Volume 35, Issue 2, 01 February 1964

Onset of Breakdown at Voltages below Ionization Potential in High‐Pressure Thermionic Diodes
View Description Hide DescriptionEarly breakdown effects have been measured in relatively high‐pressure thermionic diodes under given conditions of pressure and anode spacing. Apparent breakdown conditions are obtained at anode voltages considerably lower than the ionization potential of the gas. Measurements on xenon‐ and mercury‐filled thermionic diodes, under space‐charge conditions of operation, are presented which show that this breakdown voltage seems to be associated with the first excited state of the gas. Experimentally, early ignition occurs in diodes with a fixed cathode‐anode spacing in a limited pressure range only. A theoretical explanation for this behavior is based on the model that, in the limited pressure range, electrons traversing the cathode‐anode space can pick up almost all the energy of the applied voltage.
In addition, data are presented showing that early breakdown in an inert gas‐filled diode can occur at voltages much lower than the first excited state. This has been accomplished in thermionic diodes exposed to the radiation of a nuclear reactor. A qualitative explanation for this behavior is proposed.

Capacitance of Thin Dielectric Structures
View Description Hide DescriptionThe capacitance contribution from the space charge and associated potential distribution in the metal electrodes of a parallel‐plane capacitor has been estimated by a classical, electrostatic calculation. The electron density is assumed to be given by the absolute zero temperature, Fermi‐Dirac distribution function. The first integration of Poisson's equation and application of the boundary conditions for the electric field at the metal‐dielectric interfaces result in two nonlinear, algebraic equations (one for each electrode) for the potential distribution as a function of the dielectric thickness and applied voltage. These equations were solved numerically. The results show that for reasonable electric fields in the dielectric (<10^{8} V/cm) the lumped‐series capacitance of the electrodes is practically constant and independent of applied voltage and dielectric thickness, in good, qualitative agreement with previously reported experimental results. The total capacitance is the series combination of the capacitances of the dielectric and the electrodes. The electrodes are effectively a capacitor with a thickness 2.3 L, where L is a length characteristic of the metal. This electrode series capacitance for metals such as gold is of the order of 1 μF/cm^{2}.

Structural Effects in Thin‐Film Sandwich Emitters
View Description Hide DescriptionDirect currentmeasurements have been made of film currents in and emission currents from thin‐film sandwiches consisting of beryllium, thermally formed beryllium oxide, and overlayers of silver,barium or a combination of the two.
Silver covered oxide films formed at low temperature yield regular film current characteristics but no electron emission. With high oxidizing temperatures, electron emission can be obtained but the films are subject to frequent breakdown. An intermediate temperature range yields more reproducible film currents and, in many cases, small but steady electron emission. A discontinuous forming process generally precedes the electron emission.
Films with barium overlayers break down and heal repeatedly, accompanied by very irregular emission at film voltages considerably below those for silver covered films. Brief exposure of a silver overlayer to barium vapor induces marked changes. Initially, the film current is irregular as it is for barium alone, but as the current increases the characteristic reverts to that for silver without barium.Electron emission is more frequently obtained in this way, though it occurs in the same voltage range as without barium.
The film‐current curve shapes indicate that the current is carried mainly by electrons passing over a voltage‐dependent barrier (Schottky effect), but cooling the samples has no effect on these curves, probably because of localized heating.
A tentative model is proposed to account for the particular observations reported here. Work function differences, the porosity of the thin silverfilms, and cracks or grain boundaries in the oxide layer combine to produce highly localized currents, heating, and structural changes consistent with measurements and observed emission patterns.

Energy Dependence of Extinction Distance and Transmissive Power for Electron Waves in Crystals
View Description Hide DescriptionQuantitative measurements of the energy dependence of the extinction distance and transmissive power for electron waves in crystals have been carried out photographically and electrically with 100‐kV and 300‐kV electron microscopes. The observations show that the relativistic correction for the mass of the electron must not be disregarded in the dynamical theory of electron diffraction for either the elastic or inelastic scattering. The energy dependence of the visibility of thickness extinction contour fringes and the resolutions of the images of thick films are also discussed. Some of the features observed in electron micrographs of thin metallic films taken at 250–300 kV are illustrated.

Microwave Reflection by Nonuniform Plasmas with Exponential Electron Distribution
View Description Hide DescriptionMicrowave reflection is considered for nonuniform plasmas with an exponential electron distribution that approaches an equilibrium value far downstream. Several approximate formulas for the reflection coefficient and phase‐shift angle and the graphical results of some numerical calculations are presented. Comparison of the results with those for ``ramped'' electron distributions shows an interesting point; when piecewise linear segments are used to represent a continuous electron distribution, the wave interactions may affect the reflection and transmission of electromagnetic waves.

Efficiency Measurements on GaAs Electroluminescent Diodes
View Description Hide DescriptionAn integrating sphere was adapted to measurement of external efficiencies of GaAs injection lasers. External efficiencies rise rather sharply with current once the laser threshold is reached, attaining ultimate values of about 35% at 77°K for the more efficient diodes, as high as 60% at 20°K. On the basis of a highly simplified model, the observed external efficiencies indicate junction efficiencies near unity.

Limiting‐Current Densities in the Presence of a Magnetic Field
View Description Hide DescriptionThe limiting value of the current density in a beam compressed by an arbitrarily large magnetic field has been derived from Liouville's theorem and the adiabatic invariance theorem. For sufficiently large compression ratios the limiting values are (1+eV/kt), as a result expected from the Langmuir‐Pierce expression. Some of the effects due to the use of a finite magnetic field have been examined with particular reference to the current density obtainable at the planes where cathode images are formed. The transit time scatter imposes an upper limit on the maximum compression ratio for which these cathode images can be utilized.

Ion Acoustic Oscillations in Plasma Thermionic Energy Converters
View Description Hide DescriptionThe oscillations in low‐pressure plasma thermionic energy converters are ion acoustic oscillations that may be explained on the basis of an electron beam‐plasma interaction. A low‐energy beam is formed by the acceleration of electrons through an ion sheath at the emitting surface. The ion acoustic oscillations are observed as expected when the ordered beam velocity is greater than the phase velocity of the wave and less than the plasma electron thermal velocity. Their frequency is where the emitter to collector spacing is a multiple of the half‐wavelength. Langmuir probes indicate high effective electron temperatures, and the agreement between calculated and measured frequencies is good if γ_{ e }=3 is used, whether or not the electron‐electron collision frequency is higher than the frequency of the observed oscillations.

Efficiency of Production of Characteristic X Radiation from Pure Elements Bombarded by Electrons
View Description Hide DescriptionEquations are presented which give the intensity (photons/electron steradian) of K characteristic x radiation produced by pure elements under electron bombardment at normal incidence. For values of φ (the angle between the x‐ray beam and the specimen surface) between 0° and 90° the equations may be summarized:where v_{K} is the number of K quanta per incident electron radiated into solid angle δω at takeoff angle φ, W is the fluorescence yield factor, R is a correction for loss of ionization due to electron back scattering,f(χ) is Castaing's absorption correction function, N is the number of atoms per unit volume, Q̄ is an average over electron energy of the cross section for K shell ionization,x_{K} is the electron path length at which the energy of the electron drops to that necessary for K shell ionization, and N_{Kf } is the contribution due to secondary K shell ionization by high energy continuous x radiation. It is concluded that the predictions of this theory are in general in good agreement with available experimental data; further, that this theory shows certain advantages over those of Archard (1960) and of Green and Cosslett (1961). Values of f(χ) are calculated and compared with the experimental work of Castaing and the calculations of Archard and Mulvey.

Thermal Velocity Limit to Current Density in a Beam
View Description Hide DescriptionThe limiting current density that can be achieved with thermal velocities in the source emission is calculated by phase‐space ellipse transformations. The results check those of Pierce, and are applicable to a wider range of problems. Simple lens systems can realize the ellipse transformations which give the limiting density. Under certain conditions the maximum current density is obtained between the object plane and the image plane, a situation analogous to the ``waist'' formation in a beam transport system used with particle accelerators.

Propagation of Microwaves in a Plasma‐Filled Waveguide
View Description Hide DescriptionA reasonably homogeneous plasma slab has been produced in a waveguide in order to examine a number of features of the plasma‐microwave interaction. The usual linear theory gives expressions for the transmission and reflection coefficients in terms of the electron density and collision frequency for such a slab. Experimental values for the electron density are compared with those found from the shift in the cyclotron resonance absorption peak. In addition, an observed microwave amplification is described.

Sputtering of Metals by Hydrogen Ions
View Description Hide DescriptionSputtering‐yield data for metals are obtained by drilling holes in thin target foils with magnetically separated, dense hydrogen beams. Results for H_{2} ^{+} and H_{3} ^{+} at 7 keV are presented for Be, Al, Ti, V, Fe, Co, Ni,Cu, Zr, Mo, Pd, Ag,Ta, W, Re, Ir, Pt, and Au. Results for protons can be inferred from those for molecular ions since the hydrogen atoms sputter independently. Typical results at 7 keV in atoms/ion are for 0.032/H_{2} ^{+}, 0.046/H_{3} ^{+}, and for Cu 0.025/H^{+}, 0.065/H_{2} ^{+}, and 0.105/H_{3} ^{+}.

Low‐Temperature Elastic Moduli of Aluminum
View Description Hide DescriptionThe adiabatic elastic moduli of single crystals of aluminum have been measured from 4.2° to 300°K by using the ultrasonic pulse‐echo technique. The values obtained by extrapolation to 0°K are C _{11}=11.430, C _{12}=6.192, and C _{44}=3.162 in units of 10^{11} dyn/cm^{2}. The Debye temperature obtained from the 0°K modulus values is 430.3°K, in excellent agreement with the value from heat‐capacity measurements.

Origin of the Induced Magnetic Anisotropy in Nickel Ferrite
View Description Hide DescriptionThe magnetic anisotropy induced in nickelferrite at room temperature by an applied field has been investigated as a function of composition. The magnitude and time rate of change, or relaxation, of this anisotropy has been studied by measuring the torque produced by an applied magnetic field. Disaccommodation, or the change of permeability with time of multidomain materials, is a direct result of this relaxation. The relaxation times are independent of composition and the magnitude of the anisotropy is not simply related to the concentration of cation vacancies or Fe^{2+} ions; this behavior of the induced anisotropy cannot be explained by either electron or cation migration. A model, consistant with the experimental results, is suggested in which the induced anisotropy is the result of preferentially oriented, localized Jahn‐Teller distortions. The distortions arise from Ni^{2+} ions on the tetrahedral (A) sites of the spinel lattice.

Optical Absorption and Diffuse Reflectance of Powders
View Description Hide DescriptionTwo different models for the dependence of diffuse reflectance of powders on optical absorption coefficient, particle size, and index of refraction are discussed. For particles large compared to the wavelength of incident radiation a model in which the path of this radiation is controlled mainly by specular internal and external reflections and refraction is preferable to one in which path deviation by Lambert law scattering dominates.

Fourth‐Order Elastic Coefficients
View Description Hide DescriptionThe fourth‐order elastic coefficients form an eighth‐order tensor containing 6561 components of which 126 are independent for a triclinic system. From symmetry arguments it is shown that there are only eleven independent fourth‐order elastic coefficients for a crystal with full cubic symmetry.

Platinum as a Thermal Conductivity Standard
View Description Hide DescriptionThe thermal conductivityK of high‐purity platinum appears to be a reproducible quantity for temperatures between 75°K and the melting point at 2047°K. This behavior combined with the outstanding chemical and physical properties of platinum suggests that platinum could serve as a useful K standard in this region. From 200° to 2047°K the available data in the literature on K can be represented to within ±2% by K = (0.64_{8}+1.8_{7}×10^{−4} T) W/cm deg. Some further work in determining K by several different methods, particularly at high temperatures, appears necessary before platinum can be adopted as a standard.

Preparation of Uniaxial Permalloy Films by Cathodic Sputtering on Glass and Metal Substrates
View Description Hide DescriptionThin magnetic films have been prepared by cathodic sputtering on glass and metal substrates. The influence of the ferromagneticcathode on the orienting magnetic field, as well as on the resulting magnetic behavior of the films is discussed. Use of a ``thin'' foil cathode is shown to be a simple method for minimizing the distorting effect of the cathode. Wall coercive force,anisotropy field, skew, and dispersion values are given for these films. Pulse behavior of a high‐density bit plate in the biased mode of operation is also given.

First Approximation to Magnetization Curling in a Chain of Spheres
View Description Hide DescriptionThe nucleation field for the magnetization curling mode in an infinite chain of ferromagnetic spheres is calculated as a first‐order perturbation on the curling mode in a sphere. To this approximation, the nucleation field dependence on the radius R of the spheres is expressed as a−bR ^{−2} with a, b positive constants. This yields positive nucleation fields for rather small radii (R≥130 Å for iron). To this first approximaton, nucleation by curling is more difficult than by fanning, for R≤100 Å in iron. The packing might influence somewhat the nucleation field, but the dependence is essentially exponential and as such is negligibly small for all practical cases.

Absorption Data of Laser‐Type GaAs at 300° and 77°K
View Description Hide DescriptionOptical absorption coefficients have been determined at 300° and 77°K for samples of GaAs which were doped to concentrations comparable to those present in the n, p, and p+ regions of the GaAs injection laser. n‐type GaAs doped with Te to 3×10^{17} cm^{−3} and 9.6×10^{17} cm^{−3} have absorption coefficients of 20 cm^{−1} and 10 cm^{−1}, respectively, at 1.475 eV and 77°K. Free carrier and deep‐level absorptions contribute approximately 5 cm^{−1} to these values. At 77°K, p‐type Zn‐doped samples with N_{A}−N_{D} equal to 1.6×10^{18} cm^{−3} exhibit in the spectral region of laser emission an absorption that increases exponentially with photon energy and probably results from transitions of electrons in acceptor states to levels in the conduction band or conduction band tail. At 1.475 eV and 77°K, the absorption coefficient of this p‐type material is 225 cm^{−1} to which free carrier absorption contributes only 3 cm^{−1}. For heavier Zn‐doped p+ material, e.g., 7×10^{19} cm^{−3}, the value of K at 77°K and 1.475 eV is 250 cm^{−1} and is entirely due to free carrier absorption. For both n and p materials, a Burstein shift of the edge to higher energies is observed for the heavily doped materials in agreement with the observations of others.