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Adsorption and molecular siting of CO₂, water, and other gases in the superhydrophobic, flexible pores of FMOF-1 from experiment and simulation

Description: This article discusses the use of neutron diffraction and molecular simulations to investigate the framework expansion behaviour and the accessibility of the small pockets to N₂, O₂, and CO₂.
Date: March 9, 2017
Creator: Moghadam, Peyman Z.; Ivy, Joshua F.; Arvapally, Ravi K.; dos Santos, Antonio M.; Pearson, John C.; Zhang, Li et al.
Partner: UNT College of Arts and Sciences

Small-angle and surface scattering from porous and fractal materials.

Description: We review the basic theoretical methods used to treat small-angle scattering from porous materials, treated as general two-phase systems, and also the basic experimental techniques for carrying out such experiments. We discuss the special forms of the scattering when the materials exhibit mass or surface fractal behavior, and review the results of recent experiments on several types of porous media and also SANS experiments probing the phase behavior of binary fluid mixtures or polymer solutions confined in porous materials. Finally, we discuss the analogous technique of off-specular scattering from surfaces and interfaces which is used to study surface roughness of various kinds.
Date: September 18, 1998
Creator: Sinha, S. K.
Partner: UNT Libraries Government Documents Department

Characterization of interphase structure using neutron reflection

Description: Neutron reflection is one of only a few characterization techniques which can be used to study buried interfaces in situ. While restricted to model samples, interfacial density and composition profiles can be obtained with a resolution of {approx}5 {angstrom} using isotopic substitution (typically H/D for organic materials). We are using neutron reflection to address several problems of fundamental importance to the durability of organic/inorganic interphases. One important focus of this study is water adsorption within interphases with and without coupling agents. From the time and temperature dependence of moisture uptake and removal in vacuum, information regarding the nature of the interaction of water with the interphase species can be obtained.
Date: May 1, 1997
Creator: Kent, M.S.; McNamara, W.F. & Domeier, L.
Partner: UNT Libraries Government Documents Department

Redox behavior below 1000K of Pt-impregnated CeO{sub 2}-ZrO{sub 2} solid solutions : an in-situ neutron diffraction study.

Description: The Ce{sup 3+} {leftrightarrow} Ce{sup 4+} redox process in automotive three-way catalysts such as Ce-ZrO{sub 2}/Pt provides an essential mechanism to oxygen storage/release under dynamic air-to-fuel ratio cycling. Such a function requires a metal-support interaction which is not completely understood. We have carried out an in-situ neutron powder diffraction study to monitor the crystal structures (a mixture of a major tetragonal and a minor monoclinic phase) of 10mol% Ce-doped ZrO{sub 2} with and without Pt (1wt%) impregnation under oxidizing and reducing conditions over the temperature range of 25--7000 C. The samples were heated first in flowing 2%O{sub 2}/Ar from room temperature to 400 C and then in 1%CO/Ar to about 700 C. A discontinued increase of the tetragonal unit-cell volume, a decrease of tetragonality (c/a), and a change of color from light yellow to gray when changing from oxidizing to reducing atmosphere were observed only in the sample containing Pt. This result supports the model which assumes the formation of oxygen vacancies initially near the Pt atoms. As more Ce ions are reduced from 4+ to 3+ oxidation states at high temperatures, oxygen vacancies migrate to the bulk of the oxide particles.
Date: January 12, 1998
Creator: Loong, C.-K.; Short, S. M.; Ozawa, M. & Suzuki, S.
Partner: UNT Libraries Government Documents Department

Macrostrain measurement using radial collimators at LANSCE

Description: A series of `short` radial collimators have been implemented in the 90{degrees} scattering geometries on the neutron powder diffractometer at Los Alamos. The capability to perform macrostrain measurements has been improved by the commensurate ability to rapidly select a sampling volume appropriate to the specimen. The compact design of the collimators was dictated by the need to fit them in a cylindrical vacuum chamber as well as providing space in which to manipulate a specimen in three dimensions. Collimators of different vane lengths were fabricated to give 4 different resolutions for which 2/3 of the diffracted intensity comes form distances of 0.75, 1. 25, 2.5, and 4.0 mm along the incident beam. Qualifying scans and a demonstration of a cracked ring, containing a steep stress gradient, are included.
Date: June 1, 1996
Creator: Bourke, M.A.M.; Roberts, J.A. & Davis, D.
Partner: UNT Libraries Government Documents Department

Residual stress distribution in an Al{sub 2}O{sub 3}-Ni joint bonded with a composite layer

Description: Neutron diffraction was used to study the residual stress distribution in an axisymmetric Al{sub 2}O{sub 3}-Ni joint bonded with a 40 vol% Al{sub 2}O{sub 3}-60 vol% Ni composite layer. A series of measurements was taken along the axis of symmetry through the Al{sub 2}O{sub 3} and composite layers. It is shown that after taking into account the finite neutron diffraction sampling volume, both the trends and peak values of the experimental strain distribution were in excellent agreement with calculations of a simple finite element model, where the rule-of-mixtures approach was used to describe the constitutive behavior of the composite interlayer. In particular, the predicted steep strain gradient near the interface was confirmed by the experimental data.
Date: July 1, 1996
Creator: Wang, X.L.; Watkins, T.R.; Rabin, B.H.; Williamson, R.L. & Bruck, H.A.
Partner: UNT Libraries Government Documents Department

Instrument design program progress. Final report, May 1 1995--September 30, 1995

Description: The goals of the task for this time period were met, and the resulting tools were used to obtain significant results in a number additional studies. Six presentations will be published in the Proceedings of ICANS-XIII, the 13th Meeting of the International Collaboration on Advanced Neutron Sources, Paul Scherrer Institut, Villigen, Switzerland, October 11-14, 1995. The principle task in this time period and presented here was to make the neutron optics Monte Carlo library MCLIB as general as possible, and to collaborate in the construction of an object-oriented user interface to make it easy for relatively untrained users to use the codes to test neutron instrument designs. As of September 30, the interface could be used to define all element types necessary to study small-angle diffractometers or surface reflectometers.
Date: August 1, 1996
Creator: Seeger, P. A.
Partner: UNT Libraries Government Documents Department

Workshop on Probing Frontiers in Matter with Neutron Scattering, Wrap-up Session Chaired by John C. Browne on December 14, 1997, at Fuller Lodge, Los Alamos, New Mexico

Description: The Workshop on Probing Frontiers in Matter with Neutron Scattering consisted of a series of lectures and discussions about recent highlights in neutron scattering. In this report, we present the transcript of the concluding discussion session (wrap-up session) chaired by John C. Browne, Director of Los Alamos National Laboratory. The workshop had covered a spectrum of topics ranging from high T{sub c} superconductivity to polymer science, from glasses to molecular biology, a broad review aimed at identifying trends and future needs in condensed matter research. The focus of the wrap-up session was to summarize the workshop participants' views on developments to come. Most of the highlights presented during the workshop were the result of experiments performed at the leading reactor-based neutron scattering facilities. However, recent advances with very high power accelerators open up opportunities to develop new approaches to spallation technique that could decisively advance neutron scattering research in areas for which reactor sources are today by far the best choice. The powerful combination of neutron scattering and increasingly accurate computer modeling emerged as another area of opportunity for research in the coming decades.
Date: December 1, 1998
Creator: Mezei, F. & Thompson, J.
Partner: UNT Libraries Government Documents Department

Magnetism in BaCoS{sub 2}

Description: BaCoS{sub 2} is a layered Mott-Hubbard insulator that orders antiferromagnetically near 300 K. We report magnetic susceptibility measurements on polycrystalline BaCoS{sub 2} from 77 K to 800 K, and in- and out-of-plane measurements on single crystals from 2 K to 350 K. We also report a powder neutron refinement of the magnetic structure of BaCoS{sub 2}. The neutron measurements indicate that the moments lie in the plane, and that the magnetic unit cell is the same as the (orthorhombic) chemical cell. The ordered moment obtained from the neutron refinement is close to 3 {mu}{sub B} indicating that the Co ions are in the high-spin (S = 3/2) configuration. The effective moment inferred from the high temperature susceptibility, p = 5.26 {mu}{sub B} is also consistent with high-spin Co{sup +2}.
Date: August 1996
Creator: Mandrus, D.; Chakoumakos, B. C.; Fernandez-Baca, J. A.; Nagler, S. E.; Sales, B. C. & Sarrao, J. L.
Partner: UNT Libraries Government Documents Department

Microphase separation in a model graft copolymer

Description: We present a preliminary overview of our work on a series of graft copolymers having poly(ethyl acrylate) backbones with pendant chains of polystyrene (PS). The copolymer system appeared to be in the strong segregation limit and exhibited evidence of ordered structures. The morphology of these structures can apparently be very different from what would be expected. For instance, we observed a lamellar structure in a material containing 28 wt.% PS grafts. Samples under uniaxial strain showed either conventional (i.e., affine deformation) and anomalous ({open_quotes}butterfly{close_quotes} isointensity patterns) behavior in small-angle neutron scattering.
Date: October 1, 1993
Creator: Dozier, W.D.; Thiyagarajan, P. & Peiffer, D.G.
Partner: UNT Libraries Government Documents Department

High Temperature Structural Behavior of SrRuO{sub 3}

Description: The unusual metal SrRuO{sub 3} is perhaps the only known 4d transition metal based ferromagnet (Tc = 162K) with a sizable moment. To complement low T polarized neutron diffraction measurements of the magnetization density, high T neutron diffraction measurements are reported here. Two structural phase transitions are observed. Between 10K and 800K SrRuO{sub 3} is orthorhombic and at 800K it appears to be tetragonal until 975K, where it becomes cubic. The temperature variation of the lattice parameters are reported along with a structural description of the tetragonal phase.
Date: July 15, 1997
Creator: Chakoumakos, B.C., Nagler, S.E., Misture, S.T., Christen, H.M.
Partner: UNT Libraries Government Documents Department

Toward the solution of the inverse problem in neutron reflectometry

Description: The authors show that the chemical depth profile of a film of unknown structure can be retrieved unambiguously from neutron reflection data by adding to the system a known magnetic layer. Three independent reflectivities are obtained by taking measurements with the sample magnetized in a magnetic field perpendicular to the surface and subsequently parallel to it, and using in the latter geometry neutrons polarized either in the direction of the field or opposite to it. The procedure consists of two steps. First, from the three reflectivities both the real and imaginary parts of the reflection coefficient of the unknown film are extracted within the framework of the rigorous dynamical theory. Second, the neutron scattering-length density (and consequently the chemical depth profile) is obtained by a suitable numerical technique for the conventional Schroedinger inverse scattering problem. Computer experiments were conducted for selected cases: starting from the profiles the reflectivities were calculated in a limited range of q and then the original profiles were successfully recovered. The influence on the accuracy of the recovered depth profile of the counting statistics and the cutoffs at low and high q are discussed.
Date: August 1, 1995
Creator: Haan, V.O. de; Well, A.A. van; Sacks, P.E.; Adenwalla, S. & Felcher, G.P.
Partner: UNT Libraries Government Documents Department

The implications of diffraction in-situ loading macro-strain measurements in composites and finite element interpretations

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The main objective of the project is to show that diffraction strain measurement, when complemented with numerical tools, can provide insights that traditional metallurgical methods cannot offer. In this project, we concentrate on issues concerning discontinuously reinforced metal and intermetallic matrix composites. We have studied load transfer between phases during loading, internal stress distribution during and as a result of inelastic stress relaxation (creep). We also studied the signatures of reinforcement fracture on the evolution of average elastic global strain, and used neutrons to study the micromechanics of slip in intermetallic composites. Of these subjects, we have found, most importantly, that the ratio of load partition in multiphase material does not stay as a constant; internal damage in composites leaves a distinct signature on the evolution of global average elastic strain; and, in reinforced low-symmetry materials, stress relaxation is dictated by the micromechanics of crystal slip, which cannot be accounted for by the continuum theory.
Date: December 31, 1998
Creator: Shi, Ning; Bourke, M.A. & Roberts, J.A.
Partner: UNT Libraries Government Documents Department

Magnetic excitations in (VO)HPO{sub 4}{center_dot}{1/2}H{sub 2}O

Description: The magnetic excitations of an antiferromagnetic spin dimer system, (VO)HPO{sub 4}{center_dot}{1/2}H{sub 2}O, are examined using inelastic neutron scattering. A dispersionless mode is found, consistent with expectations for a dimer excitation. The intensity variation of the mode reveals a V{sup 4+} - V{sup 4+} dimer separation of 4.43{angstrom}, almost 50% larger than the originally expected length.
Date: July 11, 1997
Creator: Garrett, A.W.; Nagler, S.E. & Tennant, D.A.
Partner: UNT Libraries Government Documents Department

Excitations of the transversely polarized spin density waves in chromium

Description: Inelastic neutron scattering measurements across the TSDW satellites of chromium were performed at {Delta}E=5,20meV, both under zero field and 5T magnetic field. Analysis concerning the transverse magnetic excitations (T{sub 1},T{sub 2}) and the longitudinal magnetic excitations (L) indicates that T{sub 1} and L are equally intense while T{sub 2} dominates and becomes more so at higher energies.
Date: July 15, 1997
Creator: Lee, W.T.; Werner, S.A.; Fernandez-Baca, J.A. & Fishman, R.S.
Partner: UNT Libraries Government Documents Department

Application of 2-dimensional coordinate system conversion in stress measurements with neutron diffraction

Description: This paper will present a method and program to precisely calculate the coordinates in a positioner coordinate system from given sample position coordinates with a minimum number of neutron surface scans for three possible circumstances in stress and texture measurement using neutron diffraction.
Date: July 10, 2000
Creator: Wang, D.-Q.; Hubbard, C.R. & Spooner, S.
Partner: UNT Libraries Government Documents Department

Grazing incidence neutron diffraction from large scale 2D structures

Description: The distorted wave Born approximation (DWBA) is applied to evaluate the diffraction pattern of neutrons (or X-rays) from a 2D array of dots deposited onto a dissimilar substrate. With the radiation impinging on the surface at a grazing incidence angle {alpha}, the intensities diffracted both in and out the plane of specular reflection are calculated as a function of the periodicity of the array, height and diameter of the dots. The results are presented in the form of diffracted intensity contours in a plane with coordinates {alpha} and {alpha}{prime}, the latter being the glancing angle of scattering. The optimization of the experimental conditions for polarized neutron experiments on submicron dots is discussed. The feasibility of such measurements is confirmed by a test experiment.
Date: January 13, 2000
Creator: Toperverg, B. P.; Felcher, G. P.; Metlushko, V. V.; Leiner, V.; Siebrecht, R. & Nikonov, O.
Partner: UNT Libraries Government Documents Department

WELD APPLICATION OF A NEW METHOD FOR CROSS-SECTIONAL RESIDUAL STRESS MAPPING

Description: The new ''contour method'' was used to measure a cross-sectional map of residual stresses in a welded plate. Comparisons with neutron diffraction measurements confirm the capability of the contour method to measure complex, 2-D stress maps. Compared to other methods, the contour method is relatively simple and inexpensive to perform, and the equipment required is widely available.
Date: March 1, 2001
Creator: PRIME, M. & AL, ET
Partner: UNT Libraries Government Documents Department

IN-SITU CHARACTERIZATION OF MATRIX RESPONSE TO FIBER FRACTURES

Description: Successful application of metal matrix composites often requires strength and lifetime predictions that account for the deformation of each constituent. However, the deformation of individual phases in composites usually differs significantly from their respective monolithic behaviors. For instance, generally little is known about the in-situ deformation of the metal matrix and fiber/matrix interface region, other than that it likely differs from the bulk material response. This article describes an approach to quantifying the in-situ deformation parameters using neutron diffraction measurements of matrix failure around a fiber fracture in a model composite consisting of an Al matrix and a single Al{sub 2}O{sub 3} fiber. We also study the shear sliding resistance as it evolves through fiber fracture upon loading and unloading. Matching the stress/strain distributions predicted from micromechanical models to the measured strain distributions determined by neutron diffraction under applied tensile loading results in an estimate of the typically non-linear, stress-strain behavior of the metal matrix.
Date: March 1, 2001
Creator: HANAN, J. & AL, ET
Partner: UNT Libraries Government Documents Department

Glass-Like Heat Conduction in Crystalline Semiconductors

Description: The thermal conductivity and structural properties of polycrystalline and single crystal semiconductor type-1 germanium clathrates are reported. Germanium clathrates exhibit thermal conductivities that are typical of amorphous materials. This behavior occurs in spite of their well-defined crystalline structure. The authors employ temperature dependent neutron diffraction data in investigating the displacements of the caged strontium atoms in Sr{sub 8}Ga{sub 16}Ge{sub 30} and their interaction with the polyhedral cages that entrap them. Their aim is to investigate the correlation between the structural properties and the low, glass-like thermal conductivity observed in this compound.
Date: June 13, 1999
Creator: Nolas, G.S.; Cohn, J.L.; Chakoumakos, B.C. & Slack, G.A.
Partner: UNT Libraries Government Documents Department