Volume 31, Issue 8, 01 August 1960
Index of content:
31(1960); http://dx.doi.org/10.1063/1.1735834View Description Hide Description
The stability of neutron bombardment damage in Sb‐doped Ge has been investigated by making continuous measurements of the electrical conductivity and Hall mobility following a neutron pulse. Measurements were made in the temperature range from 77° to 308°K with a time resolution of 1 sec. At temperatures near 195°K an initial decrease in conductivity and mobility was followed by an additional decrease which exhibited nearly second‐order kinetics. At 273°K and above, an initial decrease in conductivity and mobility was observed, but was followed by a recovery consistent with an activation energy of 0.68 ev. The void region model of Gossick and Crawford has been employed to explain the initial decrease in mobility and a major portion of the initial decrease in conductivity. The transitory changes in mobility and conductivity after the neutron pulse are considered as changes in the void volumes.
31(1960); http://dx.doi.org/10.1063/1.1735835View Description Hide Description
The performance characteristics of a conventional thermoelectric generator operating under constant heat flux conditions have been computed. Expressions for power output, optimum efficiency, and optimum ratio of internal‐to‐load resistance show that optimization requires operating conditions appreciably different from those for the operation of thermoelectric generators having constant hot and cold junction temperatures. Performance curves are presented for the optimum ratio of internal‐to‐load resistance when either the internal resistance or the load resistance is under the control of the designer.
31(1960); http://dx.doi.org/10.1063/1.1735836View Description Hide Description
The stress fields of an infinite and a finite screw dislocation wall are given and from them the interactions of the screw dislocation walls with parallel and nonparallel dislocations have been studied. The nature of the penetration through a screw dislocation wall of a number of parallel screw dislocations driven by external stress and the possible stress concentration for a pileup against the wall are discussed in detail. The forces exerted by a screw dislocation wall on nonparallel edge and screw dislocations are calculated. The significance of the local torques due to these forces as a possible mechanism for the screw dislocation wall to become a source for generating dislocations is pointed out. The metastability of the screw dislocation walls is demonstrated and their strain energy is estimated and compared with that of the edge dislocation walls. It is concluded that the resistance to penetration of a dislocation wall increases in the following order: infinite edge dislocation wall, finite edge dislocation wall, finite screw dislocation wall, and infinite screw dislocation wall. It is also found that the strength of a dislocation wall increases with its surface strain energy.
31(1960); http://dx.doi.org/10.1063/1.1735837View Description Hide Description
The technique of the total reflection of x rays has been applied to the study of thin films of Cu,Ni,Ge, and Se vacuum‐deposited onto polished glass substrates. Starting with fresh films, ``smooth'' enough to exhibit pronounced x‐ray interference fringes in the region just beyond the critical angle, the effects of vacuum anneal and oxidation were studied. Changes in the reflection curves are interpreted in terms of possible structural changes in the films. Reflection from layers of particles of carbon or polystyrene latex deposited onto ``smooth'' substrates was also studied for comparison.
31(1960); http://dx.doi.org/10.1063/1.1735838View Description Hide Description
The theory of phase oscillations in the synchrotron, first given by Bohm and Foldy, is extended to cover the case where the amplitude of the accelerating voltage is not constant and where the accelerated bunch is itself sufficiently large to produce an appreciable part of the accelerating voltage. It is found that the phase oscillation is more stable when the driving frequency is less than the natural frequency of the accelerating electrode system, and that the plate characteristic of the driving amplifier has a negligible effect on the phase stability.
31(1960); http://dx.doi.org/10.1063/1.1735839View Description Hide Description
Current noise measurements on the same lightly dopedcadmium sulfide crystal having indium soldered contacts of different quality are used to demonstrate the strong influence of contacts on the observed noise spectra. The results suggest the presence of shallow trapping states distributed in energy and located near the electrodes. The concentration of these states depends on the quality of the contact. These effects are not contact noise in the usual sense since the shape of the noise spectra is only moderately affected; however, the noise level may change by orders of magnitude.
31(1960); http://dx.doi.org/10.1063/1.1735840View Description Hide Description
The solid‐liquid interface shape during growth from the melt has been determined for pure materials in the vicinity of external and internal surfaces; both the cases of isotropy and anisotropy of solid‐liquid interfacial energy have been considered. From these considerations it is possible to determine some effects associated with interface and external surfaceanisotropy. For example, a preferred direction of growth, a surface layer phenomenon and stray‐crystal formation during seeding and normal crystal growth may arise as a result of the solid‐liquid interface configuration.
31(1960); http://dx.doi.org/10.1063/1.1735841View Description Hide Description
A study was made at 4080 Mc of the partition noise introduced into a beam by a meshed grid as a function of its position along the beam. Simultaneous information concerning the beam dynamics was obtained by means of a beam analyzer. With cathodemagnetic fields somewhat below those corresponding to immersed flow, the partition noise exhibited sharp dips as the grid in moving along the beam passed through the planes which corresponded to image planes of the cathode. These observations confirm the picture of noise smoothing taking place over relatively small areas in the vicinity of the cathode and elucidate the contribution of the random interception of electrons to the partition noise. With small cathodemagnetic fields the partition noise varied sinusoidally with the cyclotron periodicity as the grid was moved along the beam. This variation with position is thought to result from the contribution to the partition noise of higher‐order mode conversion.
31(1960); http://dx.doi.org/10.1063/1.1735842View Description Hide Description
The linear thermal expansion coefficients of the a and c axes of bismuth have been measured from 80°K to 540°K by mechanical means. These same coefficients were determined over a more limited range of temperatures by single crystal and powder x‐ray methods. The mechanical and x‐ray values did not differ significantly. The expansion coefficient for the c axis was found to be 16.6±0.4×10−6 (C°)−1 at 100°K, 17.3±0.2×10−6 (C°)−1 at 150°K, and 17.6±0.2×10−6 (C°)−1 at 540°K. For the a axis the expansion coefficient was 10.8±0.3×10−6 (C°)−1 at 100°K, 11.6±0.2×10−6 (C°)−1 at 150°K, and 11.8±0.2×10−6 (C°)−1 at 540°K.
31(1960); http://dx.doi.org/10.1063/1.1735843View Description Hide Description
The dislocation structure of type 6H hexagonal siliconcarbide has been studied by etching combined with optical microscopy and by x‐ray diffractionmicroscopy. The validity of the conventional etching technique for identification of the sites of the intersection of dislocations with (0001) surfaces has been established. However, high densities of dislocations lying in (0001) planes and hence heretofore undetected by etching techniques were often observed by diffractionmicroscopy. Dislocations with [112̄0] vectors have now been found with evidence for slip both on basal planes and on a ``puckered'' pyramidal plane. Pileups formed by slip and dislocation walls formed by climb were also observed. Siliconcarbide shows many of the characteristics of more conventional plastically deformablematerials.
31(1960); http://dx.doi.org/10.1063/1.1735845View Description Hide Description
The general theory of stationary beams of prescribed shape is developed taking into account the electrostatic interaction between the particles. The Hamilton‐Jacobi equation, expressed in a particular system of coordinates, allows the determination of the potential distribution inside the beam. Compatibility conditions for the equations are obtained, as well as a general rule for the determination of the feasibility of arbitrarily chosen trajectories. A theoretical expression results for the proportionality coefficient k in Child's law (J=kV ) which relates current densityJ with the potential difference V between electrodes. The theory was applied to the particular case of a plane beam with hyperbolic boundaries. The beam was experimentally reproduced, providing values for k in close agreement with the theoretical ones.
31(1960); http://dx.doi.org/10.1063/1.1735846View Description Hide Description
Several experimental tests dealing with the effects of stress waves initiated in solids by high explosives or by impacts are described. The tests indicate that stress waves which are initiated solely as compression commonly acquire tensile components which are not due to reflections.
Spectral Emissivity, Total Emissivity, and Thermal Conductivity of Molybdenum, Tantalum, and Tungsten above 2300°K31(1960); http://dx.doi.org/10.1063/1.1735847View Description Hide Description
The emissivities and thermal conductivities of high purity arc melted molybdenum and tantalum and powder metallurgytantalum and tungsten were measured between 2300°K and their respective melting points. A method is presented for the determination of spectral and total emissivities from the determination of brightnesstemperature at the center of an electrically heated rod as a function of heat flow rate. The method does not depend on the experimental achievement of blackbody conditions. Spectral emissivities were also determined by measurement of brightnesstemperatures at the respective melting points. Thermal conductivities of electrically heated rods were determined by the Jain and Krishnan longitudinal heat flow method. The spectral emissivities of tantalum and tungsten decrease linearly with increase in temperature. The spectral emissivity of molybdenum is constant. The total emissivities of molybdenum,tantalum, and tungsten increase with increasing temperature. For any of the metals the spectral and total emissivities are closest to each other at the melting point. The melting point of tantalum is significantly lowered by small increase in impurity content. The thermal conductivities of molybdenum,tantalum, and tungsten decrease linearly with increasing temperature. The ranges of spectral emissivity, total emissivity, and thermal conductivity above 2300°K are as follows:
31(1960); http://dx.doi.org/10.1063/1.1735848View Description Hide Description
A magnetic memoryeffect is observed in a low Curie pointferrimagnetic spinel (∼NiFe0.15Cr1.85O4) containing a coherent antiferromagnetic impurity (NiO) with a high Néel point. It is explained in terms of the Meiklejohn‐Bean exchange anisotropy coupling the two phases. The past magnetic history is remembered in the spontaneous reacquisition of a fraction of the past remanent state despite prolonged storage at temperatures above the ferrimagneticCurie point. A feature of the explanation is the condition that antiferromagnetic domains in NiO rotate irreversibly in accessible fields.
31(1960); http://dx.doi.org/10.1063/1.1735849View Description Hide Description
Forces arising on ion‐bombarded electrodes have been measured with a torsion balance immersed in a low‐pressure Hg plasma. The force curves obtained for 22 metals in the range of 20 to 300 ev bombarding energy indicate that Hg+‐ions are completely accommodated (α=1) on clean metal surfaces and the forces originate essentially from the ejection of sputtered atoms. On comparing the forces with the sputtering yields, one obtains information on the average ejection velocities of sputtered atoms which are found to be much higher than those of thermally evaporated atoms. Oxide‐covered surfaces exhibit a quite different behavior inasmuch as the accommodation coefficient for Hg+‐ions seems to be less than unity.
31(1960); http://dx.doi.org/10.1063/1.1735850View Description Hide Description
The space charge theory of Langmuir has been extended to include the effects of anode emission on the performance of a vacuum thermionic power converter. The basic equation is similar to Langmuir's ξ—η equation, but it involves two additional parameters which depend on the temperatures and thermionic properties of the electrodes. An iterative technique is described for obtaining solutions in specific cases, and some sample calculations based on hypothetical diodes are presented. The calculations indicate that the effect of the anode temperature is considerably more pronounced if the cathode‐anode work function difference is large.
31(1960); http://dx.doi.org/10.1063/1.1735851View Description Hide Description
Effects of magnetoresistance and extraordinary Hall effect on the propagation of electromagnetic waves through very thin sheets of ferromagneticconductors are studied. These contributions to the conductivity produce electric fields varying like products of current and magnetization and introduce nonlinear interactions, which are particularly large in the neighborhood of ferromagnetic resonance. Detailed first‐order dcelectric fields are obtained for the configuration of a very thin ferromagnetic sheet very close to a conducting wall. The results predict signals strictly proportional to incident microwave power, and mostly inversely proportional to film thickness, becoming independent of thickness for the thinnest films. Their magnetic field or frequency dependence gives typical resonance curves modulated by slowly varying amplitudes. The maxima and zeros of these amplitudes are very sensitive to the local field configuration. Once this latter is well known, however, these effects can be employed to study conveniently the high‐frequency conduction and magnetic properties of the thinnest films.
31(1960); http://dx.doi.org/10.1063/1.1735852View Description Hide Description
The temperature dependence of Young's modulus of vitreous GeO2 has been determined by a dynamic resonance method from −195°C to 540°C. The modulus increases with increasing temperature from about −120°C to 400°C. Below and above this range the modulus decreases with increasing temperature. Young's modulus for vitreous SiO2 also increases from about −190°C to 1175°C and decreases with increasing temperature outside this range.
In view of the similarity in structures and bond energies of these two materials, the similarity in the elastic modulus‐temperature relations is believed to be significant; especially when contrasted with the lack of agreement in another commonly measured anharmonic property,thermal expansion.
31(1960); http://dx.doi.org/10.1063/1.1735853View Description Hide Description
The thermal conductivity of eight halide crystals has been measured over the temperature range 220°K to 390°K. The thermal conductivities of potassium bromide, potassium chloride, and sodium chloride vary inversely with temperature for the lower part of this temperature range, and are nearly independent of temperature for the higher temperatures of the range measured. The thermal conductivities of cesium bromide, cesium iodide, and silver chloride are nearly independent of temperature throughout the entire temperature range. The thermal conductivities of barium fluoride and calcium fluoride vary more rapidly with temperature than anticipated from a T −1 relationship; however, the accuracy of the data is not sufficient to establish the exact form of the variation of the thermal conductivity with temperature. The comparative method used for these measurements is described.
31(1960); http://dx.doi.org/10.1063/1.1735854View Description Hide Description
Dislocation densities and arrangements were studied, as a function of specimen thickness, in metal crystals grown from the melt. Aluminum crystals containing fewer than 104dislocations per cm2 were produced by this method. The results of this study are consistent with the mechanism of collapsing vacancy disks for generating dislocations in crystals grown from the melt.
It was found, in aluminum, that below a dislocation density of about 106 lines per cm2, dislocations remain in a random network and do not form sub‐boundary arrays. The range of effective interaction between dislocations, to form arrays, thus appears to be limited to the average distance between dislocations when the density is about 106 lines per cm2, or about 10 μ.