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Scientific and Technological Problems Investigated at the NCNR
SOFT MATERIALS
Neutron scattering plays a pivotal role in how scientists and engineers understand and design the molecular structure, dynamics, physical properties, and processing of soft materials. This class of condensed matter subsumes an enormous variety of products, including plastics, composites, almost all forms of biological tissue, and complex combinations of organic and inorganic compounds formulated into personal care products. The NCNR provides the nation and the international community with the neutron-scattering facilities, associated measurement techniques, and fundamental knowledge necessary to advance our understanding of how soft materials are spatially configured and how they respond temporally to forces at length scales ranging from 1 to 10,000 nm. This effort draws on an extensive suite of instruments currently in operation or under development, including five small-angle neutron scattering (SANS) machines, four reflectometers, a backscattering instrument, a spin-echo spectrometer, a disk chopper spectrometer (DCS), and an imaging station. This section highlights selected recent advancements in experimental techniques and scientific discoveries at the NCNR through neutron scattering on soft materials.
A particularly exciting area of research pursued at the NCNR deals with the structure and dynamics of lipid membranes, which form the boundary of most cellular structures in living tissue. Lipids are a category of molecules known as amphiphiles. One portion of a lipid, known as the “head group,” is polar and readily associates with water, while the tail section is a nonpolar, oil-like hydrocarbon that does not mix with aqueous media. These compounds self-assemble into sheetlike bilayer structures just 3 to 4 nm thick with the hydrocarbon tails at the center. In living cells, this bilayer membrane is decorated with a plethora of other structural elements, such as cholesterol and proteins, that mediate chemical and electrical communication inside and outside the cell. Understanding the structure and function of lipid membranes is a central challenge to molecular biology.
Neutron scattering offers unparalleled opportunities to unravel the mysteries associated with lipid bilayers. The NCNR team has provided scientists from around the nation with a robust complement of experimental tools with which to attack these problems. Neutron reflectivity plays a particularly important role. In collaboration with scientists from the National Institutes of Health (NIH), Carnegie Mellon University, the University of Chicago, the University of California, Irvine, and other institutions, NCNR staff members and postdoctoral researchers have explored how integral membrane proteins are embedded in the lipid bilayer and how peripheral membrane proteins attach to the membrane surface. By mathematically inverting the angular-dependent intensity of neutrons reflected from these structures, information regarding the precise location and configuration of such compounds within and around the lipid membranes have been exposed. Augmented by molecular theory, these results are guiding exciting new interpretations of cellular function. These studies provide the basic ingredients to target specific diseases with tailored molecular-level therapies.
Polymers, composites, and self-assembled soft materials such as surfactants inevitably are processed under conditions that require flow and deformation. Neutron scattering offers unique opportunities to explore the basic molecular configurations and molecular-scale deformations that govern the processing conditions required to optimize ultimate material properties. Polyethylene, the largest
volume synthetic polymer, is a semicrystalline material that is formed into products such as packaging films and plastic parts at elevated temperatures by melt processing. Viscoelastic fluids, essential to the oil field services industry as drilling fluids, respond to flow in complex ways that reflect the organization of surfactants into nanoscale structures. The NCNR has been a world leader in developing scattering tools that permit the in situ interrogation of molecular structure during flow.
A unique enabling factor is that cold neutrons can penetrate many structural materials such as glass and aluminum, which can be fashioned into tools that can be inserted into a SANS instrument. These rheological devices impose specified flow fields while simultaneously recording the applied forces, providing a direct link between nanoscale structural features and macroscopic viscoelastic properties. The NCNR has developed a host of complementary devices for this purpose. For example, paints are formulated with a complex array of ingredients that include colloidal polymers and inorganic particles that interact in ways that control the flow behavior. When subjected to shear, the associated structures reorganize in a manner that directly influences the formation of coatings. The Dow Chemical Company has used the SANS instrument to determine the state of dispersion in latex paints filled with titanium dioxide particles while shearing the fluid. The resulting knowledge, published in the leading polymer journal Macromolecules, enabled the development of theoretical models, providing a critical link for the company’s formulation of advanced coatings.
In another project, NCNR scientists teamed up with a research group from the University of Delaware to create a shear flow cell that allows the interrogation of surfactant-based viscoelastic fluids that find applications in myriad products, including cosmetics and drilling fluids. This class of soft materials spontaneously organizes into wormlike micelles, long threadlike moieties that entangle at the nanometer scale, resulting in a soft, gelatinous state. When subjected to shearing flows, these soft solids flow, but not homogeneously. Using a rheological tool designed at the NCNR and operated in conjunction with several SANS instruments, team members were able to unravel how the evolution of a branched micelle structure during shearing resulted in the inhomogeneous flow behavior, and they were able to connect this morphological transition to the applied forces. This discovery holds important consequences for the application of these complex fluids while drilling oil wells.
Another important set of advancements currently being developed at the NCNR is instruments that permit extreme test conditions, including unprecedented temperatures and pressures such as those encountered when drilling deep wells. Other testing devices under development will allow simultaneous application of electric fields to conducting fluids and ionic membranes used in batteries and fuel cells.
Another unique feature afforded by neutron scattering is the ability to label molecules with deuterium, a stable isotope of hydrogen. Because neutrons scatter differently from deuterium, specific structural features can be identified by isotope labeling. Because hydrogen is ubiquitous in soft materials, this technique, enabled by advanced synthetic chemical methods, is an essential tool. NIST scientists are using the NCNR to exploit this approach in order to explore the molecular configurations responsible for the mechanical properties of polyethylene, a $200 billion per year industry worldwide. By swelling semicrystalline polyethylene plastics with deuterium-labeled solvent and monitoring the resulting changes in SANS patterns, scientists were able to establish how portions of the polyethylene molecules connect the crystalline regions of the plastic. This work, conducted in collaboration with the Chevron Phillips Chemical Company, is contributing to the design of tougher and more resilient plastics.
NCNR staff should be applauded for providing an outstanding scientific environment for the diverse range of outside users from universities and industry who make use of the neutron-scattering facilities. The instruments placed at the disposal of the user community play an integral role in the pursuit of fundamental knowledge and the development of advanced and technologically sophisticated soft materials.
HARD MATTER
The Hard Matter research program at the NCNR is focused on three important areas: magnetic materials, novel superconductivity, and the structure of thin films and interfaces. The first two of these are predicated on an excellent suite of inelastic neutron-scattering instruments: MACS; a double-focusing triple-axis spectrometer (BT7); a DCS time-of-flight chopper instrument; and the Spin Polarized Inelastic Neutron Spectrometer (SPINS). Additionally, a very successful program of reflectometry studies has been enabled by NCNR’s strength in this area. The recent exploitation of sophisticated polarization analysis using polarized 3He transmission cells has contributed significantly to all programs related to magnetism in materials.
Magnetism is widely regarded as a significant application of neutron scattering, because the neutron possesses a spin magnetic moment and couples well to magnetism in solids. The magnetism program at the NCNR has focused on a range of very topical magnetic materials, including quantum, low-dimensional, and geometrically frustrated magnetism; multiferroic materials; novel magnetic superconductors; and exotic metals close to a magnetic instability. New capabilities on MACS, in particular, have enabled very important and appreciated studies of the quantum S = 1/2 Kagomé antiferromagnet, herbertsmithite. This general problem is a cause-célèbre in contemporary condensed-matter physics, and the MACS NCNR results have been an extremely high-profile contribution to the field.
NCNR staff and collaborators have carried out very comprehensive studies on several families of Fe-based superconductors and their magnetic parent compounds, mostly using the BT7 and BT1 powder diffractometer, and these have also been well appreciated internationally. The DCS time-of-flight chopper instrument continues to support a broad-ranging program in hard matter research, mainly related to magnetism and low-energy spin dynamics in new materials.
Recent impressive advances in 3He polarization analysis cells at the NCNR have led to the development of new polarization analysis capabilities with wide angular coverage on both BT7 and MACS. This technical advance is state of the art and is expected to very soon enhance the impact of the magnetism program at the NCNR generally. Large area polarization cells have been tested with very encouraging results on both BT7 and MACS. This will make spin flip and non-spin flip measurements of a ~3 dimensional S(Q, ω) relatively routine in the near future, and that would be a significant advance.
The NCNR has longstanding strengths in neutron reflectometry and SANS. In tandem with sophisticated polarization analysis, these capabilities have been exploited to address challenging hard matter problems involving long wavelength and nanoscale magnetism that are of great interest and importance in both basic and applied science. In particular, the recent polarized neutron reflectivity studies performed at the NCNR have beautifully resolved complex interfacial magnetism in oxide thin-film assemblies, which would not have been amenable to study by other techniques.
Inelastic neutron scattering is well known as a powerful probe of lattice dynamics, and measurements of phonon dispersions and densities of state in new materials on BT7 have made important contributions in this area. In particular, recent measurements on new superconducting materials as well as related materials have been used to benchmark theoretical calculations, using density functional theory. The combination of these experimental and theoretical techniques has nicely elucidated the role of electron–phonon interactions in such new materials. This work also clearly illustrates the close interaction between sophisticated theory and neutron scattering and their role in advancing our understanding of ground states in new hard materials.
All of these research programs depend critically on the availability of sophisticated sample environments such that NCNR neutron experiments can be carried out under conditions that most directly reveal the novelty of the states that the new materials display. The NCNR has developed impressive capabilities for neutron scattering at low temperatures and high magnetic fields, and it currently operates the most advanced suite of infrastructure in North America for neutron scattering at low temperatures and high magnetic fields by comparison with its only competitor, the Oak Ridge National Laboratory Spallation Neutron Source (ORNL/SNS). Low temperatures and high magnetic fields are very important
to cold neutron spectroscopy. One of the great strengths of the NCNR is that the energy scale of relevance to cold neutrons (> 0.1 meV ~ 1 K) matches the scales accessed with 3He and dilution fridges, which corresponds to the energy scale of magnetic moments in magnetic fields up to 10 T. The NCNR deserves credit for recognizing the importance of sophisticated sample environment to the broad range of materials research programs that it facilitates.
ENERGY-RELATED RESEARCH
Energy-related research is well recognized as a crucial area of research to serve critical current societal needs. Because neutron-related research can have an impact on important problems in this area, such as energy storage and energy production, of both hydrocarbon forms of energy and alternative energy sources, this is a reasonable program for NIST scientists to be involved in.
One technologically important area is the development of alternative fuels for motor vehicles. The NCNR appears to be one of the few places where serious neutron research is being carried out on metal organic framework (MOF) compounds. These have the potential to store large quantities of methane and could enable motor vehicles to be powered by natural gas (e.g., methane) in a cleaner, safer way, if the energy density storage requirements can be met to be competitive with gasoline or compressed natural gas. Work at the NCNR using neutron powder diffraction and inelastic neutron scattering, coupled with density functional methods to calculate electronic structure and energies from first principles, has given insight into the mechanisms of methane adsorption in MOFs and the importance of open metal sites in these compounds and possible modifications of the structures—for example, with linker functionalization—to begin to approach the Department of Energy (DOE) energy storage requirements. Already, researchers at the NCNR, in collaboration with University of Texas, San Antonio, have found a compound with a record high, to date, CH4 working capacity (257 cm3(STP)/cm3). The capacity nevertheless remains approximately 30 percent below DOE targets for methane storage.
Because of the ability to deuterate and therefore change the contrast of hydrocarbons, neutrons provide a unique and effective tool to image hydrocarbon flow through porous media such as shale or other porous rocks. This is being set up at the imaging facility and is likely to enable high-quality, high-resolution dynamic imaging of hydrocarbon flow in such materials. This is of importance in problems such as secondary and tertiary oil recovery. This facility should also be useful for imaging fuel cells and battery materials under actual working conditions.
FUNDAMENTAL NEUTRON SCIENCE AND APPLICATIONS
The availability of intense, well-characterized cold neutron beams at the NCNR opens the opportunity to perform studies in fundamental neutron science and important applications in imaging and trace element analysis.
Neutron Lifetime
It is remarkable that more than 80 years after the discovery of the neutron, the lifetime of this elementary constituent of matter is still uncertain at the 1 percent level, with a major discrepancy occurring between beam and bottle-type measurements. It is well within the NIST portfolio to resolve this discrepancy and to improve the precision of this fundamental parameter. Plans were presented for a group of several university partners working with NIST staff to perform the necessary measurements in a new location in the guide hall, providing an order of magnitude more intense beam than was used for the previous measurements.
aCORN
Neutrons beta-decay into a proton, an electron, and a neutrino. The standard model of particle physics predicted the angular correlation between the electron and the neutrino. Neutrinos are very hard to detect; however, their directions may be inferred from a measurement of the electron and proton momenta, assuming that the parent neutron is essentially at rest. This measurement is being pursued by a collaboration including external university groups (academic, industrial, and government researchers) working with NIST staff. The apparatus uses a more intense cold neutron beam. This will result in a more definitive measurement with smaller error bars.
Neutron Imaging
Neutrons offer several unique characteristics for imaging. The contrast between rocks and hydrogen-containing fluids opens the possibility of following fluid-penetration through porous rocks as a function of time; the superior penetration ability of neutrons offers the possibility of imaging hydrogen fuel cells in operando, of imaging turbine blades, and following the time development of the curing process in concrete. The NCNR has built a facility to perform simultaneous tomography capabilities using neutrons and x rays, enabling imaging at 10-20 micron resolution. This facility is in high demand, and it will be upgraded using Wolter mirrors. These optical elements, originally developed by the National Aeronautics and Space Administration (NASA), promise to provide magnification capabilities in neutron imaging that may extend the resolution to the 1 micron level.
Prompt Gamma
Neutron activation analysis is a time-honored technique of elemental identification. The Prompt Gamma End Station at the NCNR is used by the NIST Elemental Measurement Science Group to develop standard reference materials used by a wide variety of industries and research institutions. The Prompt Gamma End Station has a new home at the neutron beam guide NGD (neutron-gamma density), where it enjoys a higher cold neutron flux and lower background in its high-resolution gamma-ray detector. This yields lower detection limits for trace elements in the samples of interest.
