Volume 35, Issue 12, 01 December 1964
Index of content:
35(1964); http://dx.doi.org/10.1063/1.1713243View Description Hide Description
In order to explain previously observed deviations of measured plasma velocities from velocities predicted by computer solutions based on a snowplow model, the experimental and computer data are here extended over a much wider range of electrode configurations, capacitor potentials, and gas densities in the shock tube. At low densities the experimental velocity curves fall below the predicted curves because of material evaporated from insulation and electrodes. At high gas densities the experimental velocity curve for the largest center electrode coincides with the predicted curve, but for center electrodes of successively smaller diameter the measured curves lie progressively farther above, but parallel to, those predicted, up to a factor of 5.5 times for the smallest electrode used. These results are explainable in terms of a modified snowplow model postulating an effective annular region adjacent to the center electrode, whose thickness is of the same order as the electrode radius, such that the force due to the magnetic pressure inside this annulus acts on only the gas contained within it to produce the measured velocity.
35(1964); http://dx.doi.org/10.1063/1.1713245View Description Hide Description
A retarding potential technique has been employed in determining the velocity spread of electrons emerging through an aperture in the anode of a hollow cathode discharge device which is to serve as an electron source. Estimates of electron temperature for the sample stream emerging into a high‐vacuum drift space were inferred by assuming a Maxwellian distribution. Electron temperatures of ∼1 eV were obtained in this manner, and the assumption of a Maxwellian energy distribution was justified experimentally. Characteristics of the emergent beam are interpreted as being indicative of discharge properties. It was further found that the kinetic energy of the sample stream was consistently several electron‐volts greater than that corresponding to the total voltage applied across the arc.
35(1964); http://dx.doi.org/10.1063/1.1713246View Description Hide Description
The plasma resonance probe was first studied by Takayama and his co‐workers in 1960. Since that time several attempts have been made to determine precisely the experimental and theoretical behavior of the resonance. This paper reviews the various contributions made so far, and develops further the explanation advanced in 1963 by Harp. Detailed experiments have been carried out to confirm the predictions: (i) that the resonance frequency lies below the local electron plasma frequency, (ii) that it is dependent on probe potential and probe size and, (iii) that the resonance should be highly damped when probe dimensions are smaller than a few electronic Debye lengths. If the resonance probe is to be of great practical importance, a simplified theoretical model describing its behavior is desirable. The theory of such a model is presented and compared to experimental results. The paper concludes with a discussion of improved methods of applying the resonance probe, and suggests a ringing technique which provides electron density measurements in laboratory discharges in times of the order of a few cycles at the electron plasma frequency.
35(1964); http://dx.doi.org/10.1063/1.1713247View Description Hide Description
A prism reflector is described which has high reflectivity for only a very narrow angular range (∼1′). The prism makes use of multiple internal reflections near the critical angle, where the dependence of reflectivity on angle is extremely sharp. It is shown experimentally that such a prism can be used to suppress unwanted off‐axis modes in an optical maser. With a typical ruby optical maser a substantial reduction in beam width, as well as an increase in emission near the beam center, is observed. Applications to various optical masers and use as a modulator are discussed.
35(1964); http://dx.doi.org/10.1063/1.1713248View Description Hide Description
The positive ion current emitted by heated filaments of high‐purity Fe displayed sharp maxima when the metal passed through the structural transformation temperature. The height of the maxima varied with previous flashing temperatures and with the direction of approach to the transformation. Plastic deformation also produced maxima in the positive ion current after a temperature‐dependent time delay; the peak height was approximately proportional to strain. Electron emission from the same filaments was unaffected by these factors. The results suggest that the positive ion emission from flash‐cleaned Fe filaments is not surface controlled and they seem to require internal rate controlling mechanisms which involve diffusion of the impurity species to the surface along high diffusivity paths.
35(1964); http://dx.doi.org/10.1063/1.1713249View Description Hide Description
The observed positions of optical absorption bands arising from plasma resonances in small metallic particles are tabulated and compared to the positions calculated from the optical constants of the metals. For very small spherical particles the calculated positions are close to those observed; for larger or non‐spherical particles there are deviations between observed and calculated positions. Similar bands should exist in certain metals in which they have not been observed.
35(1964); http://dx.doi.org/10.1063/1.1713250View Description Hide Description
Vortex streeteddies shed from multiple cylinders located one behind the other in the plane of flow were observed to interact in a very complex fashion. Contraction, expansion, cancellation, and coalescence of vortices occurred for different values of cylinder separation and Reynolds number. The results appear to have important implications both for turbulence promotion during heat or mass transfer and for theoretical interpretation of vortex street phenomena.
35(1964); http://dx.doi.org/10.1063/1.1713251View Description Hide Description
Thermal conductivity measurements on high‐purity SiC and impure Si and SiC have been made over the temperature range from 3° to 300°K. These results show that the thermal conductivityK, of the highest purity SiC is intermediate between those of pure Si and pure diamond, and at 300°K is greater than that of copper. The heat transport in SiC is produced by phonons and these are scattered by other phonons,isotopes, and the crystal boundaries in the pure material.
In impure SiC the phonons are also scattered by the electrically active impurities Al and N. These impurities reduce the K of SiC in much the same way that B and P impurities do in Si. The N impurities in natural diamonds also reduce their K below that of ideally pure diamond, but the effect is rather different since N is not electrically active.
35(1964); http://dx.doi.org/10.1063/1.1713252View Description Hide Description
High‐temperature compression tests have revealed unsuspected high‐temperature strength in block talc. Analyses by x‐ray diffraction indicate that the increase in strength is associated with the transformation of the talc to protoenstatite, silica, and water. The bulk modulus of elasticity also increases with increasing temperature, reaches a maximum at 1100°C and then decreases at higher temperatures. These findings suggest that block talc is not as suitable for a solid pressure‐transmitting medium at elevated temperatures as is commonly supposed.
Local Regions with Biaxial Anisotropy in Thin Polycrystalline Ferromagnetic Films with Uniaxial Anisotropy35(1964); http://dx.doi.org/10.1063/1.1713253View Description Hide Description
Deposited thin ferromagneticfilms with uniaxial anisotropy have local regions with admixed biaxial anisotropy, although there is no biaxial component to the average anisotropy field of the whole film. Two methods are presented for measuring the biaxial contribution to the inhomogeneity in the anisotropy field. The effect of this biaxial inhomogeneity is investigated theoretically and found to explain much puzzling data: the various negative anisotropy effects, the relationship between α90 and Δ90 (the inhomogeneity in orientation and in magnitude of the anisotropy, respectively), the internal biasing field, the compositional dependence of the inhomogeneity and negative anisotropy, the deposition substrate temperature dependence of negative anisotropy, the position of the negative anisotropy peak, the shape of the probability density function for films with both large and small inhomogeneities, the ratio of positive to negative anisotropy, and the skew of the Δ90probability density curves. Theoretical switching threshold curves for complex (biaxial) anisotropy are drawn; theoreticalprobability density functions are plotted for various values of biaxial inhomogeneity and compared with experimental curves. Local regions of biaxial anisotropy can arise from the interaction of strongly coupled adjacent areas of the film with different anisotropy fields, as well as from magnetocrystalline anisotropy.
35(1964); http://dx.doi.org/10.1063/1.1713254View Description Hide Description
Room‐temperature microwaveferrimagneticresonancemeasurements of the large uniaxial magnetocrystalline anisotropy fields (H an) of two compositional series of hexagonal ferrimagnetic oxides are reported. H an was found to increase rapidly in the Al‐substitution series (SrO·xAl2O3·(6−x)Fe2O3) from 19.3 to 53.4 kOe as x was increased from 0 to 1.7. In the TiCo series (BaO·x[TiCoO3]·(6−x)Fe2O3), H an was found to decrease from 17.5 to 6.6 kOe as x was increased from 0 to 0.78. Experimental plots are given for the variation of H an with x. The linewidths of these oriented polycrystalline compounds were all about 2 kOe. The g value was consistently found to be about 1.9.
Variation with x of the respective saturation magnetizations, anisotropy constants, and anisotropy fields are discussed; relations are presented.
35(1964); http://dx.doi.org/10.1063/1.1713255View Description Hide Description
A study is reported of the superconductivity and microstructure which occur when small quantities of La are quenched with Rh in the arc furnace. A strikingly regular prismatic honeycomb of a superconducting compound is found when the La concentration is ≥0.5 at. %. For smaller concentrations a continuous network of the superconducting phase is no longer observed which correlates with the lowering and broadening of the superconducting transition region. Evidence is given that for such low concentrations superconductingtunneling occurs through the elemental Rh phase itself thus pointing to the superconductivity of Rh at lower temperatures.
35(1964); http://dx.doi.org/10.1063/1.1713256View Description Hide Description
High‐speed magneto‐optical experiments show that when magnetization reversal takes place in a magnetic film in a period of a few microseconds, roughly diamond‐shaped domains form with the long axis of the domain parallel to the film easy axis. Calculations show that domains of this shape form because the component of the magnetic field parallel to the easy axis is uniform along the boundary of such a domain.
35(1964); http://dx.doi.org/10.1063/1.1713257View Description Hide Description
The model (for predicting Curie temperatures of ternary alloys) developed in a previous paper is extended by removing the assumption that the fraction of plus and minus spins contributed by an element in the alloy is directly proportional to the concentration of the element. Empirical data concerning the Curie temperatures of binary Co‐Ni, Fe‐Ni, and Fe‐Co alloys is again used to predict the Curie temperature of ternary fcc Fe‐Ni‐Co alloys. The predicted temperatures agree with experimental values within 30°C over the range of concentrations where the magnetic moments of the elements in the alloy do not vary with concentration (nickel concentration greater than 0.55 and the cobalt concentration greater than 0.4).
35(1964); http://dx.doi.org/10.1063/1.1713258View Description Hide Description
Films in the thickness range 100 to 500 Å are studied. Ohmic contacts(goldelectrodes) are formed only sporadically. In the best example of Ohmic contact, space‐charge‐limited current (SCLC) occurs at a field of 3×104 V/cm. SCLC is obtained in this and other specimens at fields great enough for field emission (3 to 5×105 V/cm) where trap densities about 1023/m3 were found, greater by a factor of ten than the lower field case. In less conclusive results and in general agreement with theory, a greater dependence on thickness as compared to voltage is found. The establishing of SCLC at fields great enough for field emission suggests a causative relation, perhaps tunneling through the blocking contact at the electrode‐insulator interface. The change in the ac resistivity is lower than that using dc by about three orders of magnitude over the temperature range of 20° to 95°C. Three activation levels are found with transition points at 20° and 110°C. It is necessary to use a layer of SiO at the crossover region of the electrodes to prevent dielectric breakdown in this area. This protective layer is made sufficiently thick not to interfere significantly with the measurements on the polybutadiene.
35(1964); http://dx.doi.org/10.1063/1.1713259View Description Hide Description
The conductive properties of indiumfilms vacuum deposited at room temperature were examined in a shielded cryotron configuration for thicknesses up to 10 200 Å and temperatures down to 2.90°K. Due to agglomeration, films were conductive only for thicknesses exceeding a thickness tc about 2000 Å for large rates of deposition and 5000 Å for small rates. For thicknesses tb or greater, room‐temperature resistivity was independent of thickness and had a value of 9.2±0.3 μΩ‐cm. Thickness tb was about 2.5 and 1.5 tc for high and low deposition rates, respectively. The superconductive critical temperature was 3.420°±0.005°K in the thickness range tested, 2200 to 10 200 Å. Critical fields were described by the London theory with penetration depth having the temperature dependence suggested by the Gorter‐Casimir theory and value 700 ± 100 Å at 0°K. Film current for first appearance of resistance had a dependence on film thickness markedly different from any previously reported in the literature. This critical current exhibited a distinct peak at a thickness between tc and tb . The peak values agreed quantitatively with the London theory assuming width‐independent current density and adopting a critical current density switching hypothesis. It was postulated that structure inhomogeneities served to maintain a uniformly distributed current on a macroscopic scale for certain thicknesses between tc and tb .
35(1964); http://dx.doi.org/10.1063/1.1713260View Description Hide Description
An investigation of the magnetoresistance of single crystals of n‐type indium oxide has been made at room temperature. The general behavior of the magnetoresistance is consistent with constant energy surfaces in the conduction band corresponding to the ``warped‐spherical'' model or with the ``many‐valley'' model along the and  axes of symmetry with a small anisotropy of the conduction band.
35(1964); http://dx.doi.org/10.1063/1.1713261View Description Hide Description
Polycrystalline black phosphorus has a hydrostaticpiezoresistive coefficient π H =3.04×10−10 cm2/dyn independent of pressure and decreasing by 28% between 0° and 100°C, and a highly stress‐sensitive uniaxial piezoresistive stress coefficient π l >3×10−9cm2/dyn under appropriate conditions. The thermoelectric power is in the range 330 to 413 μV/deg, and Young's modulus=6.89×1010±15% dyn/cm2, linear to at least 4×106 dyn/cm2 tension. The ultimate tensile strength is 3.4×107 dyn/cm2 and the ultimate compressive strength is 3.5×108 dyn/cm2. These properties of black phosphorus make it potentially useful for strain and hydrostatic gauges, and other transducer use. The advantages and disadvantages relative to other materials are discussed briefly.
35(1964); http://dx.doi.org/10.1063/1.1713262View Description Hide Description
Mixed crystals of CdS‐CdSe were prepared in a sealed system by solid‐statediffusion; by starting with pure CdS crystals, a composition as high as CdS0.6Se0.4 was easily obtained. With increasing Se content, the sharp absorption edge of the compound shifted from approximately 5200 to 6200 Å. The composition of the mixed crystals prepared under equilibrium conditions agrees with values calculated from available thermo‐dynamic data and with the assumption that the mixed crystals form ideal solid solutions.