Volume 34, Issue 3, 01 March 1963
Index of content:
34(1963); http://dx.doi.org/10.1063/1.1729291View Description Hide Description
Impurity diffusion of Na ions along dislocations in LiF has been studied by both dc and transient techniques. The Na impurity is injected into the LiF by plating NaCl on one surface of the LiF and placing a potential across the crystal at an elevated temperature. Large‐scale diffusion with Na dendrite formation is observable in a dc arrangement and the diffusion can be measured by either radioactive tracer methods or measurement of charge transport. Transient experiments yield an activation energy for diffusion of approximately 0.3 eV for both edge and screw dislocations, and D 0=0.01 cm2/sec. Analysis of the charge effects in the core of the dislocationsleads one to propose a very large vacancy density in the core, and the experiments are even best understood in terms of a hollow of atomic dimensions along the dislocation core.
34(1963); http://dx.doi.org/10.1063/1.1729292View Description Hide Description
Recent interest in the application of infrared and optical masers (lasers) for range measurement, surveillance, welding, and drilling has focused attention on the problem of obtaining high peak‐power pulses of short duration and fast risetime from these quantum devices. Kerr cells, Pockel cells, rotating disks, and mirrors have been used as shutters in the past with varying degrees of success. In this paper, experiments are described that demonstrate an ultrasonic shutter suitable for obtaining giant pulses from a ruby optical maser by utilizing the refraction resulting from the passage of a plane‐parallel light beam through an ultrasonic field whose wavelength is much larger than the width of the light beam. Amplitude, risetime, and pulse duration are comparable with the values reported utilizing a Kerr cell and superior to those reported for rotating mechanical shutters. In addition, the ultrasonic shutter can be utilized to synchronize the usually random output pulses of a ruby optical maser with the ultrasonic frequency.
34(1963); http://dx.doi.org/10.1063/1.1729293View Description Hide Description
The Nernst coefficient, the Hall coefficient, the electrical resistivity, and the Seebeck coefficient have been measured on Te‐ and Se‐doped GaSb between 60° and 800°K. The two minima conduction bandmodel, as applied to the present group of measurements, provides a qualitative description of conditions under which certain scattering mechanisms will dominate the transport phenomena.
34(1963); http://dx.doi.org/10.1063/1.1729294View Description Hide Description
Various factors entering into the continuous operation of optically pumped solid‐state lasers are summarized. Numerical estimates for ruby and for Nd+++‐ activated materials in two types of optical systems suggest that several watts of output power should, ideally, be obtainable when pumping with a 1‐kW Hg arc lamp.
34(1963); http://dx.doi.org/10.1063/1.1729295View Description Hide Description
Saturationmagnetizations of V‐Fe and Cr‐Fe alloys were measured and the results used to determine the average atomic magnetic moment. The linear rate of change of the moment with solute concentration, dp̄B /dC, was found to be −3.286 Bohr magnetons per atom in V‐Fe α‐phase alloys. Transformation of an approximately equiatomic alloy to the CsCl‐ordered α′‐phase resulted in a decrease of about 0.4 Bohr magneton in the moment. Subsequent transformation to the σ‐phase lowered it by a factor of three. In Cr‐Fe solid solution alloys, dp̄B /dC was found to be −2.36 Bohr magnetons per atom.
34(1963); http://dx.doi.org/10.1063/1.1729296View Description Hide Description
Electron microscopical observations of glide dislocations lying in the basal plane of the layer structure GaSe are described. The dislocations are dissociated into partials separated by stacking fault ribbons as wide as 1 μ. Network‐like grids formed from sets of ribbons lying in different glide planes were observed and are analysed in detail. A discussion is given of the contrast effects produced by dislocation dipoles contained in the grids.
34(1963); http://dx.doi.org/10.1063/1.1729297View Description Hide Description
This paper presents a coupled mode formalism which predicts the effects of rf magnetic fields on the propagating modes of the plasma loaded waveguide. In addition, this formalism provides a simple physical interpretation of the propagation characteristics in terms of the uncoupled modes, which are taken to be the waveguide modes and quasi‐static plasma modes, respectively. The method of obtaining coupled mode equations is given and two typical calculations using the theory are discussed.
34(1963); http://dx.doi.org/10.1063/1.1729298View Description Hide Description
The coefficient of heat transfer between a smooth vertical silversurface and liquid helium is typically 0.020±0.005 W cm−2 °K−1 for convection and increases in the pool boiling range to 0.22 W cm−2 °K−1 for 0.040 °K superheat at 4.0°K. The heat transfer rate for pool boiling decreases with decreasing temperature between 4.0° and 2.5°K in proportion to the latent heat of vaporization and inversely as the molar volume of the vapor at the pressure and temperature of the experiment. The superheat in the steady state is determined by the volume rate of gas production at the surface. The hysteretic nature of the heat transfer curves in the boiling range suggests a nucleation mechanism strongly dependent on the thermal history of the specimen.
34(1963); http://dx.doi.org/10.1063/1.1729299View Description Hide Description
The frequency stability of a circularly polarized solid‐state paramagneticmaseroscillator has been observed with the objective of obtaining the information needed to construct a frequency standard with a broad microwavespectral line. The beat frequency between two such masers by using ruby can be made almost free from the variation of the static magnetic field and of the angle between the crystalline axes and the static magnetic field. The stability of the 1.5‐Mc beat frequency is approximately 10−6 for short time intervals, and is approximately 10−5 for long time intervals. The short‐time stability is limited mainly by the fluctuation of the cavity resonant frequency in the liquid helium. The long‐time stability is determined by the drift of the pump frequency, although the reason for this interrelation is not thoroughly understood. The effect of reflections from the load on frequency is considerably reduced by the unidirectional structure used here, as compared with that to be expected with a reflection type of maser. We also show that the unidirectional maseramplifier can be built with reduced cross section by utilizing dielectric‐loaded circular waveguide.
34(1963); http://dx.doi.org/10.1063/1.1729300View Description Hide Description
The shape of metal‐insulator‐metal potential barriers is derived, including the effects of space charge. For very thin insulating films, the barrier is shown to be essentially rectangular (or trapezoidal if different metals are used on either side of the insulating film). The modification in the rectangular barrier introduced by image forces is derived and shown to be significant for very thin insulating films. Finally, the effect of insulatordoping on barrier shape is considered.
34(1963); http://dx.doi.org/10.1063/1.1729301View Description Hide Description
The scattering of plane compressional waves by a spherical obstacle in an elastic solid, which was investigated by Ying and Truell is examined further. For a rigid inclusion, the boundary conditions are redefined to take into consideration the motion of the inclusion inside the solid. By a proper limiting process, it is shown that the solutions for a rigid insert, a fluid sphere, a cavity, or an obstacle in a fluid are all derivable from the general results of an elastic inclusion. The rates of energy scattering due to a small rigid obstacle (a«λ) are found to be inversely proportional to the fourth power of wavelength.
34(1963); http://dx.doi.org/10.1063/1.1729302View Description Hide Description
As pulsed light sources, exploding wires can be used to provide intense narrow spectral lines as well as the characteristic continuum of very high temperature blackbodies. The surface brightness depends on the wire material and conditions under which the wire is exploded. Optimum conditions for 100–500‐μsec long light pulses from wires exploded in air have been found for tungsten,tantalum, and molybdenumwires 0.005 to 0.010 in. in diameter for energy inputs of 3000 J or more. Wires exploded in small diameter vacuum tubes (1–20 μ of Hg) radiate as blackbodies, but narrow line spectra are produced in enclosures of larger dimensions. In the visible and ultraviolet regions of the spectrum the spectral radiance of air‐exploded wires is one to three orders of magnitude greater than that of conventional flash lamps. Vacuum‐exploded wires have spectral radiance another factor of eight greater. Excitation of various optical masers with exploding wires is discussed.
34(1963); http://dx.doi.org/10.1063/1.1729303View Description Hide Description
A self‐consistent, semiclassical treatment is given for the acoustoelectric effect based on the Cohen‐Harrison‐Harrison formalism for the ultrasonic attenuation in metals. The calculation is carried out for a simple model in which a sound wave feeds dc crystal momentum into the quasi‐free electron system by exerting an oscillating Lorentz force and a deformation‐potential force. Emphasis is placed upon the feed back of crystal momentum into the sound wave due to the collision‐drag effect and upon the magnetic‐field dependence of the acoustoelectric effect. A general relation (the Weinreich formula) between the attenuation of sound and the rate of increase of dc crystal momentum of electrons holds for each interaction of electrons with sound waves. In principle, all characteristic features of the magnetoattenuation of sound such as geometric resonances,cyclotron resonances, etc. can be expected in the acoustoelectric effect. The possibility of this experimental detection is also discussed.
34(1963); http://dx.doi.org/10.1063/1.1729304View Description Hide Description
The experimental behavior of electron beams of variable energies injected into an axially symmetric magnetic field which varies as 1/r is investigated. When the injection velocity is less than that required for circular motion, the beam spirals in toward an inner radius and reverses direction. The particle orbits are in agreement with those obtained from the equation of motion of a single charged particle injected into such a field.
34(1963); http://dx.doi.org/10.1063/1.1729305View Description Hide Description
Two experiments pertaining to oxide‐coated cathodes are described. One of these involves the drift of activators in oxide‐coated cathodes under the influence of an electric field and the other involves the diffusion of these activators in the absence of an electric field. The drift experiment revealed a temporal nonuniform increase in the activation of the conductivity. However, it failed to show any significant spatial nonuniformity. Pulse measurements of the current‐voltage characteristics showed the origin of the temporal nonuniformity to be associated with nonohmic properties of the coating which were related to the Nergaard effect. The results of the diffusion experiment indicated a diffusion coefficient at 1000°K for the crystalline defects undergoing diffusion of (1.5±0.8)×10−7 cm2/sec and an activation energy of the diffusion of 0.67±0.4 eV. Comparison of this diffusion coefficient with data obtained from the literature makes possible the identification of the mobile defects involved in activation processes in oxide‐coated cathodes as being bariumvacancies. On the bases of these results, a ``mobile acceptor model'' for oxide‐coated cathodes is presented which suggests that activation is achieved by the removal of the highly mobile acceptors (bariumvacancies) which compensate the relatively immobile donors (oxygen vacancies) and not by an increase in the density of the donors as has usually been assumed. Nergaard's theory explaining the millisecond decay of thermionic emission from oxide‐coated cathodes is modified in terms of the mobile acceptor model, and the kinetic processes involved in the activation of oxide‐coated cathodes are discussed.
34(1963); http://dx.doi.org/10.1063/1.1729306View Description Hide Description
The interaction of a prebunched beam with an anisotropic medium is considered for the generation of coherent electromagnetic energy in the low and submillimeter region. Generalized expressions are obtained for the fields created by a prebunched beam of general shape in an anistropic medium. The permeability is assumed to be a scalar constant whereas the permittivity is tensorial. Two special cases are considered: one is that of a helical prebunched beam and the other is that of a rectilinear beam, both interacting with gyrotropic gaseous plasmas. Ionization, scattering, and rf field effects on the beam are neglected.
In addition to the solutions of the inhomogeneous vector wave equation, a method is suggested whereby the free mode solutions could be directly deduced.
34(1963); http://dx.doi.org/10.1063/1.1729307View Description Hide Description
After a brief survey of methods of representing the transient response of a distributed linear system, a general analysis is presented of the temperature dependence of relaxation times of a thermally activated system. It is assumed that either the pre‐exponential factor τ d (or its inverse, the vibrational frequency vd ) or the activation energyE may be separately distributed or that they both depend linearly on a structure factor S and so are similarly distributed. Further, τ d , E, and parameters of their distributions are taken temperature independent. A two‐parameter, generalized, truncated exponential probability density function is chosen for the distribution of S and transformed to yield a power‐law distribution for G(τ), the over‐all relaxation‐time distribution function. Expressions yielding the time and temperature response of the system are then derived from G(τ). The specific transient response considered is that following the imposition of a constant forcing function at t=0 and may represent the charge and current response of a dielectric system after application or removal of a constant voltage, the strain and rate‐of‐strain response of a mechanical system after constant stress is applied or removed, or the stress and rate‐of‐stress response on application and maintenance of a constant strain. Representative results are found to agree quantitatively or qualitatively with available transient and temperature responses for a wide variety of materials under electrical or mechanical stimulation. In particular, the current transient response or stress relaxation response is constant for measuring times less than the shortest relaxation time of the system, then exhibits one or two regions with t −(1+ρ i ) behavior, and, finally, decreases very rapidly when the measuring time exceeds the longest time constant of the system. The theory leads to specific possibilities for the temperature behavior of ρ i and to reasons why it is usually found to be less than unity in magnitude. Finally, conditions are analyzed for which the time‐temperature superposition law of rheology applies.
34(1963); http://dx.doi.org/10.1063/1.1729308View Description Hide Description
Radiation patterns obtained with He–Ne masers employing mirrors external to the discharge region are described. Operation was obtained with discharge tubes having Brewster angle windows, windows perpendicular to the optical axis, two spherical, one flat and one spherical, and two flat mirrors. Mode selection by using obstacles interposed in the cavity is described. It is shown that the radiation pattern is established within a very small region of the cavity. Comparisons of lasers operating with external and internal mirrors show that the lack of azimuthal symmetry of the radiation patterns is intrinsic to the spherical mirror geometry and not to the Brewster angle windows.
Optical Properties of Powders. Part I. Optical Absorption Coefficients and the Absolute Value of the Diffuse Reflectance. Part II. Properties of Luminescent Powders34(1963); http://dx.doi.org/10.1063/1.1729309View Description Hide Description
The optical properties of powders are determined for the case of particles, large compared with the wave‐length of light. The solution for the diffusion of light in a quasi‐infinitely thick powder is applied to relate certain measurements obtained on powders with the corresponding properties of the bulk material.
In Part I, the diffuse reflectance R of a powder is first derived in general form in terms of the parameters of the individual particle, taking into account both internal and external scattering. Parameters are then obtained for a model for randomly shaped particles, and used to relate the diffuse reflectance to the optical absorption coefficient, the index of refraction, and the particle diameter. The relationship is compared with measurements on materials of different refractive indices.
Part II deals with applications to luminescent powders, the calculation of the characteristics of emission in the presence of optical absorption, and the calculation of cascade transfer yields.
34(1963); http://dx.doi.org/10.1063/1.1729310View Description Hide Description
Two kinds of mounds or hillocks have been found on BaTiO3single crystals after etching. Rounded, flat‐topped hillocks were found after etching with 1% HF for 30 sec at room temperature. These hillocks were found where a 180° domain wall had been left for several days and had been removed shortly before etching. Their behavior can be explained by the ad hoc assumption that negative charges are attracted to the 180° domain wall at the crystal surface.
Mounds shaped like Egyptian pyramids were found after etching BaTiO3single crystals for 30 min in H3PO4 at 155°C. The edges between pyramidal faces always project onto the crystal surface (a 100 plane) in 110 directions.