Previously, researchers could only print graphene in 2D sheets or basic structures. Now, Virginia Tech engineers have now collaborated on a project with colleagues at Lawrence Livermore National Laboratory that allows them to 3D print graphene objects at a resolution that is claimed to be an order of magnitude greater than ever before printed. This, they say, unlocks the ability to theoretically create any size or shape of graphene. The research is published in Materials Horizons.
Because of its strength and its high thermal and electrical conductivity, 3D printed graphene objects would be highly coveted for use in batteries, aerospace, separation, heat management, sensors and catalysis.
Graphene is a single layer of carbon atoms organised in a hexagonal lattice. When graphene sheets are stacked on top of each other and formed into a three-dimensional shape, it becomes graphite, which has poor mechanical properties. But if the graphene sheets are separated with air-filled pores, the three-dimensional structure can maintain its properties. This porous graphene structure is called a graphene aerogel.
"Now a designer can design three-dimensional topology comprised of interconnected graphene sheets," said Xiaoyu "Rayne" Zheng, assistant professor with Virginia Tech’s Department of Mechanical Engineering in the College of Engineering and director of the Advanced Manufacturing and Metamaterials Lab. "This new design and manufacturing freedom will lead to optimisation of strength, conductivity, mass transport and weight density that are not achievable in graphene aerogels."
Previously, researchers could print graphene using an extrusion process. "With that technique, there's very limited structures you can create because there's no support and the resolution is quite limited, so you can't get freeform factors," Zheng said. "What we did was to get these graphene layers to be architected into any shape that you want with high resolution."
To create these complex 3D structures, lead author Ryan Hensleigh started with graphene oxide, a precursor to graphene, crosslinking the sheets to form a porous hydrogel. Breaking the graphene oxide hydrogel with ultrasound and adding light-sensitive acrylate polymers, Hensleigh could use projection micro-stereolithography to create the desired solid 3D structure with the graphene oxide trapped in the long, rigid chains of acrylate polymer. Finally, Hensleigh would place the 3D structure in a furnace to burn off the polymers and fuse the object together, leaving behind a pure and lightweight graphene aerogel.
"It's a significant breakthrough compared to what's been done," Hensleigh said. "We can access pretty much any desired structure you want." According to Virginia Tech, the key finding of this work is that the team created graphene structures with a resolution an order of magnitude finer than ever printed. Hensleigh said other processes could print down to 100 microns, but the new technique allows him to print down to 10 microns in resolution, which approaches the size of actual graphene sheets.
"We've been able to show you can make a complex 3D architecture of graphene while still preserving some of its intrinsic prime properties," Zheng said. "Usually when you try to 3D print graphene or scale up, you lose most of their lucrative mechanical properties found in its single sheet form."
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