May 28, 2014 |
Supersonic spray delivers high quality graphene layer
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(Nanowerk News) A simple, inexpensive spray method that deposits a graphene film can heal manufacturing defects and produce a high quality graphene layer on a range of substrates, report researchers at the University of Illinois at Chicago and Korea University.
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Their study is available online in the journal Advanced Functional Materials ("Self-Healing Reduced Graphene Oxide Films by Supersonic Kinetic Spraying").
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Using a supersonic spray, graphene flakes with deformed pentagonal and heptagonal structures stretch on impact and spring into a perfect hexagonal graphene lattice. This opens the way to scale up from the microscopic to large scale applications. (click image to enlarge)
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Graphene, a two-dimensional wonder-material composed of a single layer of carbon atoms, is strong, transparent, and an excellent conductor of electricity. It has potential in a wide range of applications, such as reinforcing and lending electrical properties to plastics; creating denser and faster integrated circuits; and building better touch screens.
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Although the potential uses for graphene seem limitless, there has been no easy way to scale up from microscopic to large-scale applications without introducing defects, says Alexander Yarin, UIC professor of mechanical and industrial engineering and co-principal investigator on the study.
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"Normally, graphene is produced in small flakes, and even these small flakes have defects," Yarin said. Worse, when you try to deposit them onto a large-scale area, defects increase, and graphene's useful properties — its "magic" — are lost, he said.
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Yarin first turned to solving how to deposit graphene flakes to form a consistent layer without any clumps or spaces. He went to Sam S. Yoon, professor of mechanical engineering at Korea University and co-principal investigator on the study.
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Yoon had been working with a unique kinetic spray deposition system that exploits the supersonic acceleration of droplets through a Laval nozzle. Although Yoon was working with different materials, Yarin believed his method might be used to deposit graphene flakes into a smooth layer.
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Their supersonic spray system produces very small droplets of graphene suspension, which disperse evenly, evaporate rapidly, and reduce the tendency of the graphene flakes to aggregate.
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But to the researchers' surprise, defects inherent in the flakes themselves disappeared, as a by-product of the spray method. The result was a higher quality graphene layer, as found in the analysis by another collaborator, Suman Sinha-Ray, senior researcher at United States Gypsum and UIC adjunct professor of mechanical and industrial engineering.
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The researchers demonstrated that the energy of the impact stretches the graphene and restructures the arrangement of its carbon atoms into the perfect hexagons of flawless graphene.
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"Imagine something like Silly Putty hitting a wall — it stretches out and spreads smoothly," said Yarin. "That's what we believe happens with these graphene flakes. They hit with enormous kinetic energy, and stretch in all directions.
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"We're tapping into graphene's plasticity — it's actually restructuring."
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Other attempts to produce graphene without defects or to remove flaws after manufacture have proved difficult and prohibitively expensive, Yarin said.
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The new method of deposition, which allows graphene to "heal" its defects during application, is simple, inexpensive, and can be performed on any substrate with no need for post-treatment, he said.
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Yarin and his Korean colleagues hope to continue their successful collaboration and foster the development of industrial-scale applications of graphene.
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