05 December 2014
Graphene's mighty strength confirmed
Graphene's exceptional strength has been confirmed by a team of Rice University researchers, who have tested the material's properties by peppering it with microbullets.
Led by materials scientist Edwin Thomas, the team used less than 1lb of graphene to prove that the material is on average 10 times better than steel at dissipating kinetic energy.
Microscopic projectiles were fired at multilayer sheets of graphene, which allowed the scientists to determine just how hard it is to penetrate at the nano level – and how strong graphene could be in macroscopic applications.
The researchers tested sheets ranging from 10 to 100nm thick (up to 300 graphene layers). They then used a high speed camera to capture images of the projectiles before and after hits to judge their speed and viewed microscope images of the damage to the sheets.
In every case, the 3.7µm spheres punctured the graphene. But rather than a neat hole, the spheres left a fractured pattern of 'petals' around the point of impact, indicating the graphene stretched before breaking.
"We started writing the paper about the petals, but as we went along, it became evident that wasn't really the story," said Thomas. "The bullet's kinetic energy interacts with the graphene, pushes forward, stretches the film and is slowed down."
The experiments revealed graphene to be a stretchy membrane that, in about 3ns before puncture, distributes the stress of the bullet over a wide area defined by a shallow cone centered at the point of impact. Tensile stress cannot travel faster than the speed of sound in materials, and in graphene, it's much faster than the speed of sound in air (1,125ft/s).
"For graphene, we calculated the speed at 22.2km/s, which is higher than any other known material," Thomas continued.
As a microbullet impacts the graphene, the diameter of the cone it creates – determined by later examination of the petals – provides a way to measure how much energy the graphene absorbs before breaking.
"The game in protection is getting the stress to distribute over a large area," Thomas said. "It's a race. If the cone can move out at an appreciable velocity compared with the velocity of the projectile, the stress isn't localized beneath the projectile."
Looking ahead, the researchers believe controlled layering of graphene sheets could lead to lightweight, energy absorbing materials used in body armour or for shielding spacecraft.
"We're currently working to demonstrate to NASA and the military that these microscopic tests are relevant to macroscopic properties," Thomas concluded.