Showing papers by "National Institute of Standards and Technology published in 1994"
TL;DR: If the particles are first prepared in particular quantum-mechanically correlated states, it is found that the signal-to-noise ratio can be improved over the case of initially uncorrelated particles, and a squeezing parameter is introduced which quantifies the sensitivity of an angular-momentum state to rotation.
Abstract: We investigate the properties of angular-momentum states which yield high sensitivity to rotation. We discuss the application of these ``squeezed-spin'' or correlated-particle states to spectroscopy. Transitions in an ensemble of N two-level (or, equivalently, spin-1/2) particles are assumed to be detected by observing changes in the state populations of the particles (population spectroscopy). When the particles' states are detected with 100% efficiency, the fundamental limiting noise is projection noise, the noise associated with the quantum fluctuations in the measured populations. If the particles are first prepared in particular quantum-mechanically correlated states, we find that the signal-to-noise ratio can be improved over the case of initially uncorrelated particles. We have investigated spectroscopy for a particular case of Ramsey's separated oscillatory method where the radiation pulse lengths are short compared to the time between pulses. We introduce a squeezing parameter ${\ensuremath{\xi}}_{\mathit{R}}$ which is the ratio of the statistical uncertainty in the determination of the resonance frequency when using correlated states vs that when using uncorrelated states. More generally, this squeezing parameter quantifies the sensitivity of an angular-momentum state to rotation. Other squeezing parameters which are relevant for use in other contexts can be defined. We discuss certain states which exhibit squeezing parameters ${\ensuremath{\xi}}_{\mathit{R}}$\ensuremath{\simeq}${\mathit{N}}^{\mathrm{\ensuremath{-}}1/2}$. We investigate possible experimental schemes for generation of squeezed-spin states which might be applied to the spectroscopy of trapped atomic ions. We find that applying a Jaynes-Cummings--type coupling between the ensemble of two-level systems and a suitably prepared harmonic oscillator results in correlated states with ${\ensuremath{\xi}}_{\mathit{R}}$1.
908 citations
TL;DR: In this article, a theoretical model for calculating electron-impact ionization cross sections for atoms and molecules is presented, which combines the binary-encounter theory with the dipole interaction of the Bethe theory for fast incident electrons.
Abstract: A theoretical model, which is free of adjustable or fitted parameters, for calculating electron-impact ionization cross sections for atoms and molecules is presented. This model combines the binary-encounter theory with the dipole interaction of the Bethe theory for fast incident electrons. The ratios of the contributions from distant and close collisions and interference between the direct and exchange terms are determined by using the asymptotic behaviors predicted by the Bethe theory for ionization and for stopping cross sections. Our model prescribes procedures to calculate the singly differential cross section (energy distribution) for each subshell using the binding energy, average kinetic energy, and the differential dipole oscillator strengths for that subshell. Then the singly differential cross section is integrated over the ejected electron energy to obtain the total ionization cross section. The resulting total ionization cross section near the threshold is proportional to the excess energy of the projectile electron. We found that this model yields total ionization cross sections for a variety of atoms and molecules from threshold to several keV which are in good agreement (\ensuremath{\sim}10% or better on average) with known experimental results. The energy distributions also exhibit the expected shapes and magnitudes. We offer a simpler version of the model that can be used when differential oscillator strengths are not known. For the ionization of ions with an open-shell configuration, we found that a minor modification of our theory greatly improves agreement with experiment.
799 citations
TL;DR: Five algorithms proposed in the literature for library search identification of unknown compounds from their low resolution mass spectra were optimized and tested by matching test spectra against reference spectra in the NIST-EPA-NIH Mass Spectral Database.
702 citations
TL;DR: The first retroreflector array was placed on the moon by Apollo 11, enabling highly accurate measurements of the Earthmoon separation by means of laser ranging as discussed by the authors, which has been used for a broad range of investigations, including astronomy, lunar science, gravitational physics, geodesy, and geodynamics.
Abstract: On 21 July 1969, during the first manned lunar mission, Apollo 11, the first retroreflector array was placed on the moon, enabling highly accurate measurements of the Earthmoon separation by means of laser ranging. Lunar laser ranging (LLR) turns the Earthmoon system into a laboratory for a broad range of investigations, including astronomy, lunar science, gravitational physics, geodesy, and geodynamics. Contributions from LLR include the three-orders-of-magnitude improvement in accuracy in the lunar ephemeris, a several-orders-of-magnitude improvement in the measurement of the variations in the moon's rotation, and the verification of the principle of equivalence for massive bodies with unprecedented accuracy. Lunar laser ranging analysis has provided measurements of the Earth's precession, the moon's tidal acceleration, and lunar rotational dissipation. These scientific results, current technological developments, and prospects for the future are discussed here.
685 citations
TL;DR: A simple double-pass fiber noise measurement and control system is described, which is demonstrated to provide millihertz accuracy of noise cancellation.
Abstract: Although a single-mode optical fiber is a convenient and efficient interface/connecting medium, it introduces phase-noise modulation, which corrupts high-precision frequency-based applications by broadening the spectrum toward the kilohertz domain. We describe a simple double-pass fiber noise measurement and control system, which is demonstrated to provide millihertz accuracy of noise cancellation.
539 citations
TL;DR: In this paper, the authors measured the water density profile perpendicular to a silver surface at two applied voltages and found that the water molecules are ordered in layers extending about three molecular diameters from the electrode, and that the spacing between the electrode and first water layer indicates an oxygen-up (oxygen-down) average orientation for negative (positive) charge.
Abstract: THE arrangement of water molecules at charged, aqueous interfaces is an important question in electrochemistry, geochemistry and biology. Theoretical studies1–11 suggest that the molecules become arranged in several layers adjacent to a solid interface, with densities similar to that in the bulk, and that the molecules in the first layer are reoriented from oxygen-up to oxygen-down as the electrode charge changes from negative to positive. Few of these predictions have been verified experimentally12–16, however. Using X-ray scattering, we have measured the water density profile perpendicular to a silver (111) surface at two applied voltages. We find that the water molecules are ordered in layers extending about three molecular diameters from the electrode, and that the spacing between the electrode and first water layer indicates an oxygen-up (oxygen-down) average orientation for negative (positive) charge. Contrary to current models, however, we find that the first layer has a far greater density than that in bulk water. This implies that the hydrogen-bonding network is disrupted in this layer, and that the properties of the water in the layer are likely to be very different from those in the bulk.
534 citations
TL;DR: In this article, an atmospheric pressure sintering strategy for obtaining dense SiC-based materials with microstructures consisting of uniformly distributed elongate-shaped [alpha]-SiC grains and relatively high amounts of second-phase yttrium aluminum garnet (YAG) was presented.
Abstract: A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (1) uniformly distributed elongate-shaped [alpha]-SiC grains and (2) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of [beta]-SiC powder doped with [alpha]-SiC, Al[sub 2]O[sub 3], and Y[sub 2]O[sub 3]. The Al[sub 2]O[sub 3] and Y[sub 2]O[sub 3] aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the [alpha]-SiC seeds'' assist in controlling in situ growth of the elongated [alpha]-SiC grains. The fracture pattern in the in situ-toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si[sub 3]N[sub 4] ceramics. The elongate nature of the [alpha]-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ-toughened SiC relative to a commercial SiC.
454 citations
24 Jul 1994
TL;DR: Photographs, comparing the drape of actual cloth with visualizations of simulation results, show that the approach is able to reliably model the unique large-scale draping characteristics of distinctly different fabric types.
Abstract: We demonstrate a physically-based technique for predicting the drape of a wide variety of woven fabrics. The approach exploits a theoretical model that explicitly represents the microstructure of woven cloth with interacting particles, rather than utilizing a continuum approximation. By testing a cloth sample in a Kawabata fabric testing device, we obtain data that is used to tune the model's energy functions, so that it reproduces the draping behavior of the original material. Photographs, comparing the drape of actual cloth with visualizations of simulation results, show that we are able to reliably model the unique large-scale draping characteristics of distinctly different fabric types.
423 citations
TL;DR: In this article, the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement-based materials during hydration is discussed and features of the spectra are discussed and related to components of the micro-structure with the assistance of pixel-based computer modeling techniques.
Abstract: This work concerns the state of the art for use of impedance spectroscopy for studying the evolving microstructure of cement-based materials during hydration. Features of the spectra are discussed and related to components of the microstructure with the assistance of pixel-based computer modeling techniques. It is proposed that the enormously high relative dielectric constants (∼105) observed just after set are the result of dielectric amplification and are related to the distribution of pore sizes and the thickness of product C─S─H layers separating the pores. The conductivity is related to the volume fraction of porosity, the conductivity of the pore solution, and the interconnectivity of the porosity. The conductivity, when normalized by that of the pore solution, i.e., inverse formation factor, is a measure of this interconnectivity and can be used to predict such engineering properties as ionic diffusivity and water permeability. Composite mixing laws are employed to aid in explaining the behavior of the conductivity and to obtain a qualitative measure of the pore shape with hydration. Procedures for predicting the conductivity of the pore solution and for subtracting out electrode lead effects at high frequency are discussed.
369 citations
TL;DR: In this paper, a triacontyl polymeric surface modification of a moderate pore size (approximately 20 nm), moderate surface area (approximately 200 m2/g) silica, without subsequent endcapping, was demonstrated for the separation of structurally similar carotenoid standards, an extract of a food matrix Standard Reference Material, and a beta-carotene dietary supplement under consideration as an agent for cancer intervention/prevention.
Abstract: A variety of bonded phase parameters (endcapping, phase chemistry, ligand length, and substrate parameters) were studied for their effect on column retention and selectivity toward carotenoids. Decisions were made on how each of these variables should be optimized based on the separation of carotenoid and polycyclic aromatic hydrocarbon test probes. A column was designed with the following properties: high absolute retention, enhanced shape recognition of structured solutes, and moderate silanol activity. These qualities were achieved by triacontyl (C30) polymeric surface modification of a moderate pore size (approximately 20 nm), moderate surface area (approximately 200 m2/g) silica, without subsequent endcapping. The effectiveness of this "carotenoid phase" was demonstrated for the separation of a mixture of structurally similar carotenoid standards, an extract of a food matrix Standard Reference Material, and a beta-carotene dietary supplement under consideration as an agent for cancer intervention/prevention.
356 citations
TL;DR: Distributed irreversible deformation in otherwise brittle ceramics has been observed in Hertzian contacts in silicon carbide and micaceous glass-ceramic, and an important manifestation is an effective "ductility" in the indentation stress-strain response.
Abstract: Distributed irreversible deformation in otherwise brittle ceramics (specifically, in silicon carbide and micaceous glass-ceramic) has been observed in Hertzian contacts. The deformation takes the form of an expanding microcrack damage zone below the contact circle, in place of the usual single propagating macrocrack (the Hertzian "cone fracture") outside. An important manifestation of this deformation is an effective "ductility" in the indentation stress-strain response. Control of the associated brittle-ductile transition is readily effected by appropriate design of weak interfaces, large and elongate grains, and high internal stresses in the ceramic microstructure.
TL;DR: In this article, an experimental program was designed in which the volume fraction of sand in a mortar was varied in a systematic fashion and the resultant pore system probed using mercury intrusion porosimetry.
TL;DR: In this paper, a mathematical morphology is used to derive, for each point on an image, a corresponding bounding surface for the tip, which can be used to estimate the tip shape.
TL;DR: A review of experimental and theoretical approaches to the study of negative-ion resonances is given together with a consideration of the various schemes that are used for their classification in this paper, where the authors also highlight regularities in their behavior both within groups of the periodic table and along isoionic sequences.
Abstract: The authors attempt to give a comprehensive discussion of observations of atomic negative-ion resonances throughout the periodic table. A review of experimental and theoretical approaches to the study of negative-ion resonances is given together with a consideration of the various schemes that are used for their classification. In addition to providing, where possible, tabulated data for the energies, widths, and symmetries of these states, the authors also attempt to highlight regularities in their behavior both within groups of the periodic table and along isoionic sequences.
TL;DR: In this paper, the authors evaluated the toughness characteristics of a heterogeneous silicon carbide with an intergranular second phase relative to a homogeneous, fine-grain control using indentation-strength data.
Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.
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15 Dec 1994
TL;DR: This is the eleventh in a series of evaluated sets of rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation as discussed by the authors, with special emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena.
Abstract: This is the eleventh in a series of evaluated sets of rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation. The primary application of the data is in the modeling of stratospheric processes, with special emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena.
TL;DR: Geometry growth laws for morphological change are developed and examined for the class of dynamic problems where surface diffusion is the only transport mechanism and hence volume is conserved, attachment kinetics is treated, and the only driving force for surface motion is the reduction in total surface free energy as mentioned in this paper.
Abstract: Geometry growth laws for morphological change are developed and examined for the class of dynamic problems where surface diffusion is the only transport mechanism and hence volume is conserved, attachment kinetics is treated, and the only driving force for surface motion is the reduction in total surface free energy. The two limiting laws in the isotropic case are: motion by the Laplacian of mean curvature as originally derived by Mullins, and motion by the difference between mean curvature and the average of mean curvature. A general law linking these limiting laws is formulated, and derived both from a physical model and from gradient flows. Anisotropic laws are given. We survey possible mathematical techniques for studying interface motion under all these laws. Among these are possible applications of modified phase field methods.
TL;DR: The data suggest that the deviation of the period from the bulk value decreases with increasing separation distance, as a function of the confined film thickness.
Abstract: Symmetric diblock copolymers confined between two solid surfaces were studied by neutron reflectivity. A multilayered morphology with an integral number of layers oriented parallel to the solid interfaces was found in all cases. The period of the confined multilayers deviated from the bulk period in a cyclic manner as a function of the confined film thickness. A first-order transition occurred between the expanded and contracted states of the copolymer chains. The data suggest that the deviation of the period from the bulk value decreases with increasing separation distance.
TL;DR: In this article, the role of microstructural scale on deformation-microfracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3--48 [mu]m in grain size.
Abstract: The role of microstructural scale on deformation-microfracture damage induced by contact with spheres is investigated in monophase alumina ceramics over a range 3--48 [mu]m in grain size. Measurement of a universal indentation stress-strain curve indicates a critical contact pressure [approx] 5 GPa, above which irreversible deformation occurs in alumina. A novel sectioning technique identifies the deformation elements as intragrain shear faults, predominantly crystallographic twins, within a confining subsurface zone of intense compression-shear stress. The twins concentrate the shear stresses at the grain boundaries and, above a threshold grain size, initiate tensile intergranular microcracks. Below this threshold size, classical Hertzian cone fractures initiate outside the contact circle. Above the threshold, the density and scale of subsurface-zone microcracks increase dramatically with increasing grain size, ultimately dominating the cone fractures. The damage process is stochastic, highlighting the microstructural discreteness of the initial deformation field; those grains which lie in the upper tail of the grain-size distribution and which have favorable crystallographic orientation relative to local shear stresses in the contact field are preferentially activated. Initial flaw state is not an important factor, because the contact process creates its own flaw population. These and other generic features of the damage process will be discussedmore » in relation to microstructural design of polycrystalline ceramics.« less
TL;DR: In this article, the relative rates of these degradation processes are a function of the chemistry of the resin, temperature, length of time of exposure, degree of stress (whether cyclic or static), chemistry and morphology of coating of coupling agent on the glass fiber, and type of glass fiber.
Abstract: Durability of glass-fiber/polymer composites is dictated by the durability of the components: glass fiber, matrix, and the interface. Environmental attack by moisture, for example, can degrade the strength of the glass fiber; plasticize, swell, or microcrack the resin; and degrade the fiber/ matrix interface by either chemical or mechanical attack. The relative rates of these degradation processes are a function of the chemistry of the resin, temperature, length of time of exposure, degree of stress (whether cyclic or static), chemistry and morphology of coating of coupling agent on the glass fiber, and type of glass fiber. Several examples illustrate how the chemistry and morphology of the coatings of coupling agents that are on the glass fiber influence the strength and durability of the interfacial region.
TL;DR: In this paper, a class of water-based non-stick coatings prepared by self-assembly and immobilization of reactive polymeric surfactants is described. But they do not specify the type of polymeric polymeric surfaces to which they are applied.
Abstract: THERE is a considerable demand for materials that have surfaces to which other substances will not easily stick. Coatings with these properties might be used to make surfaces non-adhesive towards soil, ice, biological foulants, graffiti and other unwanted contaminants. Here we describe a class of water-based non-stick coatings prepared by self-assembly and immobilization of reactive polymeric surfactants1,2. These polymer surfactants contain pendant perfluoroalkyl groups which become oriented so as to yield surfaces with very low energy. Immobilization by crosslinking increases non-stick performance and film toughness. These hard, clear coatings release adhesives and cannot be wetted or attacked by solvents.
TL;DR: The gas chromatography–mass spectrometry (GC/MS) technique appears to find a major role in studies of oxidative DNA damage and its repair in the future.
Abstract: Publisher Summary Oxidative DNA damage produced by free radicals or other DNA-damaging agents has been implicated to play a role in mutagenesis, carcinogenesis, reproductive cell death, and aging. Oxygen-derived species, such as superoxide radical (O 2 − ) and H 2 O 2 are generated in all aerobic cells. Excess generation of these species by endogenous sources or exogenous sources (for example, redox-cyclic drugs and ionizing radiation) may cause damage to cellular DNA by a variety of mechanisms. The gas chromatography–mass spectrometry (GC/MS) technique has been applied to a variety of in vitro and in vivo studies of oxidative DNA damage. The technique offers the sensitivity, selectivity, speed, and versatility to solve a wide range of important measurement problems in terms of DNA base and sugar damage and DNA-protein cross-links. It also allows studying enzymatic repair of DNA damage. It appears that the GC/MS technique will continue to find a major role in studies of oxidative DNA damage and its repair in the future.
TL;DR: The results indicate an association between decreased activities of antioxidant enzymes and increased levels of DNA lesions in cancerous tissues and suggest that free radical reactions may be increased in malignant tumor cells.
TL;DR: An analysis of the island size and separation distributions of Fe islands, observed in the initial stages of growth in the homoepitaxy of Fe on Fe(001) whiskers, shows scaling properties recently predicted in nucleation and growth theories.
Abstract: An analysis of the island size and separation distributions of Fe islands, observed in the initial stages of growth in the homoepitaxy of Fe on Fe(001) whiskers, shows scaling properties recently predicted in nucleation and growth theories. A critical size of one atom, where islands greater than the critical size undergo irreversible nucleation, is supported by the measured scaling functions for the Fe on Fe system in the temperature range of 20--250 \ifmmode^\circ\else\textdegree\fi{}C.
08 Mar 1994
TL;DR: The new paradigm used in the most recent ARPA-sponsored Continuous Speech Recognition (CSR) evaluation is introduced and then the important features of the test design are discussed.
Abstract: In this paper, we introduce the new paradigm used in the most recent ARPA-sponsored Continuous Speech Recognition (CSR) evaluation and then discuss the important features of the test design.The 1993 CSR evaluation was organized in a novel fashion in an attempt to accomodate research over a broad variety of important problems in CSR while maintaining a clear program-wide research focus. Furthermore, each test component in the evaluation was designed as an experiment to extract as much information as possible from the results.The evaluation was centered around a large vocabulary speaker-independent (SI) baseline test, which was required of every participating site. This test was dubbed the 'Hub' since it was common to all sites and formed the basis for controlled inter-system comparisons.The Hub test was augmented with a variety of problem-specific optional tests designed to explore a variety of important problems in CSR, mostly involving some kind of mismatch between the training and test conditions. These tests were known as the 'Spokes' since they all could be informatively compared to the Hub, but were otherwise independent.In the first trial of this evaluation paradigm in November, 1993, 11 research groups participated, yielding a rich array of comparative and contrastive results, all calibrated to the current state of the art in large vocabulary CSR.
TL;DR: In this paper, the authors compare four surface motion laws for sharp surfaces with their diffuse interface counterparts by means of gradient flows on corresponding energy functionals, which can be defined to give the same dependence on normal direction for the energy of sharp plane surfaces as for their diffuse counterparts.
Abstract: We compare four surface motion laws for sharp surfaces with their diffuse interface counterparts by means of gradient flows on corresponding energy functionals. The energy functionals can be defined to give the same dependence on normal direction for the energy of sharp plane surfaces as for their diffuse counterparts. The anisotropy of the kinetics can be incorporated into the inner product without affecting the energy functional.
TL;DR: In this article, the primary particle size of axisymmetric diffusion flames burning methane, methane/butane, and methane/1-butene in air at atmospheric pressure was determined.
TL;DR: Results from the numerical simulation of Stokes flow through random packings of nonoverlapping or overlapping sphers and a [ital scaling] [ital ansatz] are used to obtain universal curves for the fluid permeability.
Abstract: Results from the numerical simulation of Stokes flow through random packings of nonoverlapping or overlapping sphers and a [ital scaling] [ital ansatz] are used to obtain universal curves for the fluid permeability. The scaling ansatz is motivated by previous analysis of rigorous bounds on the permeability. Excellent agreement was found for a variety of model microstructures of porous media in the low and high porosity regimes. Experimentally obtained permeabilities of several sandstones were found to agree well with our universal curve.
TL;DR: A novel model lipid bilayer membrane is prepared by the addition of phospholipid vesicles to alkanethiol monolayers on gold, which will allow the assessment of the effect of lipid membrane perturbants on the insulating properties of natural lipid membranes.
TL;DR: Neutron reflectivity measurements are made on a chemically end-grafted polymer brush swollen over a range of temperatures above and below the theta point, which resembles a swelling gel, while the brush tail expands similarly to a polymer in solution.
Abstract: Neutron reflectivity measurements are made on a chemically end-grafted polymer brush swollen over a range of temperatures above and below the theta point. Good agreement between the brush profiles and recent self-consistent field calculations and numerical simulations is obtained for temperatures in the vicinity of the theta point and in a good solvent. The expansion of the main body of the brush resembles a swelling gel, while the brush tail expands similarly to a polymer in solution.