Graphene is strong, but not very tough

New York, Feb 5 (IANS) Though touted as the strongest material known to exist, polycrystalline graphene has quite low resistance to fracture, US scientists, including one of Indian-origin, have found.

Graphene, a material consisting of a single layer of carbon atoms, has been touted as the strongest material known to exist — 200 times stronger than steel, lighter than paper and with extraordinary mechanical and electrical properties.

Yet graphene has remarkably low toughness, or resistance to fracture, researchers at a US laboratory have found.

“This material certainly has very high strength, but it has particularly low toughness — lower than diamond and a little higher than pure graphite,” said Robert Ritchie, a scientist at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

“Its extremely high strength is very impressive, but we can’t necessarily utilise that strength unless it has resistance to fracture,” Ritchie pointed out.

The senior scientist in the Materials Sciences Division of Berkeley Lab together with Ashvini Shekhawat, a Miller Research Fellow, developed a statistical model for the toughness of polycrystalline graphene to better understand and predict failure in the material.

“It’s a mathematical model that takes into account the nanostructure of the material,” Ritchie said, adding: “We find that the strength varies with the grain size up to a certain extent, but most importantly this is a model that defines graphene’s fracture resistance.”

“We simply don’t use strong materials in critical structures — we try to use tough materials. When you look at such a structure, like a nuclear reactor pressure vessel, it’s made of a relatively low-strength steel, not an ultrahigh-strength steel,” the scientist said.

“The hardest steels are used to make tools like a hammer head, but you’d never use them to manufacture a critical structure because of the fear of catastrophic fracture,” he added.

The researcher said that a soccer ball can be placed on a single sheet of monocrystalline graphene without breaking it. But for polycrystalline graphene a soccer ball is much too heavy, and the material can support only a ping pong ball.

“Still remarkable for a one-atom thick material, but not quite as breathtaking anymore,” Ritchie said.

The study was published recently in Nature Communications.

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