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【原创】International Conference on Neutron Scattering---- DAYS 3, 4
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International Conference on Neutron Scattering May 3-7, 2009 Knoxville, Tennessee DAYS 3, 4 WEDNESDAY, THURSDAY, MAY 6,7 The 2009 International Conference on Neutron Scattering (ICNS2009) concluded on Thursday, May 7, 2009, in Knoxville, Tennessee, after a successful run of five days of activities and technical sessions. The final event was a mixer followed by a closing plenary talk by Sunil Sinha (University of California, San Diego). The next American Conference on Neutron Scattering (ACNS) will be held in Ottawa, Canada, in 2010, while the next International Conference on Neutron Scattering (ICNS) will be held in Europe in four years.   CONTENTS Plenary Lecture - David Mandrus Plenary Lecture - Collin Leslie Broholm Closing Plenary Lecture - Sunil Sinha Technical Sessions Scenes from the Banquet  PLENARY LECTURE - DAVID MANDRUS A Materials Overview of the New Iron-Based Superconductors Gold is for the mistress -- silver for the maid -- Copper for the craftsman cunning at his trade. "Good!" said the Baron, sitting in his hall, "But Iron -- Cold Iron -- is master of them all." - Rudyard Kipling   The new class of iron-based superconductors has been very much in the news over the past year or so. David Mandrus (Oak Ridge National Laboratory) presented an excellent overview of these materials. Fe-based superconductors are very exciting because they represent a new family of high-Tc superconductors, they are the best model systems for unconventional superconductivity, and they represent a large materials space to look for new superconductors. Mandrus presented a brief history of these materials leading up to the report in early 2008 of 26 K superconductivity in F-doped LaFeAsO. The key feature of these materials are the square nets of tetrahedrally coordinated iron. Mandrus first discussed the 1-1-1-1 materials including the structure, theoretical density fuctional theory studies, and magnetic order. He then talked about 1-2-2 materials such as BaKFeAs. He finally discussed FeTe(1-x)Sex materials. He ended by indicating that lots of new materials will emerge in this broad class of superconducting Fe-based materials and the whole field has lots to offer to researchers involved in it. PLENARY LECTURE - COLLIN LESLIE BROHOLM Exploring Quantum Magnetism Through Neutron Scattering  Collin Broholm presented the plenary lecture on Thursday morning, on the last day of the conference. The subject of his talk was Quantum Magnetism; where do we find it, and why is it interesting? Ferromagnetism essentially is a fundamental, and collective phenomenon. Quantum magnetism is inter-atomic singlet formation, macroscoscopic singled formation, stong correlations without static order, emergent quantum particles. Broholm indicated that the fundamental question for the field is what types of collective phases are possible, and what are the corresponding quasi-particles. The elements of the research include materials synthesis (both bulk and nano-structured), advanced characterization and of course theory. In terms of applications, these systems represent model systems for quantum correlated physics. Other potential applications include sensors and filters via strong collective response and quantum computing by manipulating emergent quasi-particles. Neutron scattering plays an important role in many of these studies, as Broholm showed using a few examples. To summarize, quantum magnetism goes beyond spin to collective quantum correlations. It is stabilized by low dimensionality, frustration, and proximity to metal insulator transitions. CLOSING PLENARY LECTURE - SUNIL SINHA Neutron Scattering: Accomplishments, Challenge and Outlook for the Future  Sunil Sinha (University of California, San Diego) presented an outstanding closing lecture outlining the current state of the field of neutron scattering. There have been tremendous accomplishments in the field such as antiferromagnetic structures, rare earth spirals and spin structures, magnetism and superconductivity etc., as well as in soft matter. More recent accomplishments include quantum phase transitions, magnetic order and magnetic fluctuations in high-Tc cuprates etc. He then presented the competition - synchrotron X-rays, scanning probe microscopies, direct imaging. He strongly made the point that though several of the advances and accomplishments in neutron scattering are tremendous, it is very difficult to convey this visually to the scientific community and to the general public as compared to other techniques yielding beautiful images and videos. He noted that synchrotron X-rays and neutrons can be complementary rather than competing because of their unique advantages in different ways. He presented the example of nanomagnetism with X-rays, imaging and neutrons all yielding different but significant information. Sinha then made the connection to usefulness of the field to society including in areas such as energy, environment, health, security, culture. In terms of training young scientists in the field, he suggested that it is better to have several smaller neutron sources in addition to one "supersource," where they get a chance to try out different techniques and intruments before going on to the large sources. Finally, he reiterated the point made on the first day of the conference in needing to "sell" neutron scattering to the general public. This was an appropriate plenary talk by a stalwart in the field to end the conference on a high note.   Closing mixer TALKS Symposium A: Soft Condensed Matter Structure and Dynamics of Polymer Electrolytes Lithium-ion batteries are extremity popular today -- they are some of the most energetic rechargeable batteries available. They can be found everywhere - in iPods, laptops, and cell phones. Janna Maranas from Pennylvania State University presented an investigation on the solid polymer electrolyte PEO/LiClO4 which can be used as a conductive material in lithium-ion batteries. PEO is a wonderful choice since it is non-toxic, highly flexible and mobile at room temperature. In the talk, Maranas discussed the relationship between PEO structure, mobility and ionic conductivity. At the eutectic point composition in PEO/LiClO4, fluctuations occur which introduce a configuration reminiscent of the PEO6 crystal structure. This structure has high conductivity at low PEO molecular weight, although the formation of a lamellar morphology at high molecular weight disrupts long range transport of lithium ions. She showed that this structure formed via fluctuations, and contributes to conductivity. If the flucutations are stabilized, either by a small amount of the pure PEO crystalline phase, or by nanoparticles, the conductivity is enhanced. Both of these modifications do not influence segmental dynamics of the polymer, which is normally associated with lithium transport. Instead, rotation of the PEO6 structure is altered in the presence of pure PEO crystals or nanoparticles.  In-Situ Structural and Rheological Investigation of Fully Hydrated Fibrin Gels formed in D2O buffers Fibrinogen is a protein crucial to the formation of blood clots. In the presence of the protein thrombin, fibrinogen is converted into fibrin which then self assembles into a network of interconnected fibers. Like other biopolymer matrices the resulting hydrogel exhibits unique mechanical properties, for example the fibrin gel stiffens upon application of sufficient shear stress. Kathleen Weigandt, a graduate student from the University of Washington demonstrated that using a combination of small angle neutron scattering and various rheological techniques, she can determine the origin of the strain dependent rheological properties of fibrin gels. She also showed in her talk that the strain-hardening of silk is reversible and very sensitive to the protein concentration.  The Couette configuration of the Neutron Stroboscopic Rheometer for the investigation of entangled polymers in the bulk under shear via small-angle neutron scattering The Couette configuration of the Neutron Stroboscopic Rheometer (CNSR) has been constructed at the Los Alamos Neutron Science Center (LANSCE) to examine the molecular steady-state and dynamic responses of entangled polymeric materials in the bulk to the application of shear stress via small angle neutron scattering (SANS). This was described in a talk by Eric Yearley (Los Alamos National laboratory). The SANS measurements along with the implementation of the CNSR will provide new information on the structure and dynamic behavior at the molecular and nanoscales of polymers under shear stress. SANS results of the highly viscous sample, bulk polystyrene, are presented. The tube model for entangled polymers in the bulk suggests that the disentanglement and elongation of polymer chains due to external stress may be responsible for their unique rheological properties including shear-thinning. The observed anisotropic neutron scattering pattern of the polystyrene melt above entanglement provides evidence that the conformation of the chains are elongated in the direction of the melt flow in agreement with the current tube theories concerning linear polymers in the bulk.   Symposium B: Biological Sciences Collective Molecular Dynamics in Membranes and Proteins Understanding of molecular dynamics and mechanisms of functioning of biological membranes is one of the important questions in molecular biology and biophysics. Indeed, a cell membrane has a very complex structure and presents a primary side of interactions. in this respect, Maikel Rheinstadter from McMaster University (Canada) presented new fundamental insights into membrane functioning and protein-protein interactions. He addressed several important questions in his talk. For example, he described dynamics between molecules in membranes to be “multi-scale” with a large range of length (from microscopic to mesoscopic) and time scales. He also illustrated how membrane composition determines dynamics and membrane properties. Applying inelastic scattering measurements he studied membrane permeability and elasticity. Based on his study he found that even in simple models, biological systems present an array of units interacting trough coherent interactions indicating that coherence is a fundamental property in molecular biology.   Symposium C: Condensed Matter Physics Neutron Diffraction Studies of Negative Thermal Expansion in Metallic Conductor ReO3 Metals generally have high coefficients of thermal expansion. Conversely, highly covalent materials such as diamond show low thermal expansion, and other insulating materials such ZrW2O8 even exhibit negative thermal expansion (NTE). ReO3 appears to be unique in that it is the only known ‘simple metal’ which exhibits isotropic NTE behavior. The metallic conductor ReO3 exhibits both negative and positive thermal expansion, and past studies have varied significantly on the temperature regime at which NTE or PTE are observed. Time-of-flight (TOF) neutron diffraction and specific heat studies of various samples of ReO3 were presented in this report by Efrain Rodriguez (University of California, Santa Barbara) to explain the observed discrepancies in past studies. Local and Average Structures of the spin Glass Pyrochlore Y2Mo2O7 from Neutron Diffraction and Neutron Pair Distribution Function Analysis Pyrochlore oxide was investigated by means of temperature dependent neutron diffraction and neutron pair distribution function in order to solve the issues of positional disorder and frustration in the compound, as described by Delphine Gout (Oak Ridge National Laboratory). The measurements showed no crystallographic phase transition over the temperature range studied, the principal source of disorder comes from the Y-O1 atom pairs instead of the previously considered Mo-Mo pairs, data fit improves significantly as anisotropic displacements for all sites are considered, and becomes worse as temperature decreases.   Symposium D: Chemistry/Earth Sciences Hydrogen Storage in Microporous Materials Energy is obviously a major issue currently and in the near-term. Hydrogen of course has been touted to be an important solution to the problem, and in fact hydrogen-fueled automobiles are already making an appearance. The US Department of Energy has placed a grand challenge in terms of targets for hydrogen storage materials FOR 2010, 2015 and beyond including gravimetric and volumetric capacities. Craig Brown (NIST) in his talk on Tuesday described microporous materials for hydrogen storage and ways to improve storage capacities. These include optimized poresize (high specific areas), catalytic spillover of hydrogen, exposed metals and substituted/dopant materials. Brown showed that neutron methods are crucial for these attempts at understanding the performance of materials, including powder diffraction, vibrational and rotational spectroscopy and quasi-elastic scattering. Some of the materials discussed included MIL-53, Mn-BTT and the metalorganic framework, MOF-74. Flexible frameworks provide interesting new avenues for optimized pore sizes. Also, for MOFs with coordinatively unsaturated metal centers, the metal is the primary hydrogen absorption site and the hydrogen tends to lay down on the metal, the rotational dynamics are quasi-2D and there is no H-H bond elongation, and the enhanced interaction is primarily Coulombic. The presentation clearly validated the advantages of using neutron-based techniques for microporous materials.  Symposium E: Engineered and Applied Materials Revealing the Unseen: Residual Stress Characterization of a Fabrication Weld from the Victoria-Class Submarine Pressure-Hull The HMCS Victoria is a Canadian Navy Victoria-class submarine that was obtained from the British Navy and was formerly the HMS Unseen. When the submarine reached Canadian docks, a largish dent was observed in one panel. This section was removed in the dry dock and this section contained a length of circumferential-seam closure weld. This has allowed for the rare opportunity for detailed characterization of the as-fabricated condition of this common weld type from the original vessel assembly. Ron B. Rogge (Canadian Neutron Beam Center, Chalk River, Canada) described neutron diffraction studies to study this weld structure. Neutron diffraction is able to yield 3-D characterization of the weld and surounding structure. The data indicated that the as-formed hull plate (no weld) showed internal tensile stress peaks of around 35% yield stress in both axial and circumferential directions. The circ-seam weld yielded near-surface peak tensile stress values of up to 80% yield stress with a slight dominance of transverse direction stresses. In addition, Rogge described new 3D visualization methods to help guide modeling activities and efforts to attenuate stresses by improved fabrication techniques. In a demonstration he asked the audience to view two dots cross-eyed so that they merge into three and then look at the visualization underneath allowing for 3-D visualization. He termed this his attempt at audience participation. The results of this study will be incorporated into numerical models supporting submarine mission-ops. In-Situ Electric-Field Studies of Ferroelectric Ceramics Using Time-Resolved Neutron Diffraction on WOMBAT Ferroelectrics can couple mechanical and electrical displacements as a result of their spontaneous polarization. They are used in a number of electromechanical applications. Many of these applications involve applied fields that are dynamic, with time-dependent dielectric displacement in response to a cyclic stress. The objective of the present study, described by Jacob L. Jones (University of Florida) was to evaluate a new commercial PZT material, EC-65, under the application of dynamic electric fields, in particular using neutron diffraction measurements. These were carried out using WOMBAT which is a high-flux neutron powder diffractometer at the new OPAL reactor at ANSTO, Australia, ideal for time-resolved studies, with stroboscopic data acquisition with 31 microseconds resolution. Ferroelectric/ferroelastic domain switching, tetragonal to rhombohedral phase switching and lattice strains were also measured. The response times of these microstructural mechanisms were found to be faster than the minimum acquisition window indicating that the ferroelectric material exhibits a very soft character with high domain wall displacement. Also domain and phase switching were found to be possible origins of macroscopic nonlinear peizoelectric coefficients in this material. SCENES FROM THE BANQUET                 ABOUT THE MEETING SCENE The Meeting Scene e-mails were compiled and edited by Dr. Gopal Rao, Web Science Editor, Materials Research Society  Additional contributors include Dr. Eugenia Kharlampieva (Georgia Institute of Technology) and Dr. Alice I. Smith (Los Alamos National Laboratory)  Eugenia Kharlampieva  Alice I. Smith You have received this as a subscriber to the Meeting Scene or as a member of the NSSA. To Unsubscribe, please e-mail braughler@mrs.org. Archived Meeting Scene Issues are available online. View all free MRS e-newsletters and alertsand subscribe. |
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