Engineers at MIT have developed a manufacturing process that continuously produces long strips of graphene for the first time.
The team at MIT said the results represent the first demonstration of "an industrial, scalable method for manufacturing high-quality graphene" that is tailored for use in membranes that filter a variety of molecules, including salts, larger ions, proteins, or nanoparticles.
Such membranes should be useful for desalination, biological separation, and other applications, the team said.
"For several years, researchers have thought of graphene as a potential route to ultrathin membranes," said John Hart, the associate professor of mechanical engineering and director of the Laboratory for Manufacturing and Productivity at the university.
Hart, who is the senior author on the paper - which appears online in the journal Applied Materials and Interfaces - said it is the first study that has tailored the manufacturing of graphene toward membrane applications, which require the graphene to be seamless, cover the substrate fully, and be of high quality.
MIT said graphene is ideal for use in filtration membranes as a single sheet of it resembles atomically thin chicken wire. It is composed of carbon atoms joined in a pattern that makes the material extremely tough and impervious to even the smallest atom, helium.
Researchers have developed techniques to fabricate graphene membranes and precisely riddle them with tiny holes, or nanopores, the size of which can be tailored to filter out specific molecules.
Scientists usually synthesise graphene through a process called chemical vapor deposition, in which they first heat a sample of copper foil and then deposit onto it a combination of carbon and other gases. Graphene-based membranes have mostly been made in small batches in laboratories, where researchers can carefully control the material's growth conditions.
However, Hart and his colleagues believe that if graphene membranes are ever to be used commercially they will have to be produced in large quantities, at high rates, and with reliable performance.
"We know that for industrialisation, it would need to be a continuous process," Hart said. "You would never be able to make enough by making just pieces. And membranes that are used commercially need to be fairly big - some so big that you would have to send a poster-wide sheet of foil into a furnace to make a membrane."
The researchers therefore set out to build an end-to-end, start-to-finish manufacturing process to make membrane-quality graphene. The setup combined a roll-to-roll approach for continuous processing of thin foils with the common graphene-fabrication technique of chemical vapor deposition, to manufacture the graphene in large quantities and at a high rate.
The system consisted of two spools, connected by a conveyor belt that runs through a small furnace. The first spool unfurls a long strip of copper foil, less than 1cm wide. When it enters the furnace, the foil is fed through first one tube and then another, in a "split-zone" design.
While the foil rolls through the first tube, it heats up to a certain ideal temperature, at which point it is ready to roll through the second tube, where the scientists pump in a specified ratio of methane and hydrogen gas, which are deposited onto the heated foil to produce graphene.
"Graphene starts forming in little islands, and then those islands grow together to form a continuous sheet," Hart says. "By the time it's out of the oven, the graphene should be fully covering the foil in one layer, kind of like a continuous bed of pizza."
The researchers found that they were able to feed the foil continuously through the system, producing high-quality graphene at a rate of 5cm per minute. Their longest run lasted almost four hours, during which they produced about 10 meters of continuous graphene.
"If this were in a factory, it would be running 24-7," Hart says. "You would have big spools of foil feeding through, like a printing press."
Looking forward, Hart says he wants to find ways to include polymer casting and other steps that currently are performed by hand, in the roll-to-roll system.
"In the end-to-end process, we would need to integrate more operations into the manufacturing line," he added. "For now, we've demonstrated that this process can be scaled up, and we hope this increases confidence and interest in graphene-based membrane technologies, and provides a pathway to commercialisation."
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