Graphene is a fascinating material, consisting of a layer of carbon only one atom thick. As such, it’s not unusual for scientific journals to make it the subject of research articles. However, last month brought two vastly different publications that made an identical, dramatic point about the mechanical properties of graphene. Science featured an article by scientists and engineers at Rice University and the University of Massachusetts-Amherst that described the results of targeting micro-bullets at graphene. And in Marvel Comics’ Superior Ironman #2, someone shot Tony Stark in the face.
In the latest adventures of the armored Avenger, Tony Stark has undergone a serious personality change thanks to a magic spell cast by the Scarlet Witch and Dr. Doom (seriously, don’t ask): he’s reverted to the self-centered arrogance that he exhibited before he escaped terrorists by creating an iron suit of armor out of scraps (waaay back in 1963’s Tales of Suspense #39, an origin story reprised in the 2008 movie Iron Man). Instead of the familiar red and gold suit of armor, he has taken to wearing gleaming all white armor that looks like something available at the Marvel Universe Apple store (iRon Man?). Significantly, this new Ironman suit has no faceplate, in order to make it easy for the public to see Tony Stark’s handsome features. But vanity comes with risks---and sure enough, an assailant that Stark has driven to despair pulls a pistol and shoots the engineer-playboy in the face, point blank.
Fortunately for Stark (and for us fans, who know that our old, more heroic Tony will eventually return), he is unharmed, and reveals to his assailant that his face is actually protected by a thin, transparent sheet of graphene, invisible and yet stronger than steel. Being only one atom thick, graphene passes 97% of visible light, making it more transparent than most glasses, so we can indeed see his face through this thin, carbon “faceplate.”
So graphene, owing to its atomic thinness, is indeed invisible---but could it protect Tony Stark’s mug from a bullet? Sure enough, as if the writer of this comic book had seen an advance copy of that same week’s issue of Science, Jae-Hwang Lee, Phillip E. Loya, Jun Lou and Edwin L. Thomas of Rice and U. Mass., in “Dynamic Mechanical Behavior of Multilayer Graphene via Supersonic Projectile Penetration,” report that thin multi-layers of graphene, no more than a hundred atoms thick, are indeed ten times more “bullet-proof” than steel.
What exactly is graphene and why is it so strong? Graphene is composed of carbon, an element so flexible in its bonding arrangements that it can be considered the yoga master of the periodic table. Carbon can rearrange its electrons to connect with two, three or four other carbon atoms; materials made of carbon will have profoundly different properties depending on the number of other carbon atoms to which they are linked. Carbon can form organic molecules from long chains of atoms---liquid crystals, petroleum, and Kevlar are all examples of this---but it can also arrange its electrons to form bonds with four other atoms at a time, forming a diamond shape (if those four other atoms are carbon, then it’s actual diamond, but four hydrogen atoms would make it methane, and so on).
Because its chemical bonds are so stiff, this diamond structure leads to remarkably hard solids. Take graphite: its carbons lie on a plane, and each carbon atom is bonded to three other carbons, forming a honeycomb of hexagons on the flat surface, with a carbon atom sitting at the corner of each hexagon. Graphite is like a phylo pastry, with thousands of such sheets stacked atop each other.
However, each carbon atom in a given sheet can only form "half-a-bond" to the sheets above and below it, which means that one can easily peel the solid apart, leaving some of the graphite layers behind on the paper. If one had the lightest touch imaginable, the layer you leave behind on the page can be only one atom thick. The optical, electrical and structural properties of this ultra-thin layer are so different from bulk graphite or diamond that it warrants having its own name: graphene.
Graphene is a purely two-dimensional solid, which is the key to its great strength. If the energy of a projectile striking any surface flows away from the impact point fast enough, then the incident energy is dissipated, and not enough energy remains at the impact point to rupture the chemical bonds holding the material together. This redistribution of energy explains why lasers aren’t the death rays blasting holes in walls that we were promised in science fiction tales: the energy of the light is absorbed by the wall and converted to atomic vibrations (that is, sound waves) that flow away from the beam spot, leaving the wall unmarred. In graphene, the speed of sound is very high—more than 22 kilometers per second—which, along with very strong bonds between the carbon atoms in the sheet, makes it a remarkably strong material.
In the experiment detailed in Science, Lee and his colleagues fired tiny glass bullets at high speeds (up to 2,000 mph) at sheets of graphene. These micro-bullets penetrated the sheets, and from measurements of their exit velocity, they determined how much energy was required to create a hole of a given diameter. Their studies found that the energy to puncture layers of graphene is 8-12 times greater than that needed for a comparable mass of steel. The only other material that comes close to graphene in “bullet resistance” is kevlar, which consists of long-chain carbon molecules locked into rigid, layered sheets.
The micro-bullet speeds used in the experimental studies of Lee and co-workers are roughly twice as fast as the muzzle velocity of the small handgun used by Tony Stark’s attacker, and thus it’s not surprising that Stark was unharmed. Iron Man, the prototypical superhero as engineer (and engineer as superhero), would naturally make use of cutting edge materials and advances in technology. Real-world scientists and engineers have only begun to exploit the fascinating properties of graphene which, owing to its lightweight, great strength and transparency, would be an ideal material for constructing an invisible force barrier…as well as Wonder Woman’s invisible jet!
*Jim Kakalios is the Taylor Distinguished Professor in the School of Physics and Astronomy at the University of Minnesota and the author of The Physics of Superheroes and The Amazing Story of Quantum Mechanics, both by Gotham Books.
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