Volume 37, Issue 9, 01 August 1966
Index of content:
37(1966); http://dx.doi.org/10.1063/1.1708858View Description Hide Description
It is shown on the basis of experiments with CoSi(57Co) in a high magnetic field and with magnetically ordered Co0.5Fe0.5Si that iron does not develop a magnetic moment greater than 0.1 μB at low temperature in the system Co1−x Fe x Si. The results also indicate that the magnetic properties are not directly related to the electron and hole bands pertinent to transport properties. The isomer shift shows that the configuration of Fe is independent of composition over the entire range of composition. The electron concentration dependence of the quadrupole splitting is discussed.
37(1966); http://dx.doi.org/10.1063/1.1708859View Description Hide Description
The high‐temperature thermal properties of building material in thick slab form can be measured by a transient method reported earlier by Harmathy. The results here show the technique suitable for thin‐film polymers. Recently available ultrathin (0.00025 cm) constantan foil is shown capable of providing constant thermal flux. This foil reduces the original measurement time by two orders of magnitude, from 400 to 4 sec. Sample volume can be reduced at least 90%. Thermal diffusivity,thermal conductivity, and specific heat are measured on an absolute basis, with errors of 2%, 2.5%, and 3%, respectively. Results are given for polyethylene terephthalate, low‐density polyethylene, and cork composition, all in the range −50° to 30°C. These results support the accuracy and validity of the technique. The experimental sample can be built up from polymer films as thin as 0.0025 cm. Previously the sample consisted of slabs of approximate thicknesses 1 and 3 cm.
37(1966); http://dx.doi.org/10.1063/1.1708860View Description Hide Description
A neutron activation technique has been developed to investigate cesium‐aluminum sputtering in measured, rarefied atmospheres of oxygen. The sputtering yield was measured as a function of cesium ion energy and as a function of the ratio of oxygen molecular flux arriving at the target surface to sputteredaluminum flux leaving the surface. In all measurements the ion beam was normal to the monocrystalline aluminum target surface and to the (110) crystallographic plane. The yield increased monotonically from 0.74 (atoms/ion) at 1 keV to 2.4 at 10 keV and was unaffected by flux ratios less than 0.2. The yield decreased for higher values of the flux ratio and eventually reached a saturation value where it is conjectured that a surface film of Al2O3 is maintained.
Effect of Neutron‐Induced Defects on the Current‐Carrying Behavior of Vapor‐Deposited Niobium Stannide37(1966); http://dx.doi.org/10.1063/1.1708861View Description Hide Description
The current‐carrying capacity of vapor‐deposited Nb3Sn can be increased by introduction of structural defects by neutron irradiation. The change in current density per unit of neutron flux is a function of the initial perfection of the material. The characteristics of low‐α material (α≈J cH) are altered more than high‐α material by irradiation. The current capacity of the material is eventually decreased by continued irradiation. The current behavior of material exhibiting an α less than 15 kG·A/cm2 can be described by a simple Lorentz force model. Material exhibiting an α higher than this is not adequately described by such a model. The electrical scattering cross section of a neutron‐damaged area is calculated to be ∼4×10−12cm2.
37(1966); http://dx.doi.org/10.1063/1.1708862View Description Hide Description
Permalloythin films in the thickness range from 100 to 2530 Å and of melt composition 82% Ni‐18% Fe were deposited in an ultrahigh‐vacuum system and the standing spin‐wave modes (SSWM) of these films were measured in vacuum and in air. The SSWM shifted to higher magnetic fields as the films aged in vacuum, but shifted to lower magnetic fields as they aged in air. Since the SSWM of the films displayed a quadratic dependence on mode number at all times, the observed spectra shifts could be accounted for by changes in g, 4πM, and A/M with no observed changes in pinning. Annealing greatly altered the absorption spectra, indicating that a significant change in the pinning occurred.
It is concluded that spin pinning exists in a modest vacuum (p≃1×10−7 Torr) even before the films have been exposed to the atmosphere and that spin pinning is dependent on the substrate used in that spin pinning is greatly affected by annealing.
37(1966); http://dx.doi.org/10.1063/1.1708863View Description Hide Description
It is demonstrated that there is an error in von Laue's treatment of diffuse scattering of x rays from random alloys and that a correct formulation predicts that the diffuse intensity must vanish at reciprocal lattice points. Thus there is no reason to reconstruct the Cowley theory, as has been recently suggested. The physical origin of this effect and some of its consequences are discussed.
37(1966); http://dx.doi.org/10.1063/1.1708864View Description Hide Description
At a given magnetic field,critical currents may be induced to flow in the surface layers of a superconductor by the application of a relatively small‐amplitude alternating‐current field. Many measurements of this induced current have been made as a function of the intensity of the direct‐current field, and the frequency and amplitude of the alternating‐current field. The results of these measurements may be interpreted by assuming that there is a near‐surface layer of a few hundred angstroms thickness which is able to carry extraordinary current densities from 3 to 20 times the bulk current densities at a given field. Current densities calculated for this surface layer exceed 107 A/cm2 at low fields. Details of the observations vary with sample history, but the general features of these measurements have been observed in all samples tested.
Antiphase Domains in Ordered Au3Cu Alloys. II. Comments on ``Electron Diffraction Study of Order in the CuAu3 Alloys'' by Watanabe and Fisher37(1966); http://dx.doi.org/10.1063/1.1708865View Description Hide Description
The distinction between the long‐period superlattice and the ordinary antiphase domain structure in ordered alloys, which has often been misunderstood, is critically discussed with respect to the alloy Au3Cu. In particular, remarks are directed to the recently published paper by Watanabe and Fisher. Au3Cu is ordered, but the size of antiphase domains remains small even after prolonged annealing. Irrespective of apparent similarities, it is shown that Au3Cu is not a long‐period superlattice. Some discussions concerning peculiar properties of Au3Cu are given.
37(1966); http://dx.doi.org/10.1063/1.1708866View Description Hide Description
Fine‐grain ferrites were fabricated by hot‐pressing ball‐milled and flame‐sprayed powders of Ni1−x Co x Fe2O4 (where 0≤x≤0.04) and Ni0.973Mn0.027Fe2O4. Threshold fields for subsidiary absorption at Ku‐band were significantly increased by decreasing the grain size to 5 μ or less. Between 5 and 10 μ, the critical fieldh crit decreased gradually and above 10 μ was independent of grain size. The highest value observed was 192 Oe on a fine‐grained nickel‐cobalt ferrite vs ∼24 Oe for conventionally fired nickelferrites.Dielectric losses were reduced to values below 5×10−4 at X‐band, and low‐field magnetic losses were approximately two to four times those of conventionally fired ferrites of the same composition. No systematic variation in threshold field or magnetic loss was observed with cobalt substitution in fine‐grain samples, although threshold fields in general were found to be higher when cobalt was present. The threshold field increased linearly with cobalt substitution on large‐grain samples. A Ku‐band 90° ferritephase shifter using a fine‐grained, hot‐pressed ferrite was capable of operating at power levels in excess of 750 kW/750 W, whereas the same device with a conventional ferrite exhibited nonlinearity below 20‐kW peak power.
37(1966); http://dx.doi.org/10.1063/1.1708867View Description Hide Description
The growth and decay of infrared absorption bands in the spectra of oxygen‐doped siliconirradiated with fast neutrons or 2‐MeV electrons at −50°C have been studied. These bands at 836, 922, 932, and 936 cm−1, are associated with defect impurity complexes, and previously have been observed after electron irradiation at temperatures as low as 80°K. The gradual growth of the 836‐cm−1 band, associated with an oxygen‐vacancy complex, from the low‐intensity measured immediately after neutron irradiation to approximately four times that intensity upon annealing to 275°C is in contrast with the gradual decay of the band observed after electron irradiation over the same temperature range. The growth of the band is attributed to an increase in the concentration of the O‐V complex as additional vacancies are evolved from defect clusters produced by neutron irradiation. Shifts in the annealing curves of a band at 865 cm−1, which appears upon annealing above 0°C, and the band at 936 cm−1 suggest that the corresponding complexes also involve vacancies. The constant intensity ratio and identical annealing behavior for the 922 and 932 cm−1 bands indicate that they belong to different vibrational modes of the same complex. No shift in the annealing curves for these bands was observed for the neutronirradiated sample, suggesting that the corresponding complex does not involve vacancies but rather may involve interstitial Si atoms.
37(1966); http://dx.doi.org/10.1063/1.1708868View Description Hide Description
Fission‐spectrum neutron damage of n‐type, float‐zone silicon is compared with 60Co gamma‐ray damage. The damage was measured at 293°K by the degradation of the minority‐carrier lifetime. The room‐temperature damage coefficients, defined as K = (1/Φ) [(1/τai)−(1/τ0)], are 5.8×10−6 (neutrons/cm2)−1 sec−1 for neutron irradiation and 2.8×10−10 (photons/cm2)−1 sec−1 for gamma‐ray irradiation where τ0 and τai are the initial and after‐irradiation carrier lifetimes, and Φ is the integrated flux. The neutron damage coefficient is found to be independent of the irradiation temperature (76°–300°K) in contrast to a marked decrease with decreased irradiation temperature which has been observed in the gamma‐ray damage coefficient. Neutron damage recovery is observed over a broad temperature range from 60°–220°C, while gamma‐ray damage recovers sharply between 150° and 210°C. These results indicate that the energy transferred to the silicon recoils by the irradiating particle influences the number and kind of primary defects produced at 76°K and also affects defect reordering at higher temperatures.
37(1966); http://dx.doi.org/10.1063/1.1708869View Description Hide Description
The dynamic mechanical properties of single crystals of linear polyethylene have been measured at temperatures between −190° and 133°C. The measurements were made with a free‐oscillation torsion pendulum at a nominal frequency of 1 cps on mats of single crystals which had been annealed at various temperatures up to 133°C. Broad‐line NMR and small‐angle x‐ray measurements were also made on samples of single crystals which had undergone the same annealing treatment.
The alpha and gamma relaxations previously observed in melt‐crystallized polyethylene were found to occur also in the single crystals. Some evidence was also found for two other relaxations, one occurring below the temperature of the gamma relaxation and the other above that of the alpha relaxation. These relaxations are not discussed in detail. In the original, unannealed crystals, the gamma relaxation is about one‐third the magnitude of that in a slowly cooled, melt‐crystallized sample and also occurs at a lower temperature: −142°C as opposed to −120°C. The alpha relaxation is more pronounced in the single crystals, having about twice the magnitude of that in the melt‐crystallized sample, and also occurs at a lower temperature: 30°C as opposed to 45°C. Annealing at temperatures below 100°C has no effect on the relaxations. Upon annealing at progressively higher temperatures above 100°C, the gamma relaxation increases and the alpha relaxation decreases in magnitude and at the same time the relaxations move to higher temperatures. After annealing at 133°C the relaxations occur at the same temperature as in the melt‐crystallized sample, but are smaller in magnitude. These changes in the relaxations occur at the same annealingtemperatures as the increase in the lamella thickness and NMR mobile fraction.
The hypothesis is advanced that the gamma relaxation is due to the stress‐induced reorientation of defects within the lamellae, and the alpha relaxation to the reorientation of the folds at the surfaces of the lamellae. This hypothesis is consistent with the variation of the magnitude of the relaxations with annealing. The increase in the magnitude of the gamma relaxation is attributed to the generation of defects within the lamellae, either thermally or through the formation of dislocations and vacancies as the lamellae thicken. The magnitude of the alpha relaxation is inversely proportional to the lamella thickness, and its decrease upon annealing can be attributed directly to the reduction in the number of folds as the lamellae thicken. This hypothesis is substantiated by the effect of electron irradiation of a mat of single crystals, which reduces the magnitude of the alpha relaxation but leaves the gamma relaxation unchanged.
The gamma relaxation in the single crystals of polyethylene is shown to be identical to a low‐temperature relaxation previously reported in crystals of n‐alkanes. Evidence is also presented that the gamma relaxation in melt‐crystallized polyethylene is due to the same mechanism as that in the single crystals. This suggests that the gamma relaxation in the bulk polymer does not arise from a discrete amorphous phase, but from within the lamellar structure. It is also suggested that the alpha relaxation in the melt‐crystallized polyethylene is due to the same mechanism as that in the single crystals.
37(1966); http://dx.doi.org/10.1063/1.1708870View Description Hide Description
The charge distribution in thermally grown silicon dioxide is determined by measuring the change in the charge density in the silicon inversion layer as the oxide is gradually etched off. It is found that after prolonged annealing at relatively low temperatures the Si‐SiO2 system reaches a stable state. In this stable state the average charge depth in the oxide is a constant fraction of the initial oxide thickness. This behavior is independent of growth conditions. It is also shown that exposure to high temperatures in a neutral ambient greatly reduces the inversion layer. It is suggested that the inversion layer is controlled by two types of states in the oxide. One is a permanent electron ``donor'' state, determined by the basic oxide structure. The other is a trapping state which can be created by a short high‐temperature exposure.
37(1966); http://dx.doi.org/10.1063/1.1708871View Description Hide Description
The early growth stages of sputtered (S) vs evaporated (E) Ag/mica and Ag/NaCl films have been studied by transmission electron microscopy and electron diffraction. The observed maximum density of the islands at thicknesses less than 20 Å is slightly higher for S films, although the order of magnitude is the same (∼1011/cm2) for both the S and E films.Electrostatic charges carried by the S atoms and the point defects caused by the penetration of the energetic S atoms could account for the observed higher S‐island density.
As the film thickness increases, the S‐island density at 25°C decreases rapidly to reach a constant value before the film becomes continuous. The corresponding E‐island density decreases slowly. The higher mobility, as indicated by the rate of change of the island density, and the observed epitaxialgrowth of the S films at a temperature considerably lower than that obtained for E films, are attributed to the higher kinetic energy of the S atoms. The E‐island density decreases rapidly as the substrate temperature is increased. The decrease in S‐island density is, however, accompanied by an increase in the surface area of the islands and formation of bridges between the islands, thereby resulting in coalescence at a smaller nominal thickness than is observed in E films. These observations are explained on the basis of the effects of higher electrostatic charges and kinetic energy of the S atoms.
The activation energies for the diffusion processes, calculated from the temperature dependence of the separation between the islands, are 0.1 and 0.15 eV for S and E films, respectively, at substrate temperature below 250°C. Above 250°C, the activation energies increase rapidly to 0.5 and 0.9 eV, respectively.
37(1966); http://dx.doi.org/10.1063/1.1708872View Description Hide Description
A technique has been developed for determining the properties of deep centers in conducting, n‐type gallium arsenide. A Schottky barrier is produced at the surface by means of a blocking contact of gold or electrolyte. A reverse bias is applied and the ionization of deep centers in the dark causes the barrier capacitance to change with time. After this, electrons may be injected into the barrier by strongly absorbed light and captured by the deep centers that have previously ionized. Again, the kinetics of the process can be determined from measurements of the barrier capacitance as a function of time. The concentration, energy level, and capture cross section of the dominant center have been determined. It is a donor lying 0.74 eV below the conduction band edge and typically present in concentrations around 1016/cm3. From capture‐cross‐section data it appears to be negatively charged when occupied. This property is sufficient to limit minority‐carrier lifetimes to very small values.
37(1966); http://dx.doi.org/10.1063/1.1708873View Description Hide Description
The elastic constants of tantalum have been determined from room temperature to 500°C by means of a dynamic resonance method to obtain the adiabatic bulk modulus at elevated temperature. The adiabatic bulk modulus (Bs ) of tantalum is found to be 1897 kbar at room temperature and it decreases almost linearly with a slope (dBs/dT) of −0.11 kbar/°C as temperature rises. The calculation of the Grüneisen constant γ and a parameter δ, defined as − (dBs/dT)/αBs , from the present data shows that both parameters are almost independent of the temperature (1.57 for γ and 3.1 for δ). The constancy of γ and δ at high temperatures has also been tested for tungsten by using the recent data of Lowrie and Gonas [J. Appl. Phys. 36, 2189 (1965)]. The results indicate that both γ and δ are virtually constant at high temperature. The applicability of the following equation to estimate the bulk‐modulus‐temperature relationship for the refractory metals Ta and W has also been tested:,
where H is the enthalpy and the superscripts indicate values at 25° and T°C.
37(1966); http://dx.doi.org/10.1063/1.1708874View Description Hide Description
The distribution of dislocations in deformed calcium tungstate was studied by electron transmission microscopy. The photomicrographs confirmed earlier optical observations that (001) 〈100〉 is the slip system of this material between 500°–900°C. Screw dislocations, long edge‐dislocation dipoles, quadrupoles, and tangles of dislocations were observed in foils prepared from specimens deformed at 500°C. Specimens deformed at 700°C showed similar features, except that many of the long dipoles had split into long strings of prismatic loops. At 900°C the loops were more isolated and sometimes grew to a diameter exceeding the thickness of the foil. Many single dislocations could also be seen with nearly pure edge or screw orientation. Two models are proposed for quadrupole formation and their relative stability considered. Strain hardening is thought to be due to the interaction of dislocations with multipoles and prismatic circular loops.
37(1966); http://dx.doi.org/10.1063/1.1708875View Description Hide Description
We report the results of measurements of the low‐frequency dielectric behavior and of persistent polarization in single‐crystal LaF3. This ionic conductor exhibits unusually large polarization effects. The measurements distinguish at least two components of polarization. One component, with a time constant of the order of milliseconds, is linear in the applied voltage; the long‐term polarization is proportional to its square. The phenomena are evidently associated with surface effects, but they cannot be described by the accepted space‐charge theory of ionic conductors. They appear, however, to be qualitatively consistent with an extension of the notion of ionic surface states introduced by Frenkel.
37(1966); http://dx.doi.org/10.1063/1.1708876View Description Hide Description
The elastic constants c 33 and c 44 of hexagonal selenium have been determined and are compared with the elastic constants of glassy selenium. It is shown that the bonding forces in glassy selenium are mainly of van der Waals type. The temperature variation of these elastic constants in the region of glass transition temperature has also been studied.
37(1966); http://dx.doi.org/10.1063/1.1708877View Description Hide Description
Theoretical investigations in the area of laser‐induced damage in transparent media have been primarily directed toward explaining the mechanism by which sufficient energy can be absorbed to cause the observed fracture. However, the work considered here is based on a theoretical investigation of damage from a macroscopic point of view, with the goal of developing a simple continuum mechanical model for the processes leading up to fracture. In this case, the energy absorbed and nonlinear absorption effects are related to the beam's total energy through an experimentally determined absorption parameter. The problem is formulated in terms of dynamic thermoelasticitytheory, the energy absorbed from the laser beam being represented by a volume heat source with a physically reasonable space and time dependence based on diffraction theory and known parameters of the optical system. The nonhomogeneous, thermoelastic field equation is solved for the stress distribution by a Green's function technique. By introducing a tensile stress fracture criterion, conditions under which fracture can be initiated during the irradiation process are obtained. Initial numerical results based on an f/30 lens give a good explanation of the phenomenon of the pulverized region associated with laser damage and indicate that this region has a diameter of about 0.4 mm, which is in good agreement with experimental findings.