Volume 35, Issue 4, 01 April 1964
Index of content:
35(1964); http://dx.doi.org/10.1063/1.1713576View Description Hide Description
A low‐pressure magnetically confined argon arc discharge was used in a mass spectrometerion source to study low‐energy (0–100 eV) sputtering of polycrystallinecopper. Target bombarding ion current densities ranged from 60–200 μA/cm2. Neutral particles were studied. Cu atoms and Cu2 molecules were detected. The mass ratio of analyzed Cu2 molecules to Cu atoms increased with bombarding ion energies to about 5½% at ion energies of 100 eV. Target voltages for appearance of Cu atoms and Cu2 molecules were − 19 and − 50 V, respectively. No Cu3 molecules were detected; if they were present, it was estimated that the ratio 189Cu3 to 63Cu is less than 0.09%. The method has been found to be promising for the study of neutral particles in low‐energy sputtering. Yield curves agree well with results of other observers; sensitivities of 7×10−4 atoms/ion were attained, and this figure can be improved.
35(1964); http://dx.doi.org/10.1063/1.1713577View Description Hide Description
For an exact solution of the Bragg equation it is necessary that there be a one‐to‐one correspondence (equivalence) between the angular measure θ and the wavelength measure λ. Because the aberrations inherent in all x‐ray diffraction profile measurements distort the profile, it is necessary to correct any given measure (peak, centroid, etc.) for all aberrations. The only angular measure of powderdiffractometer profiles that can be corrected to a high degree of accuracy is the centroid. To realize the greater accuracy inherent in the centroid method, accurate centroid spectral wavelengths are required. Centroids of CuKα and FeKα two‐crystal spectrometer profiles prepared by Bearden have been calculated and corrected for axial divergence, distortions arising from variation in absorption across the spectral distributions, crystal asymmetry, and other factors. To determine the centroid wavelength it is necessary to truncate the profile; equivalence of λ and θ is maintained by truncating the powder and spectral profiles in the same manner. Four methods for systematically truncating the profiles have been published; the methods are evaluated and the superiority of one of the methods is demonstrated. It is shown that the centroid wavelength is a function of integration (wavelength) range, primarily as a result of inclusion or exclusion of the Kα satellite group within the truncated profile. The two‐crystal spectrometer profiles are compared with models based on a Cauchy distribution. The necessity for additional spectral data before reference centroid wavelengths can be established is discussed.
35(1964); http://dx.doi.org/10.1063/1.1713578View Description Hide Description
The afterglow following pulsed dc excitation of neon can be explained by the requirement that the electron temperature must decrease before electron‐ion recombination followed by radiative transitions can take place. This conclusion is supported by the suppression of afterglow by simultaneously applied rf power. The absence of negative absorption rules out energy level population inversion and stimulated emission.
35(1964); http://dx.doi.org/10.1063/1.1713579View Description Hide Description
A substantial lowering of the work function of a tungstenLangmuir probe immersed in an argon plasma in a discharge tube is observed. The mechanism is thought to be similar to the Schottky effect. The electron temperature measurement is lowered about 30%, in a particular discharge condition, as the probe goes from room temperature to 2000°C. The chargenumber densitymeasurement, by saturation electron current appears to be higher by 15%, and that by ion saturation current appears to be higher by almost one order of magnitude. This phenomenon can also account for the discrepancy of the ratio of ion and electron saturation current obtained by previous authors and this author.
35(1964); http://dx.doi.org/10.1063/1.1713580View Description Hide Description
High‐power lasers encounter increasing losses of stored energy from stimulated emission by once‐through avalanches of photons as higher powers are sought. This occurs even without resonance buildup. Even in the simple geometry of a thin rod, a satisfactory analysis would require elaborate machine calculation. A very crude model has, however, produced rough results from which some guidance is obtainable.
The model exhibits the expected linear increase of outgoing photon flux near the end of the rod and the huge depletion of excited atom population away from the rod equator. A criterion has been developed for the maximum useful excitation relative to emission parameters, rod length and rod cross section. It is concluded that this excitation level is fairly insensitive to aspect ratio (radius/length) but that internal reflection from the rod cylindrical surface can markedly lower this level by increasing the effective aspect ratio enormously.
35(1964); http://dx.doi.org/10.1063/1.1713581View Description Hide Description
A uniform magnetic field is compressed in a circuit containing two semi‐infinite slabs of constant conductivity σ. The separation l of the slabs decreases uniformly according to l=l 0−vt. An equation involving the magnetic Reynolds numberRm =σμ0 l 0 v is derived for the magnetic field in the gap. Expressions are given for the magnetic and electric fields everywhere up to the instant of collision. These expressions are evaluated for several values of Rm and it is found that the ratio of Joule energy density to magnetic energy density at the surfaces of the slabs is fairly close to unity throughout the motion. For Rm »1 it is shown that the maximum magnification of the initial field is approximately Rm /8.
35(1964); http://dx.doi.org/10.1063/1.1713582View Description Hide Description
The results and experimental details of S‐band noise figure and noise spectrameasurements made on electron beams that yield traveling‐wave‐tube noise figures below 4 dB are presented. A simple technique involving the observation of the change in noise figure with electron gun‐helix separation in a special traveling‐wave tube (TWT) is employed. A positive correlation between current and velocity fluctuations contributes heavily to the reduction of noise in the ultralow‐noise TWT. This conclusion supports the general results, but not necessarily the details of the theory of noise on multivelocity beams. In contrast to the theoretical assumption that the important source of multivelocity flow is the cathodetemperature, the measurements give evidence that, in the ultralow‐noise TWT, the flow of importance is due to transverse beam slippage in the finite beam that produces what may be termed a ``quasi‐multivelocity'' flow with a velocity spread which may be wide enough to mask the thermal spread. The measurements also raise the possibility that a cyclotron wave‐space‐charge wave interaction occurring beyond the cathode‐first anode space contributes to the noise behavior.
35(1964); http://dx.doi.org/10.1063/1.1713583View Description Hide Description
Electroluminescent GaAs diodes have been shaped as truncated cones with the p‐n junction at the apex to direct a large fraction of the radiation toward the base of the cone within the critical angle for transmission. Devices having an efficiency at 77°K of 8% have been fabricated. Efficiency is defined here as the radiant power out the base per unit electrical power input. It is calculated that improved devices of this type will have efficiencies of about 18%.
Effect of Crystallization Conditions and Heat Treatment on Polyethylene: Lamellar Thickness, Melting Temperature, and Density35(1964); http://dx.doi.org/10.1063/1.1713584View Description Hide Description
Lamellar thickness, melting temperature, and density were measured for bulk polyethylene crystallized by the application of a fixed pressure to molten polymer at different temperatures. Melting temperature varied from 135.2° to 139.2°C, and density varied from 0.9759 to 0.9842 g/cc. Small‐angle x‐ray diffraction yields two maxima which apparently are not first and second orders of diffraction. Electron micrographs of fracture surfaces exhibit lamellae, which support the suggestion that the lamellar thickness is the shorter of the two dimensions (469≤L 1≤981 Å and 193≤L 2≤440 Å) corresponding to the diffraction maxima. However, the functional relationship of both dimensions to melting temperature and density are in accord with existing or easily derived equations. Analysis of these data, together with the result obtained from paraffin data, yields an equilibrium melting temperature of 141.9±1.7°C. Dimensions of the unit cell did not vary with lamellar thickness.
Single crystals of polyethylene were collected into mats and heated at a fixed temperature under various pressures. The observed effect of pressure was to limit the fold period attained during heat treatment. The data suggest that the refolding process follows a relationship which describes crystallization from the melt.
35(1964); http://dx.doi.org/10.1063/1.1713585View Description Hide Description
The free volume theory of diffusion for hard spheres, earlier developed by Cohen and Turnbull, is modified for simple van der Waals type liquids. The modified theory fits the self‐diffusion data for argon,krypton, and xenon fairly well and predicts glass transition for these liquids at approximately one‐third of their normal boiling points. It is found that a more accurate model for the free volume diffusion in liquids should include a redistribution energy for voids arising from the nonlinear behavior of the pair potential. The agreement found by Cohen and Turnbull between their hard sphere theory and the self‐diffusion data of some large organic molecules is interpreted as evidence that the simple Lennard‐Jones potential does not accurately describe their interaction with each other. This situation is reflected in the failure of these molecules to obey the same corresponding states relationship as the simple liquids of argon,krypton, and xenon.
35(1964); http://dx.doi.org/10.1063/1.1713586View Description Hide Description
Elastic resonances in NH4H2PO4 (ADP) have been studied by an optical method which depends on the elasto‐optic effect. Displacement models of the overtone resonance modes have been inferred from the optical patterns. For a (zxt) 45° plate, the transmission patterns for the stronger resonances are consistent with the picture of two longitudinal standing waves traveling at right angles; the reduced frequency constants are independent of the mode number m, and the sample resonates as m 2 Lamé plates. For a (zxt) 0° plate, the greater part of the transmission patterns can be explained in terms of two shear standing waves traveling at right angles; the reduced frequency constants depend on the mode number n, but they tend to become constant as n increases. For both types of plate, the configuration of the electrodes is important in determining which modes are excited and to what extent.
35(1964); http://dx.doi.org/10.1063/1.1713587View Description Hide Description
This paper describes a simple technique for determining the coefficient of kinetic friction for extremely small displacements. An attempt was made to obtain more information about the transition stage between the coefficients of static and kinetic friction. If the force of staticfriction persists over small distances before falling to the lower value of the kinetic friction, such distances in the case of steel on steel are probably smaller than 5×10−6 cm. With the exception of very soft metals, higher values of the coefficient of staticfriction were obtained for impulsive forces than for slowly increasing tangential forces.
35(1964); http://dx.doi.org/10.1063/1.1713588View Description Hide Description
Electrical conductivitymeasurements between 90° and 800°K have been performed on single crystals of uranium dioxide with compositions in the range UO2.000–UO2.007. The activation energy calculated from the lnσT vs 1/T curves decreases from 0.34 to 0.19 eV with increasing excess oxygen content. Hall effect measurements by dc and ac techniques yielded no measurable response, and indicate that the Hall mobility is smaller than 0.015 cm2V−1 sec−1 at room temperature. This result and a consideration of the fundamental parameters determining the transport mechanism suggest an interpretation based on small polaron theory.
35(1964); http://dx.doi.org/10.1063/1.1713589View Description Hide Description
35(1964); http://dx.doi.org/10.1063/1.1713590View Description Hide Description
A spectrophotometric method for determining the concentration of chromium in oriented and unoriented single ruby crystals is described. It makes use of the absorption coefficients for the blue and green bands and for the red lines at ∼300°K. The coefficients are based on concentration data obtained by three independent methods.
35(1964); http://dx.doi.org/10.1063/1.1713591View Description Hide Description
When Permalloyfilms are annealed, the Hc and Hk of the as‐deposited films are observed to decrease and α, the dispersion angle, is seen to increase. Electron diffraction studies show the development of long‐range order in these films with annealing, as evidenced by the appearance of superlattice lines. The changes in Hk and α can be partly explained by this transition from short‐ to long‐range order, and a model is proposed.
35(1964); http://dx.doi.org/10.1063/1.1713592View Description Hide Description
Two variable‐state methods have been developed which both offer the advantage of producing negligible thermal disturbance in a solid during measurement. The first is a curve‐fitting method. It makes use of the fact that the initial temperature rise, due to a constant flux plane heat source, at points within a certain region of a finite solid is essentially the same as that in an infinite solid. Temperature records obtained from such points thus can be analyzed with the assumption of linear heat flow, and used for calculating all thermal properties of the solid.
The second is a pulse method. If a hot or cold pulse is applied to a plane surface of a specimen, the rise or drop of temperature at some distance from the surface will pass through a maximum; from the time of the maximum the thermal diffusivity can be calculated. The main advantage of this method is the ease of specimen preparation.
35(1964); http://dx.doi.org/10.1063/1.1713593View Description Hide Description
35(1964); http://dx.doi.org/10.1063/1.1713594View Description Hide Description
The dynamic stress‐strain relation of aluminum was measured in compression to a stress of 22 kbar using plate impacts to induce uniaxial strain. The results were compared to a uniaxial strain curve constructed from elastic moduli and quasistatically determined stress‐strain curves. The dynamically determined stress‐strain relation was not unique in that it varied as a function of the magnitude of the induced stress and was offset from the quasistatic curve. These small deviations are attributed to strain rate effects. The rate effects were further demonstrated by observing the transmission of an elastic compression stress wave in aluminum after the aluminum was dynamically stressed beyond the yield point and held at constant strain for a period of approximately 2 μsec.
The dynamic stress‐strain relation was measured upon the release of a shock induced compressive stress of 10.5 kbar to a tension of −9 kbar by observing the effects of tensile stress waves. The deviation of the unloading stress‐strain relation from that predicted by simple elastic‐plastic theory is attributed to strain rate and Bauschinger effects.
35(1964); http://dx.doi.org/10.1063/1.1713595View Description Hide Description
A brief description of an Ampère electrodynamic experiment is given and some connection with it is indicated in such effects as those of: Wiedemann, Matteucci (MS), and Hall. Experimental arrangements and procedures in the MS study of ferromagnetics is described. The essential differences in the MS of different ferromagnetics are cited. An interpretation of the MS has been based on the well‐known effect of the exchange energy in ferromagnetic metals dependent on the interatomic distances instead of a phenomenological interpretation in terms of the magnetostriction.