12th January 2017
Researchers design one of the strongest and lightest materials known
A team at MIT has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene. Tests show it could lead to materials with a density just 5% that of steel, but 10 times stronger.
Credit: Qin et al. Sci. Adv. 2017; 3:e1601536
In its two-dimensional form, graphene is thought to be the strongest of all known materials. But until now, researchers have had a difficult time translating that two-dimensional strength into useful three-dimensional materials.
New findings show that the crucial aspect of these 3-D forms is more to do with their unusual geometrical configuration than the material itself, suggesting that similar strong, lightweight materials could be made from a variety of materials, by creating similar geometric features. The research is published by the journal Science Advances, in a paper led by Markus Buehler, head of MIT’s Department of Civil and Environmental Engineering (CEE).
Other groups had suggested the possibility of such lightweight structures – but lab experiments so far had failed to match predictions, with some results exhibiting orders of magnitude less strength than expected. The MIT team decided to solve the mystery by analysing the material’s behaviour down to the level of individual atoms. They were able to produce a mathematical framework that very closely matched their experimental observations.
Two-dimensional materials like graphene are basically flat sheets, just one atom in thickness, but able to be indefinitely large in the other dimensions. They have exceptional strength, as well as unique electrical properties. But because of their flatness, “they are not very useful for making 3-D materials that could be used in vehicles, buildings, or devices,” Buehler says. “What we’ve done is to realise the wish of translating these 2-D materials into three-dimensional structures.”
The team was able to compress small flakes of graphene using a combination of heat and pressure. This process produced a strong, stable structure whose form resembles that of some corals and microscopic creatures called diatoms. These shapes, which have an enormous surface area in proportion to their volume, proved to be remarkably strong.
“Once we created these 3-D structures, we wanted to see what’s the limit – what’s the strongest possible material we can produce,” said his colleague and Professor of Engineering, Zhao Qin. To do that, they created a variety of 3-D models and subjected them to various tests. In computer simulations, which mimic loading conditions in tensile and compression tests performed in a tensile loading machine, “one of our samples has five percent the density of steel, but 10 times the strength,” Qin says.
Buehler says that what happens to their 3-D graphene material, which is composed of curved surfaces under deformation, resembles what would happen with sheets of paper. Paper has little strength along its length and width, and can be easily crumpled up. But when made into certain shapes – for example, rolled up into a tube – suddenly the strength along the length of the tube is much greater and can support substantially more weight. Similarly, the geometric arrangement of the graphene flakes after treatment naturally forms a very strong configuration.
The team demonstrated their new configurations in the lab using a high-resolution, multimaterial 3-D printer. They were mechanically tested for their tensile and compressive properties, and their mechanical response under loading was simulated using the team’s theoretical models. The results from these experiments and simulations matched accurately.
Many applications of the material could eventually be feasible, the team explains, for uses needing a combination of extreme strength and light weight. “You could either use the real graphene material, or use the geometry we discovered with other materials like polymers or metals,” says Buehler, to gain similar advantages of strength combined with advantages in cost, processing methods, or other properties like transparency or electrical conductivity. “You can replace the material itself with anything. The geometry is the dominant factor. It’s something that has the potential to transfer to many things.”
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