This number was determined fairly precisely last year by Smalley's group at
Rice. Unfortunately, I couldn't find the number on line given a few minutes
of search. I did find an estimate of 1000 GPa in a paper from the group
that apparently predates the more precise measurement:
http://cnst.rice.edu/TIPS_rev.htm.
Young's modulus is usually 30 to 100 times larger than the breaking strength
of a material. This is equivalent to saying most materials can be stretched
by 1% to 3% before they snap. Obviously, it's bigger for rubber-- more like
100%. But strong materials tend to be at the low end of the range. This is
annoying, because the larger this percentage, the less susceptible the
material is to crack propagation. That's why glass cracks easier than
rubber, even though glass is much stronger and stiffer.
Buckytubes put under stress tend to unravel into one-atom-thick carbyne
strings, which are stronger than the original tube, but also chemically
reactive (http://cnst.rice.edu/images/tube1.gif). It would be cool if
streching a rope made of buckytube would convert it into something even
stronger! Probably won't work, though-- carbyne strands will react with one
another. Even if a stretched fiber turns into a chunk of amorphous carbon,
that would be great, because that's fairly strong stuff, and it's a lot
easier doing engineering with a material that retains *some* of its strength
when its limits are exceeded.
--CarlF
