Nanotube Threads Get Stronger

pythorlh writes: "NewScientist has an article about carbon-nanotube thread. Could this be the begining of "monofilament" that sci-fi has been drooling over for years?" Well, from the sound of the article, not yet. But soon, perhaps: according to the article, "The new nanotube threads are about 10 times stronger than buckypaper, and can be tied into knots without breaking. But they are still much weaker than many other fibres, such as iron thread."

Comments

Re:Potential?

[Tofof]

The other use could be space tethers. This goal is probably sooner in the future, though the first test (Feb 25, '96) failed a few years ago. That test, though was designed to generate electricity - the tether had a copper braid around a simple nylon string, and was encased in kevlar. That one failed because air was trapped in the nylon, and the 3500 volts being generated changed the air to plasma (similar to a fluorescent tube lighting up) which subsequently melted the tether. This material would likely be far, far superior to the nylon.

TSS-1r results aren't what you claim

[Tau Zero]

Trapped air wouldn't have affected the insulating value (there is at least as much trapped air in nylon used dirtside). Instead the problem appears to have been some kind of puncture or porosity. Here is the press release on the report issued on the tether-break analysis. The most important paragraph:

The board found sufficient evidence to identify two possible causes of the breach in the insulation -- foreign object damage, or a defect in the tether itself. Debris and contamination found in the deployer mechanisms and in the tether itself could have been pushed into the insulation layer while the tether was still wound on its reel. The investigation found evidence of damage to copper wire in the tether, and also established that normal forces on the tether while on the reel could push a single copper strand or foreign debris through the insulation.

NASA's not that hard to search, you should go consult it more often.
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Re:monofilament in SF books

[AltGrendel]

I think Larry Niven wins that one, he mentions them in his universe early on, before his "Ringworld" book(s).

Re:Potential?

[NanoProf]

One can estimate theoretically the ultimate strength of a nanotube be examining the microscopic failure modes, i.e. the ways in which atoms rearrange in response to an external stress (i.e. stretching).

In the case of perfect, defect-free nanotubes, there are two modes that seem to be important. First, the rotation of a single carbon-carbon bond by 90 degrees, which converts a patch of 4 hexagons (remember that carbon atoms are arranged in a chicken-wire or honeycomb pattern on the tube wall) into two pentagons and two heptagons (relevant references are Zhang & Crespi from Penn State in Physical Review Letters and work by Bernholc at NC State and Yacobson at Rice I think, but the exact journal escapes me at the moment). This mode is a plastic distortion of the tube; the tube with the bonds rearranged is a bit longer than it was before.

The second failure mode is for one of the hexagonal rings of carbon atoms to break open, i.e. for a carbon-carbon bond to break. This is a more catastrophic event, in that the tube then quickly breaks near the point of failure. Which way a tube fails may actually depend on how the honeycomb pattern is rolled into a tube shape.

Now that's just the microscopic theory on the ideal, defect-free system. In a real tube, one expects there to be pre-existing defects in the structure. The failure under tension will then be at the defective points

But, since nanotubes are so small, it's plausible that a single tube or bunch of tubes might grow entirely defect-free, in which case one can access the ultimate theoretical failure strength. Experiments on trying to stretch and break single bundles of nanotubes (Lieber's group at Harvard) show that one can extend a nanotube by about 6% of it's length before it breaks. This is in good agreement with the theoretical predictions mentioned above (and it's a legit prediction- the theory came first!). So it appears that in small enough systems, one can attain the theoretical mechanical strength.

Now if one wants to make a space elevator, one's material has to also be resistant to radiation damage, etc. I think a back of the envelope estimate shows carbon nanotubes or diamond nanowires as being in the right ballpark, so long as one allows the structure to taper, but once one factors in the necessary engineering margins and the need to be resistant to damage over long periods (don't want it to fall apart in a year or two :-) it's much less clear if it's really possible. It's all in the very very long run, of course.

I should admit- I have not yet read this specific article (New Scientist website is crashing on me) so I can't comment specifically on this current experimental result. My guess is they did a larger-scale version of Lieber's experiment and found that the resulting thread was alot weaker (not surpirising- their structure likely has lots more defects and possibly single tubes don't extend throughout the entire length- they overlap).