bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Chain Folding in Poly(Ethylene Terephthalate) Fibers
Authors: Statton W. O. , Koenig Jack L. , Hannon M.
Published: February 2004

Annealing treatments at elevated temperatures have been characterized by a specific infrared absorption assigned to the regularly folded chain segments in poly(ethylene terephthalate). The higher the annealing temperature, the more regular is the chain folding introduced into the fiber. When a fiber is free to contract, it will have a much larger amount of regular refolding than one which is held at constant length. Stretching is able to prevent refolding at low temperatures but in these experiments stretching was not able to prevent refolding at the highest temperatures. By comparing the above infrared results with small-angle x-ray diffraction and broadline nuclear magnetic resonance techniques, it is now possible to give validity to the interpretation that chain folds provide a structure which is a major contributor to the long period intensity and to the fluid-like mobility at elevated temperatures. The external tension applied to a fiber during an annealing treatment will determine the internal shrinkage tension caused by the molecules during subsequent reheating. If chain refolding is allowed to occur during annealing, the potential for subsequent shrinkage and the amount of internal shrinkage tension are lowered, and higher temperatures are required to activate additional shrinkage. It thus appears that a major contributor to shrinkage and shrinkage tension in an oriented fiber is the refolding of molecules during thermal activation.
Alpha Relaxation in Polyethylene
Authors: Eby R. K. , Colson J. P.
Published: February 2004

Data are presented to show that a previously reported correlation of the strength of the alpha relaxation in annealed solution-grown crystals of polyethylene with the inverse of lamella thickness is not unique. Nor is a previously reported dependence of relaxation temperature on lamella thickness unique. As the lamella thickness changes with annealing temperature, the unit cell dimensions, the macroscopic density, and the residual solvent content of the crystals also change. Thus, there are apparent correlations between these parameters and the relaxation strength and temperature. Since it is not obvious which, if any, of these correlations is the fundamental one with regard to the alpha relaxation, some caution must be exerted in interpreting them. As an example of this, data are discussed to suggest that there is at least one other presently unknown parameter of importance to the relaxation.
Thermal Expansion of Polyethylene Unit Cell: Effect of Lamella Thickness
Authors: Davis G. T. , Eby R. K. , Colson J. P.
Published: February 2004

The unit cell dimensions have been measured at temperatures between 93 and 333 K for linear polyethylene samples with long periods of 385, 220, and 99 Å. The angular positions of 6 x-ray diffraction lines were obtained at 5–10 K intervals with a powder diffractometer and the positions corrected for beam penetration so as to agree with powder camera results obtained with more lines at 296 and 155 K. At lower temperatures, the cell dimensions are nearly independent of long period, but at higher temperatures, the basal area of the cell appears to vary linearly with the reciprocal of the long period. The value of the slope increases with temperature and at 293 K is nearly the same for sets of data obtained with a number of different molecular weight distributions, crystallization and annealing conditions as well as for n-paraffins. The specific volume data for all three polymer samples can be represented between 133 and 333 K with a standard deviation of 2.6×10−4 cm3 g−1 by the equation V=0.8341(1055.5−T)/(931.7−T)+(0.12−1.6×10−3T+7.8×10−6T2)/l2in which V is in cm3 g−1, T is °K and l2 is the long period expressed in angstroms. It is concluded that the interaction of the molecules at the surface of the crystal is not as important as the length of the molecular stems between the folds in affecting the dimensions at higher temperatures. The stems probably alter the dimensions through their effect on the thermal energy. The dependence of crystal specific volume on crystal size is estimated to reduce the heat of fusion of small crystals by about 2% in the normal range of long period. Neglecting this in analyzing melting-temperature-lamella-thickness data can lead to errors of the order of 3% in the surface free energy and 0.3 K in the equilibrium melting temperature. The variation of crystal specific volume accounts for about 5% of the variation of macroscopic specific volume and constant pressure specific heat with long period.
Stress Aging in Anhydrous Nylon 6–10
Authors: Kramer Edward J.
Published: February 2004

A delay time td to re-establish steady-state neck motion is observed if a partially drawn nylon 6–10 monofilament is aged for a time ta at a stress σa below the stress used to propagate the neck in a tensilecreep experiment. The delay time increases as kσatam for long ta, where 0<m<1, as long as the neck is immobile. The factor k increases steadily with the aging temperature Ta below 40°C, the glass transition temperature Tg of amorphous nylon 6–10, but much more rapidly with Ta above Tg. Aging at σa>0 is necessary to observe a delay time and, in fact, the effects of aging at σa can be erased by subsequently aging a short time at zero stress. At aging stresses just below the propagation stress σp, the neck still moves, but at a lower velocity than at σp. In this regime the delay time increases as c(σp−σa), where c is markedly increased by decreases in temperature. It is proposed that stress aging is caused by the stress-induced formation of small regions of better interchain packing (microcrystals) in nominally amorphous portions of the nylon and that the delay time is the time necessary to break up the microcrystalline structure induced by stress aging into the structure characteristic of steady-state flow. The breakup process is thermally activated with an activation enthalpy of 3.85 eV and stress activated with a shear activation volume of approximately 104 Å3. A modified Eyring model of flow is developed which assumes that the microcrystals are primary flow units which use up free volume in the amorphous regions as they are formed. This model accounts qualitatively for all, and quantitatively for some, of the features of the dependence of delay time on propagation and aging variables. Other theories, in particular free-volume collapse and dislocation dynamics theories, cannot be reconciled with all the observed characteristics of stress aging.
Observations of Deformation in Spherulitic Polyethylene
Authors: Predecki Paul , Thornton A. W.
Published: February 2004

A thin film marker technique is described for determining quantitatively from replicas the nonhomogeneous deformation modes operative in spherulitic polyethylene. Experiments were conducted at temperatures from −20° to 120°C and strain rates of the order of 0.04 sec−1. At −20°C, pronounced deformation at spherulite boundaries is observed. This results in stress concentrations which cause the accompanying nonhomogeneous deformation of the spherulites themselves to occur. At 70°C, corresponding to the α relaxation, the deformation has become almost entirely homogeneous within lamellae. At 120°C, corresponding to the α′ relaxation, lamellar boundary slip is clearly observed in addition to homogeneous deformation.
Creep of Polyethylene Above Room Temperature
Authors: Thornton A. W.
Published: February 2004

Previous work has shown that the creep data of a slow-cooled polyethylene can be superposed by means of a series of horizontal and vertical shifts to give a creep master curve at room temperature. The curve is smooth and regular with the exception of a discontinuity in the region of 2000 sec. In the time and temperature range of the measurements, polyethylene has two overlapping relaxation mechanisms, the α and α′. If it is assumed that the logarithmic decrements of the two relaxations are additive, the theory of linear viscoelasticity can be simply extended to cover this case, and it is shown that the discontinuity in the master creep curve arises quite naturally as a result of the differing activation energies of the two relaxations. This is a general conclusion and is independent of the nature of the relaxation mechanisms involved.
Rheo-Optical Properties of ABS Polymers and their Component Materials
Authors: Moore Robert S. , Gieniewski C.
Published: February 2004

This paper reports results of studies of birefringence (Δ) in ABS systems and in two component materials, measurements which reveal information about the orientation and response of these materials to deformation. Continuous measurements of (Δ) at strain rates (ε̇) from 8.5×10−5 to 3.3×10−3 sec−1 were carried out with a He–Ne laser light source on polystyrene (PS), poly(styrene-acrylonitrile) (SAN), and two clear ABS polymers. Measurements at constant ε̇ were related to mechanical response of the materials in creep, and two tests for the invariance of the stress optical coefficient were developed. These were achieved through comparison of computer-generated plots of appropriate rheo-optic functions with rheological counterparts derived from previous creep measurements via the convolution procedure of Hopkins and Hamming. Such comparisons indicated nearly identical constant time dependences for the respective logarithmic plots of (Δ/ε̇) and (stress/ε̇) vs time (t) for the PS and SAN polymers. Comparison of other functions confirmed the constant value of the stress optical coefficient (SOC) implied above, |(SOC)| being 1.0×10−12 cm2/dyn. This paper also presents results which indicate that the onset of yielding in the ABS systems can be studied by these optical techniques. It was observed that the increase in Δ in the two ABS systems was followed by the onset of a high degree of scattering which produced a maximum (M) in each plot of apparent Δ vs t, the height of M decreasing with increasing ε̇. The relevance of this latter phenomenon to the study of impact strength and yielding in ABS systems is discussed.
Infrared and Raman Spectra of the Cellulose from the Cell Wall of Valonia ventricosa
Authors: Blackwell J. , Vasko P. D. , Koenig J. L.
Published: February 2004

Comparison of the laser-Raman spectra with the polarized infrared spectra of native cellulose from the cell walls of the algae Valonia ventricosa has allowed greater confidence to be placed on the vibrational assignments than was possible previously from infrared data alone. The major details of the chain conformation and packing are already known from x-ray and electron diffraction studies, but the relative chain polarities and the nature of the intra- and intermolecular hydrogen bonding remain unknown. The present work indicates that only one rotational conformation is present for the −CH2OH side chains, which reduces considerably the number of structural possibilities.
Molecular Beam Sources Fabricated from Multichannel Arrays. IV. Speed Distribution in the Centerline Beam
Authors: Olander D. R. , Jones R. H. , Siekhaus W. J.
Published: February 2004

The collision model utilized by Giordmaine and Wang to describe the centerline beam intensity from long tubes has been extended to account for molecular-speed effects. The modification was accomplished by utilizing the velocity-dependent mean-free path instead of the Maxwellian averaged value. The analysis shows that the centerline beam intensity (or peaking factor) is practically unaffected by explicitly accounting for the distribution of molecular speeds. The speed distribution on the axis of the beam, however, is depleted of slow-speed molecules. The perturbation of the Maxwellian spectrum begins at a Knudsen number based on tube length of ∼10. For Knudsen numbers less than ∼0.1, the spectrum is no longer dependent on the Knudsen number. The average translational energy of the centerline beam in the low Knudsen number limit is 5.5% greater than that of a Maxwellian beam.
Molecular Beam Sources Fabricated from Multichannel Arrays. V. Measurement of the Speed Distribution
Authors: Siekhaus W. J. , Jones R. H. , Olander D. R.
Published: February 2004

A technique for measuring the speed distribution of molecular beams which uses symmetric modulation and phase-sensitive detection is described. Corrections for system errors on experiments of this type are developed and verified. In addition to the expected 1/v dependence of the ionizer, the extraction efficiency of the quadrupole mass spectrometer detector is shown to depend on the velocity of the molecules before ionization. The prediction of 5.5% energy enhancement of beams from channel sources operated at pressures well above the free-molecule flow limit has been verified.
Diffusion-Limited Phase Transformations: A Comparison and Critical Evaluation of the Mathematical Approximations
Authors: Aaron Howard B. , Fainstein Dora , Kotler Gerald R.
Published: February 2004

Mathematical analyses previously made for diffusion-limited growth and dissolution of spherical and planar precipitates have been reviewed. The analyses include the invariant-field (Laplace), the invariantsize (stationary interface) and the linear-gradient approximations, and where possible, exact solutions of the differential time-dependent diffusion equation and of the independent flux balance. The kinetic parameters are calculated and the different solutions compared. For virtually all cases of practical interest it is shown that the stationary-interface approximation is the best one. By considering a reversed-growth analysis and showing that it is merely an approximation to the dissolution of a spherical precipitate, it is thereby shown that growth and dissolution cannot be generally considered as simply conjugate processes.
Theory of Dislocation Cells: The Dislocation Quadrupole
Authors: Moore J. , Kuhlmann-Wilsdorf D.
Published: February 2004

The prototype of all dislocation cells composed of dislocations with two mutually perpendicular Burgers vectors is the terminated quadrupole. It consists of two similar, mutually perpendicular, rectangular glide dislocation loops which interpenetrate such that their edge dislocation segments are parallel to each other and that each of the two pairs of like-handed screw dislocation segments form a cross of fourfold symmetry. The stress components of the terminated quadrupole have been computed numerically and have been presented in the form of contour maps in, or parallel to, the midplane of the quadrupoles as well as in a cut parallel to the prism axis of the quadrupole. It is found that the intensity of the stresses falls with distance from the cell as 1/r5 for medium short or squat quadrupoles, and as 1/r4 for very elongated quadrupoles.
Extension of a Theory of Damping Due to Dislocations
Authors: Trott B. D. , Birnbaum H. K.
Published: February 2004

A theory of mechanical damping based on the model developed by Granato and Lücke was extended to small numbers of pinning points per network length and to stresses high enough to produce complete breakaway. The detailed response of a dislocation array to an applied stress was calculated using computer techniques in the low-frequency (kilocycle) range and both the viscous and hysteretic damping were computed. The amplitude dependence of the viscous and hysteretic damping was computed as a function of loop-length distribution, distribution of dislocation orientations, pinning point density, and stress distribution. The computation techniques allowed these calculations to be made without the usual mathematical approximations which are shown to result in a severe limitation on the previous analytic expressions. The results were applied to a discussion of the use of the damping theory to obtain parameters which describe the dislocation array and dislocation behavior. The time dependence of dislocation damping which results from diffusion of pins along the dislocation and in the lattice is discussed.
Motion of a Screw Dislocation in a Crystal
Authors: Celli V. , Flytzanis N.
Published: February 2004

The problem of the uniform motion of a screw dislocation in a crystal is treated from the point of view of lattice dynamics. A simple model of cubic lattice with piecewise-linear nearest-neighbor interactions is considered in detail and the extension to more realistic interatomic forces is outlined. The external stress necessary to maintain the dislocation in uniform motion is computed as a function of dislocation velocity. This stress has a minimum at one-half of the sound velocity and motion is quite possible at hypersonic velocities. The calculations are at zero temperature, hence the only dissipative mechanism present is the emission of phonons by the core of the moving dislocation. The theory is relevant to high-velocity dislocation motion, which is not of the usual thermally activated type.
Screw Dislocation Pileups and Shear Cracks in a Lamellar Composite
Authors: Chou Y. T. , Chou T. W. , Li J. C. M.
Published: February 2004

The continuous distribution of screw dislocations for a single-layered double-ended pileup in a soft lamella embedded between two rigid phases is studied. Explicit expressions are obtained for the distribution function, the number of dislocations, the relative displacement, and the strain energy. The stress concentration at the tips is calculated by using Moutier's theorem. When compared with a similar case in a homogeneous medium under the same stress, all these quantities are smaller by about 30%. The present solution is applicable also to the case of a slit crack under an antiplane shear stress.
Theory of the Schwarz-Hora Effect
Authors: Van Zandt L. L. , Meyer James W.
Published: February 2004

The theory of electrons scattered by light in the presence of a thin dielectric slab is developed and applied to some recent experiments of Schwarz and Hora. The experimental results are explained and some new effects predicted.
Chemical Diffusion in Titanium Carbide Crystals
Authors: Kohlstedt D. L. , Williams Wendell S. , Woodhouse J. B.
Published: February 2004

Single crystals of TiCx (x=0.84) were exposed to a carbon source at temperatures near 2000°C. The penetration of carbon into the structure, reflected in a gradient in the value of x, was evaluated with the electron microprobe analyzer. The chemical diffusivity D̃ was then determined using the Boltzmann-Matano method of analysis. The experimental value of D̃ was found to increase with decreasing carbon vacancy concentration, 1−x, and to have the average value of 220 exp (−97 700/RT) cm2/sec. This value of D̃ is in good agreement with one generated from Sarian's 14C tracer measurements on TiC crystals and DePoorter's semitheoretical model for activity. Because of the abundance of vacancies in the carbon fcc sublattice of TiCx, the experimental activation energy represents an energy of motion only. Lower experimental values for the diffusion activation energy have been obtained by other workers with the phase-boundary advance method. The present work shows that the difference between the activation energy measured by the tracer method and that measured by the phase-boundary advance technique cannot be ascribed to the influence of a chemical gradient. Hence it is probably due to grain-boundary short circuiting.
Creation Model of the Bostick Plasmoid in Magnetic-Field-Free Space
Authors: Butter D. A.
Published: February 2004

A creation model has been developed to interpret the behavior of Bostick plasmoids in the plane of the discharge. The plasmoids are formed by pulsed unidirectional current discharges across the electrodes of a button gun in vacuo (10−6 Torr). The model is that of a vacuum arc which expands in the form of a thin, highly conducting torus by virtue of the interaction of the surge current with its self-induced magnetic field. When the current no longer flows, the plasma of the arc continues to move by virtue of its own inertia. The plasmoid speeds directly away from the button gun have been determined experimentally from time of arrival measurements to moveable electrostatic probes, and are found to increase monotonically from 20 to 100 km/sec for a corresponding source voltage range of 2.5–17.5 kV. The measured velocity ν′ of any point on the plasma which subtends an angle θ to the direction of forward motion at the button-gun source is given by v′=〈I〉(α/πρ)1/2cosθ, where 〈I〉 is the average surge current, ρ is the measured mass per unit length (line density) of the genetic plasmoid and α is a theoretically calculated constant. The line density ρ of the plasmoid has been shown theoretically to be a constant and its magnitude has been measured to be 10−9 kg/m and hence the mass M of the plasmoid is deduced theoretically to be given by: M=〈I〉τ(απρ)1/2, where τ is the duration of the current pulse. This formula predicts the plasmoid mass to increase from 0.05 to 0.2 μg for the current range of interest and the predicted mass is verified to within 16% by a novel experimental technique.
Dynamic Behavior of a Gas Bubble in Viscoelastic Liquids
Authors: Tanasawa Ichiro , Yang Wen-Jei
Published: February 2004

The dynamic characteristics of a gas bubble situated in a viscoelastic fluid of three-parameter Oldroyd model is analytically investigated. Consideration is given to the influence of the thermodynamic process of the gas inside the bubble. An equation is derived which governs the timewise variation of the bubble size. Numerical results are obtained for the collapse of a bubble due to a sudden increase in the system pressure. The influence of the physical parameters on the collapse is investigated. It is disclosed that, in the presence of elasticity, the viscous damping effects on the bubble collapse are less in viscoelastic liquids than in pure viscous fluids. The dynamic behavior of a gas bubble in a three-parameter viscoelastic fluid is simulated by a mechanical system. The mechanism of the bubble behavior may be satisfactorily explained using the model.
Bubbles and the Rocket Effect
Authors: Chincholle Lucien
Published: February 2004

When gas or vapor bubbles of variable volume move through a liquid, they are submitted to high acceleration. This process, which we call ``the rocket effect,'' involves high energy density and provides bubbles with great autonomy. We establish that the energy transfer mechanism goes with a microjet flowing behind the bubble. Application is made to the flow of emulsions in centrifugal pumps, cavitation erosion, boiling, formation of aerosols, some electrohydrodynamic processes, machining by ultrasonics or electroerosion, etc.
Degradation of Apollo 11 Deployed Instruments because of Lunar Module Ascent Effects
Authors: O'Brien B. J. , Freden S. C. , Bates J. R.
Published: February 2004

Three similar horizontal solar cells were deployed by the Apollo 11 astronauts to measure whether the lunar module ascent caused degradation of their surface properties and resultant thermal control. The outputs of the cells at lunar module ascent decreased approximately 18%, 7%, and 0%. The average absorptivity thus decreased approximately 8%, while the average emissivity decreased approximately 12%, resulting in overheating. The results are discussed and several possible causes are suggested. Subsequently, the surface absorptivity and emissivity of each cell changed less than 1%–2% during the following month. Lunar module ascent effects for the closely deployed Apollo 11 instruments were therefore much more important than the long-term exposure to the lunar environment. The relevance of these findings to future deployment of lunar instruments is discussed.
Principal Hugoniot, Second-Shock Hugoniot, and Release Behavior of Pressed Copper Powder
Authors: Boade R. R.
Published: February 2004

Principal Hugoniot, second-shock Hugoniot, and adiabatic release data obtained by gas gun experimental techniques are presented for porous copper prepared by hydrostatically pressing copper powder to a density of about 6.4 g/cm3. The bulk of the data presented pertain to a powder with initial particle diameters of 30μ, however, sufficient data are also presented for other particle diameters of 10, 50, and 100μ to show that particle size variations over this range have no distinct effect on shock loading properties. Transmitted wave profiles for each of these materials are characterized by a single precursor wave (amplitude ∼1.4 kbar, velocity ∼1.9 mm/μsec) followed by a main shock wave. Principal Hugoniot data for stress levels above the precursor amplitude and over the entire stress range examined (up to ∼35 kbar) are well represented by the relation α2=1 + (α1−1) exp[−â(P2−P1)], where α2 is the distention ratio (density of solid copper/density of porous copper) at the stress level P2, P1 is the precursor amplitude, α1 is the value of α at P1, and â is an adjustable parameter (found to be 0.254 kbar−1). Second-shock Hugoniot data (i.e., for a Hugoniot centered at a prestressed state P2, α2) are also well represented by the above equation provided α1 and P1 are changed to correspond to the prestressed state. Release data indicate that the shock loaded porous copper unloads along a path that is essentially the same as the loading path. These data, however, extend only over the upper one-fourth of the stress range between the shock loaded state and the foot of the release adiabat and a straight-line continuation of the measured adiabat indicates that the compression process is not reversible.
Spectral Anomalies Due to Inhomogeneous Optical Pumping in the Ruby Laser
Authors: Izatt Jerald R.
Published: February 2004

The output of a ruby laser operated near threshold at liquid-nitrogen temperature normally consists of one or more longitudinal cavity modes near the center of each member of the R1 doublet. The nominal doublet spacing is 0.38 cm−1, but by masking a portion of the ruby rod from the pump radiation the separation of the laser components can be reduced significantly below that value. For example, masking ¼ of the length of a 5.08-×0.64-cm ruby rod has been observed to result in a laser component separation of only 0.20 cm−1. This paper presents typical laser spectra for masked rubies and also presents and discusses time-resolved laser spectra and fluorescence spectra observed under the same masking conditions. The latter provide insight into the mechanisms responsible for the observed anomalies, and a theoretical model is developed to explain the principal features of the observations.
Change of Eigenstate in a Superconducting rf Cavity Due to a Nonlinear Response
Authors: Halbritter J.
Published: February 2004

The studies on superconducting rf cavities have shown phenomena not familiar to normal conducting cavities. Therefore we rediscuss the phenomenological description of rf cavities at low- and high-field levels, especially nonlinear effects accompanied with high rf field strength. To handle nonlinear effects an appropriate perturbation theory is developed. Together with knowledge on the surface impedance in the superconducting state and on field-emitted electrons in the cavity this theory is solved in lowest approximation. The resulting frequency shift and the nonlinear rf losses are compared with published experimental data. This comparison shows that at present nonideal effects define the results. The intrinsic fast superconducting mechanisms causing harmonics are overshadowed by slow nonideal effects.
Luminescence in Tellurium-Doped Cadmium Sulfide
Authors: Roessler D. M.
Published: February 2004

The room-temperature emission in CdS:Te is a single broad band whose half-width and peak position provide a simple means of estimating the tellurium concentration in the range 2×1018−5×1019 atoms cm−3 with a precision of 15%. Of the two discrete bands (at 2.1 and 1.7 eV) seen at low temperatures, only the higher-energy emission can be described by a simple configuration coordinate model. The nophonon line energy, not observed experimentally, is 2.36±0.05 eV, the exciton being bound to the tellurium atom by 0.22 eV. At high tellurium concentrations the exciton can be trapped at pairs or larger groupings of tellurium atoms sited on nearest-neighbor sulfur sites, the binding energy increasing correspondingly. The emission intensity from the 2.1-eV band is reduced by a factor of about 100 as the temperature is increased from near 0° to 300°K. The quenching behavior cannot be described in detail by a single decay process but the effective activation energy is about 0.20±0.03 eV. The drastic temperature quenching above 200°K restricts the usefulness of CdS:Te as a room-temperature scintillation device but can be reduced by the addition of donors. The relative efficiency at 300°K to that at low temperature increases from less than 1% in undoped material to 15%–20% in CdS:Te containing about 1018 donors cm−3. Chlorine, bromine, or iodine appear to serve equally well as donors. Donor-doped crystals exhibit two components in decay lifetime measurements at 300°K but only one (τ∼250±50 nsec) at low temperature. The extra radiative decay time (500±100 nsec) may arise from free-electron to bound-hole transitions at high donor concentrations. Energetically one expects such a process to be near degenerate with the trapped exciton decay and no obvious change occurs in the broad emission band. Hall data show a reduction in the donor binding energy roughly proportional to ND1/3, where ND is the donor concentration. Pulse heights detected after nuclear excitation of CdS:Te are increased about fivefold when 5×1017 donors cm−3 are added, chlorine giving the cleanest pulse shapes.
Optically Pumped 199Hg NMR Shape Function
Authors: Conklin Glenn E. , De Santo Joseph R. , Greenwood Ivan A.
Published: February 2004

An experimental investigation was made to determine in detail an NMR ground state transition lineshape function of a two-level system (spin 1/2). The optically pumped vapor of 199Hg was selected as an investigative medium because of the narrow bandwidth (∼1 Hz) and large maximum signal-to-noise power ratio (>60 dB at an observation bandwidth of 1 Hz) of the associated NMR. The shape at 1.0 kHz, under the conditions of the experiment, for 199Hg was found to be describable within experimental error over a 750 linewidth spectral region using only simple combinations of Lorentzian functions. In addition, the saturation properties were found to agree with the prediction of modified Bloch susceptibility equations.
Magnetic Properties and Nuclear Magnetic Resonance of Cubic RCu5 Intermetallic Compounds
Authors: Buschow K. H. J. , van Diepen A. M. , de Wijn H. W.
Published: February 2004

The magnetic properties of the cubic RCu5 compounds TbCu5, DyCu5, HoCu5, ErCu5, and TmCu5 are reported for temperatures between 2.1° and 300°K. In the paramagnetic region the susceptibility follows a Curie-Weiss law. The type of ordering at low temperatures changes with increasing atomic number from antiferromagnetic (R = Tb) to ferromagnetic (R = Ho, Er, Tm), while DyCu5 is metamagnetic. The Knight shifts of the 63Cu NMR in these compounds have been measured between 100° and 300°K. The results are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida theory.
Linear Magnetic Birefringence in the Ferrimagnetic Garnets
Authors: Dillon J. F. , Remeika J. P. , Staton C. R.
Published: February 2004

Light propagating in a ``cubic magnetic crystal'' perpendicular to M is subject to a linear magnetic birefringence. The origin and representation of this birefringence is discussed. In the ferrimagnetic garnets several mechanisms may be identified, but the overwhelming part is apparently associated with the magnetostrictive distortion. Measurements are presented for each of the rare-earth iron garnets at 77° and 300°K.
Magnetic Forces on a Coil Moving in a Conducting Tube
Authors: Neil V. Kelvin , Cooper Richard K.
Published: February 2004

A theoretical investigation is made of the forces on a current ring moving in a conducting tube. The ring carries constant current and moves with constant speed along the tube. The ring is coplanar with the circular cross section of the tube, with the center of the ring displaced downward from the axis of symmetry of the tube so that the asymmetric image currents provide a lift force on the ring. The wall of the tube has finite thickness and finite conductivity. The lift and drag forces exerted on the ring by the wall currents are calculated. The minor radius of the ring is neglected, and within this approximation, the calculations are exact.
Generalized Equation for the Elastic Moduli of Composite Materials
Authors: Nielsen Lawrence E.
Published: February 2004

A generalized equation is proposed for the relative elastic moduli of composite materials. By the introduction of a generalized Einstein coefficient and a function which considers the maximum volumetric peaking fraction of the filler phase, the moduli of many types of composite systems can be calculated.
Resolving Power and Information Capacity in Scanning Electron Microscopy
Authors: Simon Rene
Published: February 2004

Information theory formalism has been applied to the scanning electron microscope. A theoretical model of two-dimensional secondary electron noise has been derived and the maximum capacity content per picture element of the S.E.M. has been computed as a function of its geometrical resolving power. According to the theory, it is shown that the maximum number of grey levels on the picture varies between 2 and 8 (varying with the maximum secondary electron yield) with a resolving power of 200 Å for a S.E.M. equipped with a tungsten filament gun, and 30–40 Å is equipped with a lanthanum-hexaboride gun. However, these figures are established with the assumption that the object does not present any redundant properties. The knowledge of the statistics of a general class of objects will permit the use of adapted coding to increase the information content in the object area of interest.
Optical Properties of Single Crystals of NbSe2 and Nb1.04Se2
Authors: Myers G. E. , Montet G. L.
Published: February 2004

Niobium-selenium compounds in the composition range NbSe2–Nb1.14Se2 can form layered crystals which are readily cleaved in a direction perpendicular to the c axis. For compositions in the range NbSe2–Nb1.05Se2, two structures, a high-temperature ζ phase and a lower-temperature ε phase may be formed. Single crystals of ε-NbSe2, ε-Nb1.04Se2, ζ-NbSe2, and ζ-Nb1.04Se2 were grown by the vapor transport technique and the optical absorption spectra of thin-cleaved specimens in the visible and near infrared regions were studied. An infrared absorption spectrum to λ = 15 μ was also obtained for ε-NbSe2. Accurate values of the absorption coefficient α were measured at 10 nm intervals in the spectral region 400–620 nm. For all crystal types, α was found to have peak value ∼4.5×105 cm−1 at λ = 460 nm, decreasing steadily to ∼1.3× 105 cm−1 at λ = 620 nm. The values of α for ε-NbSe2, along with bulk reflectivities R0 measured on basal plane cleavage surfaces of opaque sections of ε-NbSe2, have been used to calculate the optical constants n and k. The most notable feature of the visible and near infrared studies is the similarity of the gross features of the absorption spectra of all four crystal types. This indicates that the absorption is governed principally by a mechanism operating largely within the Se–Nb–Se ``layer molecules'' and relatively independent of structure or the presence of interstitial excess metal. An absorption minimum centered about λ = 1.2 μ has been attributed to plasma oscillation of the conduction electrons, and calculations based on this assumption have yielded a value m*≤5.9 m for the effective mass of these electrons.
Enhancement of the Vacuum-Ultraviolet-Excited Fluorescence and Photoconductivity of Polystyrene by Naphthalene Vapor
Authors: Oron N. , Ofran M. , Weinreb A.
Published: February 2004

The fluorescence intensity and photoelectric current in polystyrene under excitation with radiation in the vacuum ultraviolet are strongly enhanced by the presence of naphthalene vapor and application of an electric field.
Conductance Peaks in a Cylindrical Plasma Capacitor at the Cyclotron Harmonics
Authors: Downward J. G. , Harp R. S.
Published: February 2004

Theoretical and experimental results are presented for the conductance peaks at the cyclotron harmonics in a plasma-filled cylindrical capacitor. A theory is developed and is shown to predict peak height variation with density, harmonic number, and sheath thickness that is in agreement with experimental observation. A harmonic conductance peak is found to rise initially as density increases, only to fall to a minimum height as the upper hybrid frequency passes through that harmonic. Possible applications of this work to plasma diagnostics and further research are mentioned.
Excitation of Electrons Across Al2O3 Insulating Barriers by Proton Bombardment
Authors: Lee R. S. , Sethna P. , Crawford J. L.
Published: February 2004

Measurements have been made of the current excited across Al2O3 insulating barriers by proton bombardment. Films of Al2O3 sandwiched between thin metal electrodes were bombarded by a monoenergetic beam of protons from a low-energy accelerator. The Al2O3 was grown by anodic oxidation on films of Al which had been deposited on glass substrates. The Al2O3 films ranged in thickness from 100 to 600 Å. Ag counterelectrodes deposited on top of the Al2O3 completed the junctions. The thickness of the Ag counterelectrode was <500 Å for all of the junctions studied. Current-voltage characteristics of the junctions were measured while the junctions were in the proton beam. The current-voltage curves were approximately linear, showing a net electron current from the Ag electrode to the Al electrode at zero bias, and zero electron current when the Al electrode was biased from 2.1 to 4.75 V negative with respect to the Ag electrode. Proton beam currents ranged from 5×10−10 to 2×10−11 A. Proton energies ranged from 26 to 102 keV. Results are interpreted in terms of a model in which the insulating film is represented by a potential barrier separating a sea of electrons on either side of the barrier. Conduction takes place by passage of excited electrons across the top of the barrier. The effect of the bias is to modify the internal electric field in the oxide and to shift the momentum distribution of the excited electrons. Time-dependent effects in the current-voltage curves are observed which are believed to arise from charging of the oxide by the proton beam.
Photoconductivity of Neutron-Irradiated Gallium Arsenide
Authors: Borghi L. , De Stefano P. , Mascheretti P.
Published: February 2004

Photoconductivity spectra taken on fast-neutron-irradiated samples of GaAs show a structure which is interpreted as being due to transitions between energy levels and either the valence or the conduction band. Levels lie at approximately 0.2- and 0.7-eV above the valence band and 0.5-eV below the conduction band. Slow-neutron irradiation has been employed to compensate the acceptor impurity in p-type GaAs. A level at Ev+0.7 eV is detected in the high-resistivity material thus obtained. It is concluded that such level is associated with a defect that is present in GaAs before irradiation in an inactive form.
Determination of Low Barrier Heights in Metal-Semiconductor Contacts
Authors: Tantraporn W.
Published: February 2004

Ohmic contacts under ambient conditions and at a low current-density level may become blocking at a lower temperature and/or a higher current density. By observing the temperature dependence of the applied voltage across the sample for the two polarities under the constant-current condition, the temperature dependence of the bulk and that of the barrier, if any, can be separated. The detection of the barrier, if the presence of the latter is relevant within the range of experimental interest, is easy and unequivocal. The technique is suggested for device physics studies in which the barrier may play a dominant role but nevertheless might escape detection under usual testing procedures. The method is applicable to the higher barrier-height values as well as low. It also allows a simple study of the semiconductor effective mass through the determination of the Richardson constant.
Optical Properties and Energy Transfer in LiYF4:Nd3+, Yb3+
Authors: Miller J. E. , Sharp E. J.
Published: February 2004

The spectroscopic features of crystals of the scheelite-structure lithium yttrium fluoride (LiYF4) doped with Nd3+ and Yb3+ have been measured at 300°K. The energy-transfer properties of Nd3+→Yb3+ in this host have been studied and compared with other materials possessing the scheelite structure.
Linewidths of the R′ Optical Transitions of Chromium Ions in Ruby
Authors: Birnbaum Milton , Kisliuk Paul , Stocker Tom L. , Otaguro Wilfred S.
Published: February 2004

New experimental measurements of the temperature dependence of the R′ fluorescence of ruby (77°–450°K) have disclosed features not previously observed. Direct phonon transitions to nearby levels contribute less and indirect Raman processes contribute more to the R3′ linewidth than had previously been reported. Direct phonon relaxation from the R1′ and R2′ levels to the 2E level contributes appreciably to linewidth of their fluorescence to the ground state.
Equilibrium Structure of a Thin Epitaxial Film
Authors: van der Merwe J. H.
Published: February 2004

The dependence of the equilibrium configurations of a thin single-crystalline epitaxial film, on the strengths of substrate-overgrowth and overgrowth-overgrowth bonds and the interfacial misfit is calculated neglecting substrate deformations. Previous analyses of the system were based on a one-dimensional model, allowing for misfit, i.e., differing lattice parameters, in one interfacial direction only. The results of these analyses show (i) that interfacial mismatch is accommodated by a parallel sequence of misfit dislocations, (ii) that the mismatch decreases when appropriate overall lateral strains are introduced, (iii) that in stable and metastable film configurations both dislocations and overall strains are present, and (iv) that there exist limiting misfits 2/πl0 and 1/l0, respectively, below which the stable and metastable states are pseudomorphic; the film is homogeneously strained to coherency, i.e., exact fit, with the substrate. The transition to the noncoherent state is critical, the change in average lattice parameter being discontinuous. The parameter l0 depends on the various bonding strengths and yields, in an average case, the values 2/πl0=9% and 1/l0=14%. The present analysis allows for misfits along two interfacial directions, which may differ from each other and which could be accommodated by a cross grid of dislocations. Qualitatively, the results have many similarities in common with the one-dimensional case. Quantitatively, most of the results are significantly different, reducing for example, the forementioned limiting misfits by approximately 25%. Furthermore, the transition to noncoherency is not critical and the metastable configurations are probably absent for an island with a curved boundary. Most of the differences are due to the Poisson phenomenon. Some speculations, which deal with refinements of the calculations as to overgrowth shape, size, thickness, and substrate interaction potential are presented.
Influence of Chlorine on Au and Ag Films Evaporated on Alkali Halide Substrates
Authors: Green A. K. , Bauer E. , Dancy J.
Published: February 2004

Thin films of Ag and Au were deposited on vacuum-cleaved alkali halides in ultrahigh vacuum with and without controlled-chlorine exposures. Chlorine exposures were made (1) prior to deposition, (2) continuously during deposition, or (3) with various delay times after the start of deposition. The film structure and orientation was studied by conventional electron diffraction and electron microscopy. Results show that chlorine induces nearly perfect epitaxy of Ag and Au on NaCl and enhances epitaxy of Ag and Au on KCl and Kbr under deposition conditions that otherwise would produce polycrystalline films. For example, epitaxy of Au on NaCl is obtained at room temperature in a partial pressure of ∼5×10−7 Torr Cl2. Possible mechanisms responsible for the influence of Cl2 on the orientation of Ag and Au are discussed.
Ion Milling of Magnetic Oxide Platelets for the Removal of Surface and Near-Surface Imperfections and Defects
Authors: Schmidt P. H. , Spencer E. G. , Walters E. M.
Published: February 2004

Magnetic oxide platelets, useful for memory and logic circuits, are presently prepared by diamond abrasive polishing techniques. Processed platelets, however, usually contain thin damage layers and microscopic scratches resulting in increased domain wall coercivity. Platelets less than 1 mil in thickness and of desirable flatness are difficult to prepare using these techniques. Ion milling has been successfully used to rapidly remove abrasive and polishing induced defects and at the same time produce flat surfaces of much less than 1 mil in thickness. The measured removal rate was 0.5 μ/h/side; however 500-μ/h/side rates are attainable. Final platelets are scratch free, flat, and highly reflecting, having lower domain wall coercivity than diamond polished plates. No change in mobility was observed following ion milling. The process is adaptable to large areas and a wide variety of related applications.
Ionization Rates in (AlxGa1−x)As
Authors: Shabde S. N. , Yeh C.
Published: February 2004

The ionization rates of (AlxGa1−x) As have been measured as a function of the Al content and the results are presented here. Values of both A and b are found to increase with Al content. It is also found that for low values of Al content, the ionization rates of (AlxGa1−x) As are consistent with Baraff's theory.
Generation of High-Frequency Ultrasonic Waves by Gunn Effect
Authors: Hayakawa H. , Ishiguro T. , Takada S. , Mikoshiba N. , Kikuchi M.
Published: February 2004

The generation of the longitudinal waves in the frequency range up to the microwave region was observed in the GaAs Gunn diode when the applied field exceeded the threshold. This ultrasonic excitation in the Gunn diode is caused by the lattice deformation by the high-field domains through the piezoelectricity of GaAs. The two possible excitations, in the bulk and at the surface, are discussed using simple models. It is speculated that a properly oriented Gunn diode may be applicable to the narrow beam-like ultrasonic generator.
Simple Technique for Exciting and Probing Elastic Surface Waves
Authors: Hallermeier Robert J. , Diachok Orest I.
Published: February 2004

The liquid-wedge technique has been used to excite ultrasonic surface waves on various solids; using this technique, only a suitable noncorrosive liquid need come into contact with the solid, and no acoustic power need be coupled into the bulk of the solid. An optical probe was used to measure the surface-wave pattern excited; data obtained from this probe are presented, and results for the surface-wave attenuation on stainless steel are given.
Amorphous Carbon Films: Conduction Across Metal/Carbon/Metal Sandwiches
Authors: MacVicar M. L. A.
Published: February 2004

Very thin films of evaporated carbon have been deposited on various metallic thin film and single-crystal substrates to form metal/carbon/metal sandwiches. Electrical conductivity measurements through the carbon, over a range of temperatures from room temperature to below 1°K, show that conduction is a process of complicated mechanisms dependent on the presence of both structural and impurity defects. At low temperatures, conductivity appears to be independent of temperature, and the carbon films may act as tunneling barriers of effective heights 30–100 meV. These heights can be engineered by suitable choices of evaporation and substrate conditions. Explanation of conduction in these films remains speculative.