Porous, 3-D forms of graphene developed at MIT can be 10 times as strong as steel but much lighter

January 6, 2017
A three-dimensional graphene assembly and scanning electron microscope image of a graphene assembly (insert, scale bar, 20 µm). Credit: Qin et al. Sci. Adv. 2017;3:e1601536

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, 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.

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 recent graduate.

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.

The closely packed graphene-inclusion structure obtained after cyclic equilibrations. Credit:Qin et al. Sci. Adv. 2017;3:e1601536

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.

Tensile and compressive tests on the printed sample. Credit: Qin et al. Sci. Adv. 2017;3:e1601536

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.

Model of gyroid graphene with 20 nm length constant. Credit: Qin et al. Sci. Adv. 2017;3:e1601536

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 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.

Explore further: New study shows nickel graphene can be tuned for optimal fracture strength

More information: "The mechanics and design of a lightweight three-dimensional graphene assembly," Science Advances, DOI: 10.1126/sciadv.1601536 , advances.sciencemag.org/content/3/1/e1601536

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ta2025
1 / 5 (1) Jan 06, 2017
I wrote about this very substance in a novel last year. I'm not a scientist and I know you cannot have a 100% pure vacuum, but I disagree that we could not evacuate a chamber ENOUGH using this technique to build blocks that were lighter than air. Check out my take on the issue. My book is called "The windos of Aerathiea" and it can be found on Amazon.

https://www.amazo...f=sr_1_1
Whydening Gyre
not rated yet Jan 06, 2017
No points for trying, Ta2025, but you spelled "winds" wrong.
C'mon - misspelling the title of your own book?
As to your hypothesis of "lighter than air" - you would need to build a structure sufficiently dense enough to be sturdy enough (even at the most geometrically efficient design) to hold out the massed pressure of all other surrounding elements of "air".
Couldn't do it at sea level, that's for sure.
Oh. And notice the artiicle is more about POROSITY...
ta2025
not rated yet Jan 06, 2017
I was excited and typing fast. I would edit it if I could. I believe between this structure and graphene sheeting, it could be accomplished.
BubbaNicholson
not rated yet Jan 06, 2017
Unobtainium exists somewhere, but we have to look for it to find it, don't we? For the sake of argument, we might utilize helium as a foam injection, tiny bubbles, around which to assemble Unobtainium from graphene, eh? A mold injected with helium to produce an object of graphene would revolutionize the aviation industry, the construction industry, and many others. I'm not a materials scientist (medical), but I did invent the concept of Unobtainium for Avatar. Great work at MIT, congratulations.
rrrander
not rated yet Jan 07, 2017
This is too crude. They need to adapt graphene to aerogel for this to progress. Foamed metal is so expensive that it has amounted to almost nothing so far.
TheGhostofOtto1923
not rated yet Jan 07, 2017
How cheap might this become? Mile high skyscrapers? Freespan bridge across the straits of gibralter? Huge spherical orbital gas and liquid reservoirs? O'neill cylinders?
winthrom
not rated yet Jan 07, 2017
"Lighter than air" is relative. We live in a sea of air that thins as we ascend. "Lighter than air at sea level" is better. BUT not important!

As currently described in the article, structural parts for small aircraft seem very likely via large scale 3D printers. Many important parts in a typical aircraft are made of >>solid<< Al alloys to provide strength similar to steel. The weight loss still is a penalty of weight and cost. The small aircraft industries make aircraft priced from about $185K to $8M and represent a $109.3B contribution to the economy in 2013. https://www.nbaa....nomy.pdf

A similar argument can be made for the auto industry because the lighter a car is, the less fuel/power it needs to accelerate. Acceleration is the main cause of fuel consumption after long distance driving, Tesla should look into this as a range extender.

The team at MIT should look at the markets I note here.
Mark Thomas
not rated yet Jan 07, 2017
"one of our samples has 5 percent the density of steel, but 10 times the strength"

Regarding its tensile strength to density ratio, at potentially 200 times better than steel, I am thinking prospects for a space elevator are looking better all the time. In Kim Stanley Robinson's Mars trilogy, the most advanced state of Mars travel included a ride up a space elevator from Earth, transportation on a 1,000 passenger fusion-powered spacecraft to Mars in a couple weeks, and a ride down another elevator cable to the surface of Mars. Everything practical, reusable and extremely cost-effective. That vision always had a certain appeal.
winthrom
not rated yet Jan 08, 2017
A previous article on phys.org asked is graphene tough not just tensile strength strong:

http://phys.org/n...tml#nRlv

If it isn't tough, then it isn't very useful
Mark Thomas
not rated yet Jan 08, 2017
Can it be made strong enough to be coated or at least include toughening agents (of some undefined type) in outer, exposed portions? In other words, maybe there is a way to overcome the lack of toughness.
winthrom
not rated yet Jan 09, 2017
Mark T,
I do not know. Perhaps embedded in a resin the way carbon fibers are treated. The shatter problem could be fatal though.
smokeyjoe5761
not rated yet Jan 16, 2017
Basically these guys have discovered charcoal

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