The advance from Rice University in Texas combines epoxy with graphene foam invented in the lab of chemist James Tour and could improve on current epoxies that weaken the material's structure with the addition of conductive fillers. The new material is detailed in ACS Nano.
Epoxy is an insulator used in coatings, adhesives, electronics, industrial tooling and structural composites. Metal or carbon fillers can be added for applications where conductivity is desired, but more filler brings better conductivity at the cost of weight and compressive strength, and the composite becomes harder to process.
The Rice solution replaces metal or carbon powders with a three-dimensional foam made of nanoscale sheets of graphene.
The Tour lab, in collaboration with Rice materials scientists Pulickel Ajayan, Rouzbeh Shahsavari and Jun Lou and Yan Zhao of Beihang University in Beijing, took their inspiration from projects to inject epoxy into 3D scaffolds including graphene aerogels, foams and skeletons from various processes.
The new scheme is said to make much stronger scaffolds from polyacrylonitrile (PAN), a powdered polymer resin they use as a source of carbon, mixed with nickel powder. In the four-step process, they cold-press the materials to make them dense, heat them in a furnace to turn the PAN into graphene, chemically treat the resulting material to remove the nickel and use a vacuum to pull the epoxy into the now-porous material.
"The graphene foam is a single piece of few-layer graphene," Tour said. "Therefore, in reality, the entire foam is one large molecule. When the epoxy infiltrates the foam and then hardens, any bending in the epoxy in one place will stress the monolith at many other locations due to the embedded graphene scaffolding. This ultimately stiffens the entire structure."
According to the researchers, the puck-shaped composites with 32 per cent foam were marginally denser but had an electrical conductivity of about 14SI per centimetre. The foam did not add significant weight to the compound but did give it seven times the compressive strength of pure epoxy.
Easy interlocking between the graphene and epoxy helped stabilise the structure of the graphene too. "When the epoxy infiltrates the graphene foam and then hardens, the epoxy is captured in micron-sized domains of the graphene foam," Tour said.
The lab then mixed multiwalled carbon nanotubes into the graphene foam. The nanotubes acted as reinforcement bars that bonded with the graphene and made the composite 1,732 per cent stiffer than pure epoxy and nearly three times as conductive, at about 41SI per centimetre, far greater than nearly all of the scaffold-based epoxy composites reported to date, according to the researchers.
Tour believes the process will scale for industry. "One just needs a furnace large enough to produce the ultimate part," he said. "But that is done all the time to make large metal parts by cold-pressing and then heating them."
He said the material could initially replace the carbon-composite resins used to pre-impregnate and reinforce fabric used in materials from aerospace structures to tennis rackets.
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