bluuebunny/crossref_metadata_2025 huggingface dataset:
Capacitance Measurements on Au–GaAs Schottky Barriers
Authors: Senechal R. R. , Basinski J.
Published: February 2004

Dark capacitance measurements have been made on Au–GaAs Schottky barriers using boat-grown, oxygen-doped n-type GaAs with room-temperature carrier concentrations from 2×1013 to 4×1016 cm−3. The voltage and time dependence of the capacitance was analyzed using a modified version of Goodman's model for traps in depletion layers. The results yield a concentration of the dominant active trap of 1–4×1016 cm−3 in all the crystals examined. From the variation in the time dependence of the capacitance in the range 275°–365°K, the energy for the emission of an electron from this trap is 0.90±0.02 eV. Other aspects of the capacitance data locate this trap at around 0.7 eV below the conduction band, which is the energy previously assigned. Another, faster time-dependence observed in the higher-resistivity material was investigated in the range 225°–255°K. The activation energy associated with the latter time dependence is found to be 0.7 eV. The behavior of the capacitance at high reverse bias is explained in terms of electron capture by the dominant trap as the current increases due to impact ionization.
Sensitivity Enhancement by the Use of Acoustic Resonators in cw Ultrasonic Spectroscopy
Authors: Miller J. G. , Bolef D. I.
Published: February 2004

The role of an acoustic resonator in the study of cw acoustic magnetic resonance (AMR) effects is discussed. From the expression for the frequency response of a resonator consisting of an isolated sample, one can calculate absorption and dispersion sensitivities and determine the conditions under which a pure absorption or pure dispersion AMR signal may be observed. The calculation is extended to the case of a sample and transducer. The primary effect of the transducer is to shift the frequency of the isolated sample mechanical resonance without materially altering the mechanical resonance lineshape or associated AMR sensitivities.
Effects of Ultraviolet Irradiation on the Properties of Evaporated Silicon Oxide Films
Authors: Mickelsen R. A.
Published: February 2004

The optical and dielectric properties of silicon oxide films formed by reactive evaporation of silicon monoxide are investigated. The films are prepared at a deposition rate of 4 Å/sec in 1×10−4 Torr oxygen. The optical absorption of the films is found to increase with increasing substrate and source temperatures; these variations are consistent with previously reported chemisorbed oxygen and gas-phase oxidation film-growth mechanisms. Post-deposition exposure to uv radiation affects both the optical and dielectric properties of these films. Optical absorption in the 200 to 400-mμ wavelength region is essentially eliminated by 1 h irradiation for films deposited at 145°C or less; a residual absorption is evident for films prepared at a higher substrate temperature. The magnitude of the dielectric loss and relative dielectric constant, the variation of the dielectric loss and dielectric constant with frequency, and instabilities of the dielectric properties are decreased by the uv treatment. For example, at 10 Hz, the loss was decreased by an order of magnitude while the dielectric constant decreased by 12% for a typical experimental series. A model based on oxide defects, e.g., oxygen-ion vacancies and interstitial oxygen, is consistent with the experimental results.
Correlation of Reduction in Optically Induced Refractive-Index Inhomogeneity with OH Content in LiTaO3 and LiNbO3
Authors: Smith R. G. , Fraser D. B. , Denton R. T. , Rich T. C.
Published: February 2004

It has recently been demonstrated that crystals of LiTaO3 and LiNbO3 can be made more resistant to optically induced refractive-index inhomogeneities caused by laser irradiation by annealing the crystals in the present of an electric field. The explanation given for the improvement was that some impurity entered the crystal during the annealing cycle, modifying the conductivity of the material. Additional data presented here suggest that the impurity is hydrogen. The exact mechanism by which the susceptibility to index inhomogeneity is reduced is not understood at this time.
Transport of Electrons in a Strong Built-in Electric Field
Authors: Gunn J. B.
Published: February 2004

The problem is discussed of the transport of electrons in the presence of a strong field due to fixed space charges, as in the depletion region of a p-n junction. It is found that the effective mobility, for small departures from equilibrium, is equal to the chordal hot-electron mobility which would result if the strong field were applied by external means.
Magnetic Properties of PrAl2 and ErAl2
Authors: Nereson N. , Olsen C. , Arnold G.
Published: February 2004

The magnetic properties of PrAl2 and ErAl2 have been investigated by neutron-diffraction and susceptibility measurements on powder samples of the above compounds. The susceptibility data show that both compounds obey the Curie-Weiss law; the effective moment per molecule in the paramagnetic region is 3.5±0.05 μB for PrAl2 and 9.2±0.1 μB for ErAl2. The neutron-diffraction measurements confirm that these compounds are ferromagnetically ordered below a Curie temperature of 34°K for PrAl2 and 14°K for ErAl2; the respective saturated ordered moments at the rare earth atom sites are 2.94±0.05 μB and 8.3±0.3 μB. The magnetic form factor data from the ErAl2 coherent magnetic reflections are in good agreement with similar data obtained by other investigators on an Er single crystal. The magnetic scattering intensity of the (111) reflection from PrAl2 was measured just below the Curie temperature TC in order to determine the long-range magnetic-order temperature dependence. The results show that the spontaneous magnetization M follows a power law in the vicinity of TC of the form M ∝ (TC-T)β, where β=0.6±0.1.
Material Strength Effect in the Shock Compression of Alumina
Authors: Ahrens Thomas J. , Gust W. H. , Royce E. B.
Published: February 2004

The Hugoniot elastic limits (HEL) of a high-density aluminum oxide ceramic (Lucalox) and a slightly lower-density ceramic (porosity approximately 4%) were determined to be 112±13 and 83±5 kbar, respectively. Above the HEL, the shock-stress-volume Hugoniot curves for both materials are offset by 40 kbar or more above their hydrostatic-pressure-volume curves. These results indicate that shear stresses with magnitudes of about 30–40 kbar persist in aluminum oxide to shock-pressure levels of at least 300 kbar.
Nonlinear Interactions of Electromagnetic Waves with a Thermally Generated Cesium Plasma
Authors: Hewitt Jack G. , Boone Jack L.
Published: February 2004

The interaction of a large-amplitude disturbing wave with a thermally generated, highly ionized cesium plasma has been investigated using a small-amplitude sensing wave as a probe. The subsequent interaction between the disturbing and sensing waves is related to the plasma electron temperature. The plasma was contained in a magnetic field of 500–1000 G and had an electron plasma frequency near 10 GHz and an electron gyromagnetic frequency around 2 GHz. The sensing wave was a 10 GHz plane wave of approximately 1.5 mW input power and the disturbing wave was 7.5 GHz plane wave of about 1.2 W peak power. Measurements of the sensing-wave transmitted power were made using standard microwave techniques. A notch filter in the sensing-wave receiving circuit reduced the scattered disturbing-wave interference to an immeasurably low level. Langmuir probes were used to detect disturbing-wave modulation and to measure plasma temperatures and densities. Measurements of the interaction were correlated with a solution of the Boltzmann transport equation for a perturbed Maxwellian distribution function with binary Coulomb collisions.
Effect of Nonuniform Conductivity on the Behavior of Gunn Effect Samples
Authors: Hasty T. E. , Stratton R. , Jones E. L.
Published: February 2004

The behavior of n+-n−n+ GaAs samples biased at voltages below the threshold for the onset of Gunn effect is derived. In particular, the spatial distribution of the electric field and the electron temperature, along with the current-voltage characteristics of the sample, are calculated. The specific structure considered has a low conductivity region in the n layer near one of the n+ contacts. This occurs for an alloyed contact made to an epitaxial layer of n-type GaAs grown on an n+ GaAs substrate. The most striking result is the occurrence of Gunn oscillations for one direction of bias and avalanche breakdown for the other. This and other experimental findings are consistent with the theory.
An Aluminum Splash Generated by Impact of a Detonation Wave
Authors: Taylor R. W. , Venable Douglas
Published: February 2004

Experimental observations are presented of two-dimensional flow attending the impact of a detonation wave with an aluminum target plate. A time sequence of flash radiographs of the events provides a means for measuring the formation and growth of the resulting splash. Space-time histories of various features of the splash are presented. The radiographs also display the course of the reflected shock in the explosive products, the shock transmitted into the target, the rarefaction wave which arises when this latter shock strikes the free surface of the target, the spalling resulting from this rarefaction wave, and the flow of the gases expanding from the burned explosive. Adequate equation of state data exist for both the gaseous products of the high explosive and the aluminum whereby the gross features that are observed can be replicated by today's computer techniques. However, viscosity and dynamic shear and tensile strengths, as functions of local conditions, are needed before a complete computer representation can be made.
Radiation in a Compressible and Anisotropic Plasma Medium
Authors: Chen Hollis C.
Published: February 2004

Based on the compressivity and the permittivity tensor formulation of electrodynamics in a lossy, compressible, and anisotropic plasma medium, it is shown that when both electric and magnetic current sources are present, the longitudinal components of the electric and magnetic fields and the pressure variation satisfy the inhomogeneous coupled wave equations. A general method based on the spectral resolution of a square matrix for the solution of such an equation is presented. The complicated, difficult problem of analyzing sources radiating in a compressible and anisotropic plasma is thus reduced to the evaluation of some simple integrals. Equations for the determination of the transverse components of the electric and magnetic fields with sources are also derived. As an application of the general formulas, a complete solution for a magnetic line source is worked out in detail to demonstrate the simplicity of the method.
Role of Surface Defects in the Epitaxial Growth of Some fcc Metals on Potassium Chloride Cleaved in Ultrahigh Vacuum
Authors: Palmberg P. W. , Todd C. J. , Rhodin T. N.
Published: February 2004

It has been found that surface defects produced by electron irradiation play a critical role in the formation of single-crystalline films of typical fcc metals such as Au, Ag, and Cu on UHV-cleaved KCl. Epitaxial films of the (100)Au∥(100)KCl, [100]Au∥[100]KCl orientation were produced in a wide range of substrate temperatures on irradiated surfaces at low supersaturation levels. Surface defects produced by irradiation provide nucleation sites which strongly favor the epitaxial orientation for nuclei. The high sensitivity of film orientation to irradiation treatment of the substrate suggests that other known critical factors such vacuum condition and surface composition may also influence epitaxy through their effect on the nature of the substrate surface upon which nucleation occurs.
Self-Pulsing in Lasers
Authors: Risken H. , Nummedal K.
Published: February 2004

The origin of pulse formation in lasers is investigated. The main features of this phonomenon are explained in terms of a homogeneously broadened, two-level ring laser model. First, it is found that a stationary (cw) solution cannot exist for certain choices of laser parameters. By a numerical integration of the laser equations, it is shown next that a buildup of a pulse occurs for these parameters. The final (steady-state) pulse shape and its velocity are determined. Furthermore, it is found that a multistable operation is possible. A simple approximate equation is derived, by which most of the features of the final pulse are given analytically.
Preparation of Garnet Films by Sputtering
Authors: Sawatzky E. , Kay E.
Published: February 2004

Three different sputtering approaches are described for the synthesis of garnet films in the thickness range of 0–4 μ. The experimental requirements for producing single-phase, polycrystalline GdIG films are described in some detail, especially in the rf sputtering mode. Unexpectedly large systematic variations in the unit lattice parameter of the garnet structure as a function of film thickness are reported. Possible causes for these systematic structural changes are discussed.
Applications of a Holographic Interference Microscope
Authors: Magill P. J. , Wilson A. D.
Published: February 2004

The holographic microscope has extended the usefulness of holographic interferometry. Without the use of an eyepiece, one is basically concerned with image holography. This is herein considered to include the cases where an object is imaged near, or on, the holographic film plane. The application of image holography for interferometry is described beyond that previously given. Its usefulness for studying small vibrating objects and for real-time interferometry is demonstrated.
The Stacking-Fault Energies of Ag(Mn) and Cu(Mn) Solid Solutions
Authors: Quader M. A. , Dodd R. A.
Published: February 2004

The stacking-fault energies of the Ag(Mn) and Cu(Mn) solid solutions have been determined for various compositions up to 18 at.% solute by the method involving measurement of extended node radii. The computed energies correlate very closely with earlier x-ray measurements of stacking-fault probabilities, confirming that manganese has a different effect on the fault energy of the two solid solutions. The stacking-fault energy of silver is changed very little by manganese, possibly because of two offsetting contributions to the energy: (1) a tendency for γ to decrease due to an increase in valency-electron/atom ratio, and (2) a tendency for γ to increase because of d-shell interactions due to holes in the stable half-filled Mn 3d shell. The stacking-fault energy of copper is reduced by magnanese, probably because the Mn atoms are compressed so that the 3d shell behaves as if it were completely filled. Thus, manganese behaves as a multivalent element in reducing γ of copper.
Optical Spectra and Laser Action of Neodymium in a Crystal Ba2MgGe2O7
Authors: Alam M. , Gooen K. H. , Bartolo B. Di , Linz A. , Sharp E. , Gillespie L. , Janney G.
Published: February 2004

Absorption, fluorescence, and excitation spectra of a single crystal of Ba2MgGe2O7: Nd3+ were investigated. The lifetime of the 4F3/2 state of the neodymium ion is 450 μsec at 300°K. The width of the strongest fluorescence line at 1.054 μ is 35 and 10 Å at 300° and 80°K, respectively. Laser action was achieved at 1.05444 μ at room temperature.
Bounds on Transport Coefficients of Two-Phase Materials
Authors: Coriell S. R. , Jackson J. L.
Published: February 2004

Previously derived upper and lower bound expressions for the effective conductivity of composite materials are applied to two-phase materials. Results are presented for several simple geometrical arrangements and for certain limiting cases.
X-Ray Spectroscopic Study of Zinc Selenide
Authors: Bhide V. G. , Bhat N. V. , Rambhad K. R.
Published: February 2004

The x-ray K absorption spectra of Zn and Se have been studied in the pure metals and in the intermetallic compound zinc selenide (ZnSe), using a bent crystal x-ray spectrograph. The K absorption edge of Zn in ZnSe shifts by 0.7 eV towards the high-energy side with respect to that in the pure metal. On the other hand, the Se K absorption edge in ZnSe is found to shift by 2.4 eV towards the low-energy side as compared to that in pure Se. It has been shown that this is consistent with the valence bond model and the earlier investigations on Zn and Se.
A Search for X Rays from Helium and Air Discharges at Atmospheric Pressure
Authors: Noggle R. C. , Krider E. P. , Wayland J. R.
Published: February 2004

X-ray production in wide-gap helium and air discharges was investigated using calibrated film-badge detectors. Helium discharges were found to produce x rays with average energies in the range <11 to 13 keV. The energies were independent of applied electric field for average E/p ratios between 35 and 140 V/cm·Torr. X-ray intensities were proportional to the atomic number of the electrode material. No evidence for similar x-ray production was found in air discharges.
Thermal and X-Ray Diffraction Studies of the NaNbO3–KNbO3 System
Authors: Tennery Victor J. , Hang Kenneth W.
Published: February 2004

Thermal and x-ray diffraction measurements on compositions from the system NaxK1−xNbO3 have revealed the presence of phase boundaries at approximately 82.5, 67.5, and 52.5 mole × NaNbO3 at 25°C, in addition to one very near the NaNbO3 end member. The optimum piezoelectric properties in this system are apparently associated with compositions near the latter boundary. Calorimetric determinations of the enthalpy and entropy changes associated with the ferroelectric→ferroelectric and ferroelectric→paraelectric transformations in the system clearly revealed the presence of the phase boundaries while measurements of the transformation temperatures alone did not readily reveal the boundaries. Two different superlattice perovskite phases were observed in the system, one existing in the region ∼0.98≥x≥0.825 and another in the region 0.825≥x≥0.675. No multiple-cell structures were observed in the system at NaNbO3 concentrations below about 70 mole%.
Mode-Coupling Effects in Mode-Selection Experiments
Authors: Hong G. W.
Published: February 2004

Mode-coupling effects in a He–Ne laser are described by means of a mode-suppression technique. In the extreme case, alternate enhancement and complete quenching of modes were observed when only one mode was suppressed. These effects are explained in terms of mode competition as described by Lamb. A method is developed to measure the coupling parameters experimentally. Results are given for the three-mode case.
Thermal Noise in ir Quantum Counters
Authors: Gandrud W. B. , Moos H. W.
Published: February 2004

Because an ion's strength of interaction with the lattice vibrations is not appreciably greater than with the radiation field, for energy gaps > lim ∼103 cm−1, the 300°K background radiation field will play an important role in determining the equilibrium population of the near-infrared electronic levels. As a consequence, the Infrared Quantum Counter fluorescence produced by thermal population of the device's infrared level (a generalized form of spontaneous emission noise) cannot be reduced indefinitely by choosing arbitrarily cold cryogenic baths. It is shown that ``soft'' lattices with weak ion-lattice interactions produce the most noise of this type, yet yield the best detector sensitivity. Qualitative experimental confirmation is provided, and the IRQC is shown to be potentially useful, if laser pumping can be achieved, despite the noise limitation imposed by the thermal background.
Analog Storage in Uniaxial Magnetic Films using Nonuniform Fields
Authors: Paul M. C. , Oberg P. E.
Published: February 2004

Analog information storage characteristics have been produced in conventional Ni–Fe films having uniaxial anisotropy by the use of special magnetic operating fields. A directionally curved field applied in the hard-axis direction induces a temporary angular distribution of magnetization directions across the film which, in conjunction with a uniform applied easy-axis signal field, results in a proportionate net magnetization of the film. Various field-shaping elements are described and experimental results are shown. Another proportional storage technique utilizing opposing, nonuniform magnetic fields in the easy direction is also described, and applications of analog elements are discussed.
Superconductive Transition Temperatures of Reactively Sputtered Niobium Films
Authors: Mitszuoka Takeshi , Yamashita Tsutomu , Nakazawa Tokuro , Onodera Yutaka , Saito Yukinori , Anayama Takeshi
Published: February 2004

Niobium films have been examined with respect to their electrical and superconductive properties. The films were deposited by dc and asymmetric ac sputtering techniques in argon with addition of a small amount of nitrogen gas. The films obtained had a wide range of resistivities and transition temperatures depending on the partial nitrogen pressure in the argon atomosphere. The maximum transition temperature of films obtained was 15.3°K, which very close to those reported for bulk NbN. It was found that the asymmetric ac sputtering technique is useful to increase the transition temperature of niobium films sputtered reactively in the presence of a nitrogen gas.
Excitation Cross Section for some of the Doubly Ionized States of Argon, Neon, and Krypton by Fast Electrons
Authors: Koozekanani S. H.
Published: February 2004

Excitation cross section for the doubly ionized states of argon, neon, and krypton has been found for the process where a fast-moving electron impinges upon a neutral atom, causing the neutral atom to go from lN to (lN−3-nl'') in a very short time compared to the relaxation time of the ion. These calculations are useful when one is interested to know the excited states in which the ion is left after the atom loses some of its electrons and becomes ionized.
The Pressure and Temperature Dependence of the Shear Strength of Minerals
Authors: Towle Laird C. , Riecker Robert E.
Published: February 2004

Shear-strength measurements on diopside, labradorite, and olivine were made in the pressure range to 50 kbar and in the temperature range to 900°C. The data were correlated with a semiempirical formula which describes the pressure and temperature dependence of the shear strength within experimental error. The formulation provides a useful synthesis of the data whereby three constants characterize each material. In addition, the formulation provides a basis for extrapolation so that the shear strength of a material can be estimated at pressures or temperatures beyond the range of experiment.
Correlation of Optical Absorption with Thermoluminescence of Thermally Treated LiF Crystals
Authors: Nakajima Toshiyuki
Published: February 2004

The thermoluminescent intensity of LiF crystals thermally treated in air is smaller than that of crystals treated in argon gas, and moreover, decreases with the annealing temperature and time. It has been noted that, in comparison with the optical transmission of the original crystals, the transmission of samples treated in air decreases in the region of wavelengths of visible light, and that furthermore, the transmission declines with increasing annealing temperature and annealing time. Infrared absorption troughs corresponding to OH and CO3 anions are observed in the spectra of crystals treated in air. These anions present in the LiF crystals contribute to lowering the thermoluminescent intensity. When the LiF crystals treated in air are cooled in acid, the thermoluminsecent intensity is raised and the absorption troughs become shallower.
The Bunching of Electrons in the Meniscus Discharge
Authors: Weinreb J. , Ettinger Y.
Published: February 2004

The parameters of a meniscus discharge in mercury at low pressure were examined by means of a movable probe. The probe was also used as an antenna for the detection of high-frequency oscillations generated in the discharge. By applying the electron-bunching theory, it is quantitatively explained that strong oscillations are observed in a sheath located at a small distance from the meniscus top, at the side of the anode, while an oscillating electric field is located in a narrow region near the meniscus top. Calculations of the distance between the two sheaths, using the klystron theory of Webster, are in good agreement with the experimental data.
Laser-Excited Raman Scattering in Polystyrene
Authors: Cornell S. W. , Koenig J. L.
Published: February 2004

Atactic polystyrene was examined by argon ion laser-excited Raman scattering. Depolarization measurements of oriented and unoriented polystyrene were made. An analysis is made of the depolarization measurements in terms of the orientation of the sample. Comparisons are made with dichroic measurements made by infrared. Utilizing Raman and infrared polarization data on oriented polystyrene, some changes in band assignments have been made.
Rheo-optics of High-Speed Deformation. II. Polyethylene and Polybutene-1
Authors: Erhardt P. F. , Stein R. S.
Published: February 2004

The photographic light scattering-technique was used to study the relationship between sample deformation and spherulite deformation during rapid stretching of polyethylene and polybutene-1 films. Scattering patterns are interpreted in terms of spherulite deformation theory to give the elongation ratio of the spherulite which is compared with that of the sample. At low elongations spherulite deformation is less than or equal to that of the sample indicating that an affine deformation model is a reasonable approximation. At high elongations the spherulite elongation ratio is greater than that of the sample. This is due to inhomogeneous deformation where the local deformation of the sample in the scattering region is greater than the macroscopic deformation.
Rheo-optical Behavior of Polyacrylonitrile: Creep and Creep Recovery
Authors: Andrews R. D. , Okuyama H.
Published: February 2004

The rheo-optical behavior of PAN homopolymer film has been investigated by means of creep and creep recovery experiments with simultaneous measurements of birefringence changes. Creep experiments were carried out at temperatures of 70°, 100°, 120°, 140°, and 160°C, at a series of stress levels. This temperature range spanned the 90° and 140°C transition temperatures. A progressive change in shape of the creep curve in a log-log plot of compliance vs time is seen: from very flat and gradual to more S-shaped both with increasing temperature and with increasing stress at the higher temperatures. This S-shaped curve can be identified with the cold-drawing phenomenon, even though no neck formation was seen. Creep recovery is more complete at lower temperatures and after lower final strains in the creep experiment. Most of the recovery is instantaneous; only a very small amount of further time-dependent recovery is observed. At higher temperatures and higher strains, creep seems to involve permanent changes in the solid-state structure; these produce a permanent set which is not recoverable even on heating to higher temperatures. The creep and creep recovery, therefore, cannot be properly described in terms of the laws of simple viscoelasticity. The rheo-optical behavior of this polymer is clearly more complex than that of PVC, which has been investigated previously. Some speculations regarding the solid-state structure of PAN are presented, based primarily on the birefringence results.
Behavior of Elastic Networks of Various Degrees of Orientation in the Kinetic Theory of Fracture
Authors: von Schmeling H. H. Kausch-blecken , Hsiao C. C.
Published: February 2004

This paper describes a kinetic theory of fracture initiation using a linear elastic network as an approach to represent the strength and elastic properties of oriented materials. Emphasis has been placed on the questions as to whether the assumptions of small strains and invariant molecular orientational distribution are valid for the whole period of fracture initiation. The decrease of the modulus of elasticity resulting from the breakage of molecular elements during this period was found to be less than 1%. For brittle materials with high velocities of crack tip propagation the initiation period covers most of the lifetime of a sample. The logarithms of time to break calculated accordingly for network systems of different degrees of orientation are linear functions of applied stress over a wide range of stress. The slopes of these linear curves are inversely proportional to the modulus of elasticity of the network at zero time. Therefore, if the calculated curves of the logarithm of time are plotted versus the applied stress divided by the initial modulus of elasticity the linear portions of all curves reduce to one. For very small or large stresses the curves deviate from linearity.
Chain-Folding and Molecular-Species Segregation in the Crystallization of Linear High Polymers
Authors: Lindenmeyer P. H. , Peterson J. M.
Published: February 2004

For linear high polymers, the assumption that chain ends are excluded from the crystal plus the existence of a reasonable width in the distribution of chain lengths is sufficient to cause a minimum in the free energy-vs-crystal thickness curve at temperatures well below the melting point. The predicted crystal thickness is relatively insensitive to crystallite-surface free energy, but is sensitive to the degree of supercooling and the molecular length distribution. At temperatures lower than a few degrees below the equilibrium mp, there exists a thermodynamic driving force for molecular-species segregation. Hence according to the theory, crystals which are composed of a variety of molecular species are thermodynamically metastable.
Energy Contribution to Rubber Elasticity
Authors: Shen Mitchel , Blatz Paul J.
Published: February 2004

Stress-temperature measurements were performed for natural rubber at both ``infinitesimal'' (<10%) and finite (<100%) strains. Data are used to calculate the internal-energy contribution to the elastic stress fe by several thermoelastic equations. A unified derivation of these equations is given, which leads naturally to a new equation based on the temperature coefficient of shear modulus of the rubber. It is demonstrated that the dependence of the relative energy contribution fe/f on deformation as previously reported arises because of the difficulty in obtaining extremely accurate data in the low-strain regions. Calculation of the same data by our new equation shows fe/f to be independent of deformation for the region of strains in which the statistical theory of rubber elasticity applies. Thus the postulate of additivity of configurational free energy, basic to the derivation of the statistical theory, is vindicated. Finally it is shown that the stress-temperature measurements are sensitive to the sample geometry. For butt-jointed samples, which are believed to approach uniform stress distribution, the relative energy contribution is found to be approximately 15% for natural rubber.
Crystallization of Low-Molecular-Weight Polypropylene Fractions
Authors: Morrow D. R. , Newman B. A.
Published: February 2004

Samples of very low molecular-weight fractions of isotactic polypropylene have been crystallized by means of a solvent-evaporation technique as well as by melt recrystallization. The crystalline entities obtained by these methods have been examined in terms of their internal structure and associated morphology. The particular polymorphic form observed for these fractions is dependent on both the length of the molecule and the particular crystallization conditions employed. The morphological characteristics of the samples have been found to be qualitatively similar to those previously reported for unfractionated polypropylene and high-molecular-weight fractions, in spite of the fact that the molecular lengths of the samples investigated are sufficiently short to prevent chain folding. Selected-area electron-diffraction patterns have been obtained from the γ form and from mixed α and γ form of the isotactic polymer. The diffraction data have resulted in a modification of the previously proposed triclinic unit cell for the γ phase, and a conclusion that the γ→α transformation in isotactic polypropylene is martensitic in nature. The mechanism of this solid-solid transformation involves relative motion between the (040) planes which are found to be invariant in the two crystal phases.
Structure and Thermal Behavior of Pressure-Crystallized Polypropylene
Authors: Sauer J. A. , Pae K. D.
Published: February 2004

Samples of isotactic polypropylene have been crystallized at a pressure of 3.86 kbar and under a variety of crystallization conditions. These include isothermal crystallization at five different degrees of supercooling ranging from ΔT=70°C to ΔT=34°C and crystallization by slow cooling from 248°C. Each sample was studied by means of x-ray diffraction at both room and elevated temperature, by differential scanning calorimetry, and by electron microscope observation of replicas of the fracture surface. The x-ray diffraction results show that all the pressure-crystallized samples have formed in the γ, or triclinic, modification. Depending upon crystallization conditions, this phase may be stable or metastable. For high values of ΔT, the triclinic phase shows a partial conversion to the α, or monoclinic, phase under elevated temperature annealing. For low values of ΔT, the γ phase does not convert to α even at elevated temperatures. The DSC measurements indicate that the melting temperatures of the pressure-crystallized samples are dependent upon the degree of supercooling. For a heating rate of 40°C/min, the melting temperature is at 157°C for ΔT=70°C; it rises with decrease of the degree of supercooling and occurs in two stages at 175° and at 182°C for ΔT=34°C. Fracture-surface replicas of the slowly cooled sample show that the bulk polymer contains well-delineated, lath-shaped crystals, similar in morphology to those obtained from dilute-solution crystallization. For the samples isothermally crystallized, the fracture-surface replicas show the presence of long thin fibril-like entities. Consideration is given to the possibility that these fibrillar elements are single-crystal lamellas of low-molecular-weight polymer that have crystallized during cooling of the whole polymer from the crystallization temperature.
Dissolution of Polyethylene Single Crystals in Xylene and Octadecane
Authors: Huseby T. W. , Bair H. E.
Published: February 2004

Solution temperatures in xylene and n-octadecane of polyethylene single-crystal aggregates were measured by differential scanning calorimetry. Crystals were irradiated to suppress morphological changes during the heating process. The measurements, as a function of reciprocal lamellar thickness, yield values for the equilibrium dissolution temperatures Td0 and end surface free energy σe of Td0=113.7±1.2°C and σe=93±8 erg/cm2 for dissolution in xylene, and Td0=129.4±1.6°C and σe=92±10 erg/cm2 for dissolution in octadecane.
Variations of the Unit-Cell Dimensions of Polyethylene: Effect of Crystallization Conditions, Annealing, and Deformation
Authors: Davis G. T. , Eby R. K. , Martin G. M.
Published: February 2004

It is shown that the orthorhombic unit cell dimensions of a given polyethylene are not unique, but depend on the physical history of a sample. For polymer crystallized from a ¾% p-xylene solution, the values of a and b measured at 23°C by powder camera technique vary up to 0.8% for crystallization temperatures of 50°, 70°, and 90°C, annealing temperatures from 50° to 127°C, and deformation. The as-crystallized values of a and b as well as the effects of annealing and deformation are smaller the thicker the crystals. For ``undeformed'' crystals the changes correlate with the reciprocal of crystal thickness but for deformed crystals the changes occur before the thickness increases (even below the crystallization temperature). The dimensions undergo the greatest change within the first 2 min of annealing and change very little with time thereafter. The only exception observed is a slow decrease of b with time of annealing at 100°C. Crystals grown from other solvents exhibit very nearly identical effects, and the presence of solvents does not alter the cell dimensions at room temperature. Similar effects are observed when crystals grown from the melt by slow cooling, quenching in ice water, and quenching in a dry ice-acetone suspension are annealed at temperatures from 60° to 127°C. In these crystals also the dimensions correlate with the reciprocal of the long period and extrapolate to values of 7.372 and 4.933 Å for a and b, respectively, at infinite long period. The origins of the effects cannot be assigned with certainty but changes in the fold surfaces and their concentration probably play a role; a discussion relevant to this and other mechanisms is presented. Corresponding to the maximum changes in cell dimension are changes of 1% in cell density which corresponds to an apparent change in degree of crystallinity of 6%. These changes are too small to account for the frequently reported discrepancy between the cell density and that measured by pycnometry, etc. However, the changes can account for some of the variation with crystallization temperature of the density measured by pycnometry, etc. The effects of cell variation should be considered in the detailed interpretation of the variation of physical properties with physical history of polymer crystals.
A Statistical Analysis of the Initial Microwave Tensor Susceptibility of a Polycrystalline Ferrite
Authors: Kriz Thomas A. , Ishii T. Koryu
Published: February 2004

The microwave tensor susceptibility of an inhomogeneous ferrite medium at remanence is analyzed statistically. A multivariate Gaussian distribution is used to characterize the magnetization of the medium as a random vector quantity. The orientation of a random vector quantity for magnetic field intensity in the medium is taken to be the same as for magnetization, but its magnitude is described with a uniform distribution. The statistical parameters of these distributions are defined as functions of such material properties as (1) the ratio of magnetization at remanence to that at saturation, (2) shape demagnetization, and (3) rf demagnetization. Numerical values of average tensor component susceptibility for several different inhomogeneous media are calculated using an analytical solution of the statistical model and shown to be within 11% of experimental results reported by other investigators.
Anomalous Diffusion of 14C in TiC0.67
Authors: Sarian S.
Published: February 2004

The self-diffusion of 14C in TiC0.67 was studied in the temperature range 1745°–2720°C. The diffusivity, which exhibits a discontinuity at 2080°C, can be represented by two expressions: D0.67*=(2.85±0.20)×10−4exp[−(49 600±300)/RT] cm2/sec above 2080°C and D0.67*=(1.14±0.66)×102exp[−(109,900±2000)/RT] cm2/sec below this temperature. No discontinuity was found in carbon saturated TiC0.97 and the self-diffusion coefficient for this composition can be described to a temperature of 2720°C by the single expression D*0.97=(6.98±1.24) exp[−(95 300±700) / RT] cm2/sec.
Rotation and Translation of Islands in the Growth of Heteroepitaxial Films
Authors: Reiss H.
Published: February 2004

A simple yet realistic model is introduced for the description of islands of epitaxial overgrowth on a substrate surface. The model is employed to examine the potential energy surface for both rotation and translation of the islands as a whole. It is shown that one or more overwhelmingly easy directions of orientation exist, and that islands which manage to be rotated out of these easy directions can both rotate and translate with activation energies of the order of magnitude of the activation energy for surface diffusion of isolated atoms. The results provide semiquantitative support for Bassett's observation via the electron microscope that large islands of overgrowth (∼1000 atoms) seem to both rotate and translate. Considerations of surface nucleation indicate that although nucleation rate depends on orientation, subsequent growth of islands does not. A growth law independent of orientation is then derived. It is also shown semiquantitatively that under typical experimental conditions for slow deposition, nucleation occurs but once—islands initially formed grow, while their number remains constant. This result is also consistent with several observations. The process by means of which misoriented islands anneal by rotational diffusion into the easy orientation, while they simultaneously grow, is analyzed. A formula is developed which indicates that under certain conditions, islands of overgrowth become ``stuck'' in misoriented positions before they coalesce. The conditions effecting this phenomenon are discussed. General comments and observations are made concerning the initial origin of misoriented nuclei, and means are suggested for adjusting conditions so as to permit misorientation to anneal before coalescence. Typical densities of dislocations introduced by misorientation are estimated. Crucial experiments are suggested for examining the hypothesis of this paper.
Photosensitivity in Epitaxial PbS Films
Authors: Riedl H. R. , Schoolar R. B.
Published: February 2004

A study has been made of the photosensitivity in epitaxial PbS films which were sensitized by heat treatments in air or sulfur vapor. The sensitizing process results in the formation of narrow lines of material which are more p type than the host film. The resultant junctions are responsible for the photosensitivity. The junctions are associated with crystal defects which are faintly visible under a microscope and appear as lines running in (100) directions. Introduction of these lines can be sufficiently controlled so that only a few are introduced into a sample. This enables individual junctions to be studied. Various properties of the epitaxial films were determined and compared to the corresponding properties of the well-known polycrystalline PbS detectors.
A Simplified Orders-of-Scattering Technique for Calculating the Absorption of X Rays
Authors: Willis Robert G.
Published: February 2004

A simple, efficient technique is described for calculating the energy absorption in a planar slab of finite thickness from a planar source of x rays. Compton scattering and photoelectric absorption are considered as the attenuation processes. The computational model allows two scattering angles per Compton event, and in this form is amenable to closed-form solution. Results are given for several absorbers in the energy range from 25 to 400 keV, and comparisons are made with Monte Carlo calculations. Good agreement is obtained at depths of at least three mean free paths. In all cases, the Monte Carlo calculation required at least 100 times the computation time of the simplified orders-of-scattering technique.
Observations Concerning Radiative Efficiency and Deep-Level Luminescence in n-Type GaAs Prepared by Liquid-Phase Epitaxy
Authors: Kressel H. , Hawrylo F. Z. , Abrahams M. S. , Buiocchi C. J.
Published: February 2004

A study was made of n-type GaAs prepared by liquid-phase epitaxy doped with Si, Ge, Sn, Te, and Se by photoluminescence and Te-doped material by transmission-electron microscopy. A broad emission band centered at 1.2 eV (band B) is observed in LPE materials doped with group VI elements. Band B increases in intensity relative to the bandgap radiation with increasing dopant concentration in the 1018 cm−3 range. It is suggested that the recombination centers responsible for band B are the neutral (VGa+3 Te) complexes postulated by Vieland and Kudman, and that these represent the solid solution of Ga2Te3 in GaAs. With increasing dopant content, the solubility limit is eventually exceeded, and precipitates of this compound are then formed. These have been observed and identified by transmission-electron microscopy. The radiative efficiency falls off sharply with increasing dopant content beyond 2–3×1018 cm−3 in materials doped with Se and Te. It is suggested that this fall off is partly due to nonradiative recombination via small precipitates, each containing on the order of 1000 dopant atoms.
Crystal Structure of Vapor-Deposited Boron Filaments
Authors: Lindquist P. F. , Hammond M. L. , Bragg R. H.
Published: February 2004

The structure of boron filaments, vapor-deposited on a tungsten-wire substrate in the temperature range 870°–1260°C, is studied by x-ray diffraction and transmission-electron microscopy. The typical diffraction pattern observed with both x rays and electrons consists of six or seven broad maxima, or halos. This pattern is interpreted as arising from a microcrystalline (∼30 Å diam) material having an average unit-cell structure intermediate between the α-rhombohedral and ``simple'' tetragonal polymorphs of boron. This interpretation appears to be more satisfactory than others based on an ``amorphous'' or ``faulted fcc'' description, although several discrepancies remain to be explained. Single-crystal spot patterns are also observed in the electron microscope. These arise from inclusions of α- and β-rhombohedral boron, less than 1 μ in diameter, which are distributed throughout the microcrystalline matrix. These inclusions constitute a very small volume fraction of the vapor-deposited boron and are not detected by x-ray diffraction.
Four-Meter Sparks in Air
Authors: Uman M. A. , Orville R. E. , Sletten A. M. , Krider E. P.
Published: February 2004

Sparks of 4-m length in atmospheric air were studied, using high-speed image-converter photography, current and voltage measurements, absolute measurements of radiated light intensity, and high-speed image-converter spectroscopy. Correlated results of the various measurements are presented. The energy balance of the spark is discussed.
The Collision, Coalescence, and Disruption of Water Droplets
Authors: Adam J. R. , Lindblad N. R. , Hendricks C. D.
Published: February 2004

Using free pairs of droplets in the size range from 60 to 500 μ in radius, the effects of charge, velocity, and impact parameter on the collision and the disription of droplets have been examined. The impact parameters for collision and disruption of 300 and 60 μ radius droplets over a charge range from 10−15 to 10−12 C per droplet, and with impact velocity from 1 to 8 m/sec are reported. It is found that for droplets in this size and velocity range, electric charge on the droplets has no appreciable effect on the collision and coalescence process. However, the impact velocity and the impact parameter of the droplets dramatically affect the stability of the coalesced droplet. For 60 μ droplets, all collisions coalesce and form one droplet if the impact velocity is less than 2.2 m/sec. As the impact velocity is increased, the impact parameter for which only one droplet is produced after collision decreases and the all other collisions disrupt into several droplet fragments. For impact velocities greater than 8 m/sec, no collisions are stable.
Thermally Assisted Unpinning of a Dislocation Double Loop
Authors: Lücke K. , Granato A. V. , Teutonico L. J.
Published: February 2004

The effect of thermal fluctuations on dislocation unpinning is computed for a model case of a crystal containing dislocation double loops. The amplitude-dependent decrement as a function of strain amplitude, temperature, and frequency is found to have a maximum. The form of the resulting theory is the same as that for the zero-temperature theory, except that the mechanical breakaway stress of the zero-point theory has to be replaced by a temperature- and frequency-dependent thermal breakaway stress at finite temperatures.
Preparation of ZnO Thin-Film Transducers by Vapor Transport
Authors: Belt Roger F. , Florio Gerald C.
Published: February 2004

Thin films of ZnO were prepared for transducers and microwave acoustic delay lines. Preliminary results are reported for the latter devices. Vapor transport of ZnO in a sealed system was achieved at a source temperature of 700°C, a source-substrate distance of 10 cm, a substrate temperature of 570°C, and 2 mm O2. On (0001) sapphire substrates, x-ray diffraction proved ZnO (0002) planes were oriented parallel to the substrate. Methods were developed for the construction of predominantly longitudinal mode transducers. A one-way conversion loss of 20 dB at 1.2 GHz was measured.