MIT makes compressed graphene sponge material that is 20 times less dense than steel but 10 times stronger, light as stryofoam but stronger than steel

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 new material, a sponge-like configuration with a density of just 5 percent, can have a strength 10 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 new findings show that the crucial aspect of the new 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 variety of materials by creating similar geometric features.

Highlights

* light as stryofoam but stronger than steel
* 20 times less dense than steel but 10 times stronger
* could lead to a replacement for helium for strong yet light dirigible applications
* bringing 2D strength of graphene to 3D materials
* new range of lightness and strength combinations for different applications

Different atomistic and 3D-printed models of gyroid geometry for mechanical tests.(A) Simulation snapshots taken during the modeling of the atomic 3D graphene structure with gyroid geometry, representing key procedures including (i) generating the coordinate of uniformly distributed carbon atoms based on the fcc structure, (ii) generating a gyroid structure with a triangular lattice feature, and (iii) refinement of the modified geometry from a gyroid with a triangular lattice to one with a hexagonal lattice. (B) Five models of gyroid graphene with different length constants of L = 3, 5, 10, 15, and 20 nm from left to right. Scale bar, 2.5 nm. (C) 3D-printed samples of the gyroid structure of various L values and wall thicknesses. Scale bar, 2.5 cm. The tensile and compressive tests on the 3D-printed sample are shown in (D) and (E), respectively.

Science Advances – The mechanics and design of a lightweight three-dimensional graphene assembly

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 decided 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 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 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 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,” says Qin. To do that, they created a variety 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 5 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 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.

The new 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 new, 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 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,” 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 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, coat them with graphene by chemical vapor deposit before heat and pressure treatments, and then chemically or physically remove the polymer or metal phases to leave 3-D graphene in the gyroid form. For this, the computational model given in the current study 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 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 is an inspiring study 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 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 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.”

Discussion

3D carbon materials of desired mechanical properties by design are useful for multifunctional engineering applications. Our results reveal that, by designing the chemical synthesizing process, especially the reacting conditions, including pressure and temperature, we can fuse graphene flakes and produce stable 3D porous bulk materials with material architecture and density under control. By fully characterizing the material properties, we can derive scaling laws, which can be used to achieve a set of target bulk material properties. We have demonstrated that the scaling laws derived from the 3D graphene assembly can be used to design and predict the mechanical properties of a pristine 3D graphene structure as well as 3D-printed models with gyroid geometries and a wide density distribution, showing that the material architecture features, instead of the mechanics of its constituting materials, play a more dominant role in governing the scaling laws. As shown in the scaling law, higher density provides better mechanics. Also, the connectivity between flakes is critical for the mechanical properties. As shown in the construction of the 3D graphene assembly, refining the structure based on annealing may improve its mechanical properties. The study provides a simple way to predict the mechanics of graphene aerogels, and using graphene aerogels of different densities as building blocks can be helpful in designing larger-scale structures. This conclusion can be generally applied to guide designs of structural materials of diverse functions by using universal constituting materials.

With the scaling law, we can directly answer the question asked earlier: Can we use graphene as a building block to form a 3D bulk material that is strong yet lighter than air? In particular, can this material be designed and synthesized as a substitute for helium in unpowered flight balloons? This requires the material to satisfy σC over 1 atm and ρ less than 1.16 mg/cm3 simultaneously. According to the scaling law, these two conditions cannot be satisfied simultaneously because σC over 1 atm yields ρ over 28.7 mg / cm3 and ρ less than 1.16 mg/cm3 yields σC less than 6.39 × 10^−5 atm, suggesting either filling gas with a lower density or implementing structural materials as necessary conditions for these applications.

Although the porous graphene assembly can likely (but not directly) substitute helium, its material features, including its ultralight nature, outstanding mechanical properties, high surface area, and stable chemical and thermal properties, remain promising for many engineering applications, making products lighter and stronger, which can thereby play a profound game-changing role in broad industrial areas. Using the knowledge learned from the current study that the natural curved 2D surface of graphene is disadvantageous to the mechanics of the 3D assembly, we are working toward further designing and optimizing the structure of these porous materials by tuning the surface chemistry of graphene and combining the 2D material with other polymers for a more efficient use of the material and to derive improved mechanical scaling laws. The combination of a theoretical model and computational simulations provides a powerful tool to explore these opportunities for carbon material designs.

Abstract

Recent advances in three-dimensional (3D) graphene assembly have shown how we can make solid porous materials that are lighter than air. It is plausible that these solid materials can be mechanically strong enough for applications under extreme conditions, such as being a substitute for helium in filling up an unpowered flight balloon. However, knowledge of the elastic modulus and strength of the porous graphene assembly as functions of its structure has not been available, preventing evaluation of its feasibility. We combine bottom-up computational modeling with experiments based on 3D-printed models to investigate the mechanics of porous 3D graphene materials, resulting in new designs of carbon materials. Our study reveals that although the 3D graphene assembly has an exceptionally high strength at relatively high density (given the fact that it has a density of 4.6% that of mild steel and is 10 times as strong as mild steel), its mechanical properties decrease with density much faster than those of polymer foams. Our results provide critical densities below which the 3D graphene assembly starts to lose its mechanical advantage over most polymeric cellular materials.

SOURCES- MIT News, Science Advances

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