MIT Researchers Develop Porous, 3D Forms of Graphene

MIT researchers have developed one of the strongest lightweight materials known to man. They created the new material by compressing and fusing flakes of graphene, a 2D form of carbon. The new material has a sponge-like configuration with a density of only 5%, and is likely to have strength 10 times that of steel.

In its 2D form, Graphene is said to be the strongest of all known materials. But so far researchers have found it hard to translate that 2D strength into practical 3D materials.

The new findings show that the vital aspect of the new 3D forms has more to do with their extraordinary geometrical configuration than with the material itself, which proposes that similar strong, lightweight materials could be created from a range of materials by developing similar geometric characteristics.

The research findings are reported in the Science Advances journal, 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 recent graduate.

Other groups had suggested the likelihood of such lightweight structures, but until now lab experiments had failed to meet predictions, with a few results showing several orders of magnitude less strength than anticipated. The MIT team chose to solve the mystery by exploring the material’s behavior right to the level of individual atoms within the structure. They were able to formulate a mathematical framework that very strongly matches experimental observations.

Two-dimensional materials are flat sheets measuring just one atom in thickness but can be indefinitely large in the other dimensions. These materials possess excellent strength as well as exceptional electrical properties. But because of their unusual 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 three-dimensional structures.”

Using a combination of heat and pressure, the team was able to compress small flakes of graphene. This process produced a strong, stable structure whose form looks like some corals and microscopic creatures known as diatoms. These shapes, which have a huge surface area in proportion to their volume, proved to be extraordinarily strong.

Once we created these 3D structures, we wanted to see what’s the limit — what’s the strongest possible material we can produce.

Zhao Qin, CEE Research Scientist, MIT

To achieve that, they developed a range of 3D models and then subjected them to numerous tests. In computational simulations, which imitate the loading conditions in the compression and tensile tests conducted in a tensile loading machine, “one of our samples has 5 percent the density of steel, but 10 times the strength,” Qin says.

Buehler explains that what happens to their 3D graphene material, which is made up of curved surfaces under deformation, is similar to what would happen with sheets of paper. Paper has little strength across its width and length, and can be easily crumpled up.

But when made into specific shapes, for instance rolled into a tube, suddenly the strength across the length of the tube is much more and can support considerable weight. Likewise, the geometric arrangement of the graphene flakes after treatment naturally develops an extremely strong configuration.

The new configurations were created in the lab, with a high-resolution, multimaterial 3D printer. They were mechanically analyzed for their compressive and tensile properties, and their mechanical reaction under loading was simulated using the theoretical models formulated by the team. The results from the experiments and simulations corresponded accurately.

Based on atomistic computational modeling by the MIT team, the new, more accurate results ruled out a possibility proposed earlier by other teams: that it might be possible to create 3D graphene structures so lightweight that they would in reality be lighter than air, and could be used as a sturdy replacement for helium in balloons.

However, the existing work reveals that at such low densities, the material would not have adequate strength and would fail from the surrounding air pressure.

But several other potential applications of the material could be feasible in due course, the researchers say, for uses that require a combination of high 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 obtain similar advantages of strength combined with benefits in cost, processing techniques, or other material properties (such as electrical conductivity or transparency).

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.

Markus Buehler, Head of CEE, MIT

The extraordinary geometric shapes that graphene naturally forms under heat and pressure resemble a Nerf ball - round, but full of holes. These shapes, called 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, increased to thousands of times their natural size, for analyzing purposes.

The MIT researchers say that for actual synthesis, one option is to use the metal particles or polymer as templates, coat them with graphene by chemical vapor deposit prior to heat and pressure treatments, and then physically or chemically remove the metal or polymer phases to leave 3D graphene in the gyroid form.

For this, the computational model provided in the present study provides a principle to assess the mechanical quality of the synthesis output.

It is possible to even apply the same geometry to large-scale structural materials, they suggest. For instance, concrete for a structure such a bridge could be developed with this porous geometry, providing similar strength with a fraction of the weight. This technique would have the extra advantage of providing good insulation due to the large quantity of enclosed airspace within it.

As the shape is full of very miniature pore spaces, the material could also be applied in some filtration systems, for either chemical or water processing. The mathematical descriptions obtained by this group could allow the development of a range of applications, the researchers say.

This is an inspiring study on the mechanics of 3D graphene assembly. The combination of computational modeling with 3D-printing-based experiments used in this paper is a powerful new approach in engineering research. It is impressive to see the scaling laws initially derived from nanoscale simulations resurface in macroscale experiments under the help of 3D printing. [This research] shows a promising direction of bringing the strength of 2-D materials and the power of material architecture design together.

Huajian Gao, Professor of Engineering, Brown University

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.

A team of MIT engineers has successfully designed a new 3D material with five percent the density of steel and ten times the strength, making it one of the strongest, lightweight materials known. (Video: Melanie Gonick/MIT)