Volume 38, Issue 5, 01 April 1967
Index of content:
38(1967); http://dx.doi.org/10.1063/1.1709818View Description Hide Description
A solution is given to the stress field produced by an arbitrary periodic distribution of dislocations (or plastic distortion) in a half‐space. It is shown how the known results for discrete dislocations are obtained quite simply from this. The solution to the problem of a Frank dislocation network near a free surface is given and its stability examined. The whole‐space stress field decreases exponentially with the distance from the plane of the network, as do the stress terms due to the free surface which decrease exponentially with the distance from this surface. It is shown that the network always experiences a force attracting it to the free surface.
38(1967); http://dx.doi.org/10.1063/1.1709819View Description Hide Description
Essentially, the elastic properties of all the known crystals are anisotropic. This paper presents a convenient method to describe the degree of the elasticanisotropy in a given cubic crystal and then discusses its practical values. On the basis of the well‐known Voigt and Reuss schemes to average the single‐crystal elastic constants for polycrystalline behavior, the degree of elasticanisotropy has been defined as A* = [3(A−1)2]/[3(A−1)2+25A], where A is the usual anisotropy factor given by A = 2c 44/(c 11−c 12). It is shown that the present A* has the folowing properties of practical importance: (a) A* is zero for the crystals of the elastic isotropy, i.e., A = 1. (b) For an anisotropic crystal, A* is a single‐valued measure of the elasticanisotropy regardless of whether A < 1 or A > 1. (c) A* gives a relative magnitude of the actual elasticanisotropy possessed by a crystal.
38(1967); http://dx.doi.org/10.1063/1.1709820View Description Hide Description
The intrinsic brittle strength (i.e., without dislocation motion) was obtained for MgO bicrystals of various misorientations. Bicrystal fracture always occurred at the grain boundary. Strength values ranged between 70 000 psi and 370 000 psi. Two modes of fracture were observed: intergranular and intragranular fracture. The latter occurred only at high strain energies. Secondary cracks, believed to originate from flaws on the grain‐boundary surface, were observed for all bicrystal specimens that fractured in a brittle manner.
Tensile stresses as high as 400 000 psi were obtained on single crystals of MgO before ductilefracture occurred.
38(1967); http://dx.doi.org/10.1063/1.1709821View Description Hide Description
Experiments on the vacuum flashover of dielectric frusta subjected to 35 and 75 nsec pulses show the strong influence of cone angle on insulation strength. A theory is presented to explain the formative process leading to flashover for one polarity. It is proposed that positive surface charge grows by emission of hot electrons during the application of the electric field. When the surface field intensity is sufficient to draw other electrons into it, they will multiply by secondary emission until the critical breakdown condition is achieved. Flashover is thus initiated by secondary emission, but the formative time lag is governed by thermionic emission of electrons. The experimental data are analyzed and interpreted in the light of this theory.
38(1967); http://dx.doi.org/10.1063/1.1709822View Description Hide Description
The development of dislocation networks in gallium crystals, initially free of detectable dislocations, has been studied at room temperature by an x‐ray diffraction topographic technique. The condition required for the development of these networks is the existence of spatial periodic fluctuations in impurity content. The networks develop, in the absence of an external stress, from individual loops which are first detected 3 to 10 days after the crystal has been grown. These loops grow for days at a uniform rate of 10−8 cm/sec and reach a size of a few hundred microns. No specific nucleation mechanism for these loops has been identified. It it is assumed that the loops expand by climb (by the addition of vacancies), their rate of growth and observed densities are consistent with a formation energy for vacanciesU f in the range 0.2 to 0.3 eV and a migration energy for vacanciesU m in the range 0.4 to 0.7 eV. No values of U f, U m, or U f+U m determined from other experiments have been reported and a comparison is not possible at the present time. The change of initially straight dislocation lines into jagged lines has been observed and is interpreted as the climb of screw dislocations into irregular helical dislocations. A marked anisotropy in the growth of the loops and helices was observed and is attributed to an anisotropic coefficient of self‐diffusion.
Dependence of 50‐ and 100‐keV Bremsstrahlung on Target Thickness, Atomic Number, and Geometric Factors38(1967); http://dx.doi.org/10.1063/1.1709823View Description Hide Description
Experimental data have been obtained for the bremsstrahlung spectra produced with 50‐ and 100‐keV electrons incident on different targets. The spectrum per unit solid angle per incident electron is measured for photon emission angles of 30, 70, and 110 deg, for normal and oblique incidence of the electrons on the targets having atomic numbers of 13, 50, and 79, and for four target thicknesses equal to different fractions of the electron range in the given material. Also, results are given for the bremsstrahlung production efficiencies per unit solid angle at a given emission angle. The experimental results at normal incidence show good agreement with the theoretical predictions of Berger and Seltzer, which are based on thin‐target bremsstrahlung cross sections and which employ Monte Carlo calculations to account for multiple scattering and energy‐loss effects. Similar calculations for other geometries should produce reliable estimates of the emitted bremsstrahlung spectra in this same energy region.
38(1967); http://dx.doi.org/10.1063/1.1709824View Description Hide Description
Hall‐coefficient measurements have been made on vapor‐deposited chromiumfilms in the temperature range from −195° to 150°C. The stress in the chromiumfilms was estimated by the shift of Néel temperature and the differential thermal‐expansion coefficients between the substrate and the film. It was observed that compressive stress reduces the Hall coefficient of chromium and tensile stress increases it. A suggestion is made concerning the similarity between the effects of pressure and the effects of changing the number of 3d electrons in chromium.
38(1967); http://dx.doi.org/10.1063/1.1709825View Description Hide Description
A new phase transformation at high pressure and elevated temperature has been found in the InSb pressure‐temperature phase diagram. This transformation to a new phase designated as InSb‐III has been established by measurements of superconducting transitiontemperature (T c) as a function of annealingtemperature at several pressures and by x‐ray determinations on the new phase, both retained at one atmosphere and at high pressure and temperature. The higher T c for InSb‐III (T c = 4.1°±0.1°K) and the x‐ray results lead to the conclusion that InSb‐III is a new phase different from the orthorhombic phase (T c = 3.5°K).
38(1967); http://dx.doi.org/10.1063/1.1709826View Description Hide Description
For single‐ and polycrystallineniobium the relation between etch figures and dislocations is qualitatively demonstrated. Repeated etching allows investigation of the movement and multiplication of dislocations. Besides triangular etch hillocks there often occur etch traces or walls, respectively, and networks that may be interpreted as dislocations lying nearly parallel to the observation plane. Etch figures of helical dislocations have been found too.
38(1967); http://dx.doi.org/10.1063/1.1709827View Description Hide Description
Previous investigators have demonstrated the efficient and cataclysmal removal of material from surfaces hit by a giant‐pulse laser. Experimental evidence is presented here that very fine patterns, defined to 0.1 μ, can be engraved directly in metals or insulators by using the interference from two beams. Besides possible practical uses, the study of the structure of such patterns is expected to further our understanding of the mechanisms involved in damage by giant‐pulse laser impact.
38(1967); http://dx.doi.org/10.1063/1.1709828View Description Hide Description
The molybdates of Sm, Eu, Tb, and Dy are isotypic with ferroelectric Gd2(MoO4)3. Other compositions of this structure occur in the pseudoternary system Gd–La–Y‐molybdate and in the pseudobinary system Gd2(MoO4)3–Gd2(WO4)3. A representative number of these compositions were grown as single crystals and tested for ferroelectricity. Each was found to be ferroelectric as evidenced by a ferroelectric hysteresis loop and by visual observation of domains. The Curie temperatures (T C), spontaneous polarizations (P s), and coercive fields (E c) were determined, with values ranging as follows: T C, 148°–190°C; P s, 0.14–0.23 μC/cm2 at 25°C; E c, 5.0–14.5 kV/cm at 25°C.
38(1967); http://dx.doi.org/10.1063/1.1709829View Description Hide Description
Magnetostriction measurements on Permalloyfilms prepared by vacuum evaporation from a tungstenboat indicate an average Fe enrichment of about 2% above the original melt composition. In addition it is shown that the film composition varies continuously through the thickness from 4% to 7% per 1000 Å for typical evaporation conditions. Compositional gradients of even smaller magnitude are capable of pinning spin waves. Since filament and boat evaporation are used widely in preparing laboratory films, the anomalous spin‐wave results frequently reported in the literature may be due to thickness‐compositional gradients.
38(1967); http://dx.doi.org/10.1063/1.1709830View Description Hide Description
Track‐etching techniques were used to detectboron in austenitic steels. The technique involved placing polymers which were known to be sensitive to alpha particles in intimate contact with the polished metal surface, exposing the specimen to neutrons, and then etching the polymerdetector. Alpha tracks that result from the 10B(n, α)7Li reaction were developed in the polymer by etching in an aqueous NaOH solution. The boron concentration in the steels was quantitatively determined and the results compared with more conventional analytical techniques. It is seen that in addition to measuring the total boron content, the track‐etch method allows one to map the distribution of boron within the microstructure of the metal alloy. The detector materials used were cellulose nitrate and cellulose acetate butyrate. The technique is useful for measuring boron concentrations in the parts‐per‐million range.
38(1967); http://dx.doi.org/10.1063/1.1709831View Description Hide Description
During the imaging process the specimen in a field ion microscope is subject to a stress of 105 kg/cm2. The extent to which the elastic deformation caused by this stress appears in the image of W, Ta, and Mo specimens is investigated. Depending on the tip shape one observes an increase or a decrease in local magnification. When the electric field is increased from 220 MV/cm to 450 MV/cm the change in the projection angle does not exceed ±1.5°.
38(1967); http://dx.doi.org/10.1063/1.1709832View Description Hide Description
The problem of steady laminar flow of elasticoviscous liquids through a porous pipe in the presence of suction or injection has been studied in this paper. The perturbation method has been used to obtain the axial velocity regarding the Reynolds number `R' as the perturbation parameter. The effect of elastic elements in the viscous liquid on the axial velocity, axial pressure drop, and the shearing stress have been shown graphically.
38(1967); http://dx.doi.org/10.1063/1.1709833View Description Hide Description
The classical analysis of Eshelby, Frank, and Nabarro of a linear dislocation pile‐up is generalized to include the case in which the locked dislocation may have a Burgers vector of mb, where b is the Burgers vector of free dislocations and m is a positive real number. The equilibrium positions of (n−1) free dislocations piled up against the locked dislocation under a uniform applied stress are given by the roots of the Laguerre polynomialL n−1 (2m−1). Simple expressions for the distance between the locked and nearest free dislocation,x 1, the length of the pile‐up, L, and the stress at its tip, σtip, are obtained. Increasing m will increase x 1 and decrease σtip, while L is only slightly extended. For large n the stress field within a certain distance range around the tip is found to be independent of m.
Based on the Petch model of yielding it is shown that increasing m increases the Hall‐Petch slope by a factor of (m)1/2. The effect of m on the coalescence of leading dislocations leads to a higher‐fracture stress if m is increased.
38(1967); http://dx.doi.org/10.1063/1.1709834View Description Hide Description
The phenomenon of planar faceting of an otherwise curved interface is re‐examined and shown to provide an experimental means of determining crystal‐growth laws and magnitude of kinetic constants. Facets are often observed to form during crystal growth as a result of the large driving force required to generate new layer sources. This driving force is shown to be orders of magnitude larger than that required for layer‐edge passage. As a result, the interface must depart from its equilibrium morphology and facet in those crystallographic directions requiring source generation. This nonequilibrium departure is related to the actual interfacial undercooling through the temperature gradient at the interface. The effect of the non‐regular morphology on the steady‐state temperature gradient is considered. Pertinent experiments are described in detail and exemplary cases are cited.
38(1967); http://dx.doi.org/10.1063/1.1709835View Description Hide Description
Data for the AlK emission band, AlKα satellites and CuL III band in Al‐Cu alloys (10%–100% Al) are presented. Significant changes in the spectra are noted with changes in composition. The AlK emission band is split and the long‐wavelength component shifts to longer wavelengths and becomes dominant with a decrease in Al concentration. The short‐wavelength component shifts only slightly and maintains the typical sharp, short wavelength‐emission edge seen in pure aluminum. Separation between high‐ and low‐energy components in the AlK band is as much as 4 eV (10% Al) and extrapolates to about 4.5 eV for very dilute alloys, which is nearly the calculated excitation energy of 3s 2→3s3p in Al+. The AlK satellites Kα3 and Kα4 show corresponding intensity changes with alloy composition. The intensity ratio Kα4/Kα3 changes linearly (for 6‐kV excitation) from 0.48 in Al to 0.70 in 10 Al‐90 Cu.
The CuL III band also shows changes in shape with alloying but the changes are not as large as observed for the AlK band. The copper band shifts linearly to lower energy as more and more aluminum is added.
38(1967); http://dx.doi.org/10.1063/1.1709836View Description Hide Description
A thin layer on the surface of single‐crystal germanium produced by 60Co‐gamma irradiation has been found. This layer can only be produced on specimens which were irradiated in air. Several infrared absorption bands with peak absorptions at 3, 6, 6.85, 11.5, and 12.7 μ are associated with the layer. Of these bands, only the 11.5‐μ band has been identified as resulting from germanium dioxide which was formed on the surface by chemical reaction of germanium with oxygen and/or ozone. The origins of other bands as well as the layer remain unknown.
38(1967); http://dx.doi.org/10.1063/1.1709837View Description Hide Description
Thermoluminescence emission above room temperature and optical absorption have been studied in LiF crystals irradiated by gamma rays, with emphasis on conditions which are of greatest interest for applications to radiationdosimetry. The specimens were Harshaw crystals of differing purity, and the purpose was to correlate the electron and hole traps and recombination centers involved in the thermoluminescence with optical‐absorption bands produced by the trapped carriers. The dominant glow peak in suitably treated crystals occurs at about 200°C, and this is correlated with an absorption band at 310 mμ, through thermal and optical bleaching of the thermoluminescence and optical absorption. A smaller glow peak at 100°C is correlated with an absorption band at 380 mμ. The F center absorbing at 250 mμ appears to be the electron trap associated with both glow peaks. It is concluded that the 310‐ and 380‐mμ centers are trapped‐hole centers, and the thermoluminescence occurs after they are thermally ionized. The recombination center which produces the visible‐light emission is believed to be not the F center, however, but an impurity center which absorbs at 196 mμ. An energy‐level diagram is suggested, which can account for the observed optical and thermal transitions. It depends on electron tunneling from the F center to the 196‐mμ center. Available evidence indicates that the 310‐ and 380‐mμ centers involve Mg; the impurity responsible for the 196‐mμ center may be Al.