Many of the exceptional qualities of carbon nanotubes only become useful once the nanotubes are aligned in the same direction. Alignment can be performed during nanotube synthesis, but this requires substrates that can withstand high temperatures — a requirement that is incompatible with the anticipated applications in flexible plastic electronics. Alignment can also be performed after the nanotubes are synthesized, but this approach often requires applying high-frequency sound waves or chemicals to the nanotubes, which can degrade their useful properties. Now, Yongsheng Chen and colleagues at Nankai University in China have demonstrated a way of aligning carbon nanotubes using a weak magnetic field.1

Fig. 1: Just like the arrangement of iron filings around a bar magnet, a magnetic field can be used to align diamagnetic carbon nanotubes uniformly on almost any substrate.© iStockphoto/colematt

The technique exploits the fact that carbon nanotubes are diamagnetic, which causes the tubes to re-orient themselves parallel to an applied magnetic field. Previous efforts to exploit this property involved the use of enormously strong electromagnets. Chen and his colleagues have used a permanent magnet more than ten times weaker to achieve the same result.

The weaker field worked for a combination of reasons. First, the research team used a fabrication method that resulted in long bundles of tubes, which strengthened the magnetic field effect. Second, the nanotubes were dispersed in helium gas — rather than a liquid — during alignment, which sped up the orientation process. Finally, a low nanotube synthesis rate was enacted to avoid the formation of randomly oriented clusters of tubes that might be less influenced by the weak magnetic field.

Aligned nanotubes are particularly useful for strengthening composite materials, similar to embedding steel rods in concrete. Chen and his co-workers found that the tensile strength of acrylic glass (PMMA) containing aligned nanotubes was 13% higher in the direction perpendicular to the nanotube alignment, and 48% higher in the direction parallel to the nanotubes. The research team also used their alignment approach to create three-dimensional stacks of nanotubes in which each layer had a different orientation.

The technique can be applied to almost any substrate, and over large areas — the team demonstrated alignment over an area of 24 mm × 40 mm. Chen says the next step in his research will be to apply the approach to purely semiconducting or metallic nanotubes. “Once we can do this, our long-term goal will be to use our technique to build flexible electronic devices.”