bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric Properties of Poly(2, 6-Dimethyl Phenylene Oxide)
Authors: Karasz F. E. , MacKnight W. J. , Stoelting J.
Published: February 2004

The dielectric relaxation of amorphous bulk poly(2, 6-dimethyl phenylene oxide) (PPO) has been measured between −160° and 260°C over a frequency range extending from 0.1 to 10 kHz. Two broad loss peaks of comparable intensities are observed-a high-temperature α relaxation identified with the glass transition and a low-temperature γ relaxation of uncertain origin. The intensity of the α relaxation can be well accounted for in terms of a loss of orientational dipolar correlation in a calculation based on a consideration of a rotational isomeric model and of the geometry of the PPO chain.
Birefringence Studies of the Relaxation of Ethylene-Methacrylic Acid Copolymers and Their Salts
Authors: Kajiyama T. , Stein R. S. , MacKnight W. J.
Published: February 2004

The strain-optical coefficient of ethylene-methacrylic acid copolymers is measured in static and dynamic experiments as a function of temperature and degree of ionization. An alpha optical loss peak which corresponds to a mechanical loss peak is found for the ionized salt but not for the un-ionized acid. This peak is associated with mobility of an ionized domain. Birefringence relaxation curves are found to be superposable using horizontal and vertical shift factors. A plot of the logarithmic shift factor against (1/T) gives a broken curve exhibiting two activation energies for the salt indicating contributions by two processes.
Microwave Frequency Acoustic Surface-Wave Loss Mechanisms on LiNbO3
Authors: Slobodnik A. J. , Carr P. H. , Budreau A. J.
Published: February 2004

The temperature and frequency dependence of the attenuation of 0.5–5-GHz acoustic surface waves on LiNbO3 has been measured. For propagation in vacuum a frequency-squared dependence of the total attenuation is obtained with a value at 1 GHz of 0.9 dB/μsec. Temperature-dependence measurements using a novel three-transducer technique show the dominant loss (0.7 dB/μsec at 1 GHz) mechanism to be the interaction with thermally excited elastic waves. Propagation in air results in an additional loss linearly proportional to frequency with a value of 0.2 dB/μsec at 1 GHz. The effects of beam steering and diffraction losses are also investigated both theoretically and experimentally. Both misalignment of transducers with respect to pure mode propagation axes and misalignment of the propagation-plane perpendicular can add significantly to delay line insertion loss. This beam steering loss on Y-cut Z-propagating LiNbO3 is considerably higher than on the 41.5° rotated-cut X-propagating orientation. The loss mechanisms measured in this paper are sufficient to completely account for the insertion loss of surface-wave delay lines.
Phonon Component of Dislocation Drag
Authors: Brailsford A. D.
Published: February 2004

A model due to Eshelby for computing the drag force upon an abrupt kink moving at low velocity through a phonon gas and its extension to include the effect of the finite width of the kink are analyzed in detail. The temperature dependence of the drag force is derived and is shown to be in reasonable agreement with earlier qualitative predictions made by Lothe. This force is related to the dislocation viscous damping coefficient appearing in the string model of dislocation motion. In this form the theory is compared with recent experimental results. Good agreement is found if the kink width (in units of the appropriate Burgers vector) is ∼4 or 1 in Cu and Al, respectively. An incidental problem in attempting to unify the theory with that of the Bordoni peak is also discussed briefly.
Theory of Thermal Breakdown in Sodium Chloride
Authors: Chou S. , Brooks H.
Published: February 2004

Using the experimentally determined temperature and field-dependent electrical conductivity for NaCl, we have solved numerically the one-dimensional heat-transport equation, with and without current continuity requirements, to obtain the breakdown fields. The results are consistent with experiment.
Study of Piezoelectric Oscillations in Wideband Pyroelectric LiTaO3 Detectors
Authors: Glass A. M. , Abrams R. L.
Published: February 2004

The use of freely suspended LiTaO3 pyroelectric detectors for wide-band application is severely hampered by mechanical resonances of the detector (typically several hundred kHz) which are excited by the incident radiation. These resonances give rise to slowly decaying oscillatory piezoelectric signals superimposed on the pyroelectric signal. It is shown that by suitable mounting of the detector films these resonances can be entirely eliminated, thereby increasing the useful bandwidth of the devices to at least several hundred MHz. In addition, it is shown that when the incident radiation is modulated at a mechanical resonance frequency of a freely suspended LiTaO3 detector, considerable enhancement of the current and voltage response is obtained. The experimental results are found to be in reasonable agreement with results calculated for LiTaO3 by including thermal expansion in the equations of motion of a pyroelectric crystal.
X-Ray Diffraction Technique for the Investigation of Small Diffusion Zones
Authors: Tenney D. R. , Carpenter J. A. , Houska C. R.
Published: February 2004

A nondestructive x-ray diffraction technique has been developed to analyze small diffusion zones. This technique constitutes a new approach to the study of one-dimensional volume diffusion and only requires intensity measurements from two different reflections. It has the distinct advantages of permitting a determination of both the concentration profile and the atomic arrangements associated with the diffusion zone. Data for the Cu–Ni system is presented to provide an example of the technique. The composition profile resulting from the interdiffusion of an 8-μ Ni deposit and a Cu single crystal at 900°C for 45 min was determined. Good agreement was found between these experimental results and a composition curve calculated from DaSilva-Mehl diffusion coefficients. It has also been found that volume diffusion significantly increases the degree of crystalline misorientation along the diffusion zone. The structure of the as-plated Ni deposits on a Cu single crystal was found to undergo a large misorientation change, initially, but maintained a relatively constant degree of epitaxy for longer diffusion times.
Propagation of Electromagnetic Waves in a Partially Dielectric Filled Circular Waveguide
Authors: Chang Christopher T. M. , Dawson John W.
Published: February 2004

A circular waveguide partially filled with dielectric is considered as the deflecting structure for a relativistic particle separator. The cutoff wavelengths and normalized phase velocity versus frequency characteristics are presented. The expressions for power, losses, deflecting fields, and transverse deflecting forces are also given. Our investigation shows that the dominant (HEM11) mode of the structure can be used as a transverse deflecting mode for synchronous particles in a relativistic particle separator.
Gas Breakdown with 10.6-μ-Wavelength CO2 Laser Radiation
Authors: Smith David C.
Published: February 2004

The gas-breakdown threshold or the minimum power density required to ionize a gas with 10.6-μ-wave-length radiation has been examined using the radiation of a Q-switched CO2 laser. The studies show that the focused beam cannot initiate the breakdown process for intensities as high as 109 W/cm2. If an initial low degree of ionization is provided by an external source, the subsequent growth of the breakdown is in agreement with a cascade model as evidenced by the experimentally determined gas pressure and laser-radiation frequency dependence.
Behavior of Cesium Oxide as a Low Work-Function Coating
Authors: Uebbing J. J. , James L. W.
Published: February 2004

A detailed picture of the behavior of cesium oxide as a low work-function coating on III-V semiconductors and on silver has been obtained. Measurement of required cesium and oxygen exposure for optimum photoyield shows that the compound normally formed is close to CS2O, with variations in required exposure for very thin and very thick layers. By making simultaneous Kelvin work-function, photoyield-threshold, and thickness measurements, it was possible to establish that the CS2O, an n-type semiconductor, forms a heterojunction or Schottky barrier with its substrate. This provides a band bending which produces a gradual lowering of the vacuum level with increasing thickness to an ultimate work function of 0.6 eV. The photoyield and dark current from the substrate are limited by the interfacial barrier at the heterojunction. This barrier is 1.00±0.05 eV for a silver substrate and 1.23±0.03 eV for GaSb. The band-bending distance in the CS2O is about 50 Å and the hot electron scattering distance is 9 Å. These data have been used in an improved calculation of the maximum Γ escape probability and requisite CS2O thickness for electron emission from III-V semiconductors of different bandgaps. Electron emission from CS2O induced by an oxygen overpressure was also measured. CSOH is compared with CS2O as a work-function lowering coating.
Effect of Pressure on the emf of Chromel-Alumel and Platinum-Platinum 10% Rhodium Thermocouples
Authors: Getting I. C. , Kennedy G. C.
Published: February 2004

A differential technique has been used to measure the absolute effect of pressure on the emf of Chromel-Alumel and Pt-Pt10Rh thermocouples. The experiments were conducted in a solid pressure medium piston-cylinder apparatus to 35 kbar and 1000°C. Extrapolation of these data shows Chromel-Alumel to read as much as 28°C high at 50 kbar and 1200°C and Pt-Pt10Rh as much as 28°C low at 50 kbar and 2000°C. Graphs are presented which show correction voltage versus temperature for various pressures.
Basis for Picosecond Structure in Mode-Locked Laser Pulses
Authors: Pike H. A. , Hercher M.
Published: February 2004

In careful experimental measurements of mode-locked laser pulses, using nonlinear media, a number of workers have found evidence for what appear to be coexisting subpicosecond pulses, and pulses in the range of 10–40 psec. In this letter we describe a model of a light pulse in which a Gaussian random-radiation field (i.e., a thermal field) of limited spectral extent is modulated by a temporal pulse envelope, and we show that predictions based on such a model are in excellent detailed agreement with published experimental results. We also consider the case of a pulse of linearly chirped radiation, and show that a clear distinction can be made between this case and a pulse of Gaussian random radiation.
Gain Measurements in CO2 Isotope Lasers
Authors: Silver M. , Hartwick T. S. , Posakony M. J.
Published: February 2004

Measurements of the relative gain and loss were made in CO2 isotope lasers on the transitions terminating in the mixed states [10°0, 02°0]I and [10°0, 02°0]II. Agreement was found with the calculated relative gain for 12C16O2, 13C16O2, and 12C18O2.
Transmission Losses in Aprotic Liquid Lasers
Authors: Brecher C. , French K. , Watson W. , Miller D.
Published: February 2004

Transmission losses in phosphorus oxychloride- and selenium oxychloride-based laser solutions have been measured by four different methods. These solutions have intrinsic scattering losses in the range of 0.1%–0.3% per cm. A possible explanation of this scattering loss is presented.
Simplified Theory for Stable Gunn Domains Including Diffusion
Authors: Lampert Murray A. , Sunshine Richard A.
Published: February 2004

A simplified theory of Gunn domains is presented assuming a nonzero, but field-independent diffusion coefficient. The simplification is achieved by using an approximate, analytic solution to a well-known transcendental equation (the so-called ``area integral'') of Butcher and Fawcett. Any further computer calculations, where necessary, then involve only simple quadratures of known analytic functions. For the piece-wise linear approximation to the V(F) vs F curve the entire program can be carried out analytically with the aid of the Regional Approximation method. The use of dimensionless variables brings out the central role, in Gunn domain theory, of the critical dimensionless parameter α=εFTVT/eN0D, with FT the threshold field, VT the threshold drift velocity, N0 the thermal-equilibrium free-carrier density and D the diffusion coefficient (mks formula). For GaAs, α≈4×1015/N0 with N0 in cm−3. A particularly simple V(F) vs F curve is proposed for the study of phenomena (e.g., impact ionization across the gap) deriving from very high-field Gunn domains. A complete analytic study is made of the Gunn domains associated with this special characteristic.
Luminescence in Indirect Bandgap AlxGa1−xAs
Authors: Kressel H. , Nicoll F. H. , Hawrylo F. Z. , Lockwood H. F.
Published: February 2004

A luminescence study was made of Sn-, Te-, or Zn-doped AlxGa1−xAs in the Al-rich composition range where the bandgap transition is indirect. Several luminescent bands attributed to excitons, free-bound carrier and donor-acceptor recombinations have been observed. The experimental data are consistent with an ``optical'' ionization energy of Sn and Te donors in the range of 59±7 meV, and 56±5 meV for Zn acceptors. These values are expected to be similar in AlAs.
Double Injection in Semiconductors with Multivalent Trapping Centers
Authors: Zwicker H. R. , Streetman B. G. , Holonyak N. , Andrews A. M.
Published: February 2004

A numerical solution based on the general equations of conduction and recombination is used to calculate the current-voltage characteristics of p-``i''-n diodes in which the ``i'' region contains multivalent trapping centers. The theory is based on the assumption that the current is entirely field driven in the ``i'' region. The solution includes calculation of important properties of the conduction mechanism, such as the space charge and carrier density distributions. The results are a considerable improvement over more approximate analytical solutions, since the present method allows for important effects such as variations in the carrier lifetimes with injection level. Calculations are presented for the case of the Zn impurity in Si and are compared with previously published results for Au in Si. We discuss a space-charge neutrality approximation which gives reasonable agreement with the exact calculations for certain ranges of the J-V characteristic and which provides useful insight into the conduction process in the p-``i''-n. Comparisons are made between the present work and previous theoretical models based on more restrictive approximations.
Effects of a Magnetic Field on Double-Injection Negative Resistance in Long p+-π-n+ Structures
Authors: Otani Y. , Matsubara K. , Nishida Y.
Published: February 2004

Highly sensitive magnetic effects on the voltage-current characteristics in a long p+-π-n+ diode biased forwardly are analyzed. The solutions of this analysis are consistent with the following assumptions: The similar treatment for the Righi-Leduc effect, which is produced by the varied effect of the magnetic field on hot and cold carriers, is also applicable for the case where there is a density gradient in the sample. In addition, since the diffusion current and the effect of surface recombination are small enough to be neglected, the current flows through only the volume-recombination process. The major results are as follows: (1) Highly sensitive magnetic effect in ``double-injection regime'' mainly depends on the recombination process for the injected electrons which are minority carrier. This differs from the effect in ``Ohmic regime'' where majority carrier contributes to the current. (2) The threshold voltage, at which the negative resistance occurs, is strongly dependent on a ratio of the recombination density NR to the majority carrier density p0. (3) The current J in the double-injection regime beyond a minimum voltage follows a J ∝ V2+s law, where s is a function of the mobility ratio b, and it is given as s≈2(blog2−1).
Strain Dependence of the Resistivity of Silver Films
Authors: Verma B. S. , Juretschke H. J.
Published: February 2004

The strain dependence of the resistance of thin epitaxial films of silver on mica has been measured for film thicknesses between 300 and 1400 Å, in the temperature range 200°–350°K. Analysis of the data includes geometrical and strain corrections, and considers the possible deviation from bulk of both the mechanical and electrical properties of the films. The films show a strain dependence of the resistivity equal to that of bulk silver, with deviations in individual films up to 30%. The effect is temperature independent, and shows no size effect. It is too small to account for the observed temperature variation of the film resistivities, which is smaller than in bulk, on the basis of strain due to the mismatch of thermal expansion of films and substrate.
Excess Velocity Potential of the Platelet Crystal in a Supercooled Melt
Authors: Holzmann Ernest G.
Published: February 2004

Starting with the traditional time-invariant parabolic-cylinder model to represent the growth of a platelet-shaped crystal in a pure supercooled melt, the effect of thermal imbalance at the solid-liquid interface is calculated. In particular, the heat field equations are solved for the initial excess velocity distribution at the interface, on the assumption that the shape constraint is suddenly replaced by the constraint of heat conservation. The computed results resemble those found for the paraboloidal needle crystal. In particular, the hypothetical parabolic interface tends to bulge with a pronounced peak at one radius behind the leading edge of the platelet crystal. The maximum velocity principle, when applied to this model, yields a square-law relation between the dimensionless tip velocity and the supercooling for pure ice and water. The results parallel those calculated by Horvay and Cahn for the isothermal model. The curvature effect of surface tension and the effect of the molecular attachment kinetics are found to lower the dimensionless tip velocity by about 20% below that in the isothermal model, uniformly over the range of supercooling considered.
Color Centers and the Thermoluminescence Mechanism in LiF
Authors: Mayhugh M. R.
Published: February 2004

Color-center transformations in pure and dosimetry grade LiF have been studied over the wavelength range 105–750 nm. Both impurity- and irradiation-induced optical absorption were measured, the latter by a relatively high x-ray exposure at room temperature. Thermal and optical treatments were chosen to induce transformations which are easily related to the thermoluminescence (TL) process. The results demonstrate the following for dosimetry LiF: The TL-associated 310-nm absorption results from trapped electrons. The TL process intimately involves V3 centers (113-nm absorption band). The centers causing the 225-nm band (Z3 band) and those causing the 137-nm impurity band may both be involved in the sensitization (and supralinearity) phenomenon. A complete model for the TL is presented and discussed. Not so important to the TL is a broad apparently intrinsic absorption from 120–200 nm which is induced by irradiation in both pure and dosimetry LiF.
Electro-Optic g Coefficients of Pb-Containing Oxygen-Octahedra Ferroelectrics: Ceramic (Pb, Ba)(Zr, Ti)O3
Authors: Thacher Philip D.
Published: February 2004

Electro-optic g coefficients, which relate induced birefringence and polarization via the quadratic electro-optic effect, are found to have the value g11−g12 ≈ g44 ≈ 0.011±0.008 m4/C2 for 12 oxygen-octahedra ferroelectrics having Pb in the perovskite A site. Measurements on four such ferroelectrics were made using ceramic compositions of the series (Pb, Ba) (Zr, Ti)O3 which has a diffuse phase transition and a maximum quadratic electro-optic coefficient of R33−R13=3.4×10−16 m2/V2 at room temperature. To explain why the measured g is 10 times smaller than those compiled and computed by DiDomenico and Wemple for other oxygen-octahedra ferroelectrics, the optical-index dispersion energy εd and the birefringence dispersion constant K were determined. Values of εd=25 eV and K=−0.5 are as found for the other ferroelectrics. A smaller g must therefore arise from a smaller polarization potential coefficient β, which is defined with respect to energy shifts and polarization of the BO6 perovskite octahedra. Such a definition can lead to a smaller β in Pb-containing ferroelectrics because the higher polarizability and displacement of the Pb ion contribute polarization which is not properly accounted for. In any case, the g coefficient can no longer be considered as a universal constant for oxygen-octahedra materials.
Diffusion of Actinides in Pyrocarbons: Thin-Layer Experiments
Authors: Evans R. B. , Stiegler J. O. , Watson G. M.
Published: February 2004

The diffusion of uranium and thorium in columnar pyrocarbons between temperatures of 1450° and 2065°C has been studied using the usual thin-layer depletion technique. Constant coefficients are obtained that are in good agreement with results reported elsewhere for uranium diffusion in the 〈a〉 direction at actinide concentrations less than about 0.15-mg actinide/cm3. Diffusion coefficients increase at higher actinide concentration. The anisotropic pyrocarbons are well oriented with the basal planes aligned parallel to the deposition surface. Diffusion coefficients parallel to the deposition surfaces are about three orders-of-magnitude greater than those in the perpendicular direction. The difference is due primarily to the pre-exponential term in the usual Arrhenius equation. Autoradiographic studies of specimens in which diffusion occurred perpendicular to the deposition surface show segregation of actinides to growth cone centers or boundaries depending on whether diffusion occurs along or opposite the direction of growth of the pyrocarbon. The segregation is attributed to rapid diffusion parallel to the deposition surface coupled with the roughened nature of the surface. The results are discussed in terms of previously proposed models for actinide diffusion in columnar pyrocarbons.
New Superfluid Relativity Gyroscope
Authors: Ensley D. L.
Published: February 2004

Experiments have been performed in which mechanical radiation pressure, generated by a megahertz sound field in helium II, successfully supported a metal sphere three dimensionally. Because the field's forces are spherically symmetrical and stable, and because the isothermal environment provided by the quantum coherence properties of helium II eliminates convection currents, only extremely small stray torques act on the suspended mass. It is demonstrated that stray torques less than 10−7 dyn cm were present on the early test model, consistent with the theoretical predictions. Also presented is the predicted performance, based upon these results, of a more refined superfluid gyroscope which demonstrates the feasibility of observing general relativistic gyroscopic motion with a gyroscope rigidly connected to the earth. A proposed experiment to discriminate between Einstein's theory and competing theories of gravitation by measuring the spin-spin interaction between a gyroscope and the spinning earth is also discussed.
High-Power Laser Radiation Interaction with Quartz
Authors: Chang D. B. , Drummond J. E. , Hall R. B.
Published: February 2004

Absorption of high-power CO2 laser radiation on a block of quartz is considered. The SiO2 (quartz) is vaporized and heated and absorption of the incident laser radiation is computed. If the gas is not ionized, the laser energy heats it sufficiently to make it transparent.
Generation of Harmonics and Subharmonics of the Josephson Oscillation
Authors: Sullivan D. B. , Peterson Robert L. , Kose V. E. , Zimmerman J. E.
Published: February 2004

The observation of harmonics and subharmonics of the Josephson oscillation is shown to be in agreement with a rather simple model of the junction. The generation of harmonics provides an explanation of induced steps in the current-voltage characteristic which occur at submultiples of the usual induced step voltages. The subharmonic oscillation is seen to be a relaxation-like process which can be easily understood in terms of a mechanical analog.
Molecular Field Coefficients of Substituted Yttrium Iron Garnets
Authors: Dionne Gerald F.
Published: February 2004

By fitting the Néel theory of ferrimagnetism to previously reported magnetic moment-temperature data for several {Y3}[RxFe2−x](QyFe3−y)O12compositions, where R and Q represent diamagnetic octahedral and tetrahedral substitutions, i.e., Sc3+, In3+, Ga3+, and Al3+, the molecular field coefficients were determined to have the following linear relations with the levels of substitution: Ndd=−30.4(1−043x),Naa=−65.0(1−042y),Nad=97.0(1−0.125x−0.127y) mole/cm3,for x≤0.70 and y≤1.95. Since both intrasublattice coefficients are affected only by substitutions in the opposite sublattice, a strong similarity to Geller's random canting concepts is apparent. Below the antiferromagnetic transitions it is demonstrated that a clear correlation exists between the decrease in sublattice moment from canting and the reduction in magnitude of the molecular field constant. For both sublattices, the antiferromagnetic transition occurs when Ndd, Naa∼−20 mole/cm3. This observation lends further credence to the notion that canting sets in immediately upon substitution. To demonstrate the applicability of the above results, magnetic moment-temperature curves are computed for compositions of {Y3} [Mgx2+Fe2−x] (Six4+Fe3−x) O12 and are shown to compare very favorably with experiment, thus providing a method for predicting the behavior of a wide variety of substituted garnet compositions.
Stability of Magnets Moving Above a Conducting Plane
Authors: Bailey Paul B. , Norwood Frederick R.
Published: February 2004

The equations of motion are obtained for certain magnetic sources moving above a conducting plate and subject to the magnetic forces produced by the eddy currents in the plate. The effects of a small disturbance at time t=0 to a previously steady motion are investigated. It is shown that any small disturbance dies away, leaving the system in essentially the same state of steady motion as before.
Combinations of Fourth-Order Elastic Constants of Fused Quartz
Authors: Powell B. E. , Skove M. J.
Published: February 2004

An evaluation of experiments already performed shows that three equations involving the four fourth-order room-temperature elastic stiffnesses of fused quartz have been determined. They are: C111−1.4C1112+0.34C1122+0.23C1123=(5.5±3.5)×1012N/m2C1111+4C1112+2C1122+2C1123=(55±10)×1012N/m2C1111+2C1112−3C1123=(−0.9±0.6)×1012N/m2.
High-Temperature Creep Mechanisms in α Iron and Other Metals
Authors: Murty K. Linga , Gold M. , Ruoff Arthur L.
Published: February 2004

The temperature and the stress dependence of tensile creep has been studied in ferromagnetic α iron (Ferrovac-E 99.91% pure) in the temperature range 620°–700°C. By using the change-of-slope method, activation energies for creep were obtained. The modulus-compensated activation energy is shown to be essentially independent of the initial stress and of strain up to the necking point. The change-of-slope method gave a modulus-compensated activation energy of 65 kcal/mole while a plot of the logarithm of the modulus-compensated strain rate versus reciprocal of the absolute temperature gave a straight line yielding a modulus-compensated activation energy of 62 kcal/mole. These are in essential agreement with the value for self-diffusion in this temperature range. The logarithm of the temperature-compensated creep rate versus logarithm of stress gave a straight line with a slope of 6.6, i.e., a power-law dependence of the strain rate on the stress. The fact that the activation energy for creep is the same as that for self-diffusion indicates that the creep process is controlled by diffusion and hence by a nonconservative dislocation motion: either the climb of edge dislocations or the dragging of jogs on screw dislocations. It is also found that there is no noticeable drop in the activation energy with increased stress. This means that at the strain rates and temperatures employed here, the average vacancy concentration is but a small perturbation from the equilibrium vacancy concentration in the undeformed specimen. By utilizing the recent data of Ishida et al. which extends to considerably lower temperatures good estimates of vacancy supersaturations can be obtained from 150° to 700°C (0.23Tm–0.54Tm) at different creep rates. From the available creep data on some fcc metals it is shown to be possible to get a good estimate of energy for motion of vacancies. We obtain by this technique Emv=15 500 cal/mole for aluminum which is in essential agreement with the value obtained by other techniques and Emv=22 800 cal/mole for copper.
Intensity Interferometric Imaging Through a Diffusing Medium
Authors: Beard T. D. , Barnoski M. K.
Published: February 2004

It is shown that a correlation interferometer can be used to obtain directly an image of a coherently or incoherently illuminated object through a changing random diffusing plate. An image of a pair of coherently illuminated point sources was obtained through a rotating ground-glass plate using a correlation interferometer.
Magneto-Optical Effects of Hot Electrons. II. High Magnetic Fields
Authors: Wood Van E.
Published: February 2004

The effects of a static electric field on the dispersive free-carrier magneto-optic effects in semiconductors are discussed for the conditions of low degeneracy, deformation-potential acoustic scattering, a simple parabolic band, and (1) moderately high magnetic fields, for which sH/cE≈1 but ωcτ0<1, (2) strong, but nonquantizing, fields (ωcτ0>1), and (3) the extreme quantum limit where the carriers are confined to a single Landau level. In the last case, only the situation where the static electric and magnetic fields are perpendicular is considered. In the first case, only small electric-field-induced effects occur. In the second case, only the low-frequency Voigt effect is changed much by the electric field; it is strongly anisotropic and relatively independent of magnetic field strength. In the quantum limit, an electric field should produce a considerable increase in an already large low-frequency Voigt effect. Effects of higher-order terms in relaxation time and distribution function in the quantum limit are also discussed.
Radio-Frequency-Sputtered Films of β-Tungsten Structure Compounds
Authors: Hanak J. J. , Gittleman J. I. , Pellicane J. P. , Bozowski S.
Published: February 2004

Radio-frequency sputtering has been used to deposit films of intermetallic binary systems (AxBy) in which the composition varies continuously from about 10 at.% to 80 at.% for the B constituent. Within this composition range the following β-tungsten structured compounds were obtained: V3Si, Nb3Sn, Nb3Pt, Nb3Au, Nb3Al, Nb3Ge, and Ta3Ge. For films with thickness ranging from 3000 to 10 000 Å values of Tc were observed which were within 1°K of previously reported best bulk values. In the case of Ta3Ge a new β-W superconductor has been found with a Tc of about 8°K.
Luminescent Time Decay of Excitons Bound to Zn–O Complexes in Gap
Authors: Jayson J. S. , Bhargava R. N. , Dixon R. W.
Published: February 2004

Time-decay measurements of the photoexcited red luminescence near room temperature in a series of GaP (Zn, O) samples with varying zinc concentrations and varying thermal histories (annealing conditions) are presented. A phenomenological analysis of the electron recombination processes in GaP (Zn, O) based on semiconductor statistics and a simple kinetic model is then discussed which takes the (accurately measurable) time decays as independent variables and predicts the values of less accurately known quantities. It is demonstrated that such elusive but important parameters as the internal quantum efficiency, and the relative strengths of radiative and nonradiative centers, can be quantitatively predicted using time-decay and free-hole-concentration measurements. Curves of internal quantum efficiency are presented as functions of the experimentally measurable time decays and free-hole concentrations; as is the ratio of the capture time of the nonradiative centers to the capture time of the Zn–O complexes. These curves lead to an explanation of the dependence of the luminescent time decay of a sample on its previous thermal history. The explanation depends on (1) thermalization of electrons captured by Zn–O complexes back into the conduction band, (2) the presence of nonradiative recombination centers, and (3) a change in the relative concentrations of radiative and nonradiative recombination centers with annealing. From an analysis of the luminescent time decays it is predicted that the energy level of an electron on a Zn–O complex is 0.24±0.02 eV below the conduction-band edge at room temperature. The temperature dependences of the luminescent decay and the external quantum efficiency (measured in the range 300°–450°K using both above and below bandgap photoexcitation) can also be understood on the basis of the model. When the intrinsic radiative excitonic lifetime is allowed to vary as a function of exciton binding-energy agreement between predicted and measured photoluminescent quantum efficiencies results.
Photoluminescent Properties of Vacuum-Deposited Cadmium Sulfide Films
Authors: Bleha W. P. , Peacock R. N.
Published: February 2004

Green and blue photoluminescence has been observed below 100°K in vacuum-deposited low-resistivity CdS films given no post-deposition treatment. The films were deposited on fused quartz substrates in a heated chamber inside the bell jar on substrates held at temperatures between 140° and 180°C. The CdS evaporant containing a trace of residual chlorine gave polycrystalline films that had resistivities at 300°K in the range of 1–102 Ω cm parallel to the substrate. The green luminescence in the films at 77°K was similar to the emission reported in the literature for donor-doped CdS. The doped-CdS emission differs from the phonon-assisted ``edge emission'' starting at 5140 Å when observed in pure CdS in that the emission peak at 5220 Å, corresponding to the emission of one k=0 LO phonon, is more intense than the zero phonon peak. The relative peak intensities of the zero phonon and phonon replicas in pure CdS can be described by a Poisson distribution in N̄ where N̄ is the mean number of phonons emitted in the transition. These Gaussian-shaped component peaks also have equal halfwidths H. For pure CdS at 77°K values found in the literature are N̄≅0.8−1.0 and H≅0.03 eV. For the CdS films good fits could be obtained with N̄=1.1−1.5 and H=0.04−0.06 eV. However, it is also possible that the film luminescence is the superposition of two emission series. The peak position of the green emission spectrum observed at 10°K was close to that at 77°K, and an additional peak at 5040 Å was observed. The decay time of the green emission was less than 1.5 μsec at 77°K, and thermal quenching of the luminescence began around 100°K, with an activation energy of 0.15–0.17 eV. Baking in saturated cadmium vapor at 500°C quenched the green emission, and for films baked in H2S or H2S+HCl+H2 at 600°C, emission similar to that observed in pure CdS was obtained. Optical absorption measurements indicated that the band edge was sharpened by the baking processes. The blue emission peak observed in the films was at 4892 Å at 77°K, which is in the fundamental absorption edge. It shifted to 4875 Å and became more intense and narrower at 10°K. This emission persists to 300°K with the peak position decreasing in energy as the bandgap. The peak is quenched by baking in H2S but not by baking in saturated Cd vapor. The blue emission is probably associated with a shallow donor or Cd excess in the films.
Production of Smooth Deposit Thickness Profiles and Homogeneous Alloys from Parallel Strip Sources
Authors: Dahlgren S. D.
Published: February 2004

Deposit thickness profiles obtained from a set of parallel strip sources were investigated to determine the optimum conditions of strip spacing relative to source-substrate distance needed for the production of smooth thickness profiles. The study was motivated by a desire to produce homogeneous alloy deposits by sputtering from a target consisting of sets of interspersed parallel strips of the alloying elements. Calculated and experimental profiles revealed a rapid decrease in deposit thickness variation with decreased distance between the strips in the source and with increased distance from strip source to substrate. The percentage composition variation (VA′) of a binary alloy deposit produced from interspersed strips was found to depend on the composition for a given thickness variation according to the equation VA′≃(1−CA)2t′. CA is the average concentration of A in the deposit, and t′ is the percentage variation in the thickness of a deposit from a single set of parallel strip sources. Thus the variation in concentration is always less than twice the thickness variation because 1−CA <1 for all alloys. It was found that a strip spacing to source-substrate distance ratio of 0.5 or less would give sufficient uniformity of thickness for homogeneous alloys to be made from an easily constructed target-substrate arrangement. The maximum composition variation for a source-substrate distance ratio of 0.5 was calculated to be 0.012%.
Thermoelastic Response of Polycrystalline Metals to Relativistic Electron Beam Absorption
Authors: Perry F. C.
Published: February 2004

A model is presented for the one-dimensional thermoelastic response of homogeneous isotropic metals to pulsed relativistic electron beam absorption. The effect of the time dependence of the electron power pulse was examined for two cases: a delta function and a rectangular pulse of duration τ0. Equations were obtained in both cases which explicitly relate properties of the electron beam (energy absorption profile, total energy, and pulse duration) to properties of the material (density, dilatational wave velocity, and Grüneisen parameter). Experiments were performed to test the developed model for polycrystalline aluminum, copper, and tantalum. Electron beam dosimetry was accomplished by means of calorimetry and a passive film technique. Material response data was obtained by measuring the back surface motion by means of a laser interferometer. The experimental results indicated that an assumption of instantaneous energy deposition was incorrect except possibly for the case of the lowest Z metal, i.e., aluminum The assumption of a rectangular power pulse was adequate to correlate the experimental results for all of the metals. The experiments afforded a direct determination of the Grüneisen parameters which agree with the values calculated from thermodynamic data.
Sensitive Dilatometer for Low Temperatures and the Thermal Expansion of Copper below 10 K
Authors: Pereira F. N. D. D. , Barnes C. H. , Graham G. M.
Published: February 2004

We describe a dilatometer which is an improved version of the one previously used in this laboratory. It consists of a doubly twisted strip of beryllium copper with a mirror attached to its central region. This system is immersed in liquid helium II. Dilations of the sample are sensed by this system via a thin diaphragm, causing rotations of the mirror, which is detected by an external optical lever. The system has a sensitivity to relative length changes Δl/l of 10−11. Copper has been used as the test material to evaluate the performance of the instrument. It has been possible to make direct measurements of α, the coefficient of linear thermal expansion, down to 2 K for ΔT=0.2 deg. These results yield α=1.3×10−10 T+2.7×10−11 T3 deg−1. The ratio of these terms is much more accurately known than the absolute values because of uncertainty in the absolute calibration. The linear term leads to an electronic Grüneisen γ of 0.57. This value is compared with recent theoretical and experimental values.
Theoretical Calculation of the Electronic Thermal Expansion of Simple Metals
Authors: Wallace Duane C.
Published: February 2004

At low temperatures the excitation of conduction electrons in metals gives rise to an electronic contribution C=Γ̃T to the heat capacity, and also a contribution β∝T to the thermal expansion coefficient, where T is the absolute temperature. The electronic Grüneisen parameter is γ=VβB/C=(d lnΓ̃/d lnV), where V and B are the crystal volume and bulk modulus, respectively, at T=0. We have used a local pseudopotential model to calculate all the contributions to Γ̃ and its volume derivative, including the electron-phonon interactions in first-order perturbation. The calculated γ is 1.18 for Na, 1.01 for K, and 1.63 for Al, while the measured γ is 1.8 for Al. The free electron model predicts γ=⅔. From our calculations it appears that the Umklapp interactions of electrons with transverse phonons give important contributions to Γ̃.
Lattice parameters of ZnO from 4.2° to 296°K
Authors: Reeber Robert R.
Published: February 2004

The temperature dependence of the lattice parameters of ZnO was measured by x-ray powder-diffraction methods. Retrograde behavior was found to occur perpendicular to the c axis at 93°K. Lattice parameters at 4.2°K were a0=3.24826±0.00015Å, c0=5.2033±0.0018 Å. These results are compared with those for some other II-VI compounds.
Volume Expansion of Sodium near the Melting Point for Different Impurity Contents
Authors: Ritter M. , Fritsch G. , Lüscher E.
Published: February 2004

The volume expansion of sodium with a potassium impurity content of 100, 200, or 500 ppm (mole) was measured by a capillary method. The temperature region covered extended from 60° up to 100°C. (Melting point of pure sodium 97.83°C.) In the temperature region 60° to about 90°C the results, when combined with x-ray measurements of the lattice constant, agree well with published values and theoretical estimates. The formation energy of a single vacancy was found to be 0.40±0.05 eV, and 0.45±0.05 eV; the formation entropy, in units of Boltzmann's constant, to be 7.2±1.5, and 7.05±1.5 for the 100- and 200-ppm specimens, respectively. Extrapolation of the single-vacancy concentration up to the melting point yields 10.5±1.5×10−4 and 9.3±1.5×10−4, respectively, again for the two impurity contents, 100 and 200 ppm. The data taken between 90° and 97°C show an excess volume expansion over the extrapolated single-vacancy values. Several possible mechanisms responsible for this behavior are discussed. The starting point of melting (Tc) was found by a careful analysis of the volume-expansion curves versus temperature and time. A power law holds for the volume expansion in the region 10−6<(Tc−T)/Tc<10−4. The exponential number is 0.66≈2/3, independent of the potassium content of the sodium sample.
Low Thermal Gradient High-Temperature Furnace for X-Ray Diffractometers
Authors: McKinstry H. A.
Published: February 2004

The optimum conditions for a high-temperature x-ray diffractometer are considered. A furnace with a minimum of compromise with these optimum conditions is described in some detail. Comparisons of previously described furnaces are made. The thermal gradients in the furnace were measured by thermocouple probe at several temperatures, and a novel shielding technique (making use of the x-ray diffraction beam) was used to establish that the thermal gradients in the furnace were indeed very low. A series of polymorphic transitions which have been studied are presented as a means of obtaining a good calibration for high-temperature x-ray studies.
Thermal Expansion of NbC, HfC, and TaC at High Temperatures
Authors: Jun Choll K.
Published: February 2004

The thermal expansion of niobium monocarbide, hafnium monocarbide, and tantalum monocarbide was determined from room temperature to 2700°C by dilatometric measurements. The apparatus and experimental procedure were described earlier by Miccioli and Shaffer.1 All carbides used in the present investigation were prepared at The Carborundum Company by hot pressing at 3150°C and 3000 psi. The bulk thermal expansion results of these three monocarbides are compared with other published dilatometric data1–3 and also with x-ray lattice expansion data.4–6 The high-temperature divergence between the bulk thermal expansion data and the lattice expansion data is due to the effect of nonstoichiometry, caused by noncongruent vaporization7–9 at high temperature in these cubic monocarbides. The previously reported inversion phenomena were observed in the vicinity of 2000°C on several refractory monocarbides which were prepared by hot pressing at about 2000°C. This inversion was not detected in the present investigation. The inversion is due to the densification at the temperature apparently at which the specimens were fabricated.
Anisotropic Thermal Expansion of Pyrolytic Graphite at Low Temperatures
Authors: Bailey A. C. , Yates B.
Published: February 2004

Interferometric measurements of the linear thermal-expansion coefficients α∥ and α⊥ in the ``c'' and ``a'' axial directions of well-oriented pyrolytic graphite between approximately 20° and 270°K are reported. Grüneisen parameters γ∥ and γ⊥, defined for strain coordinates parallel and perpendicular to the ``c'' axis, respectively, have been calculated from the present thermal expansion coefficients and related data. Between 30° and 270°K γ∥ is positive, indicating the preponderance of longitudinal modes of vibration, while γ⊥ is negative, indicating the predominance of transverse modes. The quasiharmonic approximation has been applied to calculate the characteristic temperatures θ(n) corresponding to the maximum frequencies ωD(n) of the Debye distributions having the same nth moments 〈ωn〉 as the specimens, which vary between approximately 420° and 2270°K as n increases from −3 to +6. The dimensional dependence of the moments, defined by γ(n) =Σγjωjn /Σωjn corresponding to γ∥(n) and γ⊥(n), and their variations with n, are consistent with the existence of low-frequency modes of vibration between planes of atoms perpendicular to the ``c'' axis, and high-frequency modes within the planes, a result supported by calculations of the rms displacements of the atoms.
Lattice Dynamics of Argon at 4°, 40°, and 77°K
Authors: Batchelder D. N. , Haywood B. C. G. , Saunderson D. H.
Published: February 2004

Neutron scattering techniques have been used to measure phonon dispersion relations in a 1-ml single crystal of 36Ar. Data were taken along the 100 crystallographic direction for the transverse mode at 4°, 40°, and 77°K. The measurements were performed on a free-standing crystal in equilibrium with its own vapor pressure. Preliminary analysis of the results yields mean shifts of the frequency of the transverse mode of 7% at 40°K and 21% at 77°K, relative to the 4°K value. These frequency shifts appear to decrease slightly with increasing wave vector. The mean Grüneisen parameter for this mode between 4° and 77°K is then 3.2±0.4 if it is assumed that the frequency shifts are due to the change in volume alone. Good agreement is found between the experimental frequency shifts and those predicted using a classical lattice dynamical model.1