modulus of ~1TPa and tensile strengths up to 53GPa), electrical (e.g. ballistic conduction and current carrying capacities up to 4x10 9A/cm2), and thermal (e.g. thermal conductivity of 3500W/mK along the tube axis) properties.

In 2004, a unique method for the continuous production of CNT fibres and films was developed by Prof. Alan Windle’s group ¹, part of the MML. In this method, an “elastic CNT smoke” is formed in the CVD reaction zone of a vertical furnace in which a carbon source is decomposed in the presence of floating iron catalyst particles in a reducing atmosphere. This smoke can be pulled out of the reactor and continuously wound onto a spool – a process somewhat similar to making candy-floss. Winding the material up without any further densification results in a CNT film. Alternatively, the material may be densified into a yarn-like fibre by an in-line process in which a solvent is sprayed on it, generating capillary forces that significantly reduce its diameter. Fibres made by this method already show mechanical properties comparable to those of high performance fibres such as Kevlar and Dyneema, with much better resistance to bending and knotting. They also show vastly superior electrical conductivities compared with carbon fibre and have an axial thermal conductivity of 1250 W/m.K which is some three times that of copper or 25 times better per unit weight.

In the MML group we keep working on perfecting our synthesis method, aiming for electrical conductivities capable of challenging copper and mechanical properties closer to those of individual CNTs. We also investigate the use of our fibres and films to make multifunctional composite materials.

Over the past decade MML has developed a unique process for synthesizing carbon nanotubes via the floating catalyst route, and at the same time winding the entangled aerogel into a fibre. The process will work with a variety of different carbon feedstocks and generates a fibre with axial properties in the Kevlar range, but great robustness in bending.

It is thus possible to convert natural gas continuously into a high tech fibre in a single reactor. Current focus of the work is the improvement of mechanical, electrical and thermal properties for a wide range of applications, some of them multifunctional. Part of this activity is through enhanced control of the spinning process itself, other approaches rely on post spinning processes such as chemical functionalization of the fibre and heat and radiation treatments. The fibre’s unique mechanical robustness stems from the fact that it is a nanoscale yarn, which also means that when incorporated in composites, the matrix penetrates the fibre providing unique level of bonding with the matrix.