Researchers design one of the strongest, lightest materials known
Porous, 3-D forms of graphene developed at MIT can be ten times as strong as steel but much lighter. See Movie
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A team of researchers at MIT has designed one of the strongest lightweight materials known, by compressing and fusing flakes of graphene, a two-dimensional form of carbon. The fresh material, a sponge-like configuration with a density of just five percent, can have a strength ten times that of steel.
In its two-dimensional form, graphene is thought to be the strongest of all known materials. But researchers until now have had a hard time translating that two-dimensional strength into useful three-dimensional materials.
The fresh findings demonstrate that the crucial aspect of the fresh 3-D forms has more to do with their unusual geometrical configuration than with the material itself, which suggests that similar strong, lightweight materials could be made from a multitude of materials by creating similar geometric features.
The findings are being reported today in the journal Science Advances, in a paper by Markus Buehler, the head of MIT’s Department of Civil and Environmental Engineering (CEE) and the McAfee Professor of Engineering; Zhao Qin, a CEE research scientist; Gang Seob Jung, a graduate student; and Min Jeong Kang MEng ’16, a latest graduate.
A team of MIT engineers has successfully designed a fresh 3-D material with five percent the density of steel and ten times the strength, making it one of the strongest lightweight materials known.
Movie: Melanie Gonick/MIT
Other groups had suggested the possibility of such lightweight structures, but lab experiments so far had failed to match predictions, with some results exhibiting several orders of magnitude less strength than expected. The MIT team determined to solve the mystery by analyzing the material’s behavior down to the level of individual atoms within the structure. They were able to produce a mathematical framework that very closely matches experimental observations.
Two-dimensional materials — basically plane 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 extreme thinness, “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 realize the wish of translating these 2-D materials into three-dimensional structures.”
The team was able to compress puny flakes of graphene using a combination of warmth 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 desired to see what’s the limit — what’s the strongest possible material we can produce,” says Qin. To do that, they created a diversity of 3-D models and then subjected them to various tests. In computational simulations, which mimic the loading conditions in the tensile and compression tests performed in a tensile loading machine, “one of our samples has five percent the density of steel, but ten 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 lightly crumpled up. But when made into certain shapes, for example spinned into a tube, all of a sudden 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.
The fresh configurations have been made 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 the experiments and simulations matched accurately.
The fresh, more accurate results, based on atomistic computational modeling by the MIT team, ruled out a possibility proposed previously by other teams: that it might be possible to make 3-D graphene structures so lightweight that they would actually be lighter than air, and 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 from 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 light 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 build up similar advantages of strength combined with advantages in cost, processing methods, or other material properties (such as transparency or electrical conductivity).
“You can substitute the material itself with anything,” Buehler says. “The geometry is the superior factor. It’s something that has the potential to transfer to many things.”
The unusual geometric shapes that graphene naturally forms under fever and pressure look something like a Nerf ball — round, but utter of crevices. These shapes, known as gyroids, are so complicated that “actually making them using conventional manufacturing methods is very likely unlikely,” Buehler says. The team used 3-D-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, cover them with graphene by chemical vapor deposit before warmth and pressure treatments, and then chemically or physically eliminate the polymer or metal phases to leave 3-D graphene in the gyroid form. For this, the computational model given in the current examine provides a guideline to evaluate the mechanical quality of the synthesis output.
The same geometry could even be applied to large-scale structural materials, they suggest. 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. This treatment would have the extra benefit of providing good insulation because of the large amount of enclosed airspace within it.
Because the form is riddled with very little 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 diversity of applications, the researchers say.
“This is an inspiring investigate on the mechanics of 3-D graphene assembly,” says Huajian Gao, a professor of engineering at Brown University, who was not involved in this work. “The combination of computational modeling with 3-D-printing-based experiments used in this paper is a powerful fresh treatment in engineering research. It is exceptional to see the scaling laws primarily derived from nanoscale simulations resurface in macroscale experiments under the help of 3-D printing,” he says.
This work, Gao says, “shows a promising direction of bringing the strength of 2-D materials and the power of material architecture design together.”
The research was supported by the Office of Naval Research, the Department of Defense Multidisciplinary University Research Initiative, and BASF-North American Center for Research on Advanced Materials.