Sponge-shaped materials can be stronger than steel

A team of researchers at Massachusetts Institute of Technology (MIT) has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene, a two-dimensional (2D) form of carbon. The new material, a sponge-like configuration with a density of just 5%, can have a strength 10 times that of steel.

In its 2D form, graphene is thought to be the strongest of all known materials. But up to now researchers have had a hard time translating that 2D strength into useful three-dimensional (3D) materials.

The new findings show that the impressive strength of these novel 3D forms is due more to their unusual geometrical configuration than the material they are made from. This suggests that these strong, lightweight forms could be made from a variety of materials by creating similar geometric features. The findings are reported in a paper in Science Advances by a team led by Markus Buehler, professor of engineering and head of MIT's Department of Civil and Environmental Engineering (CEE). The other members of the team are: Zhao Qin, a CEE research scientist; Gang Seob Jung, a graduate student; and Min Jeong Kang, a recent graduate.

Other groups had suggested the possibility of such lightweight structures, but lab experiments had so far failed to match predictions, with some results exhibiting several orders of magnitude less strength than expected. The MIT team decided to solve the mystery by analyzing the material's behavior down to the level of individual atoms within the structure. This allowed them to produce a mathematical framework that very closely matches experimental observations.

Two-dimensional materials – basically flat sheets that are just one atom in thickness but can be indefinitely large in the other dimensions – have exceptional strength as well as unique electrical properties. But because of their extraordinary thinness, "they are not very useful for making 3D materials that could be used in vehicles, buildings or devices," Buehler says. "What we've done is to realize the wish of translating these 2D materials into 3D structures."

The team was able to compress small flakes of graphene using a combination of heat and pressure, producing 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 3D structures, we wanted to see what's the limit – what's the strongest possible material we can produce," says Qin. To do that, they created a variety of 3D models and then subjected them to various tests. In computational simulations that mimic the loading conditions found in the tensile and compression tests performed in a tensile loading machine, "one of our samples has 5% the density of steel, but 10 times the strength," Qin says.

Buehler says that what happens to their 3D 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, such as rolled into a tube, suddenly the strength along the length of the tube is much greater and can support substantial weight. Similarly, the geometric arrangement of the graphene flakes after treatment naturally forms a very strong configuration.

These new configurations have now been made in the lab using a high-resolution, multi-material 3D 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 the experiments and simulations matched accurately.

The new, more accurate results, based on atomistic computational modeling by the MIT team, ruled out a possibility proposed previously by other teams. This was that it might be possible to make 3D graphene structures so lightweight they would actually be lighter than air, and so could be used as a durable replacement for helium in balloons. The current work shows, however, that at such low densities, the material would not have sufficient strength and would collapse under the surrounding air pressure.

But many other possible applications of the material could eventually be feasible, the researchers say, for uses that require a combination of extreme strength and low weight. "You could either use the real graphene material or use the geometry we discovered with other materials, like polymers or metals," Buehler says, to gain similar advantages of strength, as well as advantages in cost, processing methods or other material properties (such as transparency or electrical conductivity).

"You can replace the material itself with anything," 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," Buehler says. The team used 3D-printed models of the structure, enlarged to thousands of times their natural size, for testing purposes.

For actual synthesis, the researchers say, one possibility is to use the polymer or metal particles as templates. They would then coat these templates with graphene by chemical vapor deposition before heat and pressure treatments, and then chemically or physically remove the polymer or metal phases to leave 3D graphene in the gyroid form. For this, the computational model given in the current study provides a guideline for evaluating the mechanical quality of the synthesis output.

The same geometry could even be applied to large-scale structural materials, the researchers suggest. For example, concrete for a structure such a bridge might be made with this porous geometry, providing comparable strength with a fraction of the weight. This approach would have the additional benefit of providing good insulation because of the large amount of enclosed airspace within it.

Because the shape is riddled with very tiny pore spaces, the material might also find application in some filtration systems, for either water or chemical processing. The mathematical descriptions derived by this group could facilitate the development of a variety of applications, the researchers say.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.