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  • richardmitnick 1:13 pm on October 18, 2016 Permalink | Reply
    Tags: , , Improving Silicon for Future Electronics, Material Sciences, Perovskite oxides   

    From Cornell: “Improving Silicon for Future Electronics” 

    Cornell Bloc

    Cornell University

    10.18.16
    Daniel Hada Harianja

    To retain its mainstay status in microelectronics, silicon must undergo improvement for advanced multifunctionality in future electronic devices.

    1
    Zhe Wang

    Silicon needs an upgrade. As innovators dream of better devices, they seek more functionalities to be built into their microelectronics. Silicon, the backbone of electronics, cannot fulfill those demands alone.

    One upgrade comes in the form of perovskite oxides. Named after the specific crystalline structure of such material, perovskite oxides have for decades captivated scientists with their vast range of electrical, magnetic, and optical properties. The objective, therefore, is to build desired perovskite oxide layers on top of silicon, granting a device of multiple functionalities.

    Growing most oxides on top of silicon is difficult to do directly, because silicon is easily oxidized into amorphous forms of itself, which then cannot accommodate the functional oxides. So the scientific community is hard at work to perfect an intermediate layer between the two—something that is sufficiently compatible with silicon and able to act as a template on top of which other oxides can be built.

    The Challenge of Growing Perovskite Oxides on Silicon

    Zhe Wang, an Applied and Engineering Physics graduate student, is part of that scientific frontier. Wang works in the research group of Darrell Schlom, Materials Science and Engineering. Together with the group, Wang hopes to improve the growing method of SrTiO3­­, a perovskite and the most widely researched candidate for that middle layer. Specifically, Wang aims to enhance the crystalline quality of the SrTiO3 layer.

    “The advantage is that if we can grow these functional material on silicon, we can reach multifunctionality on silicon,” says Wang. “This can be used in future devices, such as smartphones, sensors, antennae, photovoltaic cells, and many others.”

    Unlike most oxides, SrTiO3­­ can be feasibly formed on top of silicon by adjusting the growing conditions. To act as a good template, however, on which other functional materials can be built, the SrTiO3­­ film must be formed as a single-crystal, which means the layer has a single lattice orientation throughout its crystal structure.

    Creating or depositing such a film flawlessly is challenging. “Even though we can achieve single-crystal layers, the crystalline quality is often not very good. It has many defects,” says Wang. “If we grow other functional materials on top of it, the functional materials will also not be perfect, because the underlying layer is not perfect.”

    By studying molecular beam epitaxy, one of the most advanced thin-film deposition methods available, Wang hopes to fine-tune the conditions necessary for a good film. This method subjects the deposition process to very low pressures of below 10-8 Torr, which allows for the highest possible purity of the film. To form a layer on silicon, the constituent elements of the layer are separately heated in effusion cells until they sublime into vapor. The vapors, along with a stream of oxygen, then meet on the silicon surface and react to form a film. As the deposition occurs, reflection high-energy electron diffraction is employed to evaluate the crystal growth by firing electrons on the target materials and analyzing its diffraction pattern.

    “The parameters [of the process] are complicated to get right,” Wang says. For one, the stoichiometry of the constituent elements of the film must be extremely precise. The temperature must be high enough to allow the film deposition to occur, but not too high that it oxidizes the silicon.

    Toward Success, It Takes Collaboration

    Despite the challenges, many appreciate the progress in Wang’s work. Within the past year, collaborators from Singapore, Berkeley, and the Netherlands have published separate papers on the properties of other perovskite materials that they have grown atop of Wang’s high quality SrTiO3 films on silicon, including their applications in different microelectronic devices. Wang also plans to try integrating his own perovskite oxides onto his template in the future. It depends, however, on the ability to build good films on top of silicon, and as Wang explains, good films require a good underlying SrTiO3­­ layer.

    It is not simply the cutting-edge tools that boost Wang’s research. “We have a lot of collaboration. We are making the material, but to understand the perfection, performance, and defects at the atomic level, we collaborate with other groups at Cornell.” For instance, the research team of Lena F. Kourkoutis, Applied and Engineering Physics, has used transmission electron microscopy to help with characterizing the interface structure and film quality. Kyle Shen’s research group, Physics, has integrated their angle-resolved photoemission spectroscopy (ARPES) with the molecular beam epitaxy system to study the materials being formed without exposure to air. Other collaborations include research into utilizing density functional theory to predict novel properties of materials. Like silicon, no one researcher can fulfill all those demands alone. Through collaboration, Wang achieves more.

    See the full article here .

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    Once called “the first American university” by educational historian Frederick Rudolph, Cornell University represents a distinctive mix of eminent scholarship and democratic ideals. Adding practical subjects to the classics and admitting qualified students regardless of nationality, race, social circumstance, gender, or religion was quite a departure when Cornell was founded in 1865.

    Today’s Cornell reflects this heritage of egalitarian excellence. It is home to the nation’s first colleges devoted to hotel administration, industrial and labor relations, and veterinary medicine. Both a private university and the land-grant institution of New York State, Cornell University is the most educationally diverse member of the Ivy League.

    On the Ithaca campus alone nearly 20,000 students representing every state and 120 countries choose from among 4,000 courses in 11 undergraduate, graduate, and professional schools. Many undergraduates participate in a wide range of interdisciplinary programs, play meaningful roles in original research, and study in Cornell programs in Washington, New York City, and the world over.

     
  • richardmitnick 4:59 pm on September 26, 2016 Permalink | Reply
    Tags: , , Material Sciences, ,   

    From BNL: “Crystalline Fault Lines Provide Pathway for Solar Cell Current” 

    Brookhaven Lab

    September 26, 2016
    Karen McNulty Walsh
    kmcnulty@bnl.gov
    (631) 344-8350
    Peter Genzer
    genzer@bnl.gov
    (631) 344-3174

    New tomographic AFM imaging technique reveals that microstructural defects, generally thought to be detrimental, actually improve conductivity in cadmium telluride solar cells.

    1
    CFN staff scientist Lihua Zhang places a sample in the transmission electron microscope.

    A team of scientists studying solar cells made from cadmium telluride, a promising alternative to silicon, has discovered that microscopic “fault lines” within and between crystals of the material act as conductive pathways that ease the flow of electric current. This research—conducted at the University of Connecticut and the U.S. Department of Energy’s Brookhaven National Laboratory, and described in the journal Nature Energy—may help explain how a common processing technique turns cadmium telluride into an excellent material for transforming sunlight into electricity, and suggests a strategy for engineering more efficient solar devices that surpass the performance of silicon.

    “If you look at semiconductors like silicon, defects in the crystals are usually bad,” said co-author Eric Stach, a physicist at Brookhaven Lab’s Center for Functional Nanomaterials (CFN). As Stach explained, misplaced atoms or slight shifts in their alignment often act as traps for the particles that carry electric current—negatively charged electrons or the positively charged “holes” left behind when electrons are knocked loose by photons of sunlight, making them more mobile. The idea behind solar cells is to separate the positive and negative charges and run them through a circuit so the current can be used to power houses, satellites, or even cities. Defects interrupt this flow of charges and keep the solar cell from being as efficient as it could be.

    But in the case of cadmium telluride, the scientists found that boundaries between individual crystals and “planar defects”—fault-like misalignments in the arrangement of atoms—create pathways for conductivity, not traps.

    2
    These CTAFM images show a cadmium telluride solar cell from the top (above) and side profile (bottom) with bright spots representing areas of higher electron conductivity. The images reveal that the conductive pathways coincide with crystal grain boundaries. Credit: University of Connecticut.

    Members of Bryan Huey’s group at the Institute of Materials Science at the University of Connecticut were the first to notice the surprising connection. In an effort to understand the effects of a chloride solution treatment that greatly enhances cadmium telluride’s conductive properties, Justin Luria and Yasemin Kutes studied solar cells before and after treatment. But they did so in a unique way.

    Several groups around the world had looked at the surfaces of such solar cells before, often with a tool known as a conducting atomic force microscope. The microscope has a fine probe many times sharper than the head of a pin that scans across the material’s surface to track the topographic features—the hills and valleys of the surface structure—while simultaneously measuring location-specific conductivity. Scientists use this technique to explore how the surface features relate to solar cell performance at the nanoscale.

    But no one had devised a way to make measurements beneath the surface, the most important part of the solar cell. This is where the UConn team made an important breakthrough. They used an approach developed and perfected by Kutes and Luria over the last two years to acquire hundreds of sequential images, each time intentionally removing a nanoscale layer of the material, so they could scan through the entire thickness of the sample. They then used these layer-by-layer images to build up a three-dimensional, high-resolution ‘tomographic’ map of the solar cell—somewhat like a computed tomography (CT) brain scan.


    Assembling the layer-by-layer CTAFM scans into a side-profile video file reveals the relationship between conductivity and planar defects throughout the entire thickness of the cadmium telluride crystal, including how the defects appear to line up to form continuous pathways of conductivity.Credit: University of Connecticut.

    “Everyone using these microscopes basically takes pictures of the ‘ground,’ and interprets what is beneath,” Huey said. “It may look like there’s a cave, or a rock shelf, or a building foundation down there. But we can only really know once we carefully dig, like archeologists, keeping track of exactly what we find every step of the way—though, of course, at a much, much smaller scale.”

    The resulting CT-AFM maps uniquely revealed current flowing most freely along the crystal boundaries and fault-like defects in the cadmium telluride solar cells. The samples that had been treated with the chloride solution had more defects overall, a higher density of these defects, and what appeared to be a high degree of connectivity among them, while the untreated samples had few defects, no evidence of connectivity, and much lower conductivity.

    Huey’s team suspected that the defects were so-called planar defects, usually caused by shifts in atomic alignments or stacking arrangements within the crystals. But the CTAFM system is not designed to reveal such atomic-scale structural details. To get that information, the UConn team turned to Stach, head of the electron microscopy group at the CFN, a DOE Office of Science User Facility.

    “Having previously shared ideas with Eric, it was a natural extension of our discovery to work with his group,” Huey said.

    Said Stach, “This is the exact type of problem the CFN is set up to handle, providing expertise and equipment that university researchers may not have to help drive science from hypothesis to discovery.”

    CFN staff physicist Lihua Zhang used a transmission electron microscope (TEM) and UConn’s results as a guide to meticulously study how atomic scale features of chloride-treated cadmium telluride related to the conductivity maps. The TEM images revealed the atomic structure of the defects, confirming that they were due to specific changes in the stacking sequence of atoms in the material. The images also showed clearly that these planar defects connected different grains in the crystal, leading to high-conductivity pathways for the movement of electrons and holes.

    “When we looked at the regions with good conductivity, the planar defects linked from one crystal grain to another, forming continuous pathways of conductance through the entire thickness of the material,” said Zhang. “So the regions that had the best conductivity were the ones that had a high degree of connectivity among these defects.”

    3
    These transmission electron microscopy images taken at Brookhaven’s CFN reveals how the stacking pattern of individual atoms (bright spots) shifts. The images confirmed that the bright spots of high conductivity observed with CTAFM imaging at UConn occurred at the interfaces between two different atomic alignments (left) and that these “planar defects” were continuous between individual crystals, creating pathways of conductivity (right). The labels WZ and ZB refer to the two atomic stacking sequences “wurtzite” and “zinc blende,” which are the two types of crystal structures cadmium telluride can form. No image credit.

    The authors say it’s possible that the chloride treatment helps to create the connectivity, not just more defects, but that more research is needed to definitively determine the most significant effects of the chloride solution treatment.

    In any case, Stach says that combining the CTAFM technique and electron microscopy, yields a “clear winner” in the search for more efficient, cost-competitive alternatives to silicon solar cells, which have nearly reached their limit for efficiency.

    “There is already a billion-dollar-a-year industry making cadmium telluride solar cells, and lots of work exploring other alternatives to silicon. But all of these alternatives, because of their crystal structure, have a higher tendency to form defects,” he said. “This work gives us a systematic method we can use to understand if the defects are good or bad in terms of conductivity. It can also be used to explore the effects of different processing methods or chemicals to control how defects form. In the case of cadmium telluride, we may want to find ways to make more of these defects, or look for other materials in which defects improve performance.”

    This research was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE)—including its Sunshot Program—and the DOE Office of Science. The cadmium telluride samples were provided by Andrew Moore of Colorado State University.

    The University of Connecticut’s Institute of Materials Science serves as the heart of materials science research at the University of Connecticut, with a mission of supporting materials research and industry throughout Connecticut and the Northeast. It houses the research labs of more than 30 core faculty, with an overall membership of 120 UConn faculty whose work benefits from the available facilities and expertise.

    See the full article here .

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    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 8:30 am on September 7, 2016 Permalink | Reply
    Tags: , , Material Sciences, ,   

    From MIT: “Sponge creates steam using ambient sunlight” 

    MIT News
    MIT News
    MIT Widget

    August 22, 2016
    Jennifer Chu | MIT News Office

    1
    MIT graduate student George Ni holds a bubble-wrapped, sponge-like device that soaks up natural sunlight and heats water to boiling temperatures, generating steam through its pores.
    Photo: Jeremy Cho

    2
    Bubble wrap, combined with a selective absorber, keeps heat from escaping the surface of the sponge. Photo: George Ni

    How do you boil water? Eschewing the traditional kettle and flame, MIT engineers have invented a bubble-wrapped, sponge-like device that soaks up natural sunlight and heats water to boiling temperatures, generating steam through its pores.

    The design, which the researchers call a “solar vapor generator,” requires no expensive mirrors or lenses to concentrate the sunlight, but instead relies on a combination of relatively low-tech materials to capture ambient sunlight and concentrate it as heat. The heat is then directed toward the pores of the sponge, which draw water up and release it as steam.

    From their experiments — including one in which they simply placed the solar sponge on the roof of MIT’s Building 3 — the researchers found the structure heated water to its boiling temperature of 100 degrees Celsius, even on relatively cool, overcast days. The sponge also converted 20 percent of the incoming sunlight to steam.

    The low-tech design may provide inexpensive alternatives for applications ranging from desalination and residential water heating, to wastewater treatment and medical tool sterilization.

    The team has published its results today in the journal Nature Energy. The research was led by George Ni, an MIT graduate student; and Gang Chen, the Carl Richard Soderberg Professor in Power Engineering and the head of the Department of Mechanical Engineering; in collaboration with TieJun Zhang and his group members Hongxia Li and Weilin Yang from the Department of Mechanical and Materials Engineering at the Masdar Institute of Science and Technology, in the United Arab Emirates.

    Building up the sun

    The researchers’ current design builds on a solar-absorbing structure they developed in 2014 — a similar floating, sponge-like material made of graphite and carbon foam, that was able to boil water to 100 C and convert 85 percent of the incoming sunlight to steam.

    To generate steam at such efficient levels, the researchers had to expose the structure to simulated sunlight that was 10 times the intensity of sunlight in normal, ambient conditions.

    “It was relatively low optical concentration,” Chen says. “But I kept asking myself, ‘Can we basically boil water on a rooftop, in normal conditions, without optically concentrating the sunlight? That was the basic premise.”

    In ambient sunlight, the researchers found that, while the black graphite structure absorbed sunlight well, it also tended to radiate heat back out into the environment. To minimize the amount of heat lost, the team looked for materials that would better trap solar energy.

    A bubbly solution

    In their new design, the researchers settled on a spectrally-selective absorber — a thin, blue, metallic-like film that is commonly used in solar water heaters and possesses unique absorptive properties. The material absorbs radiation in the visible range of the electromagnetic spectrum, but it does not radiate in the infrared range, meaning that it both absorbs sunlight and traps heat, minimizing heat loss.

    The researchers obtained a thin sheet of copper, chosen for its heat-conducting abilities and coated with the spectrally-selective absorber. They then mounted the structure on a thermally-insulating piece of floating foam. However, they found that even though the structure did not radiate much heat back out to the environment, heat was still escaping through convection, in which moving air molecules such as wind would naturally cool the surface.

    A solution to this problem came from an unlikely source: Chen’s 16-year-old daughter, who at the time was working on a science fair project in which she constructed a makeshift greenhouse from simple materials, including bubble wrap.

    “She was able to heat it to 160 degrees Fahrenheit, in winter!” Chen says. “It was very effective.”

    Chen proposed the packing material to Ni, as a cost-effective way to prevent heat loss by convection. This approach would let sunlight in through the material’s transparent wrapping, while trapping air in its insulating bubbles.

    “I was very skeptical of the idea at first,” Ni recalls. “I thought it was not a high-performance material. But we tried the clearer bubble wrap with bigger bubbles for more air trapping effect, and it turns out, it works. Now because of this bubble wrap, we don’t need mirrors to concentrate the sun.”

    The bubble wrap, combined with the selective absorber, kept heat from escaping the surface of the sponge. Once the heat was trapped, the copper layer conducted the heat toward a single hole, or channel, that the researchers had drilled through the structure. When they placed the sponge in water, they found that water crept up the channel, where it was heated to 100 C, then turned to steam.

    Tao Deng, professor of material sciences and engineering at Shanghai Jiao Tong University, says the group’s use of low-cost materials will make the device more affordable for a wide range of applications.

    “This device offers a totally new design paradigm for solar steam generation,” says Deng, who was not involved in the study. “It eliminates the need of the expensive optical concentrator, which is a key advantage in bringing down the cost of the solar steam generation system. Certainly the clever use of bubble wrap and commercially available selective absorber also helps suppress the convection and radiation heat loss, both of which not only improve the solar harvesting efficiency but also further lower the system cost. “

    Chen and Ni say that solar absorbers based on this general design could be used as large sheets to desalinate small bodies of water, or to treat wastewater. Ni says other solar-based technologies that rely on optical-concentrating technologies typically are designed to last 10 to 20 years, though they require expensive parts and maintenance. This new, low-tech design, he says, could operate for one to two years before needing to be replaced.

    “Even so, the cost is pretty competitive,” Ni says. “It’s kind of a different approach, where before, people were doing high-tech and long-term [solar absorbers]. We’re doing low-tech and short-term.”

    “What fascinates us is the innovative idea behind this inexpensive device, where we have creatively designed this device based on basic understanding of capillarity and solar thermal radiation,” says Zhang. “Meanwhile, we are excited to continue probing the complicated physics of solar vapor generation and to discover new knowledge for the scientific community.”

    This research was funded, in part, by a cooperative agreement between the Masdar Institute of Science and Technology and MIT; and by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center funded by U.S. Department of Energy.

    See the full article here .

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    The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

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  • richardmitnick 7:21 am on September 7, 2016 Permalink | Reply
    Tags: , , Material Sciences,   

    From Rutgers via New Jersey Business: “Rutgers Engineers Use Microwaves to Produce High-Quality Graphene” 

    Rutgers University
    Rutgers University

    1

    New Jersey Business

    Sep 1, 2016
    Todd B. Bates, Rutgers

    Rutgers experts discover easy way to make graphene for flexible and printable electronics, energy storage, and catalysis.

    `
    No image caption. No image credit.

    Rutgers University engineers have found a simple method for producing high-quality graphene that can be used in next-generation electronic and energy devices: bake the compound in a microwave oven.

    The discovery is documented in a study published online [today] in the journal Science.

    “This is a major advance in the graphene field,” said Manish Chhowalla, professor and associate chair in the Department of Materials Science and Engineering in Rutgers’ School of Engineering. “This simple microwave treatment leads to exceptionally high quality graphene with properties approaching those in pristine graphene.”

    The discovery was made by post-doctoral associates and undergraduate students in the department, said Chhowalla, who is also the director of the Rutgers Institute for Advanced Materials, Devices and Nanotechnology. Having undergraduates as co-authors of a Science paper is rare but he said “the Rutgers Materials Science and Engineering Department and the School of Engineering at Rutgers cultivate a culture of curiosity driven research in students with fresh ideas who are not afraid to try something new.’’

    Graphene – 100 times tougher than steel – conducts electricity better than copper and rapidly dissipates heat, making it useful for many applications. Large-scale production of graphene is necessary for applications such as printable electronics, electrodes for batteries and catalysts for fuel cells.

    Graphene comes from graphite, a carbon-based material used by generations of students and teachers in the form of pencils. Graphite consists of sheets or layers of graphene.

    The easiest way to make large quantities of graphene is to exfoliate graphite into individual graphene sheets by using chemicals. The downside of this approach is that side reactions occur with oxygen – forming graphene oxide that is electrically non-conducting, which makes it less useful for products.

    Removing oxygen from graphene oxide to obtain high-quality graphene has been a major challenge over the past two decades for the scientific community working on graphene. Oxygen distorts the pristine atomic structure of graphene and degrades its properties.

    Chhowalla and his group members found that baking the exfoliated graphene oxide for just one second in a 1,000-watt microwave oven, like those used in households across America, can eliminate virtually all of the oxygen from graphene oxide.

    The Rutgers engineers’ research was funded by the National Science Foundation, Rutgers Energy Institute, U.S. Department of Education and Rutgers Aresty Research Assistant Program.

    The study’s lead authors are Damien Voiry, a former Rutgers post-doctoral associate in Chhowalla’s Nano-materials & Devices Group who is now at the University of Montpellier in France, and Jieun Yang, a post-doctoral associate in Chhowalla’s group. Other authors include Jacob Kupferberg, who will be a Rutgers senior this fall; graduate student Raymond Fullon; Calvin Lee, who graduated in 2015; Hu Young Jeong and Hyeon Suk Shin from the Ulsan National Institute of Science and Technology in South Korea; and Chhowalla.

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    Rutgers, The State University of New Jersey, is a leading national research university and the state’s preeminent, comprehensive public institution of higher education. Rutgers is dedicated to teaching that meets the highest standards of excellence; to conducting research that breaks new ground; and to providing services, solutions, and clinical care that help individuals and the local, national, and global communities where they live.

    Founded in 1766, Rutgers teaches across the full educational spectrum: preschool to precollege; undergraduate to graduate; postdoctoral fellowships to residencies; and continuing education for professional and personal advancement.

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  • richardmitnick 6:52 am on September 6, 2016 Permalink | Reply
    Tags: , Material Sciences, Nanodiamonds in an instant, ,   

    From Rice: “Nanodiamonds in an instant” 

    Rice U bloc

    Rice University

    September 6, 2016

    David Ruth
    713-348-6327
    david@rice.edu

    Mike Williams
    713-348-6728
    mikewilliams@rice.edu

    Rice University-led team morphs nanotubes into tougher carbon for spacecraft, satellites

    1
    Experiments at Rice University showed nanodiamonds and other forms of carbon were created when carbon nanotube pellets were fired at a target at hypervelocity. (Credit: Illustration by Pedro Alves da Silva Autreto)

    Superman can famously make a diamond by crushing a chunk of coal in his hand, but Rice University scientists are employing a different tactic.

    Rice materials scientists are 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 resist damage from high-speed impacts.

    The diamonds are the result of a detailed study on the ballistic fracturing of carbon nanotubes at different velocities. The results showed that such high-energy impacts caused atomic bonds in the nanotubes to break and sometimes recombine into different structures.

    The work led by the labs of materials scientists Pulickel Ajayan at Rice and Douglas Galvao at the State University of Campinas, Brazil, is intended to help aerospace engineers design ultralight materials for spacecraft and satellites that can withstand impacts from high-velocity projectiles like micrometeorites.

    The research appears in the American Chemical Society journal ACS Applied Materials and Interfaces.

    Knowing how the atomic bonds of nanotubes can be recombined will give scientists clues to develop 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 said. “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 in a two-stage light-gas gun at Rice, and then analyzed the results from 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 survived 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.”

    The orientation of 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, Ajayan said.

    “The current work opens a new way to make nanosize materials using high-velocity impact,” said co-lead author Leonardo Machado of the Brazil team.

    Machado is a graduate student at the State University of Campinas, Brazil, and the Federal University of Rio Grande do Norte, Brazil. Co-authors are Rice’s Robert Vajtai, an associate research professor, and Enrique Barrera, a professor of materials science and nanoengineering, and Pedro Alves da Silva of the State University of Campinas and the Federal University of ABC, Santo Andre, 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.

    The research was supported by the Department of Defense, the U.S. Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative, NASA’s Johnson Space Center, the Sao Paulo Research Foundation, the Center for Computational Engineering and Sciences at Unicamp, Brazil, and the Brazilian Federal Agency for Support and Evaluation of Graduate Education.

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    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
  • richardmitnick 5:35 am on September 6, 2016 Permalink | Reply
    Tags: , Carbon nanotube transistors, Material Sciences, ,   

    From U Wisconsin: “First time, carbon nanotube transistors outperform silicon” 

    U Wisconsin

    University of Wisconsin

    September 2, 2016
    Adam Malecek

    1
    The UW–Madison engineers use a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. The team’s breakthrough could pave the way for carbon nanotube transistors to replace silicon transistors, and is particularly promising for wireless communications technologies. Stephanie Precourt

    For decades, scientists have tried to harness the unique properties of carbon nanotubes to create high-performance electronics that are faster or consume less power — resulting in longer battery life, faster wireless communication and faster processing speeds for devices like smartphones and laptops.

    But a number of challenges have impeded the development of high-performance transistors made of carbon nanotubes, tiny cylinders made of carbon just one atom thick. Consequently, their performance has lagged far behind semiconductors such as silicon and gallium arsenide used in computer chips and personal electronics.

    Now, for the first time, University of Wisconsin–Madison materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors.

    Led by Michael Arnold and Padma Gopalan, UW–Madison professors of materials science and engineering, the team’s carbon nanotube transistors achieved current that’s 1.9 times higher than silicon transistors. The researchers reported their advance in a paper published Friday (Sept. 2) in the journal Science Advances.

    “This achievement has been a dream of nanotechnology for the last 20 years,” says Arnold. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”

    This advance could pave the way for carbon nanotube transistors to replace silicon transistors and continue delivering the performance gains the computer industry relies on and that consumers demand. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.

    As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors.

    Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices.

    But researchers have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties — like a short in an electronic device.

    The UW–Madison team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

    “We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 percent metallic nanotubes,” says Arnold.

    Placement and alignment of the nanotubes is also difficult to control.

    To make a good transistor, the nanotubes need to be aligned in just the right order, with just the right spacing, when assembled on a wafer. In 2014, the UW–Madison researchers overcame that challenge when they announced a technique, called “floating evaporative self-assembly,” that gives them this control.

    The nanotubes must make good electrical contacts with the metal electrodes of the transistor. Because the polymer the UW–Madison researchers use to isolate the semiconducting nanotubes also acts like an insulating layer between the nanotubes and the electrodes, the team “baked” the nanotube arrays in a vacuum oven to remove the insulating layer. The result: excellent electrical contacts to the nanotubes.

    The researchers also developed a treatment that removes residues from the nanotubes after they’re processed in solution.

    “In our research, we’ve shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors,” says Arnold.

    The researchers benchmarked their carbon nanotube transistor against a silicon transistor of the same size, geometry and leakage current in order to make an apples-to-apples comparison.

    They are continuing to work on adapting their device to match the geometry used in silicon transistors, which get smaller with each new generation. Work is also underway to develop high-performance radio frequency amplifiers that may be able to boost a cellphone signal. While the researchers have already scaled their alignment and deposition process to 1 inch by 1 inch wafers, they’re working on scaling the process up for commercial production.

    Arnold says it’s exciting to finally reach the point where researchers can exploit the nanotubes to attain performance gains in actual technologies.

    “There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook,” says Arnold. “But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials.”

    The researchers have patented their technology through the Wisconsin Alumni Research Foundation.

    Funding from the National Science Foundation, the Army Research Office and the Air Force supported their work.

    Additional authors on the paper include Harold Evensen, a University of Wisconsin-Platteville engineering physics professor, Gerald Brady, a UW–Madison materials science and engineering graduate student and lead author on the study, and graduate student Austin Way and postdoctoral researcher Nathaniel Safron.

    See the full article here .

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    In achievement and prestige, the University of Wisconsin–Madison has long been recognized as one of America’s great universities. A public, land-grant institution, UW–Madison offers a complete spectrum of liberal arts studies, professional programs and student activities. Spanning 936 acres along the southern shore of Lake Mendota, the campus is located in the city of Madison.

     
  • richardmitnick 10:40 am on August 26, 2016 Permalink | Reply
    Tags: , , Material Sciences, Researchers develop new porous materials using doughnut nanorings   

    From ICL: “Researchers develop new porous materials using doughnut nanorings” 

    Imperial College London
    Imperial College London

    22 August 2016
    Michael Panagopulos

    1
    Image: Ella Marushchenko

    Researchers propose a new design of highly open liquid-crystalline structures from geometrically unique rigid nanorings.

    Researchers from Imperial College London, University of Manchester and Cornell University have used a computational approach to identify a new class of highly porous structures. The structures could be used to produce new materials which have potential applications for the pharmaceutical and photonics industries, among others.

    When considering phases of matter, most people think of solid, liquid and gas states. A less-known phase with rather well-known applications is the intermediate between solids and liquids: the liquid crystal state. For instance, liquid crystal displays (LCD) are present in our everyday life, including calculators, phones, computers and TVs. The properties of this unique state of matter depend upon the degree of order in the material: in the smectic phase, molecules are orientationally ordered along one direction and they tend to arrange themselves in layers. Slightly closer to the liquid phase is the nematic phase, in which molecules have no positional order but are preferentially oriented along a given direction (the director).

    In a new paper, entitled Assembly of porous smectic structures formed from interlocking high-symmetry planar nanorings which was published this week in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), the authors used a molecular-simulation approach to examine several non-convex molecular geometrically different doughnut-shaped nanoring structures in order to identify the stable microstructures and their liquid-crystalline phase properties.

    The researchers investigated a particular class of frame-like particles, namely perfectly rigid and planar nanorings, by direct molecular-dynamics simulation. Starting from a circular shape, they explored ellipsoidal and polygonal geometries; these were modelled by varying the symmetry, the cavity size and the width of the rings. Three types of nanoparticles were compared in terms of various properties: doughnuts (single rings formed from different numbers of tangent beads and symmetries); bands (multi-stacked circular rings made up of identical rings bound sideways); and washers (multi-layered circular rings made up of an outer ring and smaller inner rings).

    The doughnut-like, high-symmetry nonconvex rings with large internal cavities were found to interlock within a two-dimensional layered structure leading to the formation of distinctive smectic phases which possess uniquely high free volumes of up to 95% – significantly larger than the 50% which is typically achievable with conventional convex rod- or disc-like particles whose geometries do not lead to this interlocking phenomenon therefore limiting their porosity.

    These types of self-assembled arrays are particularly interesting due to their exceptional optical, electrical and mechanical properties which are a consequence of their large surface-to-volume ratios. The highly porous structures are good candidates as adsorption and storage materials and have promising opportunities in a broad range of applications including drug-delivery and therapeutics, catalysis, optics, photonics and nanopatterned scaffolds.

    Professor Erich Muller, co-author of the paper and Professor of Thermodynamics in the Department of Chemical Engineering at Imperial College London said “For the first time, we have looked at geometrically unique nanoring structures and found that certain shapes and sizes can lead to highly porous structures with free volumes of up to 95%. This breakthrough has some exciting possible industrial applications in many areas due to their extraordinary electrical, optical and chemical properties.”

    2
    The different models explored are shown in this figure from the paper. A basic circular ring structure is shown in the first model, from which the rest of the models are derived. The other six models are ellipsoidal rings with different aspect ratios and polygonal rings with decreasing order of rotational symmetry, all of which have similar cavity size as the first model. Models h) and i) show the two extremes of the number of beads which lead to the formation of smectic phases, while model j) is a structure which does not form an ordered fluid structure). The last two models represent a band and a washer model, respectively, where the former has smectic phase properties and the latter forms nematic phase.

    See the full article here .

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    Imperial College London

    Imperial College London is a science-based university with an international reputation for excellence in teaching and research. Consistently rated amongst the world’s best universities, Imperial is committed to developing the next generation of researchers, scientists and academics through collaboration across disciplines. Located in the heart of London, Imperial is a multidisciplinary space for education, research, translation and commercialisation, harnessing science and innovation to tackle global challenges.

     
  • richardmitnick 12:24 pm on August 15, 2016 Permalink | Reply
    Tags: , Craig Arnold, Material Sciences,   

    From Princeton- “Craig Arnold: Perspective on the allure and reach of materials science” 

    Princeton University
    Princeton University

    August 9, 2016
    John Sullivan

    Scholarly and administrative focus: Craig Arnold became director of the Princeton Institute for the Science and Technology of Materials (PRISM) Jan. 1 after serving as interim director since July 2015. The institute recently installed cutting-edge imaging equipment in the new Andlinger Center for Energy and the Environment building, including a microscope that is capable of imaging individual atoms and is one of only four of its kind in the world. In the control room of a new scanning electron microscope, Arnold recently answered questions about materials science and engineering at Princeton.

    1
    Craig Arnold (pictured), professor of mechanical and aerospace engineering, became director of the Princeton Institute for the Science and Technology of Materials (PRISM) Jan. 1. Arnold recently discussed his passion for materials science and engineering, and his vision for those fields at Princeton. (Photo by David Kelly Crow)

    What is materials science?

    Materials science and engineering is the study of the stuff that makes up the world around us. Pretty much everything is composed of materials, whether it is the table that I am sitting at, or the car that you drive, or the computer that you use. What we do is we study these materials, how their atoms are arranged, how to change their properties and how to control their response to certain stimuli. Basically, the study of materials allows us to make new materials with exciting new properties or make the existing ones we have perform better.

    What is it about materials science that fascinates you?

    The interesting thing about materials science for me is that it brings together traditional disciplines in the natural sciences and engineering and tends to work at the interfaces among them. The joke that I always say is that the greatest thing about being a materials scientist is that it gives me a secret password that gets me into any club that I want. Whether it’s chemistry or physics, mechanical or electrical engineering, if you look around, you’re bound to find a materials scientist or engineer.

    You like to open your classes by mentioning your three favorite materials. What are they?

    [laughs] Silicon, steel and pre-stressed concrete. They all are things that require an incredible knowledge of materials science and each has reshaped civilizations. Steel really is the enabling technology that allowed us to become an agrarian society. What is particularly interesting about steel — compared to other forms of metalworking in the early historical period — is that to make steel you need to coordinate many different things. You can’t just dig a rock out of the ground and turn it into steel. You needed ore, and you needed trees, and you needed limestone. And you needed trade to get all of these things together. Not only the steel itself, but also the process of making it reshaped civilization. Of course, over the millennia people figured out how to make steel better and we are still doing that today; slight changes of the steel alloy make the difference between razor blades that stay sharp and ones that you have to throw away after few uses.

    Silicon, of course, has enabled the development of computers. Everyone thinks it was the transistor that made computers possible. And absolutely it was — the transistor made it possible. But what people don’t think about is that what enabled that transistor was the ability to create very pure materials. The semiconductors that you use in a computer, you have to be able to control the amount of impurities to incredibly fine levels. If you can’t do that, you can’t make a transistor that works. The materials scientists had to figure out how to make silicon really, really pure. That required many years of understanding: how to process materials, how to melt and form materials. Nowadays you can just call up a vendor and you buy a piece of silicon wafer to make a computer chip that has the exact amount of impurities you want in a 12-inch wafer with almost no flaws. This is a remarkable achievement of materials science and it is what allows us to make computers that are cheap.

    Pre-stressed concrete is a combination of materials that has allowed us to build the infrastructure of our society. Steel is very good in tension and concrete is very good in compression. Putting them together in the right way enables complex architecture, buildings, bridges and the like. You get the idea.

    I like that materials really do change society. They change the way we think about and interact with the world, how we use objects, and how we create things. That is why I like teaching it. I can take a class of students and starting from a minimal knowledge base, get them thinking about the world around them more generally and how they can use this science to engineer a better world or to understand the nature of things more fully.

    How did you get into the field?

    As a student, I always resonated with physics but it was a particular kind of physics. I always described myself as very interested in understanding the physics of objects that I could put on a table — I used to call it tabletop physics. In my second semester of a Ph.D. program in physics, I decided to take a course titled “Kinetics,” which is a part of materials science. I had almost no idea what it was about when I signed up, but I was in the class for two days and I said, “This is it; this is exactly what I want to do.” I went to the professor and asked, “Would you take on a new graduate student?” And, well, you can figure out what happened.

    What are some problems now — big science problems that people would think of as chemistry or physics or electronics problems — but at heart are really materials-science problems?

    I could answer that by saying everything. You give me an example and I will tell you how it is a materials-science problem. But let’s take ones that maybe are not as apparent. Let’s go big. You drove a car to come into work today, right? Ask yourself: What does that have to do with materials science? Well, let’s talk about electric vehicles. Can we make better batteries? That is not a pure chemistry problem because we know a lot about the basic chemistry of how a battery works already. Also, it’s not a pure electronics problem. The challenge now is in the materials — trying to figure out what materials are optimal, what materials are safe, what materials can meet the general requirements that we have in order to make better automobile batteries.

    Let’s take this idea of cars one step further — one way to make that car more fuel-efficient is to make it lighter. But if we make it lighter we don’t want to make it less safe. How do you deal with that tradeoff? That is a materials problem. If you go back 50 years, it was cast-iron engine blocks and steel everywhere else. Now you see more plastics, more fabrics, in the makeup of the body of the car, and more aluminum and lighter-weight materials in the engine. So being able to create lightweight materials that are structurally stable seems pretty basic, but it is a really important materials-science problem with major implications in a whole host of areas.

    In the relatively short term, what are a few goals for materials science and engineering at Princeton?

    My main goal is to establish Princeton as a leading place for materials science and engineering. I want us to be recognized as a top-ranked program. When people think about materials science, I want them to think about Princeton. Right now, the individual scholars who work at Princeton, and the work that they do, are well known in the field. But as an institute, we do not have the broad recognition that others have. I think we can make a more concerted effort to coordinate our message, highlight our expertise and build on our strengths so that we can attract the best students and researchers, and we can continue to be a leader in the field.

    Well, how do we do that? Firstly, the new facilities we are sitting in are an important step. Having the latest and most advanced toolset will enable researchers to perform cutting-edge research in their areas. But also, I think it is important to have a true graduate program in materials science and engineering. This will help us attract those top students who are going elsewhere because we don’t have a home for them. One of the great things about materials science is that the innovations we make enable other fields. It helps the electrical engineers make great strides in display technologies, the physicists develop new types of superconductors, or the biomedical researchers develop new ways to treat cancer. But we need to get beyond this idea of materials science being something that is behind the scenes. We need to bring it into the forefront.

    It’s like the bass player in a band.

    [laughs] I am OK with that. If you are going to be a successful band, you need to have a good bass player. But every now and again, it’s important to bring that bass player out front, and you hear the riff and you think, “That’s a really great bassist.” That is what we need to do. We want materials to be recognized, recognized for what we do on our own and also for what we do to help others be that much better. Ultimately, my feeling is that this is all one big university. And strength in any area is strength for everybody. We need materials science — it enables so many of the traditional disciplines. But it also is a deep and fascinating discipline in itself.

    See the full article here .

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    Princeton University Campus

    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

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  • richardmitnick 5:45 am on July 26, 2016 Permalink | Reply
    Tags: , Material Sciences, Washington State U, Watching a material change its crystal structure in real time   

    From phys.org: “Researchers ‘watch’ crystal structure change in real time” 

    physdotorg
    phys.org

    July 25, 2016
    Eric Sorensen

    Washington State University researchers have met the long-standing scientific challenge of watching a material change its crystal structure in real time.

    2
    Configuration for the time-resolved, x-ray diffraction measurements in silicon subjected to impact loading. A PC projectile traveling at ∼5.1  km/s impacted the Si samples. Pulsed x rays (∼23.5  keV energy, ∼100  ps duration, 153.4 ns period) passed through the PC projectile, the silicon sample, and the PC window. Diffracted x rays from individual ∼100  ps x-ray pulses were detected on a framing area detector with a 75 mm diameter field of view. Photon Doppler velocimetry (PDV) was used to record the velocity history of the Si/PC interface

    1
    X-ray diffraction results for shocked polycrystalline silicon. (a) Ambient cd phase Si diffraction image. (b)–(e) Time-resolved diffraction images with listed times relative to impact time. The images show the temporal transition from cd phase Si to sh phase Si as the shock wave travels through the material. (f) Measured and simulated (solid line) sh diffraction peaks 406 ns after impact. The broad inner ring labeled PC is from the polycarbonate window and projectile.

    While exposing a sample of silicon to intense pressure—due to the impact of a nearly 12,000 mph plastic projectile—they documented the transformation from its common cubic diamond structure to a simple hexagonal structure. At one point, they could see both structures as the shock wave traveled through the sample in less than half a millionth of a second.

    Their discovery is a dramatic proof of concept for a new way of discerning the makeups of various materials, from impacted meteors to body armor to iron in the center of the Earth.

    Until now, researchers have had to rely on computer simulations to follow the atomic-level changes of a structural transformation under pressure, said Yogendra Gupta, Regents professor and director of the WSU Institute of Shock Physics. The new method provides a way to actually measure the physical changes and to see if the simulations are valid.

    “For the first time, we can determine the structure,” Gupta said. “We’ve been assuming some things but we had never measured it.”

    Writing in Physical Review Letters, one of the leading physics journals, the researchers say their findings already suggest that several long-standing assumptions about the pathways of silicon’s transformation “need to be reexamined.”

    The discovery was made possible by a new facility, the Dynamic Compression Sector at the Advanced Photon Source located at the Argonne National Laboratory. Designed and developed by WSU, the sector is sponsored by the U.S. Department of Energy’s National Nuclear Security Administration, whose national security research mission includes fundamental dynamic compression science.The Advanced Photon Source synchrotron, funded by the Department of Energy’s Office of Science, provided high-brilliance x-ray beams that pass through the test material and create diffraction patterns that the researchers use to decode a crystal changing its structure in as little as five billionths of a second.

    “We’re making movies,” said Gupta. “We’re watching them in real time. We’re making nanosecond movies.”

    Stefan Turneaure, lead author of the Physical Review Letters paper and a senior scientist at the WSU Institute for Shock Physics, said the researchers exposed silicon to 19 gigapascals, nearly 200,000 times atmospheric pressure. The researchers accomplished this by firing a half-inch plastic projectile into a thin piece of silicon on a Lexan backing. While x-rays hit the sample in pulses, a detector captured images of the diffracted rays every 153.4 nanoseconds—the equivalent of a camera shutter speed of a few millionths of a second.

    “People haven’t used x-rays like this before,” said Turneaure. “Getting these multiple snapshots in a single impact experiment is new.”

    “What I’m very excited about is we are showing how the crystal lattice, how this diamond structure that silicon starts out with, is related to this ending structure, this hexagonal structure,” said Gupta. “We can see which crystal direction becomes which crystal direction. Stefan has done a great job. He’s mastered that. We were able to show how the two structures are linked in real time.”

    See the full article here .

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 5:25 am on July 26, 2016 Permalink | Reply
    Tags: , Material Sciences, , Spider silk, Spiders spin unique phononic material   

    From Rice: “Spiders spin unique phononic material” 

    Rice U bloc

    Rice University

    July 25, 2016
    Mike Williams

    Researchers at Rice University, in Europe and in Singapore discover band gaps in spider silk

    1
    Scientists at Rice University and in Europe and Singapore studied the microstructure of spider silk to see how it transmits phonons, quanta of sound that also have thermal properties. They suggested what they learned could be useful to create strong synthetic fibers with silk-like properties. Click on the image for a larger version. Illustration by Dirk Schneider

    New discoveries about spider silk could inspire novel materials to manipulate sound and heat in the same way semiconducting circuits manipulate electrons, according to scientists at Rice University, in Europe and in Singapore.

    A paper in Nature Materials looks at the microscopic structure of spider silk and reveals unique characteristics in the way it transmits phonons, quasiparticles of sound.

    The research shows for the first time that spider silk has a phonon band gap. That means it can block phonon waves in certain frequencies in the same way an electronic band gap – the basic property of semiconducting materials – allows some electrons to pass and stops others.

    The researchers wrote that their observation is the first discovery of a “hypersonic phononic band gap in a biological material.”

    How the spider uses this property remains to be understood, but there are clear implications for materials, according to materials scientist and Rice Engineering Dean Edwin Thomas, who co-authored the paper. He suggested that the crystalline microstructure of spider silk might be replicated in other polymers. That could enable tunable, dynamic metamaterials like phonon waveguides and novel sound or thermal insulation, since heat propagates through solids via phonons.

    “Phonons are mechanical waves,” Thomas said, “and if a material has regions of different elastic modulus and density, then the waves sense that and do what waves do: They scatter. The details of the scattering depend on the arrangement and mechanical couplings of the different regions within the material that they’re scattering from.”

    Spiders are adept at sending and reading vibrations in a web, using them to locate defects and to know when “food” comes calling. Accordingly, the silk has the ability to transmit a wide range of sounds that scientists think the spider can interpret in various ways. But the researchers found silk also has the ability to dampen some sound.

    “(Spider) silk has a lot of different, interesting microstructures, and our group found we could control the position of the band gap by changing the strain in the silk fiber,” Thomas said. “There’s a range of frequencies that are not allowed to propagate. If you broadcast sound at a particular frequency, it won’t go into the material.”

    In 2005, Thomas teamed with George Fytas, a materials scientist at the University of Crete and the Institute of Electronic Structure and Laser Foundation for Research and Technology-Hellas, Greece, on a project to define the properties of hypersonic phononic crystals. In that work, the researchers measured phonon propagation and detected band gaps in synthetic polymer crystals aligned at regular intervals.

    “Phononic crystals give you the ability to manipulate sound waves, and if you get sound small enough and at high enough frequencies, you’re talking about heat,” Thomas said. “Being able to make heat flow this way and not that way, or make it so it can’t flow at all, means you’re turning a material into a thermal insulator that wasn’t one before.”

    Fytas and Thomas decided to take a more detailed look at dragline silk, which spiders use to construct a web’s outer rim and spokes and as a lifeline. (A spider suspended in midair is clinging to a dragline.) Though silk has been studied for thousands of years, it has only recently been analyzed for its acoustic properties.

    Silk is a hierarchical structure comprised of a protein, which folds into sheets and forms crystals. These hard protein crystals are interconnected by softer, amorphous chains, Thomas said. Stretching or relaxing the interconnecting chains changes the silk’s acoustic properties by adjusting the mechanical coupling between the crystals.

    Fytas’ team at the Max Planck Institute for Polymer Research in Mainz, Germany, performed Brillouin light scattering experiments to test silk placed under varying degrees of stress. “That was George’s genius,” Thomas said. “With Brillouin scattering, you use light to create phonons as well as absorb them from the sample. BLS allows you to see how the phonons move around inside any object, depending on the temperature and the material’s microstructure.”

    They found that when silk was “super contracted,” the velocity of phonons decreased by 15 percent while the bandwidth of frequencies it could block increased by 31 percent. Conversely, when strained, the velocity increased by about 27 percent, while the bandwidth decreased by 33 percent. They first observed a band gap in native (uncontracted) silk at about 14.8 gigahertz, with a width of about 5.2 gigahertz.

    Just as interesting to the team was the “unique region of negative group velocity” they witnessed. At these conditions, even though phonon waves moved forward, the phase velocity moved backward, Thomas said. They suggested the effect may allow for the focusing of hypersonic phonons.

    “Right now, we don’t know how to do any of this in other macromolecular fiber materials,” Thomas said. “There’s been a fair amount of investigation on synthetic polymers like nylon, but nobody’s ever found a band gap.”

    Co-authors of the paper are Dirk Schneider of ebeam Technologies, Bern, Switzerland, and Nikolaos Gomopoulos of the Swiss Federal Institute of Technology in Lausanne, both formerly of the Max Planck Institute; Cheong Koh of DSO National Laboratories, Singapore; Periklis Papadopoulos of the Planck Institute and the University of Ioannina, Greece; and Friedrich Kremer of the Institute of Experimental Physics at the University of Leipzig, Germany. Fytas is a professor at the University of Crete and has an appointment at the Planck Institute. Thomas is the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, a professor of materials science and nanoengineering and of chemical and biomolecular engineering.

    The Aristeia Alliance of the Mediterranean Institute for Scientific Research, the European Research Council, the Sonderforschungsbereich/Transregio (Collaborative Research Center) and the Deutsch Forschungsgemeinschaft (German Research Foundation) supported the research.

    See the full article here .

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    Rice U campus

    In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

    This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

     
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