{"293851":{"#nid":"293851","#data":{"type":"news","title":"As Strong as its Weakest Link: Experiments Determine Real-world Limits of Graphene","body":[{"value":"\u003Cp\u003EThere is no disputing graphene is strong. But new research by Rice University and the Georgia Institute of Technology should prompt manufacturers to look a little deeper as they consider the miracle material for applications.\u003C\/p\u003E\u003Cp\u003EThe atom-thin sheet of carbon is touted not just for its electrical properties but also for its physical strength and flexibility. The bonds between carbon atoms are well known as the strongest in nature, so a perfect sheet of graphene should withstand just about anything. Reinforcing composite materials is among the material\u2019s potential applications.\u003C\/p\u003E\u003Cp\u003EBut materials scientists know perfection is hard to achieve. Researchers Jun Lou at Rice and \u003Ca href=\u0022https:\/\/www.me.gatech.edu\/faculty\/t_zhu\u0022\u003ETing Zhu\u003C\/a\u003E at Georgia Tech have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. While it\u0027s still very useful, graphene is really only as strong as its weakest link, which they determined to be \u0022substantially lower\u0022 than the intrinsic strength of graphene.\u003C\/p\u003E\u003Cp\u003E\u201cGraphene has exceptional physical properties, but to use it in real applications, we have to understand the useful strength of large-area graphene, which is controlled by the fracture toughness,\u201d said Zhu, who is an associate professor in the \u003Ca href=\u0022http:\/\/www.me.gatech.edu\/\u0022\u003EWoodruff School of Mechanical Engineering\u003C\/a\u003E at Georgia Tech.\u003C\/p\u003E\u003Cp\u003EThe researchers reported in the journal \u003Cem\u003ENature Communications\u003C\/em\u003E the results of tests in which they physically pulled graphene apart to see how much force it would take. Specifically, they wanted to see if graphene follows the century-old Griffith theory that quantifies the useful strength of brittle materials.\u003C\/p\u003E\u003Cp\u003EIt does, said Lou. \u0022Remarkably, in this case, thermodynamic energy still rules,\u0022 he said.\u003C\/p\u003E\u003Cp\u003EImperfections in graphene drastically lessen its strength \u2013 with an upper limit of about 100 gigapascals (GPa) for perfect graphene previously measured by nanoindentation \u2013 according to physical testing at Rice and molecular dynamics simulations at Georgia Tech. That\u0027s important for engineers to understand as they think about using graphene for flexible electronics, composite material and other applications in which stresses on microscopic flaws could lead to failure.\u003C\/p\u003E\u003Cp\u003EThe Griffith criterion developed by a British engineer during World War I describes the relationship between the size of a crack in a material and the force required to make that crack grow. Ultimately, A.A. Griffith hoped to understand why brittle materials fail.\u003C\/p\u003E\u003Cp\u003EGraphene, it turns out, is no different from the glass fibers Griffith tested.\u003C\/p\u003E\u003Cp\u003E\u0022Everybody thinks the carbon-carbon bond is the strongest bond in nature, so the material must be very good,\u0022 Lou said. \u0022But that\u0027s not true anymore, once you have those defects. The larger the sheet, the higher the probability of defects. That\u0027s well known in the ceramic community.\u0022\u003C\/p\u003E\u003Cp\u003EA defect can be as small as an atom missing from the hexagonal lattice of graphene. But for a real-world test, the researchers had to make a defect of their own \u2013 a pre-crack \u2013 they could actually see. \u0022We know there will be pinholes and other defects in graphene,\u0022 he said. \u0022The pre-crack overshadows those defects to become the weakest spot \u2013 so I know exactly where the fracture will happen when we pull it.\u003C\/p\u003E\u003Cp\u003E\u0022The material resistance to the crack growth \u2013 the fracture toughness \u2013 is what we\u0027re measuring here, and that\u0027s a very important engineering property,\u0022 he said.\u003C\/p\u003E\u003Cp\u003EJust setting up the experiment required several years of work to overcome technical difficulties, Lou said. To suspend it on a tiny cantilever spring stage similar to an atomic force microscopy (AFM) probe, a graphene sheet had to be clean and dry so it would adhere (via van der Waals force) to the stage without compromising the stage movement necessary for the testing. Once mounted, the researchers used a focused ion beam to cut a pre-crack less than 10 percent of the width into the microns-wide section of suspended graphene. Then they pulled the graphene in half, measuring the force required.\u003C\/p\u003E\u003Cp\u003EWhile the Rice team was working on the experiment, Zhu and his team performed computer simulations to understand the entire fracture process.\u003C\/p\u003E\u003Cp\u003E\u201cWe can directly simulate the whole deformation process by tracking the motion and displacement with atomic-scale resolution in fairly large samples so our results can be directly correlated with the experiment,\u201d said Zhu. \u201cThe modeling is tightly coupled with the experiments.\u201d\u003C\/p\u003E\u003Cp\u003EThe combination of modeling and experiment provides a level of detail that allowed the researchers to better understand the fracture process \u2013 and the tradeoff between toughness and strength in the graphene. What the scientists have learned in the research points out the importance of fabricating high quality graphene sheets without defects \u2013 which could set the stage for fracture.\u003C\/p\u003E\u003Cp\u003E\u201cUnderstanding the tradeoff between strength and toughness provides important insights for the future utilization of graphene in structural and functional applications,\u201d Zhu added. \u201cThis research provides a foundational framework for further study of the mechanical properties of graphene.\u201d\u003C\/p\u003E\u003Cp\u003ELou said the techniques they used should work for any two-dimensional material. \u0022It\u0027s important to understand how defects will affect the handling, processing and manufacture of these materials,\u0022 he said. \u0022Our work should open up new directions for testing the mechanical properties of 2-D materials.\u0022\u003C\/p\u003E\u003Cp\u003ECo-authors of the paper are graduate students Peng Zhang, Lulu Ma, Phillip Loya and Yongji Gong, and former graduate students Cheng Peng and Jiangnan Zhang, all at Rice; Feifei Fan and Zhi Zeng, graduate students at Georgia Tech; Zheng Liu, an assistant professor at Nanyang Technological University, Singapore, with a complimentary appointment at Rice; Pulickel Ajayan, Rice\u0027s Benjamin M. and Mary Greenwood Anderson Professor in Materials Science and Nanoengineering and of Chemistry; and Xingxiang Zhang, a professor at Tianjin Polytechnic University, China.\u003C\/p\u003E\u003Cp\u003ELou is an associate professor of Materials Science and Nanoengineering and of Chemistry at Rice. The Welch Foundation, the National Science Foundation, the U.S. Office of Naval Research and the Korean Institute of Machinery and Materials supported the research. \u003Cbr \/\u003E\u003Cbr \/\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EResearch News\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EGeorgia Institute of Technology\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003E177 North Avenue\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EAtlanta, Georgia\u0026nbsp; 30332-0181\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EGeorgia Tech Contacts: John Toon (404-894-6986) (\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E) or Brett Israel (404-385-1933) (\u003Ca href=\u0022mailto:brett.israel@comm.gatech.edu\u0022\u003Ebrett.israel@comm.gatech.edu\u003C\/a\u003E).\u003C\/p\u003E\u003Cp\u003ERice Contacts: David Ruth (713-348-6327) (\u003Ca href=\u0022mailto:david@rice.edu\u0022\u003Edavid@rice.edu\u003C\/a\u003E) or Mike Williams (713-348-6728)\u003Cbr \/\u003E(\u003Ca href=\u0022mailto:mikewilliams@rice.edu\u0022\u003Emikewilliams@rice.edu\u003C\/a\u003E).\u003Cbr \/\u003E\u003Cbr \/\u003E\u003Cbr \/\u003E\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003EResearchers have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. While it\u0027s still very useful, graphene is really only as strong as its weakest link, which they determined to be \u0022substantially lower\u0022 than the intrinsic strength of graphene.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Researchers have measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle."}],"uid":"27303","created_gmt":"2014-04-29 09:37:53","changed_gmt":"2016-10-08 03:16:18","author":"John Toon","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2014-04-29T00:00:00-04:00","iso_date":"2014-04-29T00:00:00-04:00","tz":"America\/New_York"},"extras":[],"hg_media":{"293781":{"id":"293781","type":"image","title":"Graphene Fracture","body":null,"created":"1449244313","gmt_created":"2015-12-04 15:51:53","changed":"1475894991","gmt_changed":"2016-10-08 02:49:51","alt":"Graphene 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Nanotechnology"},{"id":"39471","name":"Materials"}],"news_room_topics":[{"id":"71881","name":"Science and Technology"}],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003EJohn Toon\u003C\/p\u003E\u003Cp\u003EResearch News\u003C\/p\u003E\u003Cp\u003E\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E\u003C\/p\u003E\u003Cp\u003E(404) 894-6986\u003C\/p\u003E","format":"limited_html"}],"email":["jtoon@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}