Carbon nanotubes have been pegged as the wonder material that could finally allow us to build a space elevator. A discouraging new study suggests these microscopic strands aren't as resilient as we thought — and all it could take is a single misplaced atom to bring the whole thing crashing down. A conceptual image of a space elevator (Credit: Obayashi Corporation)
Illustration of the interior of a carbon nanotube (image: Michael Ströck)
Carbon nanotubes (CNTs) are tiny hollow cylinders made of interlocked carbon molecules. When woven together they exhibit extraordinary properties, including tensile strengths up to 100 gigapascals (GPa). To put that into perspective, a single strand the width of a thread could support an entire car. In theory, CNTs could be strong enough to support the tremendous strain exerted by a space elevator — a massive structure that would reach up into space from Earth's surface.
CNTs are considered one of the strongest materials around, but efforts to manufacture the material have only yielded ropes with strengths of 1 GPa. As reported in New Scientist, Feng Ding of the Hong Kong Polytechnic University wanted to find out why, so he and his colleagues simulated CNTs with a single atom out of place. This converted two of the hexagons into a pentagon and heptagon, creating a malformation in the tube. This one simple alteration was enough to reduce the ideal strength of a CNT from 100 GPa down to 40 GPa. Naturally, this effect was exacerbated when they introduced more misaligned atoms. New Scientist explains:
The team's simulations show that the kink acts as a weak point in the tube, easily snapping the normally strong carbon-carbon bonds. Once this happens, the bonds in the adjacent hexagons also break, unzipping the entire tube. The effect on CNTs spun together into fibres is similar — once one CNT breaks, the strain on the others increases, fracturing them in sequence.
This means that just one misplaced atom is sufficient to weaken an entire CNT fibre. In terms of a potential space elevator, imagine a cable running up from the Earth's surface into space, and then suddenly ripping apart like a run in a lady's stocking. That would be... bad.
Given the primitive state of CNT manufacturing at this stage, a bad tube is practically guaranteed. As Ding said, "Most mass-produced CNTs are highly defective, and high-quality CNTs are hard to produce in large quantity."
This is definitely a setback in the effort to design and build a space elevator, which will require cables with tensile strengths reaching 50 GPa. If these skylifts are ever going to happen, engineers are going to have to figure out a way to make CNTs perfect at the atomic level — and that's a daunting proposition.