The Massachusetts Institute of Technology, Cambridge, reports its researchers have found that, at least in some nuclear reactors, adding less than 2% carbon nanotubes to aluminum can dramatically slow the breakdown process that causes it to become porous and brittle.
Helium from radiation transmutation takes up residence inside metals and causes the material to become riddled with tiny bubbles along grain boundaries and to become progressively more brittle. The nanotubes, despite only making up a small fraction of the volume, can form a percolating, one-dimensional transport network, to provide pathways for the helium to leak back out instead of being trapped within the metal, where it could continue to do damage.
Testing shows that after exposure to radiation, the carbon nanotubes within the metal can be chemically altered to carbides, but they still retain their slender shape, “almost like insects trapped in amber,” Prof. Ju Li says. “It’s quite amazing — you don’t see a blob; they retain their morphology. It’s still one-dimensional.”
The huge total interfacial area of these 1-D nanostructures provides a way for radiation-induced point defects to recombine in the metal, alleviating a process that also leads to embrittlement. The researchers showed that the 1-D structure was able to survive up to 70 DPA of radiation damage. (DPA is a unit that refers to how many times, on average, every atom in the crystal lattice is knocked out of its site by radiation, so 70 DPA means a lot of radiation damage.)
After radiation exposure, Prof. Ju Li says, “We see pores in the control sample, but no pores in the new material, and mechanical data shows it has much less embrittlement.” For a given amount of exposure to radiation, the tests have shown the amount of embrittlement is reduced about five to tenfold.
The new material needs only tiny quantities of carbon nanotubes — about 1% by weight added to the metal — and these are inexpensive to produce and process. The composite can be manufactured at low cost by common industrial methods and is already being produced by the ton by manufacturers in Korea, for the automotive industry.
