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Graphene foam helps stiffen the resolve of epoxy for electronic applications

An artists rendition of the composite of graphene foam and epoxy. Image: Rouzbeh Shahsavari Group/Rice University.

Scientists at Rice University have built a better epoxy for electronic applications by combining epoxy with ‘ultrastiff’ graphene foam invented in the Rice lab of chemist James Tour. The resulting composite is substantially tougher than pure epoxy and far more conductive than other epoxy composites while retaining the material's low density. It could improve upon epoxies currently used for electronic applications, which contain conductive fillers that can weaken the material’s structure. The new material is detailed in a paper in .

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 for applications where conductivity is desired, like electromagnetic shielding. But there's a trade-off: 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 atom-thick form of carbon. The Tour lab, in collaboration with Rice materials scientists Pulickel Ajayan, Rouzbeh Shahsavari and Jun Lou, and Yan Zhao of Beihang University in Beijing, China, took their inspiration from projects that inject epoxy into 3D scaffolds like graphene aerogels, foams and skeletons from various processes.

The new scheme makes much stronger scaffolds from polyacrylonitrile (PAN), a powdered polymer resin that the scientists 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 then 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."

Puck-shaped composites with 32% foam were marginally denser than pure epoxy, but had an electrical conductivity of about 14 Siemens (a measure of conductivity, or inverse ohms) per centimeter, according to the researchers. The foam did not add significant weight to the compound, but gave it seven times the compressive strength of pure epoxy.

Easy interlocking between the graphene and epoxy helped stabilize the structure of the graphene 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 said.

The lab upped the ante by also mixing multiwalled carbon nanotubes into the graphene foam. The nanotubes acted as reinforcement bars that bonded with the graphene and made the composite 1732% stiffer than pure epoxy and nearly three times as conductive, at about 41 Siemens per centimeter. This is far greater than nearly all of the 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 said. "But that is done all the time to make large metal parts by cold-pressing and then heating them." He added that 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.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

» Publication Date: 21/11/2018

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This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [609149].

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