Volume 30, Issue 3, 01 March 1959
Index of content:
30(1959); http://dx.doi.org/10.1063/1.1735151View Description Hide Description
It is the purpose of this report to describe an instrument for the determination of densities in rarefied gas streams which utilizes the attenuation of an electron beam. The electron beam in passage through a rarefied gas becomes scattered in a number of elastic and inelastic processes, and decays to a fraction of its initial intensity in the distance of a few electron mean free paths. Where the optical aperture of the detector is suitably small, the familiar linear absorption law applies. The mass absorption coefficient in this case is higher and more favorable for work at the lowest test section densities than in other absorption techniques investigated to the present. The instrument was used in an investigation of air flows about a sphere and a wedge, and in the undisturbed stream. The flow was maintained constant for all portions of the test at Mach 1.95, Re/in. ∼200, test section static pressure ∼80 μ Hg. Plots of the original and the reduced data show adequate internal consistency and a qualitative conformity with expected configurations.
Effect of Space Charge Fields on Polarization Reversal and the Generation of Barkhausen Pulses in Barium Titanate30(1959); http://dx.doi.org/10.1063/1.1735152View Description Hide Description
The rate of generation of Barkhausen pulses in bariumtitanate when the polarization direction is slowly reversed is profoundly influenced by the form of the voltage cycling given to the crystal. It is concluded that the rate of nucleation of new domains is determined by the field near the electrodes which, in turn, is the resultant of the applied field and a relaxing space charge field. This result follows directly if the Barkhausen pulses represent individual nucleations though this assumption is not crucial since the generation rate of the pulses parallels the total current at all stages of the polarization reversal. It is concluded also that the majority of the Barkhausen pulses occur independently of each other and of their surroundings.
30(1959); http://dx.doi.org/10.1063/1.1735153View Description Hide Description
Germaniumsurfaces were bombarded with argon ions and then heat‐treated under high vacuum. Room‐temperature values of surface recombination velocity, surface conductivity, dark field effect, and field effect under illumination were measured after successive heat treatments. In addition, the temperature dependence of these quantities was determined. It was confirmed that after annealing of the bombardment damage, a large number of acceptor type surface states approximately clamped the surface potential. Two types of recombination centers were identified: type 1, located near the middle of the gap and type 2, located near the valence band. The various heat treatments produced changes in the density of the type 1 centers but did not appear to effect the density of the type 2 centers.
30(1959); http://dx.doi.org/10.1063/1.1735154View Description Hide Description
This paper describes a method for the simulation of space charge in an electrolytic tank by the introduction of currents into the electrolyte by means of sources projecting through the tank floor. The theory and design criteria for this space‐charge simulation system are presented. This method of simulation is used as a means for introducing detailed space‐charge effects into the determination of electron trajectories in axially symmetric, high‐perveance electron guns. The trajectories determined by use of this system are compared with beam characteristics measured experimentally.
30(1959); http://dx.doi.org/10.1063/1.1735155View Description Hide Description
The driving force leading to densification during sintering in the presence of a liquid phase and the material transport phenomena have been analyzed and relationships for the densification rate during the rearrangement process, the solution‐precipitation process, and the final coalescence process have been determined. These relationships allow an experimental determination of the mechanism of sintering in the presence of a liquid phase on the basis of the time, particle size and temperature dependence of the densification rate. In addition, they allow direct calculations of densification rates to be made for certain simple systems for which property data are available.
30(1959); http://dx.doi.org/10.1063/1.1735156View Description Hide Description
Experimental measurements of the densification rate and microstructure changes taking place during sintering of the iron‐copper system, which is typical of those systems showing spheroidal grain development, indicate that in this system the rate of densification during liquid phase sintering is controlled by diffusion through a liquid film between particles. Dependence of the sintering rate on time, particle size and temperature are in agreement with theoretical predictions. A theoretical estimation of the sintering rate is in good agreement with experimental measurements.
30(1959); http://dx.doi.org/10.1063/1.1735157View Description Hide Description
Diffused diodes, fabricated from plastically deformedgermanium, were examined for high‐speed switching response. The degradation of minority carrier lifetime caused by dislocations generated during plastic deformation greatly reduced the minority carrier storage effect and permitted fabrication of diodes with turnoff times of the order of 10−9 sec.
30(1959); http://dx.doi.org/10.1063/1.1735158View Description Hide Description
The temperature coefficients of resistance (TCR) of films of sputteredAu, Ir, Mo,Ni, Pd, Pt, Rh, Ta, and W, of evaporated Al, Cr, Ti, and Zr, and of the alloy films Pt–Au, Pt–Ir, and Pt–Ni have been measured in vacuo over the temperature range 25° to 600°C; the film thickness range was 75 to 2000 A. The TCR values of films of the substantially pure metals were in the range ⅓ to ⅔ those of the respective bulk metals whether deposited on glass, Vycor, or Stupalith substrates. The departure of the conditions of growth of the film from those usual for the bulk metal, i.e., rapid cooling effects and impurities present, contributed imperfections which reduced the TCR of the film. The TCR values of alloy films were low, 0.0004 per °C for Au–Pt, and resistivities were 3 to 4 times that of either constituent. The TCR values of sputteredfilms of Mo, W, and Ta and of evaporated Cr, Zr, and Ti were generally less than 0.0001 per °C. Electron diffraction examination of films of these latter metals revealed oxide inclusions in the films. The presence of the oxide of the metal reduced the TCR and increased the R/sq of the film as compared to those of the pure metal. Only films of gold, platinum, and iridium were corrosion resistant in air near 600°C. Overcoats of evaporated SiO provided partial protection for the others. Powers up to 15 watts were dissipated by refractory metal films only by ½ in. and about 750 A thick. These high values contrasted with less than ¼ w for a goldfilm of similar dimensions.
30(1959); http://dx.doi.org/10.1063/1.1735160View Description Hide Description
The maximum switching velocity (ΔP/Δt)max·1/Ps in BaTiO3single crystals, obtained by the ultrasonic method, is discussed not only as the function of the applied pulse field, but also of the pulse width. Maximum switching current response (dP/dt) measured by the single pulse method and expressed as the exponential function of the field by Merz, is also considered as the limiting case of our expression as the applied pulse width becomes infinitely wide.
30(1959); http://dx.doi.org/10.1063/1.1735161View Description Hide Description
Vacancydiffusion models which predict parabolic diffusion‐penetration kinetics implicitly assume that vacancy equilibrium is maintained everywhere. However, a nonequilibrium vacancy distribution is generally established by the pumping action of the chemical gradient in the Kirkendall effect. A more general diffusion model is, therefore, developed in which vacancies are not necessarily maintained in equilibrium. The behavior of the model is then investigated and nonparabolic kinetics are found. However, the kinetics approach those of the equilibrium model as either the vacancy lifetime approaches zero or as the diffusion time becomes very long. The applicability of the equilibrium model, therefore, depends upon the particular system and diffusion time. The results are compared with experiment, and it is concluded that nonparabolic effects are probably of little importance in usual face‐centered‐cubic metallic systems by the time diffusionmeasurements are made by conventional sectioning methods, i.e., when Dt>∼10−5 cm2.
30(1959); http://dx.doi.org/10.1063/1.1735162View Description Hide Description
The total cross section for O−photodetachment is computed using an analytical model to give agreement with the data obtained by Branscomb, Smith, and Burch. For the calculation of the electric dipole matrix element single‐electron wave functions are constructed for the initial and final states which reflect the short range of the interaction between the neutral atom and the extra electron. These are written in terms of two parameters which are determined in order to fit the experimental data. The bound state parameter β gives an indication of the weak binding of the system and also gives a measure of the ``size'' of the ion. The continuum state parameter, the s‐wave phase shift δ, gives the total cross section for the low energy elasticscattering of electrons by O atoms.
30(1959); http://dx.doi.org/10.1063/1.1735163View Description Hide Description
Measurements of the thermal conductivityK of Harshaw LiF crystals were made over the temperature range 2°–100°K. A freshly cleaved crystal exhibited K values in agreement with the measurements of Berman, Foster, and Ziman (1956). Annealing raised the maximum K by a factor of two. Dislocations were added by compression, and the dislocation density was determined by the methods of Gilman and Johnston. The K values below 30°K for the deformed specimens were approximately proportional to T 2, in agreement with Klemens' theory, but the reduction in K by introduction of dislocations was much larger than Klemens' predictions. The phonon mean free path when limited by dislocation scattering was found to be approximately (4×106)/DT, where D is the number of dislocation lines/cm2 and T is the absolute temperature.
30(1959); http://dx.doi.org/10.1063/1.1735164View Description Hide Description
The absorption of 4 mc/sec ultrasonic waves in polyisobutylene is measured as a function of temperature, for various pressures. The extreme limits of these variables are 1 to 1400 atmos and −20°C to 80°C, respectively. Approximate measurements of propagation velocity are also obtained. Pulse reflection apparatus is used. A magnetically actuated holder places the sample in the signal path or removes it. Pressure‐volume data are obtained for polyisobutylene at 30°C by means of a capillary dilatometer fitted with electrical contacts, in which the sample is placed with mercury. The expected behavior of viscoelasticrelaxation times with temperature and pressure is computed by adapting a theory due to Bueche. With additional assumptions concerning the compressibility, the theory gives a shift of the constant‐frequency absorption maximum of 0.025 C°/atmos, in approximate agreement with experiment.
30(1959); http://dx.doi.org/10.1063/1.1735165View Description Hide Description
Electrically charged iron and aluminum particles having diameters of a few microns have been contained in a confined region of space by means of alternating and static electric fields. The theory is essentially that of alternating gradient focusing; here the motion is governed by Mathieu's equation. Under certain circumstances when many particles are confined the three dimensional focusing force and the Coulomb repulsion results in a ``crystaline'' array which can be ``melted'' and reformed.
30(1959); http://dx.doi.org/10.1063/1.1735166View Description Hide Description
Linear thermal expansion coefficients of natural and artificial single ice crystals, commercial ice, and snowice were determined in the temperature range near 0 to −30°C. There was no measurable difference in the coefficients of linear thermal expansion parallel and normal to the c axis of single crystals or polycrystallineice. However a steady decrease of expansion coefficients with each succeeding measurement on the same ice specimen was observed. The total decrease was of the order of 2×10−6, while an estimated error calculation yielded a value of about 4×10−7. An entirely satisfactory explanation of this phenomenon cannot be given at this time, and further investigation on this point would be desirable. Values of Cv , the specific heat at constant volume, were calculated from thermodynamic relationships. Average Cp/Cv values were 1.030. Gruneisen's constant was found to average 0.78, and essentially independent of temperature between 0°C and −30°C.
30(1959); http://dx.doi.org/10.1063/1.1735167View Description Hide Description
A new method is proposed for determining the separate concentrations of acceptor and donor impurities in crystals of p‐type silicon, and thereby the degree of compensation of acceptors by donors. The method involves finding the total concentration of impurities in a sample from a measurement of the weak‐field transverse magnetoresistance at 77°K and combining this result with the excess of acceptors over donors determined from a room‐temperature resistivity or Hall effect measurement. An empirical ``master curve'' of magnetoresistancevs impurity density has been constructed for this purpose. The impurity densities for the master curve were measured by the usual method of fitting a theoretical carrier concentrationvs temperature equation to an experimental curve obtained from Hall effect data.
30(1959); http://dx.doi.org/10.1063/1.1735168View Description Hide Description
The line widths at the half‐power point for ferromagnetic resonance in a number of polycrystallineyttriumirongarnet samples have been measured at 16 kmc.
It has been determined that, starting with mixtures of Y2O3 and Fe2O3, sound parts with small grain size and having line widths as low as 35 to 40 oersteds can be obtained on the iron‐rich side of stoichiometry in Y3Fe5O12. However, narrower line widths (∼20 oersteds) in large grained materials are obtained by using less than the stoichiometric amount of iron and firing at temperatures higher than 1450°C. This is due to minimizing the disrupting influences of oxygen loss in iron‐rich areas at high temperatures.
Line widths do not appear to be broadened by the addition of small amounts of nickel,cobalt, or manganese to convert excess iron to high resistance forms of spinel in the iron‐rich compositions.
30(1959); http://dx.doi.org/10.1063/1.1735169View Description Hide Description
In order to determine whether ferromagnetic or antiferromagnetic exchange coupling would result from alloying the rare earth metals with cobalt or iron, a number of melts were made containing these elements. The data on saturation magnetization with composition and on saturation magnetization with temperature obtained on the cobalt‐gadolinium system indicates strongly that antiferromagnetic exchange coupling exists in this system.
30(1959); http://dx.doi.org/10.1063/1.1735170View Description Hide Description
Solutions of Poisson's equation for a space‐charge‐limited spherical diode, in which electron trajectories are great circles rather than radial lines, are developed. It is shown how Poisson's equation may be separated to yield ordinary differential equations. The radial equation is solved analytically, whereas accurate numerical solutions of the azimuthal equation are given. A method of truncating the flow so as to produce a ``bowl‐shaped'' electron gun, leading eventually to an annular electron beam, is presented, along with power‐series solutions of Laplace's equation both inside and outside the region of the flow. The latter series are useful for calculating the shapes of anodes and focusing electrodes.