The robust carbon fiber is made possible thanks to the unique properties of graphene oxide flakes created in a process patented by Rice several years ago. The flakes that are chemically extracted from graphite seem small: Their average diameter of 22 microns is approximately one-quarter the width of a human hair. But that’s huge compared with current carbon fiber precursors. For example, mesophase pitch disks are two nanometers, making graphene oxide flakes 10,000 times larger, says Xiang.
Researchers use a wet spinning process to create the carbon fiber. First, they disperse graphene oxide in water at a high concentration to form a gel solution. After mixing the solution in a high-sheer rotating apparatus, Xiang and his peers use an optical microscope to examine the dispersion and the liquid crystal phase formation. “We have to make sure no aggregates exist within the gel solution, which is very important for having a continuous fiber spinning process,” says Xiang. “If any aggregates exist, the gel has to go through a filter to eliminate them. We also make sure that liquid crystal phase was formed in order to spin fibers with good alignment.”
Next, the researchers transfer the gel to the spinning instrument and apply controlled pressure to the piston to push the gel out through a small orifice. Immediately afterward, the fibers go through a coagulation bath (ethyl acetate) to wash away the solvent within the fiber. Then the coagulated fibers are collected by a rotating drum and air dried for 24 hours.
The result is an extremely flexible carbon fiber. “The amazing thing about graphene oxide fiber is that it doesn’t degrade when knotted,” says Xiang. “This has never been seen with other carbon fiber or polymer fibers.” He notes that the tensile strength of commercial carbon fiber degrades 95 percent when knotted. Rice’s graphene oxide-based carbon fiber could prove advantageous for applications that require the fiber to bend or knot.
Xiang says that optimization work on the graphene oxide dispersion, spinning, coagulation and post-heat treatment still needs to be performed. But he’s optimistic. “The results are promising,” he says. “This serves as a proof of concept.” At least one industry supplier agrees: AZ Electronic Materials, a division of Merck, has licensed Rice’s process for making graphene oxide and is now scaling it to the ton scale.