Volume 26, Issue 10, 01 October 1955
Index of content:
Methods for the Correction of X‐Ray Intensities for Primary and Secondary Extinction in Crystal Structure Analysis26(1955); http://dx.doi.org/10.1063/1.1721871View Description Hide Description
The theory of x‐ray extinction in diffraction studies is reviewed, and application to small crystals completely bathed in the incident beam is considered. Using a suitable approximation to a sum of Bessel functions, a correction formula is obtained, valid in the region of small Bragg angle θ where the extinction is likely to be greatest. The formula is discussed; and it is shown that when the crystal structure is essentially known, as in later stages of refinement, it is possible to separate the effects of primary from secondary extinction and to correct the experimental data accordingly.
26(1955); http://dx.doi.org/10.1063/1.1721872View Description Hide Description
A matrix method is used to solve the difference equations which describe the zone refining of a bar. The matrix method is designed for direct numerical calculation of the solute distribution, after any number of passes, with any initial solute distribution. The method is illustrated with a simple example.
Streaming Birefringence as a Hydrodynamic Research Tool—Applied to a Rotating Cylinder Apparatus above the Transition Velocity26(1955); http://dx.doi.org/10.1063/1.1721873View Description Hide Description
Streaming birefringence was studied in the annular space between a fixed outer and a rotating inner cylinder at velocities on both sides of transition to Taylor vortexflow. With a well‐collimated light beam, quantitative measurements were possible as close as 0.1 mm from a wall. For an aqueous colloidal solution of bentonite, shear stress could be evaluated from birefringence measurements only when, for a given birefringence, the angle of extinction was the same in the unknown flow as in a known laminar flow. In turbulent shear flow this is not generally the case. For a pure liquid, ethyl cinnamate, the angle of extinction remained at 45° to the streamlines, within experimental error, under all flow conditions obtainable; thus even in turbulent shear flow the birefringence appears to arise primarily from the principal stresses. Assuming birefringence proportional to mean shear stress permits construction of a velocity profile across the gap, yielding the correct value of velocity of the moving wall within limits of experimental error. Thus in this type of two‐dimensional shear flow the mean shear stress can be measured by means of the streaming birefringence of a pure liquid.
Study of the Initial Stages of Sintering Solids by Viscous Flow, Evaporation‐Condensation, and Self‐Diffusion26(1955); http://dx.doi.org/10.1063/1.1721874View Description Hide Description
The mechanism of material transport in sintering can be elucidated in some cases by direct observation of the rate of interface growth and approach of centers between spherical particles. Measurements with glass, sodium chloride, and copper indicate that with these materialsviscous flow, evaporation‐condensation, and self‐diffusion are the rate‐determining mechanisms. Values of viscosity,vapor pressure, and diffusion constants have been determined, but calculations of diffusion constants from these data are subject to uncertainties of interpretation. A model is presented for the behavior of copper during the initial stages of sintering, which is in agreement with available experimental data, and which requires vacancy elimination at dislocations or grain boundaries. Data for refractory oxides indicate the importance of purity and fabrication pressure, but the sintering mechanism for these materials is not determined by the present data.
26(1955); http://dx.doi.org/10.1063/1.1721875View Description Hide Description
A theory of steady‐state creep is developed using Mott's mechanism of dislocation climb. It is assumed in the analysis that the rate‐controlling process is the diffusion of vacancies between dislocations which are creating vacancies and those which are destroying them. The concentration of vacancies along a dislocation line is determined by setting the change in the free energy caused by a decrease or increase in the number of vacancies equal to the change in the elastic energy occuring during dislocation climb. The creep equation that results from the analysis is where α is a constant , Q is the activation energy of self‐diffusion, kT has its usual meaning, and σ is the stress. This equation is valid in the stress range from the critical shear stress to a stress about equal to 108 to 109 dynes/cm2. At larger stresses the creep rate increases much more rapidly with stress.
26(1955); http://dx.doi.org/10.1063/1.1721876View Description Hide Description
It is concluded from torque measurements on single crystals of Alnico 5 that the high coercive force of these crystals depends upon the shape anisotropy of the fine precipitated plates and that the crystal anisotropy of the plates is negligible. The present physical picture of the alloy is that it consists of plates of precipitate of high magnetic saturation separated by a matrix of comparatively low magnetic saturation. The plates are made up of rods of precipitate (approximately 75 A in diameter ×400 long), but they act magnetically like single domain plates. The magnetization vector turns in the plane of the plates, and it is the length‐to‐width dimension ratio of the plates which determines the effective shape anisotropy and the resulting coercive force of the alloy.
26(1955); http://dx.doi.org/10.1063/1.1721877View Description Hide Description
The frequency‐dependence of internal friction of four α brass (62.5% Cu) reeds was measured at room temperature under conditions where contributions to the internal friction other than that arising from transverse thermal currents were small and assessable. Measurements were made by the free‐decay method, at maximum strain amplitudes of 5×10−6. By using specimens of two different thicknesses, frequencies differing from the frequency of peak damping by nearly a factor of 20 were obtained by measurements in the range 7–370 cy/sec. The results, in conjunction with measurements of Young's modulus (1.06×1012 dynes/cm2±0.5%), coefficient of linear expansion (18.7×10−6/°C±0.5%), density (8.42 g/cc±0.2%), specific heat (0.088 cal/g/°C±2%), and thermal diffusivity (0.34 cm2/sec±5%), indicate that Zener's theory of damping by transverse thermal currents is probably valid exactly. From the position of the peak, the thermal diffusivity is found more precisely as 0.346 cm2/sec±1.5%.
26(1955); http://dx.doi.org/10.1063/1.1721878View Description Hide Description
A hysteresis effect in multiplier phototube noise as a function of tube gain has been found for both RCa 5819 and DuMont 6292 tubes. The effect is a function of both temperature and the length of time that a tube has ``rested.'' The hysteresis is seen to be a noise buildup with time but not a change in dynode multiplication. The reasons for this effect are not clear. Several possible mechanisms are discussed. Conditions are determined for obtaining reproducible noise counts for liquid scintillation counting of low‐energy beta emitters.
26(1955); http://dx.doi.org/10.1063/1.1721879View Description Hide Description
This paper deals with a new kind of electron microscopy in which the specimen constitutes an electron mirror. The low velocity of the electrons in front of this mirror‐specimen enables pictorial representations of potential distributions on surfaces. A resolving power comparable to that of the light microscope has been obtained. Sample micrographs depicting surface reliefs, potential distributions on surfaces, and the distribution of electrical conductivity are shown. The peculiar kind of image formation is briefly discussed and reference is made to the potentialities of such a research tool for surface physics and metallurgy.
26(1955); http://dx.doi.org/10.1063/1.1721880View Description Hide Description
In this paper we illustrate by a simple example that when we wish to express the admittance of a cavity excited through a hole in terms of natural frequencies of oscillations which can exist when the cavity is short‐circuited, it is important to remember that the short‐circuit must conform to the impressed field. When this is done, one need not fear that the ``obvious'' set of natural modes may be incomplete.
Spatial Distribution of Thermal Neutrons from a Polonium‐Beryllium Source in Water‐Zirconium Mixtures26(1955); http://dx.doi.org/10.1063/1.1721881View Description Hide Description
The spatial distribution of neutrons emitted by a Po–Be source and moderated by water‐zirconium mixtures has been measured for four water‐zirconium volume ratios. From these data, migration areas of source neutrons to energies below the cadmium cutoff have been determined with an uncertainty of ±2%. The experimental values of the migration area M 2 are:
26(1955); http://dx.doi.org/10.1063/1.1721882View Description Hide Description
The gas evolved from glass at temperatures below the softening point, which is of interest in bake‐out problems, is primarily water. The water evolved from unit surface at constant temperature above 300°C is linear with respect to the square root of the time of bake‐out. The intercept of the linear plot, which can be altered by different surface treatments, is a measure of the easily removed water residing at the surface. The slope is a measure of the rate of evolution of water that has diffused to the surface from the interior.
Values of the diffusion constant for water and concentration gradients after bake‐out have been calculated for a soda‐lime glass. The diffusion constant is an exponential function of the reciprocal of the absolute bake‐out temperature. Values of the activation energy for the diffusion process are given for eight glasses.
A method of calculating the amount of water that will diffuse from glass for any time‐temperature conditions following any bake‐out is presented.
26(1955); http://dx.doi.org/10.1063/1.1721883View Description Hide Description
The frequency spectrum of the noise associated with prebreakdown currents in insulators has been investigated in the audio‐frequency range. Most of the noise is due to surface discharges which can be eliminated by special types of electrodes and careful experimentation. The remaining noise has a frequency spectrum with the highest amplitudes at very low frequencies; it falls rapidly to the Johnson noise level in the measuring resistor at about 104 cps. This noise apparently originates from fluctuations in the field‐emission current.
26(1955); http://dx.doi.org/10.1063/1.1721884View Description Hide Description
The frictional adhesion between projectile and target during a ballistic penetration has been measured with a torsion‐type Hopkinson bar. The apparatus allows measurement of the torsional adhesion of a spinning projectile during target penetration. By assuming the friction resisting rotation to be equal to that resisting axial penetration, the energy loss due to friction was computed. The results show that the torque‐time pattern during penetration of a ``mechanically'' clean projectile can be predicted with the assumption of a frictional energy loss just sufficient to keep the sliding surfaces at the melting temperature of the metal. Metallographic analysis of the target metal at the projectile interface gives a further indication of a molten interface. In these tests, sliding friction accounts for about 3% of the striking energy of the projectile; common surfacecontaminants, not necessarily special lubricants, reduce this loss to less than 1%.
26(1955); http://dx.doi.org/10.1063/1.1721885View Description Hide Description
A method is described for elimination of the effects of preferred orientation of platelets in a compressed powder sample having a plane surface exposed to the x‐ray beam. The powder is diluted with a low absorption amorphous resin and the results extrapolated to zero concentration. The extrapolation is justified by analysis of an idealized model of the sample. A comparison between the analytical results and the results of an experimental study using silver iodide indicates the validity of the process.
26(1955); http://dx.doi.org/10.1063/1.1721886View Description Hide Description
Conduction currents in hexane, studied as a function of spacing, metal, and heat treatment of the electrodes over the field‐strength range from 0 to 250 kv/cm, obey neither a field‐enhanced thermionic emission nor the customary field emission relation. The most important factor determining the currents appears to be a surface layer on the cathode. Exposing the liquid to gamma rays increases the current in a fashion that indicates a decrease of the effective work function rather than merely a collection of ionization products. Heat treatment of the cathode, by changing the nature of the surface layer, changes the emission. It appears that positive ions collect on the more or less insulating surface layer and build up intense local fields that cause electron emission. The size of the local field depends on the number of the ions arriving per second and on the thickness, homogeneity, and conductivity of the surface layer.
For some electrodes the current increases with gap width for constant field; previous investigators have interpreted this fact as evidence of impact ionization in the liquid. However, change of current with gap width proves to be an electrode property; some electrodes do not show it.
In one case the current was measured up to breakdown (1060 kv/cm). There is an indication that impact ionization may exist above 800 kv/cm; if so, the ionization factor is very small.
A variety of purification systems was tried but no influence of the purification treatment on the conduction currents was found.
26(1955); http://dx.doi.org/10.1063/1.1721887View Description Hide Description
The effect of water vapor on the photoresponse curves and reverse current characteristics of germanium and siliconnp junction units has been investigated. It is concluded that changes in surface recombination velocity are only important at low relative humidities. Channel conduction accounts for the increased current of a silicon unit but another mechanism must be postulated for the germanium case. The possibility of this being ionic conduction is discussed.
26(1955); http://dx.doi.org/10.1063/1.1721888View Description Hide Description
Previously described fine‐particle magnets have been based upon crystal anisotropy; this paper reports permanent‐magnet properties derived from the shape anisotropy of substantially elongated single‐domain iron particles. The shape anisotropy of these particles overcomes the limitation imposed on the energy of previous fine‐particle ironmagnets by the low crystal anisotropy of iron.
The predicted and observed properties of crystal anisotropy fine‐particle magnets are reviewed and compared with an ideal fine‐particle ironmagnet based upon the shape‐anisotropy model of Stoner and Wohlfarth. Experimental results are reported for magnets made by aligning and compacting single‐domain iron particles 150 angstrom units in diameter, with a median length‐to‐diameter ratio of three to one, and an intrinsic coercive force before packing of 1600 oersteds. The effect of particle alignment and packing fraction on magnetic properties is described, and energy products above three million gauss‐oersteds are reported. These results are compared with properties predicted from theoretical considerations, and with existing permanent‐magnet materials.
- LETTERS TO THE EDITOR
26(1955); http://dx.doi.org/10.1063/1.1721889View Description Hide Description