Roll-to-roll manufacturing has become a common industrial approach for continuous processing of thin foils. Recently, the manufacturing approach has been brought to electronics, specifically flexible printed electronics that can be used for a variety of sectors including solar cells, clothing, flexible displays and wearables.
Researchers at MIT have now created a continuous manufacturing process to produce long strips of high-quality graphene that is tailored for use in membranes that filter a variety of molecules including salts, larger ions, proteins and nanoparticles could be used for desalination, biological separation and other applications.
“For several years, researchers have thought of graphene as a potential route to ultrathin membranes,” said John Hart, associate professor of mechanical engineering and director of the Laboratory for Manufacturing and Productivity at MIT. “We believe 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.”
Graphene is useful in filtration membranes as a single sheet of graphene resembles thin chicken wire but composed of carbon atoms joined in a pattern that makes the material extremely tough and impervious to even the smallest atom, helium.
MIT was able to develop the graphene membranes with tiny holes, or nanopores, the size of which can be tailored to filter out specific molecules. Up until this point these graphene-based membranes have been made in small batches where it can be carefully controlled.
However, 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, MIT says.
Enter Roll-to-Roll Manufacturing
MIT decided to use the roll-to-roll approach for the graphene membranes that would allow for large quantities of the material to be produced at a high rate. The system 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 one centimeter wide and as it enters the furnace, the foil is fed through the first tube and then another in a “split-zone” design.
As the foil rolls through the first tube, it heats up to a certain ideal temperature and when it is ready to roll through the second tube scientists pump in a specific ratio of methane and hydrogen gas, which is then 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 said. “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.”
Researchers found that they were able to feed the foil continuously through the system, producing graphene at a rate of five centimeters per minute with the longest run lasting almost four hours and producing 10 meters of continuous graphene.
“If this were in a factory, it would be running 24-7,” Hart said. “You would have big spools of foil feeding through, like a printing press.”
The full research can be found in the journal Applied Materials & Interfaces.