Showing papers by "National Institute of Standards and Technology published in 1993"
Book•
24 Sep 1993TL;DR: In this article, the authors present a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective, focusing on the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics.
Abstract: This is an advanced text for higher degree materials science students and researchers concerned with the strength of highly brittle covalent–ionic solids, principally ceramics. It is a reconstructed and greatly expanded edition of a book first published in 1975. The book presents a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective. Particular attention is directed to the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics. These elements hold the key to the future of ceramics as high-technology materials - to make brittle solids strong, we must first understand what makes them weak. The underlying theme of the book is the fundamental Griffith energy-balance concept of crack propagation. The early chapters develop fracture mechanics from the traditional continuum perspective, with attention to linear and nonlinear crack-tip fields, equilibrium and non-equilibrium crack states. It then describes the atomic structure of sharp cracks, the topical subject of crack-microstructure interactions in ceramics, with special focus on the concepts of crack-tip shielding and crack-resistance curves, and finally deals with indentation fracture, flaws, and structural reliability.
3,550 citations
01 Jan 1993
TL;DR: The rate constant for the reaction of NO with .O2- was determined to be (6.7 +/- 0.9) x 10(9) l mol-1 s-1, considerably higher than previously reported.
Abstract: The rate constant for the reaction of NO with ·O2− was determined to be (6.7 ± 0.9) × 109 1 mol−1 s−1, considerably higher than previously reported. Rate measurements were made from pH 5.6 to 12.5 ...
2,113 citations
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TL;DR: A revolutionary new type of robot crane, the NIST ROBOCRANE, has been developed that can control the position, velocity, and force of tools and heavy machinery in all six degrees of freedom.
Abstract: The Robot Systems Division of the National Institute of Standards and Technology (NIST) has been experimenting for several years with new concepts for robot cranes. These concepts utilize the basic idea of the Stewart platform parallel link manipulator. The unique feature of the NIST approach is to use cables as the parallel links and to use winches as the actuators. As long as the cables are all in tension, the load is kinematically constrained and the cables resist perturbing forces and moments with equal stiffness to both positive and negative loads. The result is that the suspended load is constrained with a mechanical stiffness determined by the elasticity of the cables, the suspended weight, and the geometry of the mechanism. Based on these concepts, a revolutionary new type of robot crane, the NIST ROBOCRANE, has been developed that can control the position, velocity, and force of tools and heavy machinery in all six degrees of freedom (x, y, z, roll, pitch, and yaw). Depending on what is suspended from its work platform, the ROBOCRANE can perform a variety of tasks. Examples are: cutting, excavating and grading, shaping and finishing, lifting, and positioning. A 6-m version of the ROBOCRANE has been built and critical performance characteristics analyzed.
606 citations
TL;DR: In the experiments described here, a single ion or a few identical ions were prepared in well-defined superpositions of two internal energy eigenstates, and the populations of the energy levels were measured.
Abstract: Measurements of internal energy states of atomic ions confined in traps can be used to illustrate fundamental properties of quantum systems, because long relaxation times and observation times are available. In the experiments described here, a single ion or a few identical ions were prepared in well-defined superpositions of two internal energy eigenstates. The populations of the energy levels were then measured. For an individual ion, the outcome of the measurement is uncertain, unless the amplitude for one of the two eigenstates is zero, and is completely uncertain when the magnitudes of the two amplitudes are equal. In one experiment, a single $^{199}\mathrm{Hg}^{+}$ ion, confined in a linear rf trap, was prepared in various superpositions of two hyperfine states. In another experiment, groups of $^{9}\mathrm{Be}^{+}$ ions, ranging in size from about 5 to about 400 ions, were confined in a Penning trap and prepared in various superposition states. The measured population fluctuations were greater when the state amplitudes were equal than when one of the amplitudes was nearly zero, in agreement with the predictions of quantum mechanics. These fluctuations, which we call quantum projection noise, are the fundamental source of noise for population measurements with a fixed number of atoms. These fluctuations are of practical importance, since they contribute to the errors of atomic frequency standards.
525 citations
TL;DR: In this article, the authors made additional evaluations of the electron inelastic mean free paths (MFPs) and of the predictive IMFP formula TPP-2 presented in papers I1 and I11 of this series.
Abstract: We have made additional evaluations of the electron inelastic mean free paths ([MFPs) and of the predictive IMFP formula TPP-2 presented in papers I1 and I11 of this series. Comparisons have been made with other formulae for the IMFPs and electron attenuation lengths (ALs). We find substantial differences between our IMFP results for 27 elements and 15 inorganic compounds and the AL formulae of Seah and Dench; these differences include different dependences on electron energy and on material parameters. We present IMFP calculations for A1,0, and GaAs from TPP-2 in which each parameter of the formula is varied in some physically reasonable range about the true value for each compound; these results show the sensitivity of the computed IMFPs to the choices of parameter values. Finally, we give a summary of sources of uncertainty in the IMFP algorithm, in the experimental optical data from which IMFPs are calculated, and of the TPP-2 formula. We conclude that TPP-2 is robust and useful for predicting IMFPs for electron energies and material parameter values in ranges for which the formula was developed and tested.
393 citations
TL;DR: The results show that, under appropriately selected control gains, a stable (nonoscillatory) operation of store-and-forward packet switching networks with feedback congestion control is possible.
Abstract: Addresses a rate-based feedback approach to congestion control in packet switching networks where sources adjust their transmission rate in response to feedback information from the network nodes. Specifically, a controller structure and system architecture are introduced and the analysis of the resulting closed loop system is presented. Conditions for asymptotic stability are derived. A design technique for the controller gains is developed and an illustrative example is considered. The results show that, under appropriately selected control gains, a stable (nonoscillatory) operation of store-and-forward packet switching networks with feedback congestion control is possible. >
357 citations
TL;DR: Calculations ofmass energy-transfer and mass energy-absorption coefficients for photon energies from 1 keV to 100 MeV have been developed, based on a re-examination of the processes involved after the initial photon interaction.
Abstract: Calculations of mass energy-transfer and mass energy-absorption coefficients for photon energies from 1 keV to 100 MeV have been developed, based on a re-examination of the processes involved after the initial photon interaction. The probabilities for the initial interaction are from the current photon interaction cross-section database at the National Institute of Standards and Technology. The calculations then take into account (1) electron binding effects on the Compton-scattered photon distribution; (2) the complete cascade of fluorescence emission after ionization events in any atomic subshell, including those associated with incoherent scattering and triplet production; and (3) the radiative energy losses of the secondary electrons and positrons slowing down in the medium, including the emission of bremsstrahlung, characteristic X rays from impact ionization, and positron in-flight as well as at-rest annihilation quanta. Consideration of the processes in (3) goes beyond the continuous-slowing-down approximation and includes the effects of energy-loss straggling. Results for the mass energy-absorption coefficient are compared with those from recent tabulations.
343 citations
01 Jun 1993
TL;DR: In this article, a lattice-plane range parameter is proposed as a critical scaling dimension in the brittle crack description, and the critical crack-opening dimensions using this same model confirm that the intrinsic separation process indeed operates at the atomic level.
Abstract: Until now we have approached crack propagation from the continuum viewpoint. Nonetheless, repeated allusions have been made in chapters 3 and 5 to the fundamental limitations of any such approach that disregards the atomic structure of solids. There we argued for the incorporation of a lattice-plane range parameter as a critical scaling dimension in the brittle crack description. We noted that the Barenblatt cohesion-zone model avoids reference to the atomic structure by resorting to the Irwin slit description of cracks; yet estimates of the critical crack-opening dimensions using this same model confirm that the intrinsic separation process indeed operates at the atomic level. The Elliot lattice half-space model of sect. 3.3.2 represents one attempt to incorporate an essential element of discreteness. The phenomenological kinetic models of sect. 5.5, with their presumption of energy barriers, represent another. However, those models are at the very least quasi-continuous. In brittle fracture, as in any thermodynamic process, the final answers must be sought at the atomic or molecular level. On the other hand, while an atomistic approach provides greater physical insight into the crack problem, it inevitably involves greater mathematical complexity. Classically, solids may be represented as manybody assemblages of point masses (atoms) linked by springs (bonds). We will see that the mass–spring representation can lead us to a deeper understanding of brittle cracks. But even this representation is oversimplistic. In some cases, particularly when the crack interacts with environmental species, it is necessary to consider atoms as elastic spheres rather than point masses, to allow properly for molecular size effects.
325 citations
TL;DR: Rate constants for the reactions of several organic peroxyl radicals with nitric oxide in aqueous solution are measured and it is found that they are all fast, with k = 1-3 x 10(9) L mol-1 s-1.
320 citations
TL;DR: The ability to fabricate nanometer-sized structures that are stable in air has the potential to contribute significantly to the advancement of new nanotechnologies and the understanding of nanoscale systems.
Abstract: The ability to fabricate nanometer-sized structures that are stable in air has the potential to contribute significantly to the advancement of new nanotechnologies and our understanding of nanoscale systems. Laser light can be used to control the motion of atoms on a nanoscopic scale. Chromium atoms were focused by a standing-wave laser field as they deposited onto a silicon substrate. The resulting nanostructure consisted of a series of narrow lines covering 0.4 millimeter by 1 millimeter. Atomic force microscopy measurements showed a line width of 65 +/- 6 nanometers, a spacing of 212.78 nanometers, and a height of 34 +/-+ 10 nanometers. The observed line widths and shapes are compared with the predictions of a semiclassical atom optical model.
309 citations
TL;DR: This review will discuss the general preparation and characterization of silane coupled proteins with special emphasis on enzymes and describe in detail the actual methods for the silanization and specific chemical coupling of proteins to thesilanized carrier.
Abstract: Enzymes were first immobilized on inorganic supports through silane coupling agents over 25 yr ago. Since that initial report, literally hundreds of laboratories have utilized this methodology for the immobilization of enzymes, antigens, antibodies, receptors, and other high and low mol wt compounds. Today silane coupling is one of the commonly used techniques in the arsenal of the biochemist for the binding of material of all sorts to inorganic surfaces. Inorganic materials come in a variety of shapes, sizes, and characteristics. Today silane coupling is one of the most used coupling methods for the preparation of biosensing devices. Sol-gel entrapped enzymes are also produced by the application of silane technology by the polymerization of the silane to form glass-like materials with entrapped protein. This review will discuss the general preparation and characterization of silane coupled proteins with special emphasis on enzymes and describe in detail the actual methods for the silanization and specific chemical coupling of proteins to the silanized carrier.
TL;DR: To test this and related models, the extremal spanning vectors and the associated Fermi-surface geometrical factors have been calculated for a large set of spacer-layer materials and interface orientations.
Abstract: Many structures consisting of magnetic layers separated by a nonmagnetic spacer layer show an oscillatory exchange coupling. This behavior is explained in terms of a simple model that shows that the Fermi surface of the spacer-layer material is responsible for the oscillatory coupling. The periods of the oscillatory coupling are set by extremal spanning vectors of the Fermi surface of the spacer-layer material. The strength of the coupling depends both on the geometry of the Fermi surface and on the reflection amplitudes for electrons scattering from the interfaces between the spacer layers and the magnetic layers. To test this and related models, the extremal spanning vectors and the associated Fermi-surface geometrical factors have been calculated for a large set of spacer-layer materials and interface orientations. These models are at least consistent with the experimental data. All measured oscillation periods are consistent with the calculated periods, but particularly for transition metals there are many more periods calculated than are seen experimentally.
TL;DR: The first investigation of the substrate specificity of this repair enzyme in the context of a large number of pyrimidine- and purine-derived lesions in DNA provided unequivocal evidence for the excision by E. coli endonuclease III of a number of thymine- and cytosine- derived lesions from DNA.
Abstract: The excision of modified bases from DNA by Escherichia coli endonuclease III was investigated. Modified bases were produced in DNA by exposure of dilute buffered solutions of DNA to ionizing radiation under oxic or anoxic conditions. The technique of gas chromatography/mass spectrometry (GC/MS) was used to identify and quantify 16 pyrimidine- and purine-derived DNA lesions. DNA substrates were incubated either with the native enzyme or with the heat-inactivated enzyme. Subsequently, DNA was precipitated. Pellets were analyzed by GC/MS after hydrolysis and derivatization. Supernatant fractions were analyzed after derivatization without hydrolysis. The results provided unequivocal evidence for the excision by E. coli endonuclease III of a number of thymine- and cytosine-derived lesions from DNA. These were 5,6-dihydrothymine, 5-hydroxy-5-methylhydantoin, thymine glycol, 5-hydroxy-6-hydrothymine, 5,6-dihydrouracil, alloxan, uracil glycol, and 5-hydroxy-6-hydrouracil. None of the purine-derived lesions was excised by endonuclease III. The present work extends the substrate specificity of E. coli endonuclease III to another thymine-derived and four cytosine-derived lesions. It is the first investigation of the substrate specificity of this repair enzyme in the context of a large number of pyrimidine- and purine-derived lesions in DNA.
TL;DR: Wythoff et al. as discussed by the authors provided a short history of neural network research, and a review of chemical applications with a clear and detailed introduction to the theory behind backpropagation neural networks along with a discussion of practical issues facing developers.
TL;DR: In this article, force measurements between silica surfaces in aqueous solutions of NaCl are reported, and the force at short range is dominated by a strong repulsion which is attributed to hydration of the surface.
TL;DR: In this article, the authors present high-resolution optical and microwave spectra of the benzene-ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benene plane and undergoes free or nearly free internal rotation.
Abstract: AMINES have long been characterized as amphoteric (acting as both donor and acceptor) in terms of their hydrogen-bond interactions in the condensed phase. With the possible exception of (NH_3)_2, however, no gas-phase complexes exhibiting hydrogen-bond donation by ammonia, the ‘simplest amine’, have been observed. Here we present high-resolution optical and microwave spectra of the benzene–ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benzene plane and undergoes free or nearly free internal rotation. In the vibrationally averaged structure, the C_3 symmetry axis of NH_3 is tilted by about 58° relative to the benzene C_6 axis, such that the ammonia protons interact with the benzene π-cloud. Our ab initio calcula-tions predict a 'monodentate' minimum-energy structure, with very low barriers to rotation of ammonia. The larger separation of the two molecular components, and the smaller dissociation energy, relative to the benzene–water dimer reflect the weak hydrogen-bond donor capability of ammonia, but the observed geometry greatly resembles the amino–aromatic interaction found naturally in proteins.
TL;DR: Nonideal film growth, including agglomeration of Co and Fe and surface segregation of Cu, is the rule rather than the exception.
Abstract: The growth and structure of Fe and Co thin films on single-crystal Cu(111), Cu(100), and Cu(110) substrates have been investigated using x-ray-photoelectron and Auger electron forward scattering, CO-titration, low-energy electron diffraction, and reflection high-energy electron diffraction The motivation for this study is to understand the role of surface structure and kinetics in the growth of metal films on metal substrates The effect of varying substrate growth temperatures between 80 and 450 K plays a prominent role in determining both the film morphology and crystalline phase Nonideal film growth, including agglomeration of Co and Fe and surface segregation of Cu, is the rule rather than the exception
TL;DR: The first observation of interference effects in the light scattered from two trapped atoms is reported, which is possible to selectively demonstrate either the particle nature or the wave nature of the scattered light.
Abstract: We report the first observation of interference effects in the light scattered from two trapped atoms. The visibility of the fringes can be explained in the framework of Bragg scattering by a harmonic crystal and simple ``which path'' considerations of the scattered photons. If the light scattered by the atoms is detected in a polarization-sensitive way, then it is possible to selectively demonstrate either the particle nature or the wave nature of the scattered light.
TL;DR: Real space views of the homoepitaxial growth of Fe on Fe(001) whiskers observed by scanning tunneling microscopy and measurements of reflection-high-energy-electron diffraction during growth allow a comparison of real and reciprocal space techniques.
Abstract: We report real space views of the homoepitaxial growth of Fe on Fe(001) whiskers observed by scanning tunneling microscopy. A measure of the surface diffusion of the Fe atoms is obtained over the temperature range of 20--250 \ifmmode^\circ\else\textdegree\fi{}C. The effect of the diffusion kinetics is observed in the surface morphology as a decrease in the interface width with temperature. Measurements of reflection-high-energy-electron diffraction during growth allow a comparison of real and reciprocal space techniques.
TL;DR: The NIST experience in the use of liquid chromatography for the determination of polycyclic aromatic hydrocarbons (PAHs) in environmental samples is summarized in this paper, including selection of the appropriate column, approaches to analyzing complex PAH mixtures, and accurate quantification of PAHs.
TL;DR: In this paper, an experimental investigation was designed to test the hypothesis that all axisymmetric turbulent free jets become asymptotically independent of the source conditions and may be described by classical similarity analysis.
Abstract: An experimental investigation was designed to test the hypothesis that all axisymmetric turbulent free jets become asymptotically independent of the source conditions and may be described by classical similarity analysis. Effects of initial conditions were studied by varying jet exit boundary conditions and the global density ratio. The exit velocity profile and turbulence level was changed by using both pipe and nozzle flow hardware. Initial density differences were imposed by using three gases: helium, methane, and propane. The scalar field (concentration) in the momentum-dominated regime of the far field (10 to 60 jet exit diameters downstream) of turbulent free jets was characterized using Rayleigh light scattering as the diagnostic. The results show that regardless of the initial conditions axisymmetric turbulent free jets decay at the same rate, spread at the same angle, and both the mean and r.m.s. values collapse in a form consistent with full self-preservation. The means and fluctuations follow a law of full self-preservation in which two virtual origins must be specified. The two displacements are required to account for the effects of a finite source of momentum and different development of the velocity and mass distributions in the near fields of the jets. The memory of the jet is embodied in these two virtual origins.
TL;DR: In this article, the optical absorption edge is red shifted with respect to that of an epitaxially grown single-crystal film of γ-Fe2O3, attributed to lattice strain in the small particles.
Abstract: γ-Fe2O3 nanocrystals with a mean radius of 4.2 nm have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The optical absorption edge is red shifted with respect to that of an epitaxially-grown single-crystal film of γ-Fe2O3. The red shift is attributed to lattice strain in the small particles.
TL;DR: In this paper, a single chemisorbed monolayer was used to study contact electrification and adhesion between two silica surfaces, one coated with an amino-silane.
Abstract: ELECTRICAL charge separation following contact between two materials (contact electrification or the triboelectric effect) is well known to occur between different materials as a consequence of their different electronic structures1,2. Here we show that the phenomenon occurs between two surfaces of the same material if one is coated with a single chemisorbed monolayer. We use the surface force apparatus3 to study contact electrification4 and adhesion between two silica surfaces, one coated with an amino-silane. The presence of this monolayer results in significantly enhanced adhesion between the surfaces, owing to electrostatic attraction following contact electrification, in accord with Derjaguin's electrostatic theory of adhesion5. At the same time, the observed increase in adhesion is consistent with Fowkes' acid–base model6 (in which acid–base interactions between surface groups are considered to be the predominant factor determining adhesion), as the monolayer converts the originally acidic silica surface to a basic (amine-terminated) one. These observations demonstrate a link between acid–base interactions and contact electrification.
TL;DR: Putative pathways for the introduction of this base modification at specific positions in the DNA are discussed and the possible contribution of this modification to repression of surface antigen gene expression is discussed.
TL;DR: The temperature dependent thicknesses of ultrathin polystyrene films under vacuum on Si(111) substrates were investigated via x-ray reflectivity in situ and the contraction of Ultrathin polymer films was directly observed for the first time to the authors' knowledge.
Abstract: The temperature dependent thicknesses of ultrathin polystyrene films under vacuum on Si (111) substrates were investigated via x-ray reflectivity in situ. The contraction of ultrathin polymer films was directly observed for the first time to the author's knowledge. The degree of contraction depends on the initial thickness of the ultrathin polystyrene film, with the magnitude of contraction increasing with decreasing initial film thickness. This contraction ranged from 0--17 % and occurred at temperatures well below the reported bulk polystyrene glass transition temperature.
TL;DR: A liquid chromatographic (LC) method has been developed for the quantitative measurement of the six major carotenoids in human serum (lutein, zeaxanthin, beta-cryptoxanthin), as well as retinol, retinyl palmitate, alpha-tocopherol, gamma-tocoperol, and delta-tocouserol).
01 Jan 1993
TL;DR: An overview of various modeling methods used to understand small angle neutron scattering (SANS) data from homogeneous polymer systems is presented in this article, which is meant to be a guide to help build up models in order to understand SANS data from many homogeneous polymers.
Abstract: An overview of various modeling methods used to understand small angle neutron scattering (SANS) data from homogeneous polymer systems is presented. First, calculations of single macromolecule structure factors are reviewed for many chain architectures and monomer block configurations such as linear, ring, star branched, comb grafted chains and regular “starburst” dendrimers either in the homopolymer or copolymer forms. Then, the different methods used to model “concentration” effects in polymer solutions (dilute, semidilute, concentrated), polymer melts and blend mixtures are summarized on the basis of the random phase approximation. Polymer chain stiffness is also included in the formalism so that mixtures of liquid crystals and flexible polymers in the single-phase region can be described. Specific examples are included along with various SANS data that were analyzed within this framework. This overview is meant to be a guide to help build up models in order to understand SANS data from many homogeneous polymer systems. It is not meant to be complete and is not an exhaustive review of the literature in the field. Most of the results discussed have been previously published and are brought together here in a unified self-contained approach.
TL;DR: In this article, a review of the factors that affect retention and selectivity with alkyl-modified sorbents, particularly for the separation of solutes with well defined, rigid structure (e.g., polycyclic aromatic hydrocarbons), is presented.
TL;DR: In this article, the elastic collision cross section for spin polarized atomic cesium was measured and the scattering rate was inferred from the rate at which anisotropies in the initial energy distribution were observed to relax.
Abstract: We have measured the elastic collision cross section for spin polarized atomic cesium. Neutral cesium atoms are optically cooled, then loaded into a dc magnetic trap. We infer the scattering rate from the rate at which anisotropies in the initial energy distribution are observed to relax. The cross section for F=3, ${\mathit{m}}_{\mathit{F}}$=-3 on F=3, ${\mathit{m}}_{\mathit{F}}$=-3 is 1.5(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{cm}}^{2}$, and is independent of temperature from 30 to 250 \ensuremath{\mu}K. This determination clarifies the technical requirements for attaining Bose-Einstein condensation in a magnetically trapped Cs vapor. We also study heating due to glancing collisions with 300 K background Cs atoms.
TL;DR: A numerical study examining cross-property relations linking fluid permeability to diffusive and electrical properties proposes two hybrid relations based on diffusion which provide more accurate estimates than either of the rigorous permeability bounds.
Abstract: Results from a numerical study examining cross-property relations linking fluid permeability to diffusive and electrical properties are presented. Numerical solutions of the Stokes equations in three-dimensional consolidated granular packings are employed to provide a basis of comparison between different permeability estimates. Estimates based on the \ensuremath{\Lambda} parameter (a length derived from electrical conduction) and on ${\mathit{d}}_{\mathit{c}}$ (a length derived from immiscible displacement) are found to be considerably more reliable than estimates based on rigorous permeability bounds related to pore space diffusion. We propose two hybrid relations based on diffusion which provide more accurate estimates than either of the rigorous permeability bounds.