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  • richardmitnick 5:01 pm on February 21, 2017 Permalink | Reply
    Tags: , Material Sciences, , When Rocket Science Meets X-ray Science,   

    From LBNL: “When Rocket Science Meets X-ray Science” 

    Berkeley Logo

    Berkeley Lab

    February 21, 2017
    Glenn Roberts Jr.
    glennemail@gmail.com
    510-486-5582

    Berkeley Lab and NASA collaborate in X-ray experiments to ensure safety, reliability of spacecraft systems.

    1
    Francesco Panerai of Analytical Mechanical Associates Inc., a materials scientist leading a series of X-ray experiments at Berkeley Lab for NASA Ames Research Center, discusses a 3-D visualization (shown on screens) of a heat shield material’s microscopic structure in simulated spacecraft atmospheric entry conditions. The visualization is based on X-ray imaging at Berkeley Lab’s Advanced Light Source. (Credit: Marilyn Chung/Berkeley Lab)

    Note: This is the first installment in a four-part series that focuses on a partnership between NASA and Berkeley Lab to explore spacecraft materials and meteorites with X-rays in microscale detail.

    It takes rocket science to launch and fly spacecraft to faraway planets and moons, but a deep understanding of how materials perform under extreme conditions is also needed to enter and land on planets with atmospheres.

    X-ray science is playing a key role, too, in ensuring future spacecraft survive in extreme environments as they descend through otherworldly atmospheres and touch down safely on the surface.

    Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and NASA are using X-rays to explore, via 3-D visualizations, how the microscopic structures of spacecraft heat shield and parachute materials survive extreme temperatures and pressures, including simulated atmospheric entry conditions on Mars.

    Human exploration of Mars and other large-payload missions may require a new type of heat shield that is flexible and can remain folded up until needed.


    Streaking particles collide with carbon fibers in this direct simulation Monte Carlo (DSMC) calculation based on X-ray microtomography data from Berkeley Lab’s Advanced Light Source. NASA is developing new types of carbon fiber-based heat shield materials for next-gen spacecraft. The slow-motion animation represents 2 thousandths of a second. (Credit: Arnaud Borner, Tim Sandstrom/NASA Ames Research Center)

    Candidate materials for this type of flexible heat shield, in addition to fabrics for Mars-mission parachutes deployed at supersonic speeds, are being tested with X-rays at Berkeley Lab’s Advanced Light Source (ALS) and with other techniques.

    LBNL/ALS
    LBNL/ALS

    “We are developing a system at the ALS that can simulate all material loads and stresses over the course of the atmospheric entry process,” said Harold Barnard, a scientist at Berkeley Lab’s ALS who is spearheading the Lab’s X-ray work with NASA.

    The success of the initial X-ray studies has also excited interest from the planetary defense scientific community looking to explore the use of X-ray experiments to guide our understanding of meteorite breakup. Data from these experiments will be used in risk analysis and aid in assessing threats posed by large asteroids.

    The ultimate objective of the collaboration is to establish a suite of tools that includes X-ray imaging and small laboratory experiments, computer-based analysis and simulation tools, as well as large-scale high-heat and wind-tunnel tests. These allow for the rapid development of new materials with established performance and reliability.


    NASA has tested a new type of flexible heat shield, developed through the Adaptive Deployable Entry and Placement Technology (ADEPT) Project, with a high-speed blow torch at its Arc Jet Complex at NASA Ames, and has explored the microstructure of its woven carbon-fiber material at Berkeley Lab. (Credit: NASA Ames)

    This system can heat sample materials to thousands of degrees, subject them to a mixture of different gases found in other planets’ atmospheres, and with pistons stretch the material to its breaking point, all while imaging in real time their 3-D behavior at the microstructure level.

    NASA Ames Research Center (NASA ARC) in California’s Silicon Valley has traditionally used extreme heat tests at its Arc Jet Complex to simulate atmospheric entry conditions.

    Researchers at ARC can blast materials with a giant superhot blowtorch that accelerates hot air to velocities topping 11,000 miles per hour, with temperatures exceeding that at the surface of the sun. Scientists there also test parachutes and spacecraft at its wind-tunnel facilities, which can produce supersonic wind speeds faster than 1,900 miles per hour.

    Michael Barnhardt, a senior research scientist at NASA ARC and principal investigator of the Entry Systems Modeling Project, said the X-ray work opens a new window into the structure and strength properties of materials at the microscopic scale, and expands the tools and processes NASA uses to “test drive” spacecraft materials before launch.

    “Before this collaboration, we didn’t understand what was happening at the microscale. We didn’t have a way to test it,” Barnhardt said. “X-rays gave us a way to peak inside the material and get a view we didn’t have before. With this understanding, we will be able to design new materials with properties tailored to a certain mission.”

    He added, “What we’re trying to do is to build the basis for more predictive models. Rather than build and test and see if it works,” the X-ray work could reduce risk and provide more assurance about a new material’s performance even at the drawing-board stage.

    2
    Francesco Panerai holds a sample of parachute material at NASA Ames Research Center. The screen display shows a parachute prototype (left) and a magnified patch of the material at right. (Credit: Marilyn Chung/Berkeley Lab)

    Francesco Panerai, a materials scientist with NASA contractor AMA Inc. and the X-ray experiments test lead for NASA ARC, said that the X-ray experiments at Berkeley Lab were on samples about the size of a postage stamp. The experimental data is used to improve realistic computer simulations of heat shield and parachute systems.

    “We need to use modern measurement techniques to improve our understanding of material response,” Panerai said. The 3-D X-ray imaging technique and simulated planetary conditions that NASA is enlisting at the ALS provide the best pictures yet of the behavior of the internal 3-D microstructure of spacecraft materials.

    The experiments are being conducted at an ALS experimental station that captures a sequence of images as a sample is rotated in front of an X-ray beam. These images, which provide views inside the samples and can resolve details less than 1 micron, or 1 millionth of a meter, can be compiled to form detailed 3-D images and animations of samples.

    This study technique is known as X-ray microtomography. “We have started developing computational tools based on these 3-D images, and we want to try to apply this methodology to other research areas, too,” he said.

    Learn more about the research partnership between NASA and Berkeley Lab in these upcoming articles, to appear at :

    Feb. 22—The Heat is On: X-rays reveal how simulated atmospheric entry conditions impact spacecraft shielding.
    Feb. 23—A New Paradigm in Parachute Design: X-ray studies showing the microscopic structure of spacecraft parachute fabrics can fill in key details about how they perform under extreme conditions.
    Feb. 24—Getting to Know Meteors Better: Experiments at Berkeley Lab may help assess risks posed by falling Space rocks.

    The Advanced Light Source is a DOE Office of Science User Facility.

    See the full article here .

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  • richardmitnick 12:39 pm on January 30, 2017 Permalink | Reply
    Tags: , , Material Sciences, , , Symmetry in patterns, symmorphic and nonsymmorphic patterns of atoms, Theorists propose new class of topological metals with exotic electronic properties, tungsten telluride (WTe2)   

    From Princeton: “Theorists propose new class of topological metals with exotic electronic properties (Physics Review X)” 

    Princeton University
    Princeton University

    January 30, 2017
    Tien Nguyen, Department of Chemistry

    1
    A new theory explains the behavior of a class of metals with exotic electronic properties. Credit: Muechler et al., Physics Review X

    Researchers at Princeton, Yale, and the University of Zurich have proposed a theory-based approach to characterize a class of metals that possess exotic electronic properties that could help scientists find other, similarly-endowed materials.

    Published in the journal Physical Review X, the study described a new class of metals based on their symmetry and a mathematical classification known as a topological number, which is predictive of special electronic properties. Topological materials have drawn intense research interest since the early 2000s culminating in last year’s Nobel Prize in Physics awarded to three physicists, including F. Duncan Haldane, Princeton’s Eugene Higgins Professor of Physics, for theoretical discoveries in this area.

    “Topological classification is a very general way of looking at the properties of materials,” said Lukas Muechler, a Princeton graduate student in the laboratory of Roberto Car, Princeton’s Ralph W. *31 Dornte Professor in Chemistry and lead author on the article.

    A popular way of explaining this abstract mathematical classification involves breakfast items. In topological classification, donuts and coffee cups are equivalent because they both have one hole and can be smoothly deformed into one another. Meanwhile donuts cannot deform into muffins which makes them inequivalent. The number of holes is an example of a topological invariant that is equal for the donut and coffee cup, but distinguishes between the donut and the muffin.

    “The idea is that you don’t really care about the details. As long as two materials have the same topological invariants, we can say they are topologically equivalent,” he said.

    Muechler and his colleagues’ interest in the topological classification of this new class of metals was sparked by a peculiar discovery in the neighboring laboratory of Robert Cava, Princeton’s Russell Wellman Moore Professor of Chemistry. While searching for superconductivity in a crystal called tungsten telluride (WTe2), the Cava lab instead found that the material could continually increase its resistance in response to ever stronger magnetic fields – a property that might be used to build a sensor of magnetic fields.

    The origin of this property was, however, mysterious. “This material has very interesting properties, but there had been no theory around it,” Muechler said.

    The researchers first considered the arrangement of the atoms in the WTe2 crystal. Patterns in the arrangement of atoms are known as symmetries, and they fall into two fundamentally different classes – symmorphic and nonsymmorphic – which lead to profound differences in electronic properties, such as the transport of current in an electromagnetic field.

    2
    a) Symmorphic symmetry b) Nonsymmorphic symmetry Credit: Lukas Muechler

    While WTe2 is composed of many layers of atoms stacked upon each other, Car’s team found that a single layer of atoms has a particular nonsymmorphic symmetry, where the atomic arrangement is unchanged overall if it is first rotated and then translated by a fraction of the lattice period (see figure).

    Having established the symmetry, the researchers mathematically characterized all possible electronic states having this symmetry, and classified those states that can be smoothly deformed into each other as topologically equivalent, just as a donut can be deformed into a cup. From this classification, they found WTe2 belongs to a new class of metals which they coined nonsymmorphic topological metals. These metals are characterized by a different electron number than the nonsymmorphic metals that have previously been studied.

    In nonsymmorphic topological metals, the current-carrying electrons behave like relativistic particles, in other words, as particles traveling at nearly the speed of light. This property is not as susceptible to impurities and defects as ordinary metals, making them attractive candidates for electronic devices.

    The abstract topological classification also led the researchers to suggest some explanations for some of the outstanding electronic properties of bulk WTe2, most importantly its perfect compensation, meaning that it has an equal number of holes and electrons. Through theoretical simulations, the researchers found that this property could be achieved in the three-dimensional crystalline stacking of the WTe2 monolayers, which was a surprising result, Muechler said.

    “Usually in theory research there isn’t much that’s unexpected, but this just popped out,” he said. “This abstract classification directly led us to explaining this property. In this sense, it’s a very elegant way of looking at this compound and now you can actually understand or design new compounds with similar properties.”

    Recent photoemission experiments have also shown that the electrons in WTe2 absorb right-handed photons differently than they would left-handed photons. The theory formulated by the researchers showed that these photoemission experiments on WTe2 can be understood based on the topological properties of this new class of metals.

    In future studies, the theorists want to test whether these topological properties are also present in atomically-thin layers of these metals, which could be exfoliated from a larger crystal to make electronic devices. “The study of this phenomena has big implications for the electronics industry, but it’s still in its infant years,” Muechler said.

    This work was supported by the U.S. Department of Energy (DE-FG02-05ER46201), the Yale Postdoctoral Prize Fellowship, the National Science Foundation (NSF CAREER DMR-095242 and NSF-MRSEC DMR-0819860), the Office of Naval Research (ONR-N00014-11-1- 0635), the U.S. Department of Defense (MURI-130-6082), the David and Lucile Packard Foundation, the W. M. Keck Foundation, and the Eric and Wendy Schmidt Transformative Technology Fund.

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    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 2:01 pm on December 9, 2016 Permalink | Reply
    Tags: An old meteor yields a new surprise: a never-before-seen material, , Khatyrka meteorite, Material Sciences, Quasiperiod crystals   

    From Astronomy: “An old meteor yields a new surprise: a never-before-seen material” 

    Astronomy magazine

    astronomy.com

    December 08, 2016
    Mika McKinnon

    1
    Quasicrystals provide a new perspective on solids. Luca Bindi, Chaney Lin, Chi Ma & Paul J. Steinhardt

    It’s not every day scientists discover an entirely new material adrift in the universe, but in pouring over the Khatyrka meteorite, researchers led by Luca Bindi found an all-new type of quasicrystal never seen before. The research was published in Scientific Reports.

    Quasicrystals are an odd geological material residing somewhere between the tidy order of classic crystals like quartz and diamonds, and the utter chaos of amorphous messes like obsidian and glass. Crystalline solids have a regular chemical composition building a repeating pattern with very clearly-defined symmetry. Amorphous solids have no pattern or symmetry at all. Quasiperiod crystals – quasicrystals for short – have patterns without repeating. The result is more akin to medieval Islamic mosaics than kitchen tiles, patterns following mathematics rules without the symmetry fundamental to crystals.

    Quasicrystals have a dramatic history. Dan Shechtman grew the first quasicrystal in 1982, a discovery so controversial he was asked to leave his research lab. But the evidence was overwhelming: this was a new type of material. Schechtman was honored for the discovery with the 2011 Nobel Prize in Chemistry.

    More synthetic crystals followed, but the next breakthrough was finding two matching quasicrystals in tiny fragments of the Khatyrka meteorite in 2009. The first wild quasicrystals’ structures were familiar, a variation on a synthetic quasicrystal grown from aluminum, copper, and iron in the lab. Today’s discovery of a new quasicrystal from the same meteorite is even more exciting, marking not just the third quasicrystal ever found in nature, but also the first new type of quasicrystal found the wild before it was ever grown in a lab.

    In the lab, quasicrystals need very particular conditions for growth or the same mix of chemicals will default into a symmetrical crystal structure instead. So how do they form in the chaotic real world?

    A research group led by Paul Asimow and including Bindi tried to experimentally recreate scenarios that could spawn quasicrystals. Taking a hint from Khatyrka’s evident history of violent collisions leading to shock-melting at over 5GPa of pressure at 1,200C, they fired bullets of mock-meteorites into a larger stationary fragment in a high-energy dead-on impact. The results announced earlier this year proved that, at least in theory, asteroids ramming into each other with enough energy followed by the rapid cooling enabled by deep space creates, briefly, an environment where quasicrystals naturally spawn.

    Encouraged, Bindi, Shechtman, and the rest of the team started reanalyzing their precious rock, searching for any more evidence to support their theories. They methodically searched for quasicrystals just a few microns in size, the proverbial needle in the haystack but harder to find. Their painstaking search paid off: on a meteorite chip less than half a millimeter across, they found their needle, an new quasicrystal. Like the previous discovery, it formed from a mix of aluminum, copper, and iron, but in different proportions to create an all-new structure never seen before.

    Nobody is quite sure how to apply these new materials, nor where else they might be found. But with three discoveries of two quasicrystals behind them, scientists have good reason to keep up their painstakingly-systematic search to discovery.

    See the full article here .

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  • richardmitnick 1:13 pm on October 18, 2016 Permalink | Reply
    Tags: , , , 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.

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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.

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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.

    See the full article here. .

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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.

    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.

     
  • 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.

     
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