The limited operating lifetimes of nuclear structural materials is due to embrittlement and porosity that occur in these materials under long-term radiation exposure near a reactor core. Carbon nanotube (CNT)-reinforced aluminum composite materials provide a possible solution to this problem. The addition of small quantities of CNTs to aluminum dramatically improves the material’s irradiation tolerance, suggesting potential applications in nuclear reactors, nuclear waste containers, nuclear batteries, and spacecraft. The dispersion of multiwalled carbon nanotubes in a metal matrix effectively mitigates radiation damage through additional internal interfaces for self-healing of radiation defects, and improves the mechanical properties by inhibiting dislocation propagation.

In a recent issue of Nano Energy (doi:10.1016/j.nanoen.2016.01.019), a research team led by Ju Li from the Departments of Nuclear Science Engineering and of Materials Science Engineering at the Massachusetts Institute of Technology investigated the basic materials science of an aluminum/CNT composite. “The key technology of our research is the dispersion of the CNTs inside metal grains,” says Kang Pyo So, the lead author of this work. These CNTs facilitate the recombination of atomic-level defects, such as vacancies, interstitials, and dislocation loops, and may provide pathways for releasing helium instead of trapping helium along grain boundaries to cause embrittlement. CNTs transform to aluminum carbide when radiated to a high dose of 72 dpa, but the one-dimensional nanomorphologies survive along with the self-healing interfaces that catalyze defect recombination.

In addition, this composite material exhibits improved mechanical strength and toughness as compared to the reference metal. The high-aspect ratio of CNTs creates obstacles for dislocation and crack propagation in the metal matrix. The standard tensile test of aluminum with 1% volume CNTs shows significant improvement in tensile strength without decreasing ductility as compared to the bulk Al specimen. The manufacturing process is also scalable. At less than two times the price of aluminum alloys, the team mass-produced over 100 kg of this composite material in the laboratory. “We can use this nanocomposite as a lightweight and long-term sustainable material in automotive vehicles, airplanes, and ships,” Kang Pyo says.

Dispersion of carbon nanotubes (CNTs) inside Al grain improves tensile strength without sacrificing ductility, where (a) shows a transmission electron micrograph of the CNT inside an Al grain and (b) shows the stress–strain curve (inset: 100 kg of the Al+CNT composite). Credit: Nano Energy.

“This work provides an insight to explore a new regime for designing materials under radiation applications, from previous approaches such as dispersing oxide nanoparticles into an Al matrix,” says Shimin Mao, an expert studying irradiation effects of materials at the University of Illinois at Urbana-Champaign. “This will encourage other researchers in the field to implement more fundamental studies on nanocomposite-type radiation resistance materials.”