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So-called ‘nanoarchitected materials’ – designed from precisely patterned nanoscale structures, could offer a route to lightweight armour, blast shields and other impact-resistant materials.
The researchers created an ultralight material made from nanometre-scale carbon struts to give the material toughness and mechanical robustness. Then they shot it with microparticles at supersonic speeds and discovered that the material – thinner than a human hair – was capable of stopping the miniature projectiles.
The material is better at absorbing impacts than steel, Kevlar, aluminium and other materials of comparable weight. "The same amount of mass of our material would be much more efficient at stopping a projectile than the same amount of mass of Kevlar," said the study's lead author, Carlos Portela, assistant professor of mechanical engineering at MIT.
If it’s able to be produced at a large scale, it could offer a replacement for these products. "The knowledge from this work... could provide design principles for ultra-lightweight impact resistant materials [for use in] efficient armour materials, protective coatings, and blast-resistant shields desirable in defence and space applications," says co-author Julia R. Greer, a professor of materials science, mechanics, and medical engineering at Caltech, whose lab led the material's fabrication.
Caltech researchers created the material using two-photon lithography, a process that uses a fast, high-powered laser to solidify microscopic structures in a photosensitive resin. It consists of a repeating pattern of microscopic struts in a shape known as a tetrakaidecahedron. "Historically this geometry appears in energy-mitigating foams," says Portela. "While carbon is normally brittle, the arrangement and small sizes of the struts in the nanoarchitected material gives rise to a rubbery, bending-dominated architecture."
The impact experiments were conducted at MIT. An ultrafast laser was aimed through a glass slide coated with a thin film of gold, which itself was coated with 14-micron-wide particles of silicon oxide. The laser generated plasma from the gold, which pushed the silicon oxide particles towards the target. By adjusting the laser’s power, the researchers were able to explore speeds ranging from 40 to 1100 metres per second. “Supersonic is anything above approximately 340 metres per second, which is the speed of sound in air at sea level," Portela says. "So, some experiments achieved twice the speed of sound, easily.”
When they sliced the materials open, they found the region below the embedded particle was crumpled and compacted, but the surrounding material remained intact. "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 nanoarchitected," Portela says.
This is the first time that nanoarchitected materials have been tested under such high-speed impacts. "We only know about their response in a slow-deformation regime, whereas a lot of their practical use is hypothesised to be in real-world applications where nothing deforms slowly," Portela says.
In future, Portela plans to explore different nanostructured configurations and materials beyond carbon, as well as exploring ways to scale up production. "Nanoarchitected materials truly are promising as impact-mitigating materials," Portela says. "There's a lot we don't know about them yet, and we're starting this path to answering these questions and opening the door to their widespread applications."
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