A team of researchers at MIT have designed one of the lightest and strongest materials ever using graphene.
They made it by compressing and fusing flakes of graphene, a two-dimensional form of carbon.
The new material has just five per cent density and ten times the strength of steel, making it useful for applications where lightweight, strong materials are required.
The key factor that makes this new material strong is its geometrical 3-D form rather than the material itself, suggesting that other similar strong, lightweight materials could be made from a range of other substances by creating similar geometric structures.
The research, published in the journal Science Advances, has been attempted by other research groups, but experiments had failed to match predictions, with some results exhibiting less strength than expected.
The MIT team decided to analyze the material’s behavior down to the level of individual atoms within the structure.
Two-dimensional materials – flat sheets that are just one atom in thickness but can be indefinitely large in the other dimensions – are very strong and have electrical properties.
But because of their thinness, ‘they are not very useful for making 3-D materials that could be used in vehicles, buildings, or devices,’ said Dr Markus Buehler, head of MIT’s department of Civil and Environmental Engineering (CEE) and one of the lead authors of the research.
‘What we’ve done is to realize the wish of translating these 2-D materials into three-dimensional structures,’ said Dr Buehler.
To make the material, the team compressed small flakes of graphene using heat and pressure.
This produced a strong, stable structure that resembles that of some corals and a tiny type of algae called a diatom.
‘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 Zhao Qin, a CEE research assistant and one of the authors of the study.
To test how strong the material was, the researchers created a variety of 3-D models and then subjected them to various tests.
The new 3-D graphene material, which is composed of curved surfaces under deformation, reacts to force in a similar way to sheets of paper.
Paper doesn’t have much strength along its length and width, and can be easily crumpled up.
But when its folded into certain shapes, for example rolled into a tube, the strength along the length of the tube is much greater and can support more weight.
In a similar way, the geometric arrangement of the graphene flakes naturally forms a very strong structure.
The material was made using a high-resolution, multimaterial 3-D printer.
Tests conducted by the MIT team ruled out a possibility proposed previously by other teams that it might be possible to make 3-D graphene structures lighter than air and used as a replacement for helium in balloons.
Instead, the material would not have enough strength and would collapse from the surrounding air pressure.
The researchers say that the material could have many applications in situations that require 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,’ Dr Buehler said, to gain similar advantages of strength combined with advantages in cost, processing methods, or other material properties (such as transparency or electrical conductivity).
‘You can replace the material itself with anything,’ Dr Buehler says.
‘The geometry is the dominant factor. It’s something that has the potential to transfer to many things.’
The unusual geometric shapes that graphene naturally forms under heat and pressure look something like a Nerf ball — round, but full of holes.
These shapes, known as gyroids, are so complex that ‘actually making them using conventional manufacturing methods is probably impossible,’ Dr Buehler said.
The team used 3-D-printed models of the structure, enlarged to thousands of times their natural size for testing.
To actually make the material, the researchers suggest that one possibility would be to use the polymer or metal particles as templates, coat them with graphene by chemical vapor deposit before heat and pressure treatments, and then remove the polymer or metal phases to leave 3-D graphene in the gyroid form.
The same geometry could even be applied to large-scale structural materials.
For example, concrete for a structure such as a bridge might be made with this porous geometry, providing comparable strength with a fraction of the weight.
The material would also provide the added benefit of good insulation because of the huge amount of enclosed airspace within it.
Because the shape has very tiny pore spaces, the material might have applications in filtration systems for either water or chemical processing.
‘This is an inspiring study on the mechanics of 3-D graphene assembly,” says Dr Huajian Gao, a professor of engineering at Brown University, who was not involved in this work.’
This work, Dr Gao says, ‘shows a promising direction of bringing the strength of 2-D materials and the power of material architecture design together.’