Wednesday, August 22, 2012

Space Elevators, Carbon Nanotubes and Toxicity

This weekend I'm going up to Seattle for the ISEC Space Elevator Conference. (I went last year, which spawned this feature article for Physics World.) A realistic space elevator -- one that isn't ridiculously wide at geostationary orbit -- requires a material with an extremely high tensile strength-to-density ratio -- something on the order of 30-60 GPa/(g/cm3) [Gigapascals per gram-per-cubic-centimeter]. Carbon nanotubes (CNTs) are the prime (and, so far, only) candidate that meets this requirement in principle, but so far no one knows how to make them into extended structures like a rope or ribbon, let alone braid them into long pieces (a total of about 100,000 kilometers would be needed for a space elevator).

So I especially noticed this press release that just arrived: "Super-Strong, High-Tech Material Found to be Toxic to Aquatic Animals by Researchers at MU and USGS"
CNTs are microscopically thin cylinders of carbon atoms that can be hundreds of millions of times longer than they are wide, but they are not pure carbon. Nickel, chromium and other metals used in the manufacturing process can remain as impurities. Deng and his colleagues found that these metals and the CNTs themselves can reduce the growth rates or even kill some species of aquatic organisms. The four species used in the experiment were mussels (Villosa iris), small flies’ larvae (Chironomus dilutus), worms (Lumbriculus variegatus) and crustaceans (Hyalella azteca).

“One of the greatest possibilities of contamination of the environment by CNTs comes during the manufacture of composite materials,” said Hao Li, associate professor of mechanical and aerospace engineering at MU. “Good waste management and handling procedures can minimize this risk. Also, to control long-term risks, we need to understand what happens when these composite materials break down.”
A space elevator, if it can ever be built, would likely be built on an base in the ocean, for a few reasons. First, you want to built it very near the equator because you get maximum centrifugal force there (an elevator is basically a taut string hanging off a spinning object), and because few storms cross the equator, and if the elevator was ever severed and fell back to Earth you'd want to avoid killing people. (After all, the thing has a mass of about 105 kg, or over 100 tons at the Earth's surface.)

I don't have a copy yet of the above paper, so I don't know at what concentration they found CNTs to be toxic. It's a big ocean, after all. Still, you might not want to have carbon nanotubes flaking off it, or big pieces falling off and dissolving in the ocean. Maybe you coat the CNT ribbon with some polymer or something, but that adds mass and reduces your strength-to-density ratio.

And if these are toxic to marine animals, what about other animals, including human animals? I honestly don't know. I do seem to see a relatively steady stream of press releases about nanotechnology safety, and my (mostly uninformed) impression is that this area is understudied and behind the curve of nanotech R&D. I'm not even sure about nanoparticles in sunscreens, to be honest -- information like this isn't completely reassuring.

At the very least, environmental groups will likely have another target when/if a space elevator is ever seriously proposed.

We might never get off this damned planet.


guthrie said...

The space elevator thing is interesting; there's actually a lot of research been done on various aspects of it, and the wikipedia article on space elevator safety references what appear to be genuinie works by the European space agency and others.

Suffice it to say, as a lowly chemist/ materials science/ inorganic person, there's no way bits of it are going to flake off unless you've just nuked the elevator. There'll be anti-corrosion coatings, since apparently oxygen can eat away carbon and other things quite easily in the high atmosphere. There'll be conductive or non-conductive coatings trying to prevent interesting and complex thigns happening regarding lightning and the interaction between a long conductive cable and the earths EM field.

I recall seeing people querying nanotech safety 5 or 6 years ago, it is beign taken seriously in some quarters, but may end up like GM crops - not a lot of safety stuff done until after the fact, when many of the accusations were disproven, but its kind of sensible to do the work before you unleash it upon the world, not after.

nano said...
This comment has been removed by the author.
nano said...

The space elevator is the positive outcome form this article but what really saddens m,e is that the Carbon nanotubes are harmful to aquatic animals.But still we have became such blind in nanotechnology that we are ignoring this warning may be it will become more dangerous in future.