Volume 39, Issue 1, 01 January 1968
Index of content:
39(1968); http://dx.doi.org/10.1063/1.1655712View Description Hide Description
Measurements of total pressure and static pressure were taken in a magnetically driven shock tube at an Alfvén Mach number of M 1≈0.66 in air at an initial pressure range of 50–200 μHg. Total pressure measurements ranged from 0.3–0.8 of the Newtonian pressure based on an equilibrium real gas density ratio. A small separation between the shock and drive current sheet was detected over the range of initial pressures, but was not always repeatable. The static pressure was measured and found to be approximately equal to the drive magnetic pressure.
39(1968); http://dx.doi.org/10.1063/1.1655780View Description Hide Description
39(1968); http://dx.doi.org/10.1063/1.1655787View Description Hide Description
Changes in the form of the electron velocity distribution with time in the active discharge and in the afterglow of a low‐energy nitrogen plasma immersed in a magnetic field, have been investigated. A gated microwaveradiometer was used to monitor the radiationtemperature of the plasma electrons which drops rapidly from 15 000° to 5000°K following termination of the active discharge. The radiationtemperature then remains an order of magnitude higher than that of the neutral gas molecules for a time an order of magnitude greater than the calculated relaxation time of the electron temperature. The magnetic field was swept through a range around the value which produces an electron cyclotron frequency equal to the radiometer frequency. Observation of the radiationtemperature as a function of magnetic field strength shows that, for certain values of the discharge parameters, the electron velocity distribution in the active discharge is Maxwellian. In the afterglow, the form of the distribution function is found to change rapidly.
39(1968); http://dx.doi.org/10.1063/1.1655722View Description Hide Description
An investigation of the extensional mechanical properties of a polyester‐based polyurethane (Texin 480 A Opaque 149004 B) and a polyether‐based polyurethane is described.
Dynamic steady‐state tests were performed over a frequency range of 1000–8000 Hz and a temperature range of 50°–110°F for the polyester material held at a constant elongation of 105%. The experiments on the polyether were conducted at 110% initial elongation over a range of frequency of 200–1600 Hz, and a temperature range of 50°–110°F.
The complex modulus was found for both materials and the reduced‐variable concept applied. Master curves for the moduli at a reference temperature of 510°R (50°F) are presented.
Relaxation tests were performed on the same materials and the relaxation modulus determined. Finally, the relaxation spectrum was determined by using the data from the dynamic and relaxation experiments.
Paramagnetic Resonance Study at Millimeter Wavelengths of an Axial Fe3+ Spectrum in Strontium Titanate39(1968); http://dx.doi.org/10.1063/1.1655737View Description Hide Description
Paramagnetic resonance measurements have been made on an axially symmetric spectrum of Fe3+ (configuration 3d 5) in SrTiO3. The SrTiO3 sample contained a nominal 0.3% Fe impurity. The results were found to be consistent with the assumption that the spin‐Hamiltonian is
39(1968); http://dx.doi.org/10.1063/1.1655746View Description Hide Description
The method of determining the thickness of thin metallic films from measurements of the conductivity and the isothermal Hall voltage is examined with a detail made feasible by implementing the analysis as a portion of a computer program designed for the reduction of the measured data. It is observed that the deviation of the computed data points from the values obtained by direct measurement of the thicknesses of the films, is not due predominantly to errors in the assumed values of the bulk mean free path for electrons. It is demonstrated that errors in the measurement of the geometry of the films will produce the same type of deviation in the results but to a larger extent than does the error in the electronic mean free path.
39(1968); http://dx.doi.org/10.1063/1.1655750View Description Hide Description
Most past studies of mode locking are concerned with conditions near laser threshold. In this paper we use numerical analysis to study large‐signal conditions. Specifically, we restrict ourselves to a traveling wave cavity, a homogeneously broadened line, and a small spatial extent of the medium. The density matrix equations characterizing the medium and Maxwell's equations are solved by computer. It is found for small pump power that mode locking and spike formation occur as previously predicted by the small‐signal theory. As the pump intensity is increased, the spike amplitude increases to a certain critical level and then upon further increasing the pump power, the spike repetition frequency increases discontinuously to twice, three times, four times, etc., its original value. The critical spike amplitudes are not simply related to 180° pulses.
39(1968); http://dx.doi.org/10.1063/1.1655771View Description Hide Description
Permanent axial extension is produced by reversed cyclic straining in tension‐compression at ultrasonic frequency and in torsion at mechanically induced (low) frequency. It is shown that in both ranges the axial extension is a quadratic function of the cyclic strain amplitude, which is characteristic of second‐order effects that exist independently of the frequency of the cyclic strain.
39(1968); http://dx.doi.org/10.1063/1.1655775View Description Hide Description
The shock compression of the ceramics BaTiO3(5%CaTiO3) and Pb(Zr0.95Ti0.05)O3(1 wt%Nb2O5) was measured in the ranges 5–200 kbar and 2–140 kbar, respectively. Bariumtitanate exhibits a two‐wave structure above 30 kbar; the first wave has a velocity of 6.27 mm/μsec. The cusp in the Hugoniot at 30 kbar is interpreted as a dynamic elastic limit. Comparison of the first wavevelocity with the measured longitudinal sound speeds of the tetragonal (ferroelectric) and cubic (paraelectric) phases (5.4 and ∼6.2 mm/μsec, respectively) suggests, as does other evidence, that the material begins to transform to the cubic phase in the neighborhood of 7 kbar. Below 7 kbar, subsonic velocities are observed and it is speculated that this phenomenon is associated with domain reorientation.
The particular lead zirconate composition studied has a two‐wave structure above about 40 kbar; the position of the cusp in the Hugoniot depends sensitively on initial density. This cusp is presumably the Hugoniot elastic limit. A weak cusp is also observed at about 2 kbar. The wavevelocity is essentially sonic below 2 kbar and subsonic above 2 kbar, increasing to about sonic in the neighborhood of 40 kbar.
39(1968); http://dx.doi.org/10.1063/1.1655776View Description Hide Description
The longitudinal optical phonon‐plasmon interaction theory of Varga is shown to be applicable to Gadoped CdS through the region of the first reflectivity minumum (2.5 to about 30 μ). The theoretical reflectivity vs wavelength for several concentrations of Ga‐doped CdS are compared with experiment. The agreement between theory and experiment is particularly good for high dopant concentrations. A plot of the wavelength at minimum reflectivity vs impurity concentrations shows that the Varga theory does well in explaining the data. When phonon damping is introduced into the Varga dielectric function, the reflectivity over the whole region of resonance is more realistic. Finite plasmon lifetime is not considered since no data are available for CdS.
39(1968); http://dx.doi.org/10.1063/1.1655777View Description Hide Description
The use of a line‐source transient‐heat‐transfer technique to determine the thermal conductivities of He, Ne, Ar, and of their mixtures is reported along with results. The method employs existing rigorous analytical theory for a finite line source possessing heat capacity. The experimental and analytical techniques minimize the effects of instrument size, radiation, free convection, and thermal separation of the mixtures. The linesource diameter was 0.00763 cm with a central 0.00203‐cm nickel temperature sensing coil to provide temperature data. Experimental times of 270 msec or less and total temperature rises of less than 10°C were used to minimize radiation, free convection, and thermal separation effects. The data show excellent agreement with existing data and an internal consistency and accuracy believed to demonstrate elimination of undesirable effects upon over‐all thermal‐conductivity determinations.
39(1968); http://dx.doi.org/10.1063/1.1655778View Description Hide Description
Magnetic behavior of antiferromagneticnickel oxide single crystals, both in normal and reduced state, has been studied by magnetic susceptibility,anisotropy in susceptibility, and torque measurements. The observed magnetic behavior is interpreted in terms of T‐ and S‐type domains, which are normally present in a nickel oxide single crystal.
39(1968); http://dx.doi.org/10.1063/1.1655779View Description Hide Description
Diodes of 50% alloy of GaAs: GaP have been studied in which the material has been made semi‐insulating by compensation with iron. An electrode of evaporated gold on this material is blocking for electrons.Electrons may be injected from a base wafer of n‐type GaAs. The material is slightly p‐type due to ionization of deep acceptor levels. When electrons are injected there is a space‐charge‐limited current in which the electron concentration is controlled by a small concentration of holes which are present. Analysis of the data gives estimates of the concentration and capture cross section for recombination centers. Using a blocking contact, impact ionization multiplication of carriers injected by light is observed. The impact ionization coefficient is measured.
39(1968); http://dx.doi.org/10.1063/1.1655781View Description Hide Description
Photoelectric delayed coincidence measurements of an optical field with a time‐to‐amplitude converter are now becoming common, and yield results which are related to certain correlation functions of the field. The equations governing the measurements are obtained and some computational methods are developed which allow the correlations to be derived from measured data. The methods are illustrated by an example.
39(1968); http://dx.doi.org/10.1063/1.1655782View Description Hide Description
In the region in which no phase transition occurs, the temperature variations of some properties (e.g., density, viscosity, sound velocity,specific heat,thermal conductivity) of a simple substance appear to be tied together in a remarkable way: the ratios of their values corresponding to any pair of temperatures of the region are all uniquely fixed when any two of them are given. This assumption implies a simple correlation of three such properties at four temperatures. It represents experimental data almost if not quite in the limits of experimental errors, so that it permits extrapolation, e.g., the anomalous behavior of sound velocity and specific heat of water with a maximum deviation of 0.02% and 0.04%, respectively. Similar extrapolations for gaseous CO2 and solid Fe are also in excellent agreement with experimental data.
Dielectric Properties of Solid Solutions of BiFeO3 with Pb(Ti, Zr)O3 at High Temperature and High Frequency39(1968); http://dx.doi.org/10.1063/1.1655783View Description Hide Description
Solid solutions of BiFeO3 with PbTiO3, PbTi0.5Zr0.5O3, and PbZrO3 were prepared. The crystallographic data on these solutions, which are basically perovskitic, are given. The dielectric constants of the materials were determined at a frequency of 0.53 GHz and at temperatures up to 800°C. DielectricCurie points were found in solutions containing up to 90 mole % BiFeO3. These results leave little doubt that BiFeO3 is ferroelectric or antiferroelectric. The extrapolated Curie point for BiFeO3 is above 850°C. BiFeO3 appears more likely to be ferroelectric than antiferroelectric, but the distinction between the two classifications may not be sharp.
39(1968); http://dx.doi.org/10.1063/1.1655784View Description Hide Description
Hall measurements have been made on epitaxially grown, zinc‐doped gallium phosphide, from which the gallium arsenide substrate had been removed. The measurements were made in the temperature range from 300° to 4°K on samples whose room‐temperature carrier concentration varied from 7×1015 to 7×1018 cm−3. Using both van der Pauw and conventional Hall techniques on those samples which show a high degree of compensation, the variation of the ionization energy of zinc with concentration was determined. Room‐temperature hole mobility varied from 60 to 100 cm2· V−1· sec−1. These observed hole mobilities are in good agreement with the calculated values of mobility due to reduced scattering by acoustical and optical phonons, and are shown to be too small to be accounted for by the mechanism of impurity scattering below 100°K. Resistivity measurements made in the temperature range 77°–4.2°K show that in the impurity band conduction range, gallium phosphide behaves in a manner similar to impure germanium.
39(1968); http://dx.doi.org/10.1063/1.1655785View Description Hide Description
Continuous spectra of resonances are observed when some ferromagnetic substances are examined with cw or spin‐echo rf spectrometers. These resonances have previously been assigned to domain‐wall motion; evidence is presented here to show rather that they are magnetically excited acoustic vibrations. An explanation of the manner in which a nonlinear magnetoacoustic interaction allows the production of echoes in an otherwise linear system is presented. A means of suppressing these resonances to facilitate the study of nuclear magnetic resonances in magnetic materials is demonstrated.
39(1968); http://dx.doi.org/10.1063/1.1655786View Description Hide Description
The equation of motion for a domain wall in a canted, orthorhombic antiferromagnet has been solved. The analysis shows that the mobilities for Bloch and Néel walls are nearly alike for small applied fields in materials like YFeO3 and that a great similarity exists with wall motion in ordinary magnetic materials. The main difference between the present and previous expressions for wall velocity is in the functional dependence for the value of the applied field above which deviations from the simple low‐field solution occur. However, this field turns out for YFeO3 to be numerically about the same that is obtained by considering the YFeO3 to be simply a weak ferromagnet.
39(1968); http://dx.doi.org/10.1063/1.1655788View Description Hide Description
A diffusion process has been developed for the preparation of Nb 3Sn ribbon which can subsequently be stabilized to any desired level. The preparation process is described in detail with emphasis on the effect of reaction time, reaction temperature, and contamination on the critical current of the Nb 3Sn. Under optimum conditions, short samples of the ribbon showed a current density of approx 105 A/cm2 in a field of 100 000 G.
The superconducting ribbon, after the diffusion reaction, is partially stabilized by a thin layer of tin which remains on the surface. To further stabilize the ribbon to any desired level, another metal can be bonded to the Nb 3Sn through this layer of tin.
A small copper‐stabilized test magnet was constructed to determine the degree of degradation occurring when the ribbon is wound into magnet. This magnet was found to have approximately the same current‐carrying capacity as short samples of the ribbon.