Strong, Super-Tough Carbon Sheets Fabricated at Low Temperature

High-strength, super-tough sheets of carbon have been developed by an international research team led by scientists at Beihang University in China and The University of Texas at Dallas. These sheets can be economically fabricated at low temperatures.

This false-color, scanning electron microscope image shows the fractured surface of a sequentially bonded graphene sheet. An international research team led by scientists at Beihang University in China and The University of Texas at Dallas developed the high-strength, super-tough sheets, which can be inexpensively fabricated at low temperatures. (Image credit: Image courtesy Beihang University)

The sheets were created by chemically stitching together platelets of graphitic carbon, which is quite like the graphite found in the soft lead of a standard pencil. The fabrication process brought about a material whose mechanical properties surpassed those of carbon fiber composites that are presently used in various commercial products.

"These sheets might eventually replace the expensive carbon fiber composites that are used for everything from aircraft and automobile bodies to windmill blades and sports equipment," said Dr. Ray Baughman, the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas and director of the Alan G. MacDiarmid NanoTech Institute. Baughman is a corresponding author of an article detailing the material published online recently in the Proceedings of the National Academy of Sciences.

Presently, carbon fiber composites are costly in part because the carbon fibers are fabricated at very high temperatures, which can exceed 2,500 °C (about 4,500 °F).

In contrast, our process can use graphite that is cheaply dug from the ground and processed at temperatures below 45 degrees Celsius (113 degrees Fahrenheit). The strengths of these sheets in all in-plane directions match that of plied carbon fiber composites, and they can absorb much higher mechanical energy before failing than carbon fiber composites.

Dr. Qunfeng Cheng, Professor of Chemistry at Beihang University & Corresponding Author

Graphite is made up of platelets composed of stacked layers of graphene. Graphene is just a single layer of carbon atoms, set in a pattern that resembles a chicken wire mesh fence, where each hexagon in the mesh is created by six carbon atoms.

While scientists can continuously make large sheets of graphene by high-temperature processing, and have shown these sheets to have remarkable strength, it is impractical to make thick plates of graphite by merely stacking large-area graphene sheets. One would need to stack about 150,000 graphene sheets to make a graphite sheet having about the thickness of a human hair.

Dr. Qunfeng Cheng, Professor of Chemistry at Beihang University & Corresponding Author

The research team drew inspiration from natural nacre, also called mother-of-pearl, which gives certain seashells their toughness and strength. Nacre is made up of parallel platelets that are bound together by thin layers of organic material, quite like the way bricks in a wall are sealed together by mortar.

"Instead of mechanically stacking large-area graphene sheets, we oxidize micron-size graphite platelets so that they can be dispersed in water, and then filter this dispersion to inexpensively make sheets of oriented graphene oxide," Baughman said. "This process is akin to hand-making sheets of paper by filtering a slurry of fibers.

"At this stage, the sheets are neither strong nor tough, meaning they cannot absorb much energy before rupturing," he said. "The trick we use is to stitch together the platelets in these sheets using sequentially infiltrated bridging agents that interconnect overlapping neighboring platelets, and convert the oxidized graphene oxide to graphene. The key to this advance is that our bridging agents separately act via formation of covalent chemical bonds and van der Waals bonds."

Sheets that integrated the bridging agents were 7.9 times tougher and 4.5 times stronger than agent-free sheets, said Beihang University PhD student Sijie Wan, who is the journal article’s lead author. "Unlike carbon fiber composites, no polymer matrix is needed," he said.

While sheets of expensive carbon fiber composites can provide a similar strength in all sheet-plane directions, the energy that they can absorb before fracture is about one-third that of our sequentially bridged graphene sheets. Because our sheets are fabricated at low temperatures, they are low cost. In addition to exhibiting high sheet strength, toughness and fatigue resistance, they have high electrical conductivity and are able to shield against electromagnetic radiation. These properties make these sequentially bridged graphene sheets quite attractive for possible future applications.

Sijie Wan, PhD Student at Beihang University

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