Scientists at Rice University (Houston, Texas, USA) have built a new epoxy for electronic applications by combining it with graphene foam to make a tough, conductive composite.
According to the school, their epoxy combined with “ultra-stiff” graphene foam invented in the lab of chemistry professor James Tour has proven to be tougher than pure epoxy and more conductive than other epoxy composites, all while retaining its low density. They say it could improve upon epoxies in current use by adding conductive fillers.
By itself, epoxy is an insulator, and is commonly used in coatings, adhesives, electronics, industrial tooling, and structural composites. Metal or carbon fillers are often added when conductivity is desired, like in electromagnetic shielding. But the fillers bring better conductivity at the cost of weight and compressive strength, and the composite becomes harder to process.
To address this, the Rice solution replaces metal or carbon powders with a three-dimensional foam made of nanoscale sheets of graphene, the atom-thick form of carbon. The new scheme makes scaffolds from polyacrylonitrile (PAN), a powdered polymer resin used as a carbon source, 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 says. “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."
The puck-shaped composites with 32% foam were marginally denser, but had an electrical conductivity of about 14 S/cm. The foam did not add significant weight, but gave it seven times the compressive strength of pure epoxy. Easy interlocking with the epoxy helped stabilize the graphene’s structure, as well. “When the epoxy infiltrates the graphene foam and then hardens, the epoxy is captured in micron-sized domains of the graphene foam,” Tour says.
The team then mixed multiwalled carbon nanotubes into the graphene foam. The nanotubes acted as reinforcement bars, bonding with the graphene to make the composite 1,732% stiffer than pure epoxy and nearly three times as conductive, at about 41 S/cm. This is greater than most scaffold-based epoxy composites reported to date, according to the researchers.
Tour expects the process will scale for industry. “One just needs a furnace large enough to produce the ultimate part,” he says. “But that is done all the time to make large metal parts by cold-pressing and then heating them.” Tour believes the material could initially replace the carbon-composite resins used to pre-impregnate and reinforce fabric used in various materials.
Source: Rice University Department of Materials Science and NanoEngineering, msne.rice.edu.