Carbon nanotubes structural applications (overpromised research)

"Nanotechnology will never be a structural material" — kinetic-theory-of-gases argument bounds vapor-phase deposition at ~1 mm/hr.

A lot of times people don't even run the numbers to say how fast can I grow it, how fast can I make it. If you take my casting lecture, I prove why nanotechnology will never be a structural material. Nanotechnology has to be formed from the vapor phase, and the kinetic theory of gases says the fastest you can go is about a millimeter an hour. How many Golden Gate bridges or Oakland Bay bridges are you going to build at a millimeter-an-hour growth rate with million-dollar equipment?

Modern nanotube strength claims (3 million PSI) treated as a recapitulation of 125-year-old theoretical bond-strength calculations, with the same fatal flaw — vacancies at any non-zero temperature destroy the strength.

So did we ever make any material that had that strength? In the 1940s we made iron whiskers, which had a screw dislocation right up the axis of this whisker. Anybody who knows anything about dislocations — if you pull in the same direction as the screw dislocation, then there's no stress on it. A screw dislocation moves by shear through the material. They pulled these whiskers experimentally, they measured 2 million PSI. So now, forty years later, I hear about people discovering carbon nanotubes with these tremendous strengths. Except the problem is, the physicists assume a perfect crystal with no dislocations. At what temperature is that true? Absolute zero. At any temperature above absolute zero, you're going to have some vacancies in those nanotubes. And guess what's going to happen to the strength of that nanotube when you have a vacancy? I've already ripped a piece of paper for you to show you what a notch does.

Used to make the point that the 3-million-psi theoretical strength of carbon nanotubes is unrealizable because of vacancy defects, and that the calculation was done in the 1930s. Buckyball Nobel laureate (presumably Richard Smalley at Rice) gets credit for rediscovery.

If you calculate what the strength of that bond is going to be, you get about 3 million psi. You know about this because you've read in the New York Times and the Wall Street Journal all about carbon nanotubes, and they have a strength of 3 million psi. Well, that's a calculation that was done in the 1930s when we first learned quantum mechanics and learned something about bond energy. If you take my solid-state welding course, I spend half a lecture on this. We're limited in our material properties by the strength of the chemical bonds between the atoms. There's a fundamental limitation. You can't get a material with a modulus higher than 60 million psi — about 400 gigapascals if you're metric. Unless you come up with new elements that have stronger bonds, we're not going to get anything stronger.

Tom's argument that carbon nanotubes' theoretical 2–3 million PSI strength is irrelevant because any real nanotube contains atomic vacancies (guaranteed by the second law of thermodynamics above 0 K), which act as defects and drop strength by ~10×. Physicists "getting billions of dollars to study carbon nanotubes" called "money down a rat hole" for structural use; allows for functional/display applications.

And why imperfections control the strength of materials. Anyone ever heard that bucky balls and nano graphene and stuff are super strong? Absolute garbage. A physicist's pipe dream. I can take a sheet of paper — you'll see me doing this on the History Channel when I talk about the Titanic — and pull on that piece of paper with pounds of force. But if I put a flaw in, that takes ounces. The inherent strength of a material may be millions of PSI, and you can do the calculation. I do it in the joining lectures. A carbon nanotube should have two or three million PSI strength. That's only true if you don't have a vacancy. If you have an atomic vacancy, then it's just like having a defect. If you really made a real carbon nanotube, it would fail at 1/10 the theoretical strength.

Twenty-five years and a billion dollars of research on carbon nanotubes (and later graphene) for structural applications, promising 3 million PSI carbon-carbon bond strength. Nobody has measured the strength of an actual carbon nanotube. Vacancies forbidden by statistical mechanics at any T > 0 K mean real nanotubes will be 1/5 to 1/10 the predicted strength. Tom: "Don't invest in carbon nanotubes for structural applications."

Today we have great physicists who wanted to win the Nobel Prize for discovering bucky balls — three-dimensional carbon structures. [Tom holds up a diamond-structure model.] This is the structure of diamond. Bucky balls are essentially hollow spheres with sp3 hybrid bonding. They calculated the strength of carbon-carbon bonds and found these should be 3 million PSI, and we're going to be able to make composites out of bucky fibers — nano fibers. That's true — as long as you don't have any defects in those carbon nanotubes. What's the probability of having a defect in a carbon nanotube? Pretty high. If you make them at absolute zero, you can make them without defects, but at any temperature above absolute zero, statistical mechanics will prove to you you'll have some vacancies. And if you have some vacancies, you're going to find they're not as strong as you thought.