Index of content:
Volume 37, Issue 6, 01 May 1966
37(1966); http://dx.doi.org/10.1063/1.1708788View Description Hide Description
A theoreticaltreatment of the Hall voltage developed in a gaseous plasma immersed in a uniform magnetic field is presented. The plasma considered is collision dominated and is confined by parallel planes with insulating walls. The Hall voltage is found to depend on electron‐ion volume recombination and on the tensor character of the electron mobility. The cases to be discussed are: (i) negligible diffusion, (ii) negligible electron‐ion volume recombination, and (iii) diffusion with small but not negligible volume recombination.
37(1966); http://dx.doi.org/10.1063/1.1708789View Description Hide Description
A general method suitable for computerized solution is described for finding the electric and/or current density fields due to time‐invariant sources. A charge distribution satisfying the boundary conditions is specified as the solution of an integral equation. The latter is approximated as a system of linear algebraic equations and solved by the computer. The field at any point is then obtained by Coulomb's law and superposition from the boundary charge distribution and any impressed fields that exist. The method will, within the practical restrictions imposed by the computer's capacity to solve simultaneous equations, solve any inhomogeneous, linear, isotropic, steady current flow and/or static field problem with any of the various boundary conditions which uniquely define a field. While the derivation is given here in terms of electric field quantities, it applies as well to the several analogous fields of engineering and physics. In addition to the theoretical formulation, data are given to guide in the selection of the most efficient and accurate formulas with which to approximate the integral equation. Several examples and results are presented together with data bearing on questions of accuracy and convergence. Two example results are combined to derive a quasi‐empirical formula for the capacitance of widely spaced square parallel plates.
Effect of Thermal‐Neutron Irradiation on the Superconducting Properties of Nb3Al and V3Si Doped with Fissionable Impurities37(1966); http://dx.doi.org/10.1063/1.1708790View Description Hide Description
The superconducting critical temperatures and current carrying capacities of Nb3Al and V3Si doped with small amounts of uranium and boron and subsequently irradiated with thermal neutrons are reported. While the critical temperatures are substantially unaffected, the critical current densities are dramatically increased by the internal fission of uranium in both materials. Samples of Nb3Al containing 0.321 at.% uranium and V3Si containing 0.19 at.% uraniumirradiated with 1.7×1018 neutrons/cm2 give superconductingcritical current densities over 106 A/cm2 at 30 kOe. The boron‐doped samples, however, showed no effects arising from fission of boron. Two new experimental techniques have been used in this investigation. First, uranium and boron analyses as well as information on homogeneity were determined by use of the Price‐Walker nuclear track detectors. Secondly, the critical currentmeasurements were made on 50‐mg samples of 70‐μ powders by a technique that measures the generation of odd‐harmonic voltages caused by small alternating magnetic fields applied to the sample.
37(1966); http://dx.doi.org/10.1063/1.1708791View Description Hide Description
It has long been known that transmission electron‐diffraction patterns appropriate to low‐index zone axes in thin crystals exist over several degrees of tilt from the exact orientations. In this work the magnitude of the errors in determining crystal orientation from spot patterns has been investigated using precisely oriented crystals prepared by vapor deposition and an electron microscope equipped with a goniometer specimen stage. It was observed that patterns of low‐index orientations existed over ranges as high as 20° tilt, and were insensitive to the size of the area diffracted. Tilt ranges were observed to increase noticeably when the foils were plastically deformed. However, orientations accurate to 0.5° may be determined by weighting the pattern according to the intensities of the diffraction spots.
Elastic bending of the foils and ``polygonization'' bending due to dislocations help to explain why the relrods are so much ``longer'' than predicted on the basis of relaxation of the Laue conditions, using kinematictheory.
37(1966); http://dx.doi.org/10.1063/1.1708792View Description Hide Description
From x‐ray diffraction intensity measurements it is possible to evaluate strain gradient in a single crystal. Transmission diffraction techniques were applied to investigate strain in the film evaporated on a quartz plate. Topographs showed qualitatively the locations of high strain gradient. Quantitative measurements were made through the thickness of the plate with a narrow well‐collimated x‐ray beam and a scintillation counter. It was found that the strain gradient was high at the film edge on the surface of the crystal plate.
37(1966); http://dx.doi.org/10.1063/1.1708793View Description Hide Description
The nonuniform emission of thermionic cathodes has been studied with an anode‐hole scanning technique by means of which ``effective'' work‐function distributions could be obtained. Using these ``effective'' work‐function distributions, information has been gained about the fraction of the total cathode surface that actually determines the saturated emission. The knowledge of this fraction facilitates the physical interpretation of the emission constant AR , appearing when the experimental Richardson formula is used.
It is proved that different types of practical cathodes show marked differences in emission uniformity under space charge as well as under saturated emission conditions and as a function of their lifetime.
At several hundred hours of life, approximated values of the fraction of the surface area that determines the saturated emission and of its work function were found to be 0.35 and 1.81 V at 1350°K for an Lcathode, 0.18 and 1.72 V at 1350°K for an impregnated cathode, 0.067 and 1.26 V at 1025°K for a (BaSr)O cathode.
37(1966); http://dx.doi.org/10.1063/1.1708794View Description Hide Description
Electrical conductivity and thermoelectricmeasurements of the intermediate phases in the Er‐Te system over the temperature range 170°K to 850°K show that in the intrinsic region the concentration of erbium atoms in the solid solution series Er2Te3‐ErTe is directly related to the number of carriers present. The erbium atoms act as impurity donors in the extrinsic region as their concentration increases from the terminal composition Er2Te3, which is p type, to ErTe which is n type.
37(1966); http://dx.doi.org/10.1063/1.1708795View Description Hide Description
It has been found that an electric field applied in the plane of a metal film during deposition induces coalescence of the three‐dimensional island‐like structure at an earlier stage of the film growth than is obtained without the field. The surface area of the three dimensional islands is increased under applied electric field. The critical thickness, at which the electrically continuous film is obtained, decreases with the increase of electric field. The decrease is more rapid at higher substrate temperatures. Enhanced orientation effects and fewer structural defects have been observed in films deposited under applied field and are attributed to the recrystallization during induced coalescence.
The observed short range order in the coalescence pattern indicates that the applied field transfers and redistributes the electrostatic charges on the islands. It is shown that the presence of a small number of electrostatic charges increases the surface energy and thus the surface area considerably. The electrostatic forces between two islands which either come very close to each other or carry significantly different charges of either polarity are predominantly attractive and are of sufficient magnitude to account for the physical merger. Coalescence as well as breakup of metal islands due to the presence of electrostatic charges on them have been observed on suitably prepared single‐crystal lead films irradiated with the imaging electron beam inside an electron microscope.
37(1966); http://dx.doi.org/10.1063/1.1708796View Description Hide Description
A short (5 mm) helium arc between a ⅜‐in.‐diam thoriated tungstencathode and a 1‐in.‐diam copperanode was examined spectroscopically. Electron densities were determined from the Inglis‐Teller relation for a range of gas pressures and arc currents from 500 Torr, 50 A to 5000 Torr, 250 A. At 850 Torr, 100 A, the electron density was 1.2(+2.1/−0.2)×1016 per cm3. At 850 Torr, 100 A (2.4‐kW power level) the atomic excitationtemperature was measured using a line intensity ratio technique. Self‐absorption corrections were determined experimentally using a tandem arc arrangement and were found to be significant. A numerical computational technique was solved for the spatial dependence of emission and absorption of the HeI 23 S‐N 3 P series. The state populations followed a Boltzmann distribution with an excitation temperature of 4800°K. A best straight line fit to the data was obtained with spontaneous decay coefficients computed by Bates and Damgaard and by Schiff and Perkeris. This excitation temperature is too low to account for the observed electron density assuming equilibrium so that local thermodynamic equilibrium (LTE) does not exist in the arc at this power level.
Vacuum Thermionic Work Functions of Polycrystalline Be, Ti, Cr, Fe, Ni, Cu, Pt, and Type 304 Stainless Steel37(1966); http://dx.doi.org/10.1063/1.1708797View Description Hide Description
Experimentally measured effective vacuum thermionic electron work functions are reported for well outgassed polycrystallinesurfaces of beryllium (3.67 eV), titanium (3.6 to 4.3 eV), chromium (3.90 eV), iron (4.47 eV), nickel (4.41 eV), copper (4.42 eV), platinum (5.6 to 5.8 eV), strongly (210) oriented, and 304 stainless steel (4.3 eV) in a clean, 10−9‐Torr vacuum environment. Special techniques were employed to insure accurate current, area, and temperature values. Purities, heat treatment histories, outgassing conditions, and x‐ray crystallography measurements of the surfaces are included.
37(1966); http://dx.doi.org/10.1063/1.1708798View Description Hide Description
The temperature dependence of the electron lifetime and of the time for decay of an internal applied electric field in the silver bromide microcrystals of a simple photographic emulsion has been measured by the technique employing flash exposure of the emulsion in pulsed electric fields.Activation energies were found to be 0.236±0.012 eV for the electron lifetime and 0.245±0.008 for the field‐decay time. The agreement of the temperature dependence of the two processes, together with the observation that the room‐temperature values of these two times are, without exception, nearly equal in measurements made on a variety of other emulsions, supports the hypothesis that the mechanism of permanent electron‐trapping involves the motion of the silver ion to a temporarily trapped electron to form a stable center. These results are not consistent with the alternative suggestion, namely, that the electron is captured at a preexisting deep trap consisting of a silver ion associated with a crystal imperfection.
37(1966); http://dx.doi.org/10.1063/1.1708800View Description Hide Description
A technique for experimentally determining the hydrodynamicparticle velocity in gaseous detonations is described. The technique is based on the magnetohydrodynamic interaction of the ionized reactants and products with an externally applied magnetic field.Particle velocities in 2H2+O2 and 2H2+2O2 detonations were found to be 1.3 mm/μsec and 0.96 mm/μsec, which are in good agreement with those calculated from detonation rate and pressure.
37(1966); http://dx.doi.org/10.1063/1.1708801View Description Hide Description
The coupled electron‐hole cascade in an electron gas is described by a space‐ and angle‐independent transport equation. Analytic cross sections employed for the various processes are appropriate to a high density electron gas and for low to moderate excitation energies. The solution of the integral transport equation may be expressed in analytic form. It is found that the cascade process results in the accumulation of extremely large numbers of electrons and holes close to the Fermi surface.
37(1966); http://dx.doi.org/10.1063/1.1708802View Description Hide Description
Changes in the cavityQ of GaAs lasers, aside from changing the threshold, cause large changes in the laser wavelength. The wavelength changes by about ±0.6 percent when the threshold is varied by a factor of two to three by changing the surfacereflectivity. For comparable fractional changes in Q (or threshold) the wavelength changes are larger at 300°K than at lower temperatures. The wavelength dependence on Q is specific to semiconductor lasers and is based on the continuous nature of the energy levels involved in stimulated emission. The wavelength shifts are theoretically related to the changes in Q using the ``band‐filling'' model and an expotential density in ``tail'' states. Fair agreement with the data is obtained at low temperature as well as at 300°K. The wavelength shift with Q in conjunction with changes in the shape of the threshold vs temperature curve, leads to a correction to the usual relation between threshold and losses. The wavelength dependence on Q can be used for frequency control and possibly for frequency modulation.
37(1966); http://dx.doi.org/10.1063/1.1708803View Description Hide Description
Experimental success in reflecting and focusing explosive waves and thus fracturing the neighborhood of a focus in a prolate spheroid is obtained. The shape and size of the internal fracture is essentially spherical whose volume ranges from a pinpoint to several millimeters in diameter. On the basis of a simple consideration of the transmission of elastic strain energy, corresponding sizes of fracture can be analyzed and estimated. Comparison of the theoretical results with experimental data indicates that the strain energy transmitted is directly proportional to the amount of explosive charge used.
37(1966); http://dx.doi.org/10.1063/1.1708804View Description Hide Description
If intense radiation (coherent light, secondary ionization, or sustained fission) is put into a solid in a time δ short compared with the elasticrelaxation time, a resulting internal ``explosion'' may cause multiple fracturing and other damage. We postulate that at t=δ≃0, the radiation causes a uniform dilation within a cylindrical region of radius a. Solution of the relevant wave equation (continuity preserved at r=a) indicates that a rarefaction wave of increasing amplitude moves inward from a with velocity v during the time interval, 0<t<a/v. The transient pressure variation at the center demonstrates a negative divergence at t=a/v. The pressure field at various times has been found by numerical integration. This model accounts for some features of the damage caused by laser beams and other damage processes.
37(1966); http://dx.doi.org/10.1063/1.1708805View Description Hide Description
Photographs and descriptions of the phenomena are given. Most of the observations were of magnesium oxide, a few were of spinel and corundum. The fringe systems arising from a blister (two films on a substrate) are described. Results are given for miscellaneous H+, D+, He+, and He++ bombardments, 0.025–4 MeV. Measurements are given of blister heights, diameters, shell thicknesses, and depths of cavities left after blister shells break away (exfoliate). Effects arising from superficial deposited films are described. Blistering was found on polycrystallinecopper bombarded with H+ or He+ and on polycrystallinenickel with He+ only. Blistering was not found on several other metals bombarded similarly. For the oxides, blistering was first noted at ∼3×1017, while for the metals at ∼8×1017. The effects of gas permeability, radiation expansion, and elastic moduli on blister growth are discussed. The dispersion of refractive index of magnesium oxide is given.
37(1966); http://dx.doi.org/10.1063/1.1708806View Description Hide Description
The activation field α of bariumtitanatesingle crystals has been measured over a range of thicknesses from 350 μ to 4 μ using liquid electrodes. From these measurements it has been deduced that bariumtitanate has a surface layer of layers with a total thickness of 25±5 μ. The activation field of the bulk material is 5 kV/cm and that of the surface layer is 15±5 kV/cm.
37(1966); http://dx.doi.org/10.1063/1.1708807View Description Hide Description
Low‐energy electron diffraction was used to study the cleaved surfaces of PbS,PbSe, and PbTe for the purpose of obtaining information about the factors that influence the intensities of the diffracted beams. The addition of the successively heavier anion in the three compounds produced no significant changes in the intensities that could be attributed to the atomic scattering factors of the anions. Prominent features in the dependence of the intensity of the diffracted beams on the energy of the incident electrons are attributed to multiple scattering. The structure of the surfaces in two dimensions parallel to the surface is the same as the (001) substrate planes. A complete analysis of structure normal to the surface is prevented by strong multiple scattering. Single scattering events are identified at low energies by measurements on the angular dependence of the intensities.