Volume 41, Issue 8, 01 July 1970

Dislocation Cell Formation in Metals
View Description Hide DescriptionIt is shown by an analysis similar to that for the spinodal decomposition of a supersaturated solution that an array of dislocations, modelled by parallel screw dislocations, of uniform density, is unstable; the dislocations move to form a structure having a modulated dislocation density. It is suggested that the instability grows ultimately into a dislocation cell structure and that the cell size is given by the dominant wavelength of the density modulation. This wavelength λ_{ m } is found to be proportional to ρ^{−1/2} and furthermore the wavelength is given by λ_{ m } ≈ K_{c} ·ρ^{−1/2}=r_{c} , where K_{c} is a constant, ρ is the dislocation density and r_{c} is defined as a dislocation‐dislocation interaction distance. Data in the literature relating to cell size are shown to support this result. Restrictions on the applicability of the analysis are discussed.

Diffusion of Copper in Nickel Single Crystals
View Description Hide DescriptionThe lattice diffusion coefficient of copper in high‐purity nickel single crystals is measured between 775° and 1050°C. The diffusion coefficient satisfies the equation D=0.27 exp(−61.02/RT) cm^{2} sec^{−1}, with an error in D _{0} of ±22% and in Q of ±1%. A comparison with the results of other authors is given, showing that the dc measured is considerably lower than in previous investigations. This effect is referred to grain‐boundary diffusion and chemical impurities in the specimens used earlier.

Diffusion in Wide Grain Boundaries
View Description Hide DescriptionThe rigorous solution of the grain‐boundary diffusion problem has been approximated by a series expansion method. The calculations show that higher‐order terms may be neglected in the bulk adjacent to the grain boundary. Thus, in this region Whipple's and Suzuoka's solutions represent a close approximation to the problem. Inside the grain boundary, however, higher‐order approximations have to be taken into account. These approximations gain importance in the case of wide grain boundaries. The solutions obtained for an instantaneous source have been fitted to available grain‐boundary diffusion data of Ni^{2+} in MgO at 1200°C. Numerical calculations give for the bulk diffusion coefficient D=2.9×10^{−12} cm^{2} sec^{−1}, the ratio of diffusion coefficient Δ=1.5 and for the grain‐boundary width a=75 μ.

Ferrite‐Filled Elliptical Waveguides. I. Propagation Characteristics
View Description Hide DescriptionThis paper deals with electromagnetic wave propagation in a longitudinally magnetized, ferrite‐filled elliptical waveguide. Using the wave equations and boundary conditions for a ferrite‐filled waveguide of arbitrary cross‐sectional shape, it is shown that for an elliptical cross section the characteristic equations for the propagation constant take the form of even and odd infinite determinants. Solutions for the characteristic equations are obtained for various values of applied field strength, saturation magnetization, and waveguide ellipticity. The existence of three types of cutoff conditions is demonstrated.

Ferrite‐Filled Elliptical Waveguides. II. Faraday Effects
View Description Hide DescriptionDetailed analyses were carried out for the Faraday effect of electromagnetic waves propagating in longitudinally magnetized ferrite‐filled elliptical waveguides. Few resemblances were found between the Faraday rotation effect in a circular waveguide and that in an elliptical waveguide. The circular waveguide features strong coupling between the dominant vertically and horizontally polarized modes, whereas in the elliptical ferrite‐filled guide very little mode coupling occurs for large eccentricities.

Photoemission from Amorphous Selenium
View Description Hide DescriptionPhotoemission yield measurements were made on amorphous selenium from about 6 to 21 eV. The yield curve exhibits a change in slope at about 7.8 eV corresponding to a maximum in the imaginary dielectric constant. The photoemission threshold is obtained by using a threshold law equation derived by Kane, Y = C (E − E_{t} )^{ n }, where Y is the yield, E and E_{t} are the photon energy and threshold energy, and C and n are constants. The yield data up to 8 eV are best represented by the equation with resulting in an extrapolated photoemission threshold of 5.86 eV for amrophous selenium.

Voltage‐Current Characteristics for Electrical Conduction Through Thin MgO Films
View Description Hide DescriptionThe voltage‐current characteristics of Au–MgO–Au thin‐film emission diodes formed by electron beamevaporation of Au and MgO are consistent with an expression derived for Schottkyfield emission into the conduction band of the oxide. These results however indicate a potential barrier height of about 0.72 eV when determined from the zero‐field intercept of the straight line logI vs V ^{1/2} plot and about 0.33 eV when obtained from an Arrhenius plot. These apparent barrier heights were different for devices fabricated by depositing the MgO film in an oxygen atmosphere. The apparent barrier height determined from the zero‐field intercept went up for increasing oxygen pressure and the barrier height determined from the Arrhenius plot went down. Emission of electrons into vacuum was observed from each device at very low sample biases indicating that preferential emission through pinholes in the Au overlayer was occurring.

Mechanisms Controlling the Positive Ion Emission from Heated Fe Filaments
View Description Hide DescriptionPositive ions emitted from heated iron filaments consist mainly of the alkali‐metal atoms, such as lithium,sodium, and potassium, which are present in iron as trace impurities. Changes in the work function with different applied partial pressures of oxygen and hydrogen have been recorded. From the results of measurement it is concluded that the effect of oxygen on the work function of iron is much stronger than that of hydrogen. Kinetic data representing the emission of Na and K ions from Fe were obtained in the temperature range of 1050°–1350°K. Diffusion and ion desorption are discussed as possible rate‐controlling processes. Activation energies, believed to represent the diffusion of Na and K atoms in Fe, are found to be approximately 38 kcal/mole (1.7 eV) and 53 kcal/mole (2.3 eV), respectively. The Richardson work function of iron varies from φ≃4.0 eV at P _{02 }≃10^{−9} Torr to φ≃5.6 eV at P _{02 }≃10^{−5} Torr. When a diffusion‐controlled process is assumed, the difference between the activation energies, for the above two oxygen pressure conditions, is found to be the same as the difference (I ‐ φ); that is, the difference between the first ionization potential of the atom and the work function of the metal surface. The experimentally determined values of (I ‐ φ)_{exD} for K and Na agree within experimental error with the calculated values.

Cavity Mode of a High‐Pressure Mercury Arc
View Description Hide DescriptionThe conditions necessary to establish a stable hollow‐cathode or cavity mode in a high‐pressure mercury vapor discharge have been studied by analysis of the physical processes at the electrodes. The electron emission is primarily Schottky‐enhanced thermionic, so the cavity mode is favored by a thermally shielded cavity design, at low currents. Techniques have been developed for measurement of the temperature of the arc terminus of the probe contact potential relative to the electrode, of the cathode fall of potential, and of the power input to the electrode. Positive ion current densities,work functions of the dispenser‐type electrode at the arc terminus and other interesting quantities have been obtained by computation. The analysis giving these results has been partially verified by experiment.

Effect of Radio‐Frequency Fields on the Electrical Breakdown of Vacuum‐Insulated Electrodes
View Description Hide DescriptionFive possible mechanisms of dc vacuum breakdown are analyzed for rf electric fields. The theories which are considered are the vaporization of a micron‐sized cathode irregularity due to joule heating by field‐emitted electrons, vaporization of the anode due to bombardment by field‐emitted electrons, field‐emission current instability created by the presence of anode ions, exchange of electrode ions, and microparticle vaporization. The analysis indicates that mechanisms which do not involve anode ions in gaps of 0.2–1.0 cm will not be strongly influenced by rf fields in the frequency range of 20–120 MHz. Under similar conditions, the theories involving anode ions predict a very sizable increase in the voltage capability of vacuum‐insulated electrodes. Experimental ratios of vacuum breakdown voltage at 21.5 MHz to that for 60 Hz were experimentally determined for highly polished tungsten electrodes with separations between 0.18 and 0.61 cm. The rf voltages obtained experimentally were less than the values predicted by theory, although the cathode whisker vaporization and ion perturbation by light ions were within experimental uncertainties. From the differences seen in spark conditioning of electrodes, it would appear that micron‐sized particles are involved in the spark threshold at 60 Hz.

Measurement of the Effective Dissociation Potential of Seed Compounds in MHD Experiments
View Description Hide DescriptionA method is developed for the determination of the effective dissociation potential (V_{d} ) of an alkali compound used as a seed in the low‐temperature and high‐pressure working fluid of a magnetohydrodynamic(MHD)generator. For a simple compound like potassium chloride (KCl) V_{d} can be derived directly by measuring the ion density in the plasma from the current‐voltage characteristics observed in MHDgenerator. For complex seed compounds like potassium sulphate (K_{2}SO_{4}), the method leads to an effective V_{d} as referred to an assumed dissociation reaction. Additional experimental information as to the specific radical involved would be required to establish the true dissociation energy. In the experiments presented here, the working gas is argon at atmospheric pressure and the temperature ranges from 2000° to 3000°K. In the temperature range used and for KCl undergoing the reaction KCl⇄K+Cl, the actual dissociation potential V_{d} is found to be 3.6±0.4 eV. For K_{2}SO_{4} the assumed dissociation reaction, K_{2}SO_{4}⇄2K+S+4O leads to the effective V_{d} =10.7±3.1 eV.

Phenomenological Approach to Low‐Frequency Electrical Noise
View Description Hide DescriptionA phenomenological description of excess noise in a nonequilibrium system is presented. The development involves two basic physical ideas. Firstly, the excess noise is ascribed to the fluctuation of the local reference level to which equilibrium and other white fluctuations are compared. Secondly, it is assumed that excess low‐frequency noise involves a nonlinear process. This implies the noise power in different frequency regions are coupled. A further simple assumption requires the integral of the power spectral density over specially chosen intervals to be a constant. This leads to a 1/f spectrum for the power spectral density. Possible generalizations that provide for other spectral laws are also mentioned.

Collapse of a Hollow Inviscid Cylinder
View Description Hide DescriptionIn the production of a hollow inviscid cylinder, it is necessary to maintain an internal pressure to prevent collapse. If this pressure is too low, collapse ensues. The process of collapse is herein examined and equations are given which enable the shape of the initially cylindrical surfaces to be calculated. This collapse mechanism is compared with that arising from capillary instability.

Optical Index Damage in LiNbO_{3} and Other Pyroelectric Insulators
View Description Hide DescriptionLaser‐induced optical index damage in LiNbO_{3} and other pyroelectrics of ABO_{3} structure is discussed in the light of recent results of stoichiometry investigations. The damage is attributed to the presence of B‐excess stacking‐fault defects. The dc photovoltaiceffect in LiNbO_{3} as well as the detailed spatial distribution of refractive‐index variation is explained in terms of photoinduced variation of the macroscopic spontaneous polarization density. This variation results in a polarization charge‐density distribution, and an approximately compensatory space‐charge distribution. The index damage results from the polarization‐based electroopticeffect.

Study of the Quenching Propagation of Bare and Insulated Semistabilized Superconducting Wires
View Description Hide DescriptionThe minimum propagating current, the velocity of propagation, and the temperature rise in semistabilized superconducting wires with and without insulation have been measured by heat pulse tests in magnetic fields up to 70 kG. These experiments have shown that insulated wire can be more stable under external heat perturbation than bare wire and that its temperature rise after quenching is slower. The tests also show that the velocity of propagation is strongly affected, at any current level, by the cooling conditions and by even a very thin surface coating on the wire.

Stability Analysis of a Levitated Superconducting Current Ring Stabilized by Feedback and Eddy Currents
View Description Hide DescriptionA stability analysis is presented for the superconducting levitron stabilized partly by feedback and partly by magnetic coupling to metal plates placed symmetrically above and below the current ring. The system is so designed that eddycurrents in the plates provide sufficient damping to limit the velocity of the ring to safe values in case of loss of control. The magnetic field associated with the feedback system must diffuse through the plates, which tends to handicap the feedback process and complicate the stability analysis. While levitated current rings stabilized by feedback normally require a component of stabilizing current proportional to the ring velocity to produce artificial damping, the present system is found to be stable without such a velocity‐dependent component.

Correlation between Photoluminescence and Electroluminescence Time Decay in Red‐Emitting GaP Diodes at Room Temperature
View Description Hide DescriptionIt is shown that photoluminescent and electroluminescent decay times are equal in a wide variety of red‐emitting GaP diodes grown by liquid‐phase epitaxy. In general, the luminescent decay times depend strongly on the Zn concentration in the bulk p region. For high Zn concentrations (≥5×10^{17} cm^{−3}) the decay times vary approximately inversely with Zn concentration. The equality of the photoluminescent and electroluminescent decay times for the wide variation of decay times and Zn concentrations provides additional evidence that the luminescence occurs in the bulk Zn‐ and O‐doped p region.

Factors Controlling Infrared‐Pumped Visible Emission of Yb^{3+}–Er^{3+} in the Scheelites
View Description Hide DescriptionThe rate equations for green fluorescence of the Yb^{3+}–Er^{3+} doubly doped quantum counter are solved for the case of an infrared pump at the Yb^{3+} ^{2} F _{7/2}−^{2} F _{5/2} transition. The successive transfer of excitation from two Yb^{3+} ions to the Er^{3+} produces green emission from the Er^{3+} ^{4} S _{3/2} level. The increase of the ^{4} S _{3/2} lifetime with increasing Yb^{3+} concentration in Na_{0.5}(Y, Yb, Er)_{0.5}WO_{4} confirms the consecutive transfer model of Auzel. Emission from Er^{3+} in a solid solution of divalent cation tungstates or molybdates (having the scheelite structure) under long‐wave ultraviolet excitation is proportional to the inverse of the cation‐anion separation, and hence the binding energy. The emission under infrared excitation is relatively independent of this separation.

Application of Internal‐Stress‐Distribution Theory to ΔE Effect, Initial Permeability, and Temperature‐Dependent Magnetomechanical Damping
View Description Hide DescriptionThe internal‐stress‐distribution model of magnetomechanical hysteresis is extended to a calculation of the ΔEeffect, the irreversible low‐field permeability, and temperature‐dependent magnetomechanical damping. The calculation of the strain dependence of the ΔEeffect agrees well with experimental results for Fe‐7.65% Ge. The model for permeability yields results for the product λσ_{ i }(λ=magnetostriction, σ_{ i }=average internal stress) in reasonable agreement with values from the Becker‐Kersten model for reversible initial permeability μ_{0} and coercive forceH_{c} . The temperature‐dependent damping observed in Ni by Roberts and Barrand is ascribed to an increase in domain‐wall thickness as anistropy decreases. The model gives reasonable agreement with the measuredtemperature dependence.

Measurement on Misalignment of the Axis of a Magnetic Field for Electron‐Beam Focusing
View Description Hide DescriptionThe method, which was previously proposed by the authors, of locating the magnetic‐field axis has been applied to measure the misalignment of the axes of a uniform magnetic field produced by a solenoidal coil and of a periodic one produced by a permanent‐magnet stack. In the former the tilt of field axis has been detected with a resolution of the order of 10^{−3} rad. In the latter the displacement of the field axis has been detected with a resolution of the order of 10^{−2} mm, which is equal to about one part in 10^{3} of the inner diameter of magnets. It has been found experimentally that even a slight local misalignment considerably influences electron‐beam transmission.