A graphene sheet made up of a crystal lattice one atom thick may seem very fragile, but MIT engineers have found conditions under which an ultra-thin material is exceptionally strong while remaining intact when pressures of at least 100 atmospheres are applied. This is about 20 times the pressure in your kitchen faucet.

The researchers found that the key to high strength is the junction of graphene with the porous substrate. More precisely, graphene holds the load the better, the smaller the pores located under it. This pattern has an understandable analogy in the macrocosm – the longer the bridge, the less its strength, other things being equal, are. But in the case of an object one atom thick, this pattern required checking or, at least, specifying the values.

The researchers grew sheets of graphene using a technique called chemical vapor deposition, then placed individual graphene layers on thin sheets of porous polycarbonate. Each backing sheet was fabricated with pore sizes ranging from 30 nanometers to 3 microns in diameter.
The researchers focused their attention on what they called “micromembranes” – the regions of graphene that were directly above the pores. The team placed graphene-polycarbonate membranes in a chamber, into the upper half of which argon was pumped under pressure. It turned out that graphene, placed over pores with a diameter of 200 nanometers, withstood a pressure of 100 atmospheres. In other words, a film of monatomic carbon held 100 atmospheres above a hole, the diameter of which was about three orders of magnitude larger than the thickness of the film itself. Recall that the diameter of a carbon atom is approximately 0.154 nm.

Rohit Karnik, an associate professor in the Faculty of Mechanical Engineering at MIT, says the team’s findings, presented in Nano Letters, could serve to create tough graphene-based membranes, especially for applications such as desalination, where efficient removal of salt from seawater filtration membranes must withstand high pressures.

“We are showing here that graphene can raise the pressure limits for membranes,” says Karnik. “If graphene membranes can be used for high pressure desalination, this opens up many interesting possibilities for energy-efficient desalination at high salinity.”

Currently existing membranes desalinate water using reverse osmosis, a process in which salt water is pumped under pressure from one side of the membrane to keep salt and other “excess” molecules out. The ultimate pressure for many commercial membranes ranges from 50 to 80 atmospheres, and when it is exceeded, the structure quickly degrades. Raising the ultimate pressure to 100 atmospheres or more means an overall increase in desalination performance.

“It is clear that the lack of water sources will not be eliminated in the foreseeable future, and desalination will become the main source of fresh water,” says Karnik. Reverse osmosis is one of the most efficient desalination methods in terms of energy. If the membranes could operate at higher pressures, this would provide better performance at high energy efficiency. ”

“We’re showing that graphene can withstand high pressure,” says lead author Luda Wang. Another part of the experiment, which has yet to be carried out, will answer the question of whether we can desalinate the water. “