The search for high-strength, low-density materials is unending. In particular, cables and ropes are good candidates for a particular combination of high tensile strength, flexibility, and low density. Since the discovery of carbon nanotubes, there has been considerable interest in producing cables from them, but, there are many scientific barriers that prevent the manufacture of pure carbon nanotube fibers. On the other hand, a good portion of the high strength properties of carbon nanotubes can be obtained when they are used in combination with polymers to manufacture a composite fiber.

Even these fibers have their problems, though. For instance, carbon nanotubes are graphite-like, so the nanotube is very strong along its length but the tubes slip along each other very easily. To reduce the slippage, adjacent tubes need to be in contact over the maximum possible cross-section, which means that the nanotubes need to be aligned, densely packed, and squeezed so that they are oval-shaped. When these conditions are not met, the resulting fibers tend to be less flexible and have low tensile strength¡ªneither of which is desirable.

Recent research, published in ScienceExpress, shows the potential of these fibers when manufactured correctly. The researchers used a mixture of an aerogel polymer and carbon nanotubes as the source material for the fiber. The fiber was created by pulling the polymer and the researchers found that if it was pulled sufficiently fast, the carbon nanotubes tended to align along the axis of the fiber. The resulting low density material consists mainly of gas, aerogel, and carbon nanotubes, aligned along their axes. The gas is removed and the fiber compressed during post processing, making a dense material.

As with all things, the proof is in the testing, and the results were mixed. Some of the fibers performed extremely well and had the highest tensile strength-to-weight ratio reported. Other fibers were on a par with results published by other labs and some were just plain bad. The dominating factor in the performance was attributed to the presence of defects such as misaligned nanotubes and density variation. The defect rate was quite high, since the tested fibers were only a few millimeters in length.

Despite the variation in the results, these are very promising. Even the worst fibers performed on a par with current commercial fibers and, if researchers can iron out some of the processing problems, a very strong fiber will result. This will be nowhere near the strength of an individual carbon nanotube, of course, but it is still very good.