Quality control at the nanoscale.

This article discusses new research doubting nanotubes could provide sufficient strength for a space elevator:

Nanotubes Might Not Have the Right Stuff. By Bill Christensen posted: 02 June 2006

However, this paper from doing actual measurements found a highest strength of 63 GPa, within the range to permit the building of a space elevator:

Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load. SCIENCE, VOL 287, p. 637-640, 28 JANUARY 2000

And this paper showed actual measured tensile strengths up to 150 GPa:

Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. B.G. Demczyk et al. Materials Science and Engineering A334 (2002), 174, 173-178.

Both of these studies were done on multiwalled tubes since they are larger and it's easier to make attachments with them. In the earlier study in Science, the authors from SEM imaging noted that it was actually the outer single-walled nanotube that broke first therefore it was carrying the load. This would make sense from the way the attachments were formed which could only form a bond with the outer surface of the multiwalled tube. Therefore the numbers quoted were for the strength of this outer single-walled nanotube using as thickness only that of this single-walled nanotube. However, in the later study in Materials Science and Engineering, the authors believed the attachments were made to all the layers of the multi-layered nanotube, which would explain their higher measured strength. The prevailing theory is that the range of strengths is due to the number of imperfections in the nanotubes. So we should be able to look at the nanotubes at the nanoscale using SEM,'s, STM's or AFM's and find which ones have the least imperfections. These should be the strongest tubes. In the Science study, 1 out of 21 of them, 5%, have the best strength,

63 GPa. At a production of millions of tubes at a time this should still be feasible economically and technically. The lengths of the nanotubes in these studies were however at the micron scale. Nanotubes have been created at the centimeter length scale, but as far as I know the strength of these have not been tested. Note that the reported strengths of centimeter long or longer "fibers" made of nanotubes being less than 1 GPa are not measuring the strength of individual nanotubes at these lengths. This is because the fibers are composed of the nanotubes stuck together end to end by weaker Van der Waals forces, rather than the much stronger carbon-carbon bonds that prevail in individual nanotubes. Here is one study that detects, characterizes defects in the nanotubes at the nanoscale:

Resonant Electron Scattering by Defects in Single-Walled Carbon Nanotubes. Science 12 January 2001, Vol. 291. no. 5502, pp. 283 - 285.

Methods such as this might make it possible to find the nanotubes with the least defects beforehand and therefore automatically select those of the highest strengths.

Bob Clark