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News Article | January 19, 2016
Site: http://phys.org/nanotech-news/

The same slip-and-stick mechanism that leads to earthquakes is at work on the molecular level in nanoscale materials, where it determines the shear plasticity of the materials, according to scientists at Rice University and the State University of Campinas, Brazil.


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Site: http://www.materialstoday.com/news/

Materials scientists at Rice University have succeeded in making nanodiamonds and other forms of carbon by smashing nanotubes against a target at high speeds. Nanodiamonds won't make anyone rich, but the process of making them will enrich the knowledge of engineers who design structures that can resist damage from high-speed impacts. The diamonds are the result of a detailed study into the ballistic fracturing of carbon nanotubes at different velocities. The results, which are reported in a paper in ACS Applied Materials and Interfaces, showed that such high-energy impacts caused atomic bonds in the nanotubes to break and then sometimes recombine to form different structures. Led by the labs of materials scientists Pulickel Ajayan at Rice and Douglas Galvao at the State University of Campinas in Brazil, the study is intended to help aerospace engineers design ultralight materials for spacecraft and satellites that can withstand impacts from high-velocity projectiles like micrometeorites. Knowing how the atomic bonds of nanotubes can recombine will give scientists clues for developing such lightweight materials by rearranging those bonds, said co-lead author and Rice graduate student Sehmus Ozden. "Satellites and spacecraft are at risk of various destructive projectiles, such as micrometeorites and orbital debris," Ozden explained. "To avoid this kind of destructive damage, we need lightweight, flexible materials with extraordinary mechanical properties. Carbon nanotubes can offer a real solution." The researchers packed multiwalled carbon nanotubes into spherical pellets and fired them at an aluminum target from a two-stage light-gas gun at Rice, and then analyzed the results of impacts at three different speeds. At what the researchers considered a low velocity of 3.9 kilometers per second, a large number of nanotubes were found to remain intact. Some even survived higher velocity impacts of 5.2 kilometers per second. But very few were found among samples smashed at a hypervelocity of 6.9 kilometers per second. The researchers found that many, if not all, of the nanotubes split into nanoribbons, confirming earlier experiments. Co-author Chandra Sekhar Tiwary, a Rice postdoctoral researcher, noted the few nanotubes and nanoribbons that did survive the impact were often welded together, as observed in transmission electron microscope images. "In our previous report, we showed that carbon nanotubes form graphene nanoribbons at hypervelocity impact," Tiwary said. "We were expecting to get welded carbon nanostructures, but we were surprised to observe nanodiamond as well." According to Ajayan, the orientation of the nanotubes, both to each other and in relation to the target, and the number of tube walls were as important to the final structures as the velocity. "The current work opens a new way to make nanosize materials using high-velocity impact," said co-lead author Leonardo Machado from the State University of Campinas. 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.


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Site: http://www.cemag.us/rss-feeds/all/rss.xml/all

The same slip-and-stick mechanism that leads to earthquakes is at work on the molecular level in nanoscale materials, where it determines the shear plasticity of the materials, according to scientists at Rice University and the State University of Campinas, Brazil. The Rice lab of materials scientist Pulickel Ajayan found that random molecules scattered within layers of otherwise pristine graphene affect how the layers interact with each other under strain. Plasticity is the ability of a material to permanently deform when strained. The Rice researchers, thinking about future things like flexible electronics, decided to see how graphene oxide “paper” would handle shear strain, in which the sheets are pulled by the ends. Such deep knowledge is important when making novel advanced materials, says Chandra Sekhar Tiwary, a lead author of the new paper in the American Chemical Society journal Nano Letters and a Rice postdoctoral research associate. “We want to build three-dimensional structures from two-dimensional materials, so this kind of study is useful,” he says. “These structures could be a thermal substrate for electronic devices, they could be filters, they could be sensors or they could be biomedical devices. But if we’re going to use a material, we need to understand how it behaves.” The graphene oxide paper they tested was a stack of sheets that lay atop each other like pancakes. Oxygen molecules “functionalized” the surfaces, adding roughness to the otherwise atom-thick sheets. In experiments and computer models, the team found that with gentle, slow stress, the oxides would indeed catch, causing the paper to take on a corrugated form where layers pulled apart. But a higher strain rate makes the material brittle. “The simulation performed by our collaborators in Brazil provides insight and confirms that if you pull it very fast, the layers don’t interact, and only one layer comes out,” Tiwary says. “After this study, we now know there are some functional groups that are useful and some that are not. With this understanding we can choose the functional groups to make better structures at the molecular level.” Rice graduate student Soumya Vinod is a lead author of the paper. Co-authors are Rice graduate student Sehmus Ozden and undergraduates Juny Cho and Preston Shaw; postdoctoral researcher Leonardo Machado and Professor Douglas Galvão of the State University of Campinas, Brazil; and Robert Vajtai, a senior faculty fellow in materials science and nanoengineering at Rice. Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering, and a professor of chemistry. The Department of Defense and Air Force Office of Scientific Research supported the research.


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Site: http://www.cemag.us/rss-feeds/all/rss.xml/all

An adaptive material invented at Rice University combines self-healing and reversible self-stiffening properties. The Rice material called SAC (for self-adaptive composite) consists of what amounts to sticky, micron-scale rubber balls that form a solid matrix. The researchers made SAC by mixing two polymers and a solvent that evaporates when heated, leaving a porous mass of gooey spheres. When cracked, the matrix quickly heals, over and over. And like a sponge, it returns to its original form after compression. The labs of Rice materials scientists Pulickel Ajayan and Jun Lou led the study that appears in the American Chemical Society journal ACS Applied Materials and Interfaces. They suggested SAC may be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component. Other “self-healing” materials encapsulate liquid in solid shells that leak their healing contents when cracked. “Those are very cool, but we wanted to introduce more flexibility,” says Pei Dong, a postdoctoral researcher who co-led the study with Rice graduate student Alin Cristian Chipara. “We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of flowing everywhere.” In SAC, tiny spheres of polyvinylidene fluoride (PVDF) encapsulate much of the liquid. The viscous polydimethylsiloxane (PDMS) further coats the entire surface. The spheres are extremely resilient, Lou said, as their thin shells deform easily. Their liquid contents enhance their viscoelasticity, a measure of their ability to absorb the strain and return to their original state, while the coatings keep the spheres together. The spheres also have the freedom to slide past each other when compressed, but remain attached. “The sample doesn’t give you the impression that it contains any liquid,” Lou says. “That’s very different from a gel. This is not really squishy; it’s more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers.” Ajayan said making SAC is simple, and the process can be tuned — a little more liquid or a little more solid — to regulate the product’s mechanical behavior. “Gels have lots of liquid encapsulated in solids, but they’re too much on the very soft side,” he says. “We wanted something that was mechanically robust as well. What we ended up with is probably an extreme gel in which the liquid phase is only 50 percent or so.” The polymer components begin as powder and viscous liquid, said Dong. With the addition of a solvent and controlled heating, the PDMS stabilizes into solid spheres that provide the reconfigurable internal structure. In tests, Rice scientists found a maximum of 683 percent increase in the material’s storage modulus — a size-independent parameter used to characterize self-stiffening behavior. This is much larger than that reported for solid composites and other materials, they say. Dong said sample sizes of the putty-like material are limited only by the container they’re made in. “Right now, we’re making it in a 150-milliliter beaker, but it can be scaled up. We have a design for that.” Co-authors are Rice postdoctoral researchers Bo Li, Yingchao Yang, Hua Guo, Liehui Ge, and Liang Hong; graduate student Sidong Lei; undergraduate students Bilan Yang and Qizhong Wang; alumnus Phillip Loya; Emilie Ringe, an assistant professor of materials science and nanoengineering and of chemistry; Robert Vajtai, a senior faculty fellow in materials science and nanoengineering, and Ming Tang, an assistant professor of materials science and nanoengineering; Mircea Chipara, an assistant professor of physics and geology at the University of Texas-Pan American, and postdoctoral researchers Gustavo Brunetto and Leonardo Machado and Douglas Galvao, a professor at the State University of Campinas, Brazil. Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry. Lou is a professor of materials science and nanoengineering and of chemistry and associate chair of the Department of Materials Science and NanoEngineering. The Air Force Office of Scientific Research and the Department of Defense supported the research.


Home > Press > Graphene oxide 'paper' changes with strain: Rice researchers say material is more or less brittle, depending on how hard it's pulled Abstract: The same slip-and-stick mechanism that leads to earthquakes is at work on the molecular level in nanoscale materials, where it determines the shear plasticity of the materials, according to scientists at Rice University and the State University of Campinas, Brazil. A video shows computer models of graphene oxide paper under strain. At top, under more pressure, the material remains brittle as one layer of graphene oxide is pulled away. Under less strain, the layers separate more easily as oxygen molecules on the surfaces stick and slip against each other. Credit: Ajayan Research Group/Douglas Galvão The Rice lab of materials scientist Pulickel Ajayan found that random molecules scattered within layers of otherwise pristine graphene affect how the layers interact with each other under strain. Plasticity is the ability of a material to permanently deform when strained. The Rice researchers, thinking about future things like flexible electronics, decided to see how graphene oxide "paper" would handle shear strain, in which the sheets are pulled by the ends. Such deep knowledge is important when making novel advanced materials, said Chandra Sekhar Tiwary, a lead author of the new paper in the American Chemical Society journal Nano Letters and a Rice postdoctoral research associate. "We want to build three-dimensional structures from two-dimensional materials, so this kind of study is useful," he said. "These structures could be a thermal substrate for electronic devices, they could be filters, they could be sensors or they could be biomedical devices. But if we're going to use a material, we need to understand how it behaves." The graphene oxide paper they tested was a stack of sheets that lay atop each other like pancakes. Oxygen molecules "functionalized" the surfaces, adding roughness to the otherwise atom-thick sheets. In experiments and computer models, the team found that with gentle, slow stress, the oxides would indeed catch, causing the paper to take on a corrugated form where layers pulled apart. But a higher strain rate makes the material brittle. "The simulation performed by our collaborators in Brazil provides insight and confirms that if you pull it very fast, the layers don't interact, and only one layer comes out," Tiwary said. "After this study, we now know there are some functional groups that are useful and some that are not. With this understanding we can choose the functional groups to make better structures at the molecular level." ### Rice graduate student Soumya Vinod is a lead author of the paper. Co-authors are Rice graduate student Sehmus Ozden and undergraduates Juny Cho and Preston Shaw; postdoctoral researcher Leonardo Machado and Professor Douglas Galvão of the State University of Campinas, Brazil; and Robert Vajtai, a senior faculty fellow in materials science and nanoengineering at Rice. Ajayan is chair of Rice's Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry. The Department of Defense and Air Force Office of Scientific Research supported the research. About Rice University Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,910 undergraduates and 2,809 graduate students, Rice's undergraduate student-to-faculty ratio is 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for best quality of life and for lots of race/class interaction by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger's Personal Finance. Follow Rice News and Media Relations via Twitter @RiceUNews. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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