bluuebunny/crossref_metadata_2025 huggingface dataset:
A Continuum Mechanical Model for Laser-Induced Fracture in Transparent Media
Authors: Conners Gary H. , Thompson Robert A.
Published: April 2005

Theoretical investigations in the area of laser-induced damage in transparent media have been primarily directed toward explaining the mechanism by which sufficient energy can be absorbed to cause the observed fracture. However, the work considered here is based on a theoretical investigation of damage from a macroscopic point of view, with the goal of developing a simple continuum mechanical model for the processes leading up to fracture. In this case, the energy absorbed and nonlinear absorption effects are related to the beam's total energy through an experimentally determined absorption parameter. The problem is formulated in terms of dynamic thermoelasticity theory, the energy absorbed from the laser beam being represented by a volume heat source with a physically reasonable space and time dependence based on diffraction theory and known parameters of the optical system. The nonhomogeneous, thermoelastic field equation is solved for the stress distribution by a Green's function technique. By introducing a tensile stress fracture criterion, conditions under which fracture can be initiated during the irradiation process are obtained. Initial numerical results based on an f/30 lens give a good explanation of the phenomenon of the pulverized region associated with laser damage and indicate that this region has a diameter of about 0.4 mm, which is in good agreement with experimental findings.
Effect of Stress on the Low-Temperature Phase Transformation in V3Si
Authors: Patel J. R. , Batterman B. W.
Published: April 2005

The influence of stress on the low-temperature structural transformation in V3Si was investigated. A large stress can completely suppress the transformation as observed by x-ray Berg-Barrett and lattice parameter measurements. A lower stress can affect the morphology and extent of the transformation but leaves the transformation temperature Tm unchanged.
Penetration Parameter for an Adsorbed Layer of Polarizable Ions
Authors: MacDonald J. Ross , Barlow C. A.
Published: April 2005

We present several improvements in the calculation of the penetration parameter f, which is related to ionic and electronic work functions. A regular, hexagonal array of adions which are imaged in a conducting adsorbent plane is considered. The first improvement is the treatment of discrete adion charges and their images as non-ideal, finite-length dipoles instead of ideal dipoles as in almost all earlier treatments. Next, nonzero polarizability of adions is considered, discrete induced dipole moments are calculated in a self-consistent manner, and the effect of induced-dipole images is included. When the polarizability is nonzero, previous expressions for f are inadequate even when generalized in an obvious way, and an improved method of calculating f is presented. The inclusion of polarization effects leads to significant changes in the dependence of f on surface coverage of adions. Finally, f is calculated with and without polarization contributions for the situation where redistribution of the adsorbed array upon removal of an adion affects the desorption energy. Inclusion of redistribution also leads to appreciable changes in f. Under some circumstances, either with or without redistribution contributions, it is found to be easier to desorb an adion from an array than to desorb an isolated adion. In this case, the potential at the site of a removed adion may exceed that far away from the surface. To illustrate this phenomenon, which can lead to the establishment of a potential barrier against electron emission, or to a potential well enhancing emission, we give some curves of potential vs distance from the electrode along a line perpendicular to the electrode and through a removed adion site.
Combined Effect of Ultrahigh Pressure and Nuclear Irradiation on Solid—Solid Phase Transformations
Authors: Kulcinski Gerald L. , Maynard Charles W.
Published: April 2005

A special Bridgman opposed diamond anvil apparatus was constructed to generate useful static pressures up to 50 kbar at ambient temperatures. It was designed to function in the environment of a nuclear reactor to thermal fluxes of 1018 nvt. The bulk of the study deals with ZrO2; other materials investigated to varying degrees were PbO2, BN, B2O3, B, silicic acid, SiO2, BaTiO3, graphite, P, Ni, Zr, and U. The samples were exposed while under pressure to fast neutron fluxes (>2.8 MeV) of 4.1×1015 nvt. The boron-containing samples suffered up to 2×1018 fissions/cc while under pressure. PbO2, BN, B2O3, silicic acid, graphite, SiO2, and Zr were bombarded by uranium fission fragments to 6.5×1011/cm2. ZrO2 was bombarded with 2×1016 nvt epithermal neutrons and 1.6×1014 fission fragments/cm2. There was no evidence of any irradiation-pressure-promoted phase change. Pressure alone converted tetragonal ZrO2 to the monoclinic form and cubic ZrO2 transformed to tetragonal plus monoclinic phases. A PbO2 rutile → orthorhombic transformation occurred in seconds at room temperature along with a pressure-sensitive optical absorption edge above 20 kbar.
Temperature and Pressure Dependence of the Index of Refraction of CdS
Authors: Langer D. W.
Published: April 2005

By transmission measurements on CdS platelets the locations of interference maxima were determined between the absorption edge and 6500 Å for E⊥C and E∥C. As temperature and pressure around the sample were changed the displacement of these maxima was followed. Subsequently the dispersion curves were established. The temperatures were varied between 4.2° and 358°K at atmospheric pressure and between 77° and 358°K at hydrostatic pressures up to 10 kbar. The quantities dn⊥/dλ and dn∥/dλ which might be useful for laser applications are presented.
Magnetic Decay in a Hollow Elliptic Cylinder. II
Authors: Weinstein M. A.
Published: April 2005

Earlier results for the relaxation time of an axial magnetic field in a conducting hollow elliptic cylinder are extended to the case of eccentricities near unity.
Ferromagnetic Particles in a Type-II Superconductor
Authors: Alden T. H. , Livingston J. D.
Published: April 2005

A dispersion of fine, single-domain, iron particles in a superconducting Hg-13% In alloy is found to produce considerably magnetic hysteresis that is dependent on the sense of the applied field. When the applied field is parallel to the remanent particle magnetization, and particles attract and pin flux threads, the observed hysteresis is always larger than in the antiparallel case, where particles repel flux threads. Significant sense-dependent properties have been observed for particles from 20 to 200 Å in diameter, and for volume fractions from 0.01% to 1%. The results demonstrate the effectiveness of ferromagnetic particles in increasing superconducting critical currents, the superiority of attractive centers over repulsive ones, and the possibility of ``directional superconductors'' whose properties depend on the sense of the applied magnetic field.
Effect of Spectral Hole-Burning and Cross Relaxation on the Gain Saturation of Laser Amplifiers
Authors: Cabezas Amado Y. , Treat Richard P.
Published: April 2005

A theory is presented for the effect of cross relaxation (CR) on the saturation with driving-signal intensity of the steady-state gain of a laser amplifier with an inhomogeneously broadened linewidth. The condition of steady-state gain results when the amplifier excitation level W and the driving-signal intensity are constant for about 3τ or longer, where τ is the fluorescence lifetime of the upper laser level. The driving signal is derived from an oscillator operated in the normal mode and which does not saturate the amplifier inversion density over times short compared with τ. We assume that CR, at a rate F, restores to thermal equilibrium any distortion of the spectral inversion density caused by the driving signal. Formulas are derived for steady-state gain saturation with arbitrary F for two cases: the spectral width of the driving signal is (1) narrow and (2) broad relative to the ionic or homogeneous line. The ionic linewidth Δvi is assumed to be much less than the inhomogeneous linewidth Δva and refers to the minimum frequency interval for which infinitely fast CR is operative. The theory also yields the CR coefficients f=(Δvi/Δva) [F/(τ−1+W)] for the narrow-band case and f*=(Δvs/Δva) [F/(τ−1+W)] for the broadband case, where Δvs is the driving-signal bandwidth. These coefficients significantly parametrize the CR effect: for f, f*>1, hole-burning is not severe, whereas for f, f*<1, considerable hole-burning takes place and the gain saturates faster. Finally, the theory is applied to experimental results obtained for Nd3+:glass broadband and narrow-band amplification; an approximate lower bound of 4×105 sec−1 is determined for F.
Low-Temperature Magnetic Susceptibility and Specific Heat of Constantan
Authors: Yee Roy , Zimmerman George O.
Published: April 2005

We have measured the magnetic susceptibility of constantan in the temperature range between 0.1° and 4.2°K and the specific heat in the temperature range between 0.05° and 0.15°K. The results of these measurements are reported here. They show that both the magnetic susceptibility and the specific heat are rather large in the low-temperature region, the specific heat having a maximum at 0.08°K.
Photoemission of Electrons from Metals into Silicon Dioxide
Authors: Goodman Alvin M. , O'Neill John J.
Published: April 2005

Metal-insulator contacts have been prepared by evaporating partially transparent metal electrodes onto a thermally grown silicon dioxide layer. The metals employed were Mg, Al, Ag, Cr, Cu, Au, Ni, Pd, and Pt. Photoemission of electrons from the metal electrode into the oxide conduction band has been observed in each case. The photoelectric threshold energy φT for this process depends primarily upon the metal employed and is consistent with the relation φT=φM−χ, where φM is the vacuum value of the metal work function and χ, the electron affinity of the oxide, is 1.0 eV.
Spontaneous and Stimulated Emission from GaAs Diodes with Three-Layer Structures
Authors: Pilkuhn M. , Rupprecht H.
Published: April 2005

The electrical and optical properties of GaAs diodes with three-layer structures are described. The center layer was either n-type (n+-n-p+ diodes), semi-insulating (n+-i-p+ diodes) or p-type (n+-p-p+ diodes). The spontaneous emission lines from n+-n-p+ diodes were studied in detail. For a low doping level of the n layer they can be interpreted by assuming hole recombination on the n side of the p+-n junction. Lasing occurred in a 1.5-eV line and in second-order transverse modes. Small vertical beamspreads were observed. Optical gain factor and laser losses are discussed. Stimulated emission was not obtained when the n layer was very thick. n+-i-p+ diodes had a Cr-doped middle region and showed a negative resistance at 300°K. Lasing occurred in the 1.48-eV line. n+-p-p+ diodes showed emission lines attributed to tunneling. Lasing occurred in the 1.48-eV line. The temperature dependence of the threshold current density is discussed.
Nuclear Magnetic Resonance in Poly (Normal α-Olefins)
Authors: Slichter W. P. , Davis D. D.
Published: April 2005

The spin-lattice nuclear magnetic resonance relaxation has been measured in a series of poly(normal α-olefins) in which the side chains ranged in length from methyl to hexadecyl. The studies were carried out as a function of temperature, from −160° to 150°C, and as a function of the radiofrequency, specifically at 10, 20, 30, and 50 Mc/sec. The dependence of the NMR relaxation upon temperature and frequency shows that increase in the length of branches contributes to increased segmental mobility. Effects of thermal history are examined. The rate of side-chain crystallization of polydodecene can be measured from the time dependence of the spin-lattice relaxation.
Visual and Photographic Study of Electron Beams
Authors: Ausburn K. J.
Published: April 2005

A method for visual observation and photographic recording of electron beams that involves the dropping of fluorescent powder through the beam is illustrated by photographs of convergence in Pierce guns, an electron beam profile, and electron trajectories in an Einzel lens. It has the following advantages: (a) The photographic exposure time under all normal circumstances is much less than 1 sec. (b) It is applicable in any degree of vacuum. (c) Spatial resolution of less than 100 μ is readily obtained. (d) The electron beam is not perturbed. (e) It is applicable to electron beams in less accessible regions—between closely spaced electrodes for example. (f) Satisfactory photographs may be obtained for electron beams of any perveance provided the beam potential exceeds 100 V.
Dielectric Susceptibility Tensor for an Atomic Vapor
Authors: Fork Richard L.
Published: April 2005

The dielectric susceptibility of an atomic vapor in the presence of a magnetic field is expressed as a three by three tensor. The gain coefficient in the absence of external fields and the g and J values of the atomic levels are shown to be sufficient to determine all the tensor elements. An illustrative example for a J=1 to J=0 transition and formulae for generalizing the results are given.
Shear Wave Propagation in a Birefringent Viscoelastic Medium
Authors: Thurston George B. , Schrag John L.
Published: April 2005

A theoretical and experimental study is described for the optical birefringence associated with the propagation of a plane shear wave in a viscoelastic medium. The shear wave propagation is characterized by a complex propagation constant from which the complex coefficient of shear viscosity of the medium may be derived. The optical birefringence is related to the mechanical action of the shear wave by a complex mechano-optic coefficient. In the experiment the birefringence due to the shear wave is analyzed by transmitting circularly polarized light through the material at right angles to the direction of shear wave propagation and to the direction of particle displacement. The modified, circularly polarized light is then analyzed by a plane analyzer and a photomultiplier detector. The mechanical and mechano-optic constants for the medium are determined for an aqueous milling yellow solution and for a polystyrene solution and are compared with independent determinations of these same factors from other experiments.
Sonic Degradation of Polyisobutylene in Solution
Authors: Porter Roger S. , Cantow Manfred J. R. , Johnson Julian F.
Published: April 2005

Narrow, well-defined molecular weight fractions of polyisobutylene were irradiated in n-hexadecane as 3% concentrations by a 10-kc/sec sonic oscillator. The irradiated polymers were recovered by precipitation and evaporation; the molecular weight distributions were determined by column chromatographic fractionation. Narrow fractions for study had viscosity molecular weights of 15 100, 40 000, 74 400, and 137 000. The changes in distribution are compared with those from a broad distribution polymer with a molecular weight of 40 000. The limiting degree of polymerization below which degradation will not take place is at least as low as 200, and probably 100, for polyisobutylene in solution, considerably lower than the 1000–2000 reported for several polymers.
Effect of Pressure on Young's Modulus and the Glass Transition in Rubbers
Authors: Paterson M. S.
Published: April 2005

Measurements of Young's modulus as a function of pressure up to 10 000 bars at 20°C show that the glass-transition temperature in natural and synthetic rubbers is raised by pressure. For natural gum rubber, the transition occurs at 5100 bars at 20°C and Young's modulus increases 1000-fold on passing through the transition.
Ultrasonic Relaxation near the Curie Temperature of Ferroelectric Triglycine Sulfate
Authors: O'Brien E. J. , Litovitz T. A.
Published: April 2005

According to a theory developed by Landau and Khalatnikov, the approach of long-range order to equilibrium can be characterized by a relaxation time with a temperature dependence of the form: τ=β/(Tc − T), where β is constant and Tc is a second-order phase transition temperature. The ferroelectric material, triglycine sulfate, is known to undergo a second-order phase transition at its Curie temperature. The long-range order is identifiable with the spontaneous polarization directed along the crystallographic B axis. Ultrasonic waves traveling perpendicularly to the B axis coupled to the spontaneous polarization and exhibited marked relaxational effects for T<Tc. Detailed absorption measurements verified the Landau-Khalatnikov relation, yielding β=2.25×10−10 sec/deg C and Tc=49.09°C. Using this value of β, the kinetic coefficient γ was calculated to be 8.0×1010 cgs. With the absorption results, a calculation of the accompanying rise in the sound velocity as T increases through Tc was performed. This rise was only 40% of the rise actually measured, indicating that long-range order is not sufficient to explain the temperature dependence of sound velocity for T near Tc. Substantial scattering losses were observed for temperatures above Tc. These are attributed to needle-shaped domains, which retain polarization for T>Tc. These domains, which point along the B axis, have cross sections comparable in size to the sound wavelengths.
Influence of Spin Waves upon Harmonic Generation in Ferrites
Authors: Richards P. M. , Shaw H. J.
Published: April 2005

Near the Suhl second-order threshold for spin-wave excitation in ferrimagnetic resonance the spin waves have, in general, important influences on harmonic generation which can be predicted from the theory of spin waves in ferrites. These influences are potentially useful for experimental studies of the properties of spin waves, and their understanding is important for the operation of practical frequency multipliers that use ferrites. The second-harmonic generation from the uniform mode itself is described by a simplified theory in which the only influence of spin waves is to alter the loss parameter of the resonant component. Two further effects are then predicted which are of interest: (1) A strong enhancement of the third and fourth harmonics of the uniform mode occurs above threshold, resulting from interaction of a nonresonant uniform mode component with the spin-wave dipolar field. (2) Random inhomogeneities or crystalline anisotropy can lead to a direct radiation of spin-wave energy into an external second-harmonic circuit, since the spin waves are elliptically polarized and the proper coupling mechanisms to produce coherence exist. Experimental results in certain instances give quantitative agreement with the theory of direct spin-wave contribution to second-harmonic generation, and a study of harmonic generation can lead to conclusions as to the nature of spin waves excited in resonance experiments.
Optimization of the Magnetic Triode-Type Thermionic Converter
Authors: Zgorzelski Maciej
Published: April 2005

A theoretical analysis of the efficiency of a magnetic triode thermionic converter is presented, and expressions for maximum obtainable efficiencies at given emitter temperature and collector work function are evaluated. A comparison with the thermionic vacuum diode efficiency shows similar efficiencies, but at considerably lower values of optimum current density for the triode.
A Description of Magnetic Switching in Disperse Anisotropy Films
Authors: Coren R. L.
Published: April 2005

The model of a magnetic film as one domain with a single anisotropy axis is often an inadequate approximation to true film behavior. A better approximation considers the film to consist of many uniaxial regions, with a range of anisotropy constants and of axis orientations in the plane of the film. The dispersion of values results in considerable confusion and awkwardness when using the single domain representation to understand magnetic switching. It is shown here that the total anisotropy distribution can be displayed on a polar plot in which each film region is characterized by a single point with the coordinates: Hk, its anisotropy field, and α, its axis orientation. In this (Hk,α) plane the switching effectiveness of the applied field is represented by a curve which is fixed in size and orientation relative to the field vector. Switching occurs in those parts of the film whose anisotropy points fall within the curve. The field figure is extended to include magnetization reversal by domain wall motion. Inverted switching behavior and switching time measurements are briefly discussed. Interactions between regions are not considered.
Magnetoresistance of InSb at a Microwave Frequency
Authors: Sun S. F.
Published: April 2005

The paper describes the measurement of magnetoresistance in the semiconducting compound InSb at a frequency of 10 Gc/sec. The microwave value of |dR/μdH|H0 is the same order of magnitude as the dc value.
Theory of the Thermal Breakaway of a Pinned Dislocation Line with Application to Damping Phenomena
Authors: Teutonico L. J. , Granato A. V. , Lücke K.
Published: April 2005

Although the mechanical theory of Granato and Lücke for the strain-amplitude-dependent internal friction and modulus changes of solids containing dislocations gives a fair account of many of the observed effects, simple theoretical considerations show that the effect of thermal fluctuations should be very important. To extend the theory to finite temperatures, a detailed study of the possible static equilibrium configurations of a pinned dislocation line as a function of external stress is required. This is done here for the two specific examples of a dislocation with a single pinning point and a dislocation with a continuous pinning agent. It is shown in a general way how the damping and modulus change can be computed from these results, and a qualitative discussion of expected behavior is given. For high concentrations of pinning points, the theory is also applicable in the field of yield point phenomena, and the results found here are compared with previous calculations.
An Evaluation of the Archard Electron Diffusion Model
Authors: Brown D. B. , Ogilvie R. E.
Published: April 2005

The predictions of the Archard diffusion model for electron trajectory within a specimen are compared with experimental data. The predicted distributions in depth of primary x-ray emissions are calculated. The applicability of the model to the absorption corrections in electron microanalysis is discussed in light of these calculations.
Electrical Conductivity and Growth of Single-Crystal Indium Sesquioxide
Authors: Remeika J. P. , Spencer E. G.
Published: April 2005

Single crystals of In2O3 were grown by a flux technique that is described in detail. The temperature dependence of the electrical conductivity was measured down to 4.2°K; an anomaly in the conductivity is discussed.
Thermionic Emission from Hot Particles in a Plasma
Authors: Johnson Curtis C. , Bullock C. Kent
Published: April 2005

Some plasmas contain condensed species as liquid or solid particles which may be hot enough to emit electrons thermionically into the plasma. The charge and voltage distributions in the vicinity of the particle are determined from the Poisson—Boltzmann equation and from a plasma sheath theory that is modified to account for thermionic emission. There is a critical temperature T0 at which the sheath changes from an ion sheath to an electron sheath. At temperatures greater than T0 an electron cloud is injected into the plasma, but it is confined near the particle surface within a distance less than the Debye shielding distance. Some typical voltage and charge density profiles are given for a rocket exhaust plasma.
Operation of the Field Ion Microscope with Neon
Authors: Nishikawa Osamu , Müller E. W.
Published: April 2005

The image quality obtained with neon as an imaging gas was studied. Neon can give stable images of non-refractory metals because its ionization field is lower than the evaporation field of these metals. Field evaporation occurs in helium and in neon at slightly lower fields than in vacuum when the incoming nonionized gas molecules which have both thermal and dipole attraction energy collide with the surface atoms. Localized field evaporation leads to dynamic end forms of the emitter tip which are characteristic for vacuum, neon, and helium. The ion image obtained with helium containing a few percent neon has a reduced contrast and shows more details around the low-field areas. Field evaporation in this mixed gas produces surface vacancies inside closed net planes. These effects indicate that adsorbed neon atoms act as intermediate collision partners to allow a more efficient transfer of the energy from the light helium atom to the heavy metal atom.
Coupled-Mode Laser Oscillation
Authors: Fleck J. A. , Kidder R. E.
Published: April 2005

A set of general equations describing the time-dependent behavior of a laser is derived. The derivation is based on a density matrix description of a homogeneously broadened laser transition and a semiclassical treatment of the radiation field, consisting of an arbitrary number of modes. The resulting equations resemble ordinary rate and energy conservation equations but contain additional interference terms which, in general, couple all of the modes. The strength of this coupling depends on the inhomogeneity of the population inversion as well as on the mode frequencies. The energy conservation equations imply that absorption and stimulated emission in such a system can apply, strictly speaking, only collectively to the system of modes as a whole. In addition, the threshold inversion is affected by mode coupling. Stability analysis carried out in the two-mode case indicates that a spatial inhomogeneity in the pumping rate is required for undamped oscillations. It is also concluded that coupling of off-axis modes is more likely to lead to undamped spiking than coupling of axial modes.
Thermionic Work Function of Thin-Oxide-Coated Aluminum Electrodes in Vacuum and in Cesium Vapor
Authors: Chapman Richard A.
Published: April 2005

The thermionic work function of aluminum electrodes with anodized aluminum oxide coatings of 30 and 2000 Å was determined for each electrode from the contact potential difference of each electrode with a well outgassed and aged polycrystalline tungsten wire emitter. In vacuum, the thermionic work function of the 30-Å coated electrode was 4.3±0.1 eV, relative to a polycrystalline tungsten work function of 4.6 eV and a tungsten emission constant of 120 A/cm2· °K2. In cesium vapor at reservoir temperatures up to 70°C, the 30-Å coated electrode had a minimum observed work function of 1.4±0.1 eV relative to the same tungsten reference electrode. The resistance of the 2000-Å film coated collector electrode was too large to allow accurate determination of the work function of this electrode. In both vacuum and cesium vapor, however, there was an indication that the work function of the 2000-Å coated electrode was a few tenths of an electron volt lower than that for the 30-Å coated electrode. It is suggested that similar but perhaps less stable oxide-coated electrodes could occur naturally on the collector of a thermionic energy converter due to oxidation of the collector surface or buildup on the collector surface of outgassing products from the emitter.

Auxillary experiments were performed on the secondary-electron emission from these oxide-coated electrodes. No lingering secondary electron emission current corresponding to a Malter effect was observed. The true secondary yield from the 2000-Å coated electrode was 3 in vacuum and 7 in cesium vapor for a primary electron energy of 150 eV. The secondary yield for the 30-Å coated electrode was similar in cesium vapor but was not studied in vacuum.
Dissociation of Vibrationally Excited H2+ Ions by an Electric Field
Authors: Ehler A. W.
Published: April 2005

H2+ ions produced by a Philips ionization gauge (PIG) type discharge in hydrogen were injected into a cylindrical electric field which was varied in magnitude to give a maximum strength ranging from 65 to 450 kV/cm. H2+ ions in highly excited vibrational states were dissociated by the electric field. The measured fraction of the H2+ ions produced by the PIG discharge which were dissociated was 0.0011±0.0002 at a field of 200 kV/cm. Numerous experimental checks demonstrated that the dissociation fraction could not be attributed to effects other than electric field dissociation. The fraction is several times greater than that measured by other investigators. This difference is attributed to the method used in the preparation of the H2+ ions.
Cesium-Impregnated Inert Gas Diodes
Authors: Forman R. , Reiss J. R.
Published: April 2005

Thermionic diodes containing xenon at 1 Torr and cesium at 10−6−10−3 Torr can lead to higher diode currents than those with either gas alone at the given pressures. Negative resistance thermionic diodes result from xenon in the appropriate pressure range plus cesium.
Temperature Dependence of 180° Domain Wall Velocity in BaTiO3
Authors: Stadler H. L. , Zachmanidis P. J.
Published: April 2005

The velocity of 180° domain walls in BaTiO3 crystals with saltwater electrodes has been measured from 3° to 75°C at applied fields from 0.2 to 200 kV/cm by repeated partial switching and etching. Heating the crystal was found to lower the field required to attain any given wall velocity by a factor which is substantially the same for all wall velocities (or for all applied fields) and is determined wholly by the temperature change. The factor is about 3 for a change from 3° to 75°C. This behavior agrees with the Miller-Weinreich mechanism of thermally activated wall movement. Domain shapes are found to be very rounded at 50° and 75°C, becoming almost exactly circular at 75°C around 1 kV/cm, in marked contrast to the well-known square domains at 25°C and below.
Representation of a Function by Its Line Integrals, with Some Radiological Applications. II
Authors: Cormack A. M.
Published: April 2005

A method is described for determining a variable gamma-ray absorption coefficient in a sample from measurements made outside the sample, and the applicability of the method to other radiological problems (e.g., positron scanning) is pointed out. An experimental test of the method is described, and it is concluded that the method works well.
Anelastic Effects of Alkali Ions in Crystalline Quartz
Authors: Fraser David B.
Published: April 2005

The anelastic behavior of natural quartz has been modified by doping with alkali ions. Three blocks cut from a single crystal of Brazilian quartz were doped with lithium, sodium, and potassium, respectively, by electrodiffusion at 500°C using a vapor-plated anode of a halide containing the desired alkali ion. Fundamental 1-Mc/sec, AT-cut shear mode resonators were fabricated from each block. Measurements of Q−1 and resonant frequency were made from liquid-helium temperatures to near the quartz inversion point at 573°C. For a resonant frequency of 5 Mc/sec internal friction peaks were evident in the low-temperature region with sodium doping giving peaks at 50° and 140°K, lithium a peak near 100°K, and potassium a peak near 200°K. The activation energies ranged from 0.06 to 0.2 eV and the relaxation times were all the order of 10−12 sec. The low-temperature behavior induced by sodium doping is of special interest since it is identical to that of certain synthetic quartz crystals. All three doped samples exhibited an exponentially increasing loss above room temperature. In each case the activation energy of the exponential loss corresponded to the activation energy for diffusion of the particular alkali ion, approximately 1 eV. Although one would infer that the loss is due only to alkali ion diffusion, it is felt that a more complex mechanism is involved at high temperatures.
Square-Loop Polycrystalline Garnets with and without Magnetic Field Heat Treatment
Authors: Nesbitt E. A. , Geller S. , Espinosa G. P. , Williams A. J.
Published: April 2005

This is a preliminary report on square hysteresis loops obtained on polycrystalline yttrium iron garnets that have Tb3+ or Eu3+ partially replacing the Y3+ ions. Reasonably square hysteresis loops that are technologically important can be produced. In addition, a cobalt-containing-garnet responds to a magnetic annealing treatment and has substantial values of uniaxial anisotropy at room temperature. This work indicates a method of producing other garnets which will respond to the magnetic annealing process.
A Relationship Between the Schottky Emission Mechanism and the Energy Bands of Thin-Film Metal-Insulator-Metal Configuration
Authors: Hacskaylo Michael
Published: April 2005

The room-temperature current-voltage characteristics for thermally grown Al2O3 dielectric films sandwiched between the Al base electrode and the Zn, Sn, or Au counter electrode were studied. Experimentally, voltage ranges were found for which the Schottky mechanism appears to be dependent on the energy band structure of the metal-insulator-metal configuration. The film current changes from a log J-V½ (the Schottky mechanism) dependency to a log J-V dependency at a particular applied voltage VB. Empirically, the sum φ+eVB=ψ, where φ is the metal-insulator barrier, is in agreement with the work function ψ of the positive metal electrode. In consideration of this ad hoc agreement, the contact potential ψCP=Δφ+ΔeVB, where Δφ is the difference (asymmetry) in the calculated metal-insulator barrier for positive and negative polarities, and ΔeVB is the difference in the voltages for the change of mechanisms for the corresponding polarities. The mechanism by which the film currents change from the Schottky emission to the log J-V dependency at a particular voltage VB is not understood.
Infrared Photoconductive Characteristics of Boron-Doped Germanium
Authors: Shenker H. , Moore W. J. , Swiggard E. M.
Published: April 2005

Boron-doped germanium having very low levels of compensation is shown to be useful for detection of far infrared radiation in the 50–125 μ range. The results of measurements of time constant, dc characteristics, photoresponse, and spectral response of several Ge(B) detectors is presented and discussed. Detectors having a peak detectivity of 2.1×1011 cm-cps½-W−1 at 108 μ and a low dynamic impedance have been achieved. The cross section for the capture of a free hole by an ionized boron impurity in germanium at 4.2°K, about 4×10–11cm2, is significantly larger than the corresponding cross section for n-type impurities in germanium.
Study of the Filamentary Growth of Silicon Crystals from the Vapor
Authors: Wagner R. S. , Ellis W. C. , Jackson K. A. , Arnold S. M.
Published: April 2005

The preparation from vapor and the structure of filamentary crystals of silicon have been studied in detail. It was found by chemical etching, by examination for a twist associated with a screw dislocation, and by observations in the electron microscope, that both ribbons and needles of small dimensions are free of dislocations and imperfections. Certain impurities such as gold, nickel, or platinum, however, are essential for the growth of filamentary crystals.

The growth of micron size and larger whisker crystals from the vapor takes place in two stages. The first is a rapid extension in length of a leader-like crystal of small cross section; the second, a slow thickening of the leader through deposition on lateral faces. Initial growth is associated with impurities and does not require an axial screw dislocation. Subsequent growth may be explained by classical nucleation at a step and lateral translation of the step.
Unsupported Single-Crystal Films of Germanium
Authors: Skaggs C. W. , Jones J. R.
Published: April 2005

A method of obtaining unsupported single-crystal films of Ge has been developed which involves the sublimation of a NaCl substrate from the Ge film after vacuum deposition. This allows compatibility of the film and NaCl substrate previously thought impossible due to the large difference in their coefficients of thermal expansion.

Evaluation of this endeavor has been made with microscopic, x-ray diffraction, and electron diffraction examinations. This evaluation has disclosed that the films are (100) oriented monocrystalline films which contain, in varying amounts, surface contaminants consisting of NaCl, BaCl2, Ta2O5, TaC, and amorphous C.
Si Epitaxial Growth by Iodide Transport Between Piled-Up Pellets
Authors: Lieth R. M. A. , Eggels A. G. M.
Published: April 2005

Vapor phase transport of Si over a short distance by means of the iodide and subsequent epitaxial growth on a Si substrate is described. Both the source and the substrate were biconcavely shaped pellets piled up in a silica ampule. The furnace was vertical and the temperature ranged from 1100°–1200°C, giving rise to a slight temperature gradient over the length of the ampule.
Self-Diffusion in Pb at 300°C by an Absorption Tracer Technique
Authors: Lawless W. N.
Published: April 2005

A diffusion technique is described that does not involve the usual long-time approximation for a thin specimen. Activity measurements are programmed on a digital computer to generate a large number of D values from the analysis of the data. These D values are then treated statistically, the resulting distribution curve yielding information on the experiment itself. The technique is fairly insensitive to errors in determining the scattering characteristics of the system or in taking the data.

A version of this technique applied to Pb at 300°±2°C yielded average and rms values for D, the self-diffusion coefficient, of 1.78±0.15×10−10 cm2/sec and 1.92±0.16×10−10 cm2/sec, respectively, from 40 D values generated from the data. The results compare favorably with the published data on self-diffusion in Pb at 301°C by von Hevesy, 1.85×10−10 cm2/sec. The data were also treated according to a long-time approximation method of Zhukhovitsky, yielding 1.70×10−10 cm2/sec.
Accelerated Convergence, Divergence, Iteration, Extrapolation, and Curve Fitting
Authors: Macdonald J. Ross
Published: April 2005

This paper discusses some applications of the epsilon algorithm (EA), a sequential procedure for calculating Padé approximants. The EA may be used to: (1) accelerate the convergence of slowly converging series and iterations; (2) obtain useful results from divergent series and iterations; (3) obtain the limits of iterated vector and matrix sequences; (4) aid in the solution of differential and integral equations; (5) carry out numerical integration in a new way; (6) extrapolate; (7) fit a curve to a polynomial or to a constant plus sum of exponentials.

As an illustration of curve fitting and extrapolation, we present results obtained with exact polynomial data plus random noise combined additively or proportionately. For such nonstationary data, the results are comparable, and in some cases superior, to least squares in yielding good estimates of the exact polynomial coefficients. One important advantage of the EA is that it builds up polynomials whose lower-order coefficients are independent of higher-order ones. This property is valuable when the degree of the polynomial is unknown. Finally, a simple empirical equation is given relating the precision of least-squares-calculated polynomial coefficients to the degree of the fitted polynomial and the number of effective decimal digits carried in the calculation.
Fracture of Amorphous Polymeric Solids: Reinforcement
Authors: Halpin J. C. , Bueche F.
Published: April 2005

Although the tensile stress at break (σb) and the corresponding ultimate elongation (αb) of a reinforced elastomer depend markedly on temperature and test rate, characteristic time-to-break curves can be obtained utilizing the Leaderman-Tobolsky-Ferry superpositioning techniques. In addition, a distinctive failure envelope is obtained when the log σbT0/T is plotted against log (αb−1), where T and T0 (°K) are the test temperature and an arbitrary reference temperature. Delayed ruptured experiments yield qualitatively the same results as forced rupture experiments. Furthermore, an analysis of the Leaderman-Tobolsky-Ferry shift parameters indicate a concomitant increase in the apparent activation energy with increasing reinforcement concentration. These observations are explicable by utilizing the failure theory of Bueche and Halpin. It is now apparent that failure is the result of the propagation of tears or cracks within the viscoelastic body and that the time and temperature dependence of the ultimate properties is directly related to the time-temperature dependence of the modulus. A preliminary picture of the salient molecular events is presented.
Polyisobutene Degradation in Laminar Flow: Composition and Shear Variables
Authors: Porter Roger S. , Johnson Julian F.
Published: April 2005

The mechanical degradation of polyisobutene solutions in laminar flow under known shear stress conditions was performed in a thin-film, high-shear, rotational viscometer. Polyisobutenes with molecular weights from 40 000 to over a million were studied at several temperatures and concentrations in n-hexadecane. Reduced variables, of a form similar to those used in studies of temporary viscosity losses, can be used successfully to correlate permanent viscosity losses due to degradation. The variables which superimpose are stress, temperature, and concentration. The range in which reduced variables apply corresponds to concentrations and molecular weights where degradation is commonly observed and to the region where polymer entanglements exist.
Fatigue Fracture in Fibrous Polymers as a Brittle, Crack-Nucleation Process
Authors: Prevorsek Dusan , Lyons W. James
Published: April 2005

An interpretative analysis of experimental results on the failure of synthetic fibers in fatigue under cyclic tension leads to the conclusion that rupture is of a brittle nature. Although molecular rearrangement undoubtedly takes place during longitudinal fatiguing, rupture itself cannot be regarded as a viscous, laminar flow process. Failure is localized; the undamaged sections of the specimens show relatively less important effects of the fatiguing. Lifetime cannot be predicted from measurements of fiber tenacity, modulus, or rate of creep. A theory of fatigue rupture, based on the observation of cracks in specimens strained to breakage, is formulated. The treatment involves the assumption that a crack grows by a nucleation process up to a critical size. At a slightly larger size, the crack becomes unstable and propagates spontaneously, to produce rupture. The theory leads to an expression for lifetime as a function of stress that is in substantial agreement with experimental data on a number of samples.
Gas Transport in Thermally Conditioned Linear Polyethylene
Authors: Michaels Alan S. , Bixler Harris J. , Fein Harvey L.
Published: April 2005

The permeability, diffusion, and solubility constants of helium, argon, and ethane have been measured in a series of linear polyethylene films which had been subjected (1) to various rates of cooling from the melt and (2) to subsequent annealing near the melting temperature. While solubility constants of argon and ethane showed the normal variation with amorphous content of the polymer irrespective of thermal history, helium solubility constants appeared to be anomalously high and strongly dependent on thermal history. The presence of voids or defects within the crystalline phase has been postulated to account for this behavior of helium; argon and ethane being excluded from these defects because of their size. Anomalous helium diffusion constants were also observed which have been satisfactorily explained in terms of a slow diffusion of gas into these intracrystalline defects.

The monatonic decrease in geometric diffusional impedance with increasing amorphous content of the polymer has again been observed in films directly cooled from the melt. However, upon annealing these films at elevated temperatures the impedance decreases markedly, despite an increase in crystallinity, suggesting the formation of thicker lamellas with a high concentration of intralamellar imperfections in a manner similar to that observed in annealing polyethylene single crystals.

The apparent activation energies for diffusion of argon and ethane were essentially constant irrespective of thermal history. This suggests the so called ``chain immobilization factor'' is not due to restricted chain mobility in the amorphous polymer, but is rather a penetrant-size-dependent impedance to diffusion arising from the near-molecular dimensions of the amorphous channels in the polymer.
Scattering of Light by Gel Structures. I. Regenerated Cellulose
Authors: Beebe E. , Marchessault R. H.
Published: April 2005

A low-angle light scattering photometer is described and applied to the study of regenerated cellulose gels. The scattering patterns were recorded over the angular range from 0.5° to ∼75° for the fresh gel in water (F film) and for the same sample solvent exchanged to an imbibition liquid of refractive index 1.543 (PF film). A sharp drop in over-all turbidity was recorded in going from F to PF film, indicating an intimate penetration of the nonswelling imbibition liquid into the cellulose fine structure. The observed scattering for these samples was interpreted in terms of the Debye-Bueche correlation function using the graphical integration method. The size of the regions of correlated variation in refractive index was 5800 Å for the F film and 6500 Å for the PF film; this corresponded to refractive index fluctuations of 0.005 and 0.001, respectively. It is calculated that these fluctuations in refractive index could arise from about a 3% variation in the degree of order over the respective correlation distances. Such structure heterogeneities would then be associated with a second level organization of the fine structure.
Birefringence Effects in Acrylonitrile Polymers. I. Effects at Different Temperatures
Authors: Andrews R. D. , Kimmel R. M.
Published: April 2005

Birefringence measurements of several types have been carried out as a function of temperature on films of polyacrylonitrile and a 7% vinyl acetate copolymer. The stress-optical coefficient is positive with a value of about +2 Br in the glassy state, and becomes negative as temperature increases through the glass transition region. Pronounced nonlinear stress-optical behavior is seen in the homopolymer at room temperature and above. Drawn samples have a negative orientation birefringence which changes gradually toward positive values as temperature is increased; the homopolymer and copolymer curves cross zero at about +150° and +195°C, respectively. Temperature cycling of undrawn homopolymer samples under moderate dead load gives birefringence curves which show striking hysteresis effects. The heating curve goes through a minimum at about +158°C, then rises and crosses to positive values at higher temperatures. The cooling curve of samples heated to temperatures above 158°C shows a steep drop to birefringence values of about −0.006 at room temperature (comparable to values obtained by cold-drawing, even though the sample extension in these experiments is only 10%–15%). Birefringence changes with time in samples under dead load at temperatures below 158°C are in a negative direction, and appear to be associated with mechanical creep. Above 158°C, the time effects are in a positive direction and are associated with the chemical reaction causing discoloration of the polymer. In this region, birefringence changes linearly with time (zero-order kinetics). Rate of birefringence change is not proportional to applied stress, but its temperature dependence gives an activation energy of 29 kcal/mole which is independent of stress level. At 250°C and above, in samples under dead load, the effects of a second chemical reaction are observed as a rapid increase in birefringence to very high positive values.