But how could you make a 100 000 km-long structure from carbon nanotubes? Unfortunately, no-one knows, or at least not yet. Although the longest such tube made to date is 55 cm in length and just a nanometre wide – created by Weizhong Qian and colleagues at Tsinghua University in Beijing – their growth is difficult to control. However, no-one envisions an elevator made from a single carbon nanotube, which would be too fragile on its own. Instead, individual tubes would probably be woven or braided together, thermally annealed and embedded in a composite.
Speaking at a space-elevator conference in August, Mark Haase, a chemical-engineering PhD student at the University of Cincinnati, discussed how to select and bundle carbon-nanotube threads, weaving a kind of rope with up to a trillion nanotubes per square centimetre. Attended by about 65 elevator enthusiasts, academics and even a few entrepreneurs, the conference is an annual event held at Microsoft’s headquarters in Redmond, Washington. The key, Haase believes, and also the bottleneck, is maximizing the intramolecular forces that bind nanotubes along their long dimension, which are a balance between short-range, repulsive forces arising from the quantum-mechanical Pauli exclusion principle and longer-range, attractive Van der Waals electrical forces.
Many factors remain to be perfected in order to make stronger, more durable nanotube materials. Defects along the tubes – be they from impurities, dopants or the occasional pentagon or heptagon where a six-sided hexagon would ordinarily be – can reduce attractive forces. Radiation from solar storms or cosmic rays could also induce serious tether-damaging defects, which is why the nanotubes would ideally be self-healing. Another problem is that the tether could be weakened if made from tubes with a non-uniform diameter. Indeed, Haase believes a key to making better spinnable nanotube arrays is finding a catalyst that slows down the tendency of metal particle clusters to attach themselves to nanotubes during synthesis – an unwanted effect that stops them growing any further. (Known as “Ostwald ripening”, this process also explains why partially thawed ice cream becomes crunchy after refreezing.)
