For several years, researchers have thought of graphene as a potential route to ultrathin membranes. MIT scientists now demonstrated 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.
Scientists have developed a continuous manufacturing process that produces long strips of high-quality graphene. According to scientists, this 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.
Scientists fabricated graphene membranes and drilled them with tiny holes to filter out specific molecules. They even synthesized graphene with chemical vapor deposition, where they heated a sample of copper foil and then deposit onto it a combination of carbon and other gases.
To do so, they used a system which consists 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 1 centimeter wide. When it enters the furnace, the foil is fed through first one tube and then another, in a “split-zone” design.
John Hart, associate professor of mechanical engineering and director of the Laboratory for Manufacturing and Productivity at MIT said, “We know that for industrialization, it would need to be a continuous process. 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 team’s setup combines a roll-to-roll approach — a common industrial approach for continuous processing of thin foils — with the common graphene-fabrication technique of chemical vapor deposition, to manufacture high-quality graphene in large quantities and at a high rate.
When the foil starts rolling 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.
Hart while explaining the process: “Graphene starts forming in little islands, and then those islands grow together to form a continuous sheet. 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.”
“As the graphene exits the furnace, it’s rolled onto the second spool. The researchers found that they were able to feed the foil continuously through the system, producing high-quality graphene at a rate of 5 centimeters 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. You would have big spools of foil feeding through, like a printing press.”
After the graphene is created, scientists unwound it cut small samples out. They then cast the samples along with polymer mesh and later etched away from the underlying copper.
When diffusing membranes in several trails, scientists found that the membranes were able to withstand the flow while filtering out molecules. Their performance was comparable to graphene membranes made using conventional, small-batch approaches.
The team also ran the process at different speeds, with different ratios of methane and hydrogen gas, and characterized the quality of the resulting graphene after each run. They drew up plots to show the relationship between graphene’s quality and the speed and gas ratios of the manufacturing process.
The study includes first author Piran Kidambi, a former MIT postdoc who is now an assistant professor at Vanderbilt University; MIT graduate students Dhanushkodi Mariappan and Nicholas Dee; Sui Zhang of the National University of Singapore; Andrey Vyatskikh, a former student at the Skolkovo Institute of Science and Technology who is now at Caltech; and Rohit Karnik, an associate professor of mechanical engineering at MIT.
Kidambi said, “The system gives you a great degree of flexibility in terms of what you’d like to tune graphene for, all the way from electronic to membrane applications.”
Scientists are now searching for the ways to include polymer casting and other steps that currently are performed by hand, in the roll-to-roll system.
The paper appears online in the journal Applied Materials and Interfaces.