Carbon nanotubes are often imagined to be structures of great perfection and regularity, but the reality is that, like virtually all materials we encounter, they will have defects – places where there’s a mistake in the crystal structure, like a missing atom or a wrongly connected bond. Defects are tremendously important in materials science, because they’re what stop materials from being anything like as strong as you would estimate they ought to be from a simple calculation. A recent paper in Nature Materials (abstract here, subscription required for full paper) provides what is, I think, the first accurate measurement of defect densities in single walled carbon nanotubes. For typical nanotubes, produced by chemical vapour deposition, one finds one defect every four microns of nanotube length.
It’s these atomic-level flaws that will, in practise, limit both the electronic and the mechanical properties of carbon nanotubes. The study, by Philip Collins and coworkers, at UC Irvine, uses a new technique for decorating the defects electrochemically. It’s not able to distinguish between different types of defects, which could include a substitutional dopant, a broken bond passivated by further chemical group or a mechanical strain or kink, as well as what is perhaps the theoretically best studied nanotube defect – the Stone–Wales defect. The latter occurs if, in a group of four hexagons of carbon, one bond is rotated, leading to two hexagons, a pentagon and a heptagon.
The figure of one defect per 4 microns of tube is, in one way, rather impressive – it translates into there being only one defect for every 10 thousand billion atoms. This is a similar level to the best quality silicon, which is pretty much the most perfect crystalline material available. But, on the other hand, given the essentially one-dimensional nature of a nanotube, it’s pretty significant, since a single defect in a length of nanotube being used in an electronic device would dramatically change its characteristics. And the presence of all these weak spots are likely to mean that it’s going to be difficult to make a macroscale nanotube cable whose strength approaches the theoretical estimates people have been making, for example in connection with the proposed space elevator.
CNT defects put a “kink” in SE plans. I wonder if there might be a way of fixing these defects. Endcaps of carbon nanotubes are under more strain in part because they contain 4 pentagons; when extreme heat is applied these pentagon are toasted before all the hexagons, and the endcap is quickly compromised. But when you heat or otherwise react the defect point, might there be a way of quickly substituting in a carbon atom(s) from the surrounding environment before the carbon atom(s) removed from the defect point causes the defect to become even more pronounced? Is there a way to render the environment of the defect point inert somehow so more time can be bought to fix the defect?
Fixing CNTs is 1/2 to a hydrogen economy. From what I can tell, it won’t be too hard to uncap CNTs and fill them with hydrogen (or metals, or whatever). It is re-capping that will be hard. And restoring CNT endcaps is only a scaled-up version of sidewall defect fixing. Perhaps the CNTs can be filled with something organic that will affix to the inner defect sites to promote “healing”?
(12 pentagons for CNT endcaps, 4 for nanohorns. Same idea.)
I am writing in support of Phillip’s ideas and beyond.
In my opinion, fixing CNT’s will require similar ideas to that in biology. Essentially, some sort of error correction will be required!
Now, the big problem in CNT is that resolving these defects is quite difficult to model. However, I believe that lcls imaging facility will allow scientists to resolve these errors, calculate effective 3-D Density functional maps, and provide clues to controling these effects. Maybe, some sort of enzyme could be manufactured (distant future!) which supresses defect formation.
An amateur mathematician.
The paper actually indicates that you can reduce the number of imperfections by improvements to the synthesis process, and maybe it’s possible to “heal” some more of them by some kind of annealing process. Of course, there will be a finite concentration of imperfections at thermal equilibrium, but I suspect this on calculation would turn out to be quite a lot lower than what’s observed. It would be interesting to do the sum.
I know its alot to ask Richard, but would you or anyone else be able to sketch out how one would go about attempting to calculate this sum (asked by a mechanical engineering novice)? I’ve just started a CNT progress tracker thread on a Space Elevator Forum
http://www.liftport.com/forums/showthread.php?t=331
At 45GPa tensile strengths the SE isn’t economically feasible but at 60GPa ribbon launch becomes cheap. I know most current tests show 30GPa seems to be a plateau, but I assume this is based upon one defect every 4 microns. If a method to figure this out could be supplied, I might be able to find some in the SE community who might be willing and able to actually rent supercomputer time or whatever. Thx and Happy 2006 to all.
That’s a difficult question to answer, because tensile strength isn’t really a single material property. The key point to remember is that the tensile strengths of brittle materials (and in this context nanotubes count as brittle) are statistically distributed. There’s a well known piece of analysis called the Weibull distribution which tells you that if you test a large number of (macroscopically) identical test pieces you’ll get a distribution of results. The shape of this distribution depends on how big the samples are. If you only test small lengths of fibre, then the probability of a given fibre having a defect will be less, and the number of fibres that have higher tensile strengths will be higher. No-one’s done a proper Weibull analysis of fracture in single wall nanotubes; the situation for multiwall nanotubes is reviewed here (subscription required for full article, I’m afraid).
At first sight this will seem discouraging for space elevators, since necessarily you’re going to need a very long nanotube for such a purpose. The probability of having such a long length without a single defect (you only need one nick in a rope for it to break at the weak point!) looks like it’s going to be rather small. But maybe there’s going to be some cunning way of bundling lots of nanotubes together that will get round this.
Thx. I didn’t realize defect distributions were irregular. I guess there’s nothing to do for now but keep an eye out for any sidewall defect improvements and/or repair methods.