WHY THIS MATTERS IN BRIEF
As our ability to engineer materials at the nano level improves we can create new materials with amazing properties.
If you’ve ever fancied a bullet proof shirt like you sometimes see in the movies then this newest technology breakthrough could very well be for you, eventually anyway, after engineers at Caltech, MIT, and ETH Zürich developed a nano-architected material that’s made from tiny carbon struts and that’s thinner than a human hair, and which is pound for pound, more effective at stopping a projectile than steel and Kevlar – two materials commonly used in personal protective gear.
Pioneered by Caltech materials scientist Julia R. Greer, nano-architected materials have a structure that is designed at a nanometer scale and exhibit unusual, often surprising properties – for example, exceptionally lightweight nanoceramics that spring back to their original shape, like a sponge, after being compressed.
Tiny but mighty. Source: MIT
“The knowledge from this work could provide design principles for ultra-lightweight impact resistant materials for use in efficient armoured materials, protective coatings, and blast-resistant shields [that are] desirable in defense and space applications,” says Greer whose lab led the material’s fabrication. Greer is co-corresponding author of a paper on the new material that was published in Nature Materials on July 24.
The material, which is thinner than a human hair, consists of interconnected tetrakaidecahedrons made out of carbon struts that have been formed under extreme heat, known as pyrolytic carbon.
Tetrakaidecahedrons are structures with 14 faces: six with four sides and eight with eight sides. They are also called “Kelvin cells” because in 1887 Lord Kelvin suggested that they would be the best shape to fill an empty three-dimensional space with equal-sized objects using minimal surface area.
“Historically this geometry appears in energy-mitigating foams, says Carlos Portela, assistant professor of mechanical engineering at MIT and lead author of the Nature Materials paper. Portela and his lab investigated the use of the foam-like structures to lend flexibility to the stiff carbon.
“While carbon is normally brittle, the arrangement and small sizes of the struts in the nano-architected material gives rise to a rubbery, bending-dominated architecture,” he says.
While the strength of nano-architected materials has been studied using slow deformation Portela wanted to know how such a material might survive a high-speed impact so he first fabricated the material out of photosensitive polymer using two-photon lithography, a technique that uses a fast high-powered laser to solidify and sculpt microscopic structures.
His team then pyrolized the structures; that is, they burnt them in a furnace at a very high temperature to convert the polymer to pyrolytic carbon. The scientists created two versions of the material: a denser and a looser one. Portela’s lab then blasted both versions with 14-micron-diameter spherical silicon oxide particles, one at a time. The particles travelled at between 40 and 1,100 meters per second or four times faster than the speed of sound.
The researchers found that the denser version of the material was more resilient to supersonic impacts, with the microparticles tending to embed in the material rather than tearing straight through, as would be the case with either fully dense polymers or carbon sheets of the same thickness.
Under closer examination, they discovered that individual struts directly surrounding the particle would crumple, but the overall structure remained intact until the projectile stopped – therefore pound for pound the new material outperformed steel by more than 100 percent and Kevlar composites by more than 70 percent.
“We show the material can absorb a lot of energy because of this shock compaction mechanism of struts at the nanoscale versus something that’s fully dense and monolithic, not nano-architected,” Portela says.
For the material to be used in real-world applications, researchers next will need to find ways to scale up its production and to explore how other nano-architected materials, including those made out of materials other than carbon, hold up under high-speed impacts. In the meantime, the study has demonstrated the viability of nano-architected materials for impact resistance, opening up a new avenue of research.
The Nature Materials paper is titled “Supersonic Impact Resilience of Nanoarchitected Carbon.”