Cambridge Team Spins Nanotube Yarn

FridayBob writes "They say it's bound to happen soon, although nobody knows exactly how and when. Well, perhaps the answer has arrived. It now seems as though some bright folks at the Cambridge-MIT Institute have figured out a way to continuously spin carbon nanotubes into a fiber. Will it be strong enough for a space elevator?" They're getting closer to commercialization (see older story) but not there yet...

Re:"Electrical current", eh?

[breadbot]

Unfortunately, the cable is too long to send power through efficiently, since it has to reach up at least to geosynchronous orbit. Estimates of conductivity for a composite fiber are in the range of copper or other good metallic conductors. You'd get a heck of a lot of resistance through 25,000 miles of cable. Gotta beam it down or something.

According to this calculator [allmeasures.com], 25,000 miles of copper with 1 cm^2 cross section (probably an over-estimate), would have a resistance of about 6700 Ohm.

Re:Chain Mail

[breadbot]

Very interesting. The goal of a space elevator tether is longitudinal strength, which means that the links would be stretched along one axis. I wonder if the folds and pinches at the top and bottom would cause any problems, and if you'd have to make a chain link out of a large number of nanotubes to prevent pinching. And what about rubbing?

You mention cutting an individual fiber and thereby causing the whole fabric to unravel. For a space elevator, the prevaling thinking is to make a composite material (say, fibers and epoxy) with a matrix strong enough to redistribute load among nearby fibers in case of a localize break within the tether.

Re:"Electrical current", eh?

[Orne]

The longest land based transmission line [abb.com] is in the Congo at 1700 km, running at 500 KV DC. For the rest of us, that's 1,056 miles. So basically, we'd need 25x the longest transmission line ever built to date... so we could carry less energy than building one medium size fossil fuel plant on the ground.

Now there are several tricks to power transmission. One, raise the voltage and lower the current, and you'll have less heating of the line, as temperature is proportional to current and resistance. Needless to say, this would incredibly complicate your anchoring system on earth. Next, current flows on the surface of a wire, so transmission lines are actually bundles of smaller "threads" wound together in parallel. This evenly distributes the energy, reducing the net resistance of the "wire".

For a wire that long, you have to work with the old RCLG formulase for losses across the line... reactive charging losses and resistive heat losses. The line would be so long that the voltage would decay long before you'd reach the other end, and no power could be transferred.

Re:"Electrical current", eh?

[justanyone]

Yah, but beaming microwaves in significant power ranges would be lethal or at least significantly toxic to humans near the beam path.

This would be okay for unmanned loads such as resupply, but obviously a bad idea for manned transport.

I don't want to stand within sight of the transmitter either, without standing in a nicely sealed thick metal room with lots of faraday cage layers outside as well for redundancy. I've seen what microwaves can do to meat (last night at dinner).

Re:"Electrical current", eh?

[Thauma]

Pictures and more info about the helicopter are at http://www.mtt.org/awards/WCB's%20distinguished%20 career.htm [mtt.org]

Re:Chain Mail

[Goldsmith]

As a solid state physicist, working with nanotubes, who is also a member of the SCA, I found your post quite interesting.

The first problem is that nanotubes don't grow into toroids. They can form spirals that look like toroids under any but the most powerfull microscopes, but these spirals will unravel very quickly. That point is a weak rebuttal, because we could probably close those rings with an electron or ion beam if we really wanted to.

Also, keep in mind that there are very, very few molecules which are "stiff" all by themselves. Carbon nanotubes are definitely not one of them. It would be like making chainmail out of very strong cooked noodles. Really what you want is more than the 4 links provided by chainmail. By tangling these up, we can link up to many times more other nanotubes than by controllable copying of a two dimensional nearest-neighbor lattice. For example, if we have a three dimensional cubic lattice of interlocking rings, we have 6 links in a nearest neighbor (chainmail) case, and 14 in a nearest and next-nearest neighbor case, increasing redundancy and bonding energy. We could keep going by weaving these things together. In a really tight weave (or a huge tangle like what these nanotube fibers really are), you may have one fiber connected to hundreds of others.

Except for one more issue, all the weaving done right now might still theoretically be made stronger by closing the ends of the nantubes to avoid unravelling (so your general idea is good). If stress is put on a bend in a nanotube, it will lead to a "5-7" defect where two hexagonal rings become one ring of 5, and one of 7, inducing a 15 degree permanant bend in the tube. These defects lead to nanotubes which have at best 50% the tensile strength of a non-defective tube, but often more like 15%. That's why so much work is being put into aligned nanotube fibers. These fibers have been measured to be stronger than any other known material. If these aligned fibers are then woven, they are lighter and stronger than Kevlar. Coincidentally, the molecularly woven (tangled) nanotube fibers made by these guys at MIT are not much stronger than generic clothing fiber. The key is to weave the fibers macroscopically and allow the nanotubes to bend less than 15 degrees on a molecular scale.

Hope this was helpful.