Volume 36, Issue 8, 01 August 1965
Index of content:
36(1965); http://dx.doi.org/10.1063/1.1714485View Description Hide Description
A method has been developed for taking thermal diffusivity data at elevated temperatures with a cylindrically symmetric sample consisting of a stack of disks. An axial heater is turned on and the temperatures at three radii in the midplane are recorded as functions of time. The temperature data from the inner and outer radii are used as empirical boundary conditions, and the temperature data from the middle radius are used to obtain the thermal diffusivity. A computer has been programmed to make the calculations by the method of finite differences. Thermal diffusivity measurements have been made from 26° to 895°C on Armco iron. The values found for the thermal diffusivity by this radial method agree well with the values found by Shanks et al. who used a longitudinal method with a finite rod. The radial method is particularly advantageous when samples in the form of disks, but not in the form of long rods, are available.
36(1965); http://dx.doi.org/10.1063/1.1714486View Description Hide Description
Crystallographic properties of the spinel systems Li0.5Ga2.5−x Mn x O4 (0≤x≤0.7), Li0.5Fe2.5−x Mn x O4 (0≤x≤0.5), and Li0.5Al2.5−x Mn x O4 (0≤x≤0.5) have been investigated in order to determine the effects of trivalent manganese substitution on the crystal chemistry of the ordered spinel hosts. In each system relatively small concentrations of Mn3+ removed long‐range ordering of lithium ions on the octahedral sites. This loss of order was accompanied by anomalies in the dependence of lattice parameters on composition. These observations can be qualitatively interpreted by assuming that Mn3+ ions tend to cluster in order to reduce the elastic energies associated with Jahn‐Teller stabilization. These effects may provide indirect evidence for lattice imperfections such as have been invoked to account for square B‐H hysteresis loops in ferrite materials containing Jahn‐Teller ions.
36(1965); http://dx.doi.org/10.1063/1.1714487View Description Hide Description
It is shown that in ferrites exhibiting spontaneously square B‐H loops, reverse‐domain nucleation may occur either at a grain boundary or at a chemical inhomogeneity having a volume roughly 0.1 of a grain volume, a magnetization about 2% different from that of the parent matrix, and no static crystallographic discontinuity at the inhomogeneity‐matrix interface. It is pointed out that such chemical inhomogeneities may occur in spinels having Jahn‐Teller ions in concentrations that are too small to produce a cooperative, static distortion of the entire crystal from cubic symmetry. It is postulated that the formation of such inhomogeneities is responsible for the sharp knee in spontaneously square B‐H loops found in many ferrites containing Mn3+ and Cu2+ ions, in particular the magnesium‐manganese composition used in memory‐core ferrites.
36(1965); http://dx.doi.org/10.1063/1.1714488View Description Hide Description
The addition of a second component to the cesium gas phase of a thermoionic generator could increase its efficiency by reducing the work functions of the electrodes.
The different possible additives were screened and hydrogen, since it appeared to be the most promising, was chosen for further study.
The work function reduction of cesium‐covered nickel,molybdenum, and tungsten due to the presence of H2 in the gas phase ranges from 0 to 0.5 eV. An equilibrium condition can be established between the surface and the gas phase to make the low work function surface stable at elevated temperatures.
36(1965); http://dx.doi.org/10.1063/1.1714489View Description Hide Description
The work function of cesium‐covered nickel has been measured by the thermionic and photoelectric methods. A modulated light beam technique allowed the measurement of the photoelectric emission currents in the presence of thermionic currents that were 106 times larger. The minimum work function of cesium‐covered nickel was determined to be 1.42±0.05 eV by the thermionic method at a nickel temperature of 530°K and a cesium reservoir temperature of 333°K. The results obtained by the photoelectric method were, on the average, 0.05 eV lower than the values determined by the thermionic method. The discrepancy is expected because of the bias introduced to both methods by local variations in work function. The agreement of the data obtained by the two methods indicates that the Richardson constant is close to the assumed value of 120 A/cm2·deg2. The work was undertaken as a first step toward the improvement of cesium‐filled thermionic generators by lowering the collector work function.
36(1965); http://dx.doi.org/10.1063/1.1714490View Description Hide Description
Single crystals of bismuth germanate Bi4(GeO4)3 having the eulytine structure, space group Td 6, were grown. They were crucible‐free pulled from a pool of molten material produced in its own powder at 1040°C by a thermal imaging technique. The material has an index of refraction of 2.07 and transmits light between 0.4 and 6.0 μ. The electro‐optic coefficient r 41 was determined to be 1.03·10−10cm/V. Although materials with higher r 41 values are known, Bi4(GeO4)3 may find applications because the growth of large crystals appears possible by classical pulling techniques. A large variety of chemical substitutions are possible in the eulytine structure and might lead to improvements in the magnitude of r 41.
36(1965); http://dx.doi.org/10.1063/1.1714491View Description Hide Description
This paper is concerned with the experimental determination of the arrangement of the double‐plane probes and of an upper limit of a magnetic field for valid double‐probe measurement. One of the most reliable arrangements is that of two probes facing each other and situated parallel to the field. Also, the spacing of the probes must be significantly larger than 2λ D . The upper limit of the magnetic field, below which the double‐probe method remains reliable, is determined mainly by the diffusion‐controlled collecting mechanism.
36(1965); http://dx.doi.org/10.1063/1.1714492View Description Hide Description
A new electrostatic probe method utilizing the triple probe is proposed in which no voltage or frequency sweep (or switching) is required. This feature enables us to determine the instantaneous values of the electron temperature (Te ), as well as the electron density (ne ), within a short time which is of the order of the intrinsic response time of the probe itself (⪝1μsec). Moreover, the system allows the direct display of the Te values as well as the semidirect display of the ne values on appropriate display units, thus permitting us to eliminate almost all procedures usually required for data processing. In view of the features mentioned, this system may be a useful diagnostic tool not only for stationary plasmas but also for rapidly varying time‐dependent plasmas of various types.
This paper presents theoretical considerations for the instantaneous direct‐display system using a symmetrical triple probe. Discussions are also presented for the estimation of errors caused by the variation of ion saturation current, for the application to magnetoplasmas, and for the time and spatial resolutions. Experimental confirmation was made through the measurements of stationary magneto‐ as well as non‐magnetoplasmas. As an example of the application to time‐dependent plasmas, the electron temperaturewaves accompanied by the moving striations in hydrogen discharge were observed.
36(1965); http://dx.doi.org/10.1063/1.1714493View Description Hide Description
The thermal conductivities of both the solid sulfur and the liquid sulfur were continuously measured with an accuracy of ±1.0%. The temperature range in this experiment is from 0° to 160°C. In the temperature range from 0° to 160°C, two transition points exist; one of them is the transition point from rhombic sulfur to monoclinic sulfur at 95.5°C, and the other is the change point from monoclinic sulfur to liquid sulfur at 118.9°C. At each transition point, discontinuous and conspicuous changes in thermal conductivities can be observed by this experiment.
Moreover, a new result was obtained: The thermal conductivity of monoclinic sulfur decreases with increasing temperature, although the conductivity which was obtained by Kaye's experiment increases with temperature increment.
36(1965); http://dx.doi.org/10.1063/1.1714494View Description Hide Description
Densities of metalfilms were measured using multiple beam interferometry in combination with an oscillating crystal microbalance. Film densities of Al, Au,Ag, Cr, and Cu were found to be independent of thickness within experimental accuracy of about 5% in the thickness range from 200 to 5000 Å for fixed deposition conditions. Measurements indicate that film densities were equal to bulk densities within measurement accuracy for Al, Au,Ag, and Cu for deposition at about 10−6 Torr. The film density of Cr was found to be strongly dependent on oxygen partial pressure above 10−6 Torr, decreasing sharply as the oxygen partial pressure increased. The effect of condensation coefficients of metalfilms on the optical measurements is discussed.
36(1965); http://dx.doi.org/10.1063/1.1714495View Description Hide Description
A method of computing demagnetizing factors using a resistance network analog or an electrolytic tank has been devised, and the demagnetizing factors for spheres, rods, disks, and tubes have been computed for the case μ = ∞. When the size of the model on the analog is appropriate, the value obtained for a sphere is within 1% of the theoretical value of ⅓. Although the demagnetizing factor is not a precise concept for rods, disks, or tubes, in which the induced magnetization is not uniform, this term is often used for convenience. In this paper, the ``ballisticdemagnetizing factor'' (defined in terms of the mean magnetization over the median cross section of the object) is considered. The values obtained for rods and disks are compared with those found in the literature, principally with Bozorth and Chapin's compiled values. Typical results are 0.37 (0.47), 0.223 (0.27), 0.117 (0.14), and 0.052 (0.056) for (L/d) = ½, 1, 2, and 4, respectively, where (L/d) denotes the length‐to‐diameter ratio and the values in the brackets are the previously used values. The axial nonuniformity of the magnetization in rods and tubes is also measured and discussed.
36(1965); http://dx.doi.org/10.1063/1.1714496View Description Hide Description
Nonlinear rf effects in plasmas are discussed in this paper. It is shown that the strong type of harmonic generation mechanism is due to the equation of motion for the plasma electrons being nonlinear, as is the relationship between current and velocity. A theory of plasma harmonic generators and parametric oscillators is given. Input reaction is taken into account and it is shown that a resonance at the output frequency tends to lead to high efficiency. Such a resonance is the electrostatic resonance of a plasma sphere. Experiments carried out with a small spherical rf discharge are described. A 25% conversion efficiency to the second harmonic of an S‐band signal has been obtained. Measurements have been made up to the seventh harmonic. For maximum efficiency a plasma resonance at the harmonic of interest is required. By tuning the system to be resonant at half the input frequency, parametric oscillations were obtained at this frequency, thus proving that the nonlinearity must be of a reactive type. It is believed this is the first time that parametric oscillations have been observed in a plasma.
36(1965); http://dx.doi.org/10.1063/1.1714497View Description Hide Description
We have measured the absolute Raman scattering cross section for the 1345‐cm−1 shifted Stokes line in nitrobenzene using a ruby laser. We have also measured the efficiency of conversion of radiation from a giant pulse ruby laser into stimulated Raman scattered radiation for this line. We have found that the average intensity needed to produce a given conversion is one or two orders of magnitude less than that predicted by the theory of Hellwarth using our measured cross section. We discuss some possible explanations for this disagreement.
36(1965); http://dx.doi.org/10.1063/1.1714498View Description Hide Description
The stability of extended dislocation barriers in face‐centered cubic metals is determined using the theory of anisotropicelasticity. It is found that the results typically differ from the isotropic elasticity solution by 25%, but the difference can in special cases amount to as much as a factor of four. The anisotropic results are in qualitative agreement with earlier isotropic results in predicting the relative strength of the barriers to dislocation glide. The results suggest that the measurement of barrier extension should provide the most accurate available means of estimating the stacking‐fault energy in fcc metals.
36(1965); http://dx.doi.org/10.1063/1.1714499View Description Hide Description
The possibility of generating high‐frequency acoustic waves by illuminating the negative end of a cadmium sulfide rod with light of periodically varying intensity exists. The strains produced and the acoustic gain present under different conditions of drift current and resistivity are calculated for three decades of frequency.
36(1965); http://dx.doi.org/10.1063/1.1714500View Description Hide Description
Thermal radiation from a fully ionized gas near the plasma frequency is investigated theoretically with consideration given to boundary effects. It is shown that the emission spectrum is a relatively complicated function of frequency, temperature, density, and geometry, but that the absorptionspectrum can be approximated by a simple function of the three parameters representing frequency, collisions, and geometry. The sensitivity of the absorption curve to these parameters is investigated, and the results are used to interpret recently measured infrared radiation from a theta pinch.
36(1965); http://dx.doi.org/10.1063/1.1714501View Description Hide Description
Steady‐state creep of polycrystallinesodium choride having grain size stabilized by the addition of an aluminum oxide dispersion and containing about 4% porosity was investigated at 740°C over a range of grain sizes from 65–130 μ and at stresses from 2.1–8.4 kg/cm2. Deformation was viscous (strain rate directly proportional to stress) at a rate in good agreement with the Nabarro—Herring equation, assuming the deformation rate to be limited by chlorine ion diffusion. Grain‐size dependence and absolute creep rates were satisfactorily predicted with Laurent and Bénard's diffusion data, including the influence of grain size on diffusion coefficients.
36(1965); http://dx.doi.org/10.1063/1.1714502View Description Hide Description
On the basis of a small‐signal, one‐dimensional analysis, a set of basic macroscopic differential equations, governing the fluctuations in quantities such as the electron‐beam temperature, the mean velocity, and the current density, has been derived by taking moments of the Liouville equation with respect to the velocity variable. This set of differential equations expresses the conservations of charge, momentum, and energy, and is valid for an arbitrary amount of velocity spreading and includes the effect of heat conduction.
A system of differential equations, governing the correlation among the fluctuations in the mean velocity, current density, and beam temperature, is also derived. The relationship among the various noise parameters along the electron beam is obtained in the form of a system of differential equations whose solution gives detailed information on the variation of the noisiness parameter along the beam. The solution of the system of differential equations thus derived is also discussed.
Dielectric Constant and Magneto‐Optical Kerr Rotation of Ferromagnetic Chromium Tribromide above the Absorption Band Edge36(1965); http://dx.doi.org/10.1063/1.1714503View Description Hide Description
By extending the optical and magneto‐opticalmeasurements of the insulating ferromagnet CrBr3 (hexagonal layer structure, Tc =36°K, and 4πMs =3520 G) beyond the absorption band edge near 22 000 cm−1 with reflection techniques, three absorption bands representing the π and σ processes of electron transfer transitions and an internal transition of Cr3+ have been observed. The room‐temperature dielectric constant was measured by means of an ellipsometer. The σ process and the internal transitions give rise to the two absorption bands at 24 310 cm−1 and 29 580 cm−1 with oscillator strengths of 0.30 and 1.61, respectively. They account for most of the dielectric constant. While the π process is too weak to be resolved in the dielectric constant data, it accounts for the main peak of the normal incidence polar Kerr rotation at 1.5°K of 3.5° at 23 500 cm−1. The σ process accounts for the relatively sharp negative peak in the Kerr rotation of −2.5° at 26 700 cm−1. The data further suggest that the π process component of the off‐diagonal element of the electric polarizability tensor has opposite sign to the σ process and the internal transitions.
36(1965); http://dx.doi.org/10.1063/1.1714504View Description Hide Description
An analysis is presented of the propagation of magnetoelastic waves in transversely magnetized rods. Efficiencies for longitudinal wave‐spin wave and shear wave‐spin wave conversion are evaluated for the case of a [1̄10] rod magnetized along . A longitudinal to shear wave converter in yttriumirongarnet is discussed which is estimated to have 20‐dB conversion efficiency at 3 Gc/sec.