摘要:

Measurements of minor loops of SmCo5powders have revealed that wall motion is predominant in the magnetization process. A relation between the reversible susceptibility &khgr; of the minor loops and the coercivityHchas been found, showing that the coercivity of the investigated material is determined by the pinning of domain walls, rather than nucleation. The relation &khgr;Hc2=const has been found in a wide range of coercivities obtained by aging of the material. The aging process is interpreted as a decrease of the density of pinning sites, each site retaining its pinning force. Hysteresis loops measured on single particles of a few microns size confirm these interpretations and allow for a refined model of pinned‐wall coercivity of SmCo5powders.

ISSN:0021-8979    

DOI:10.1063/1.1658556

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

High‐voltage transmission electron microscopy has been used to study the structure of thin films of cobalt ferrites of various compositions. Small defects have been identified to result from a combination of chemical polishing used to prepare foils and subsequent radiation damage in the electron microscope. Such defects must be distinguished from the characteristic defects, such as small precipitates, since they are representative only of the preparation and examination techniques.

ISSN:0021-8979    

DOI:10.1063/1.1658557

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The equations of motion are obtained for certain magnetic sources moving above a conducting plate and subject to the magnetic forces produced by the eddy currents in the plate. The effects of a small disturbance at timet=0 to a previously steady motion are investigated. It is shown that any small disturbance dies away, leaving the system in essentially the same state of steady motion as before.

ISSN:0021-8979    

DOI:10.1063/1.1658558

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The strength of anisotropic polycrystalline carbon fibers was investigated. Theoretical estimates of the strength of graphite are far higher than observed values for carbonized rayon fibers, and we have searched for an explanation. Internal ``grain boundaries'' in the fibers can account for the reduced strength inasmuch as the fracture appears to be intergranular. The ``grains'' are believed to be axially oriented fibrils, ap‐approximately 500‐Å across. Experimental evidence for some inelastic behavior in carbon fibers at room temperature was obtained from bending experiments on single filaments. The results were interpreted in terms of local ``yielding'' or rupture of cross‐link bonds between adjacent fibrils. The stress at which such ``yielding'' takes place in bending is approximately equal to the failure stress in a tensile test. Although a Griffith model of completely brittle fracture in tensile tests was considered, the possibility that local ``yielding'' might precede tensile fracture seemed more likely. Consideration of the brittle failure model and the localized plastic flow model both lead to the same conclusion: The tensile strength of carbonized rayon fibers is limited by the fibrillar microstructure and cannot be expected to approach the theoretical estimates for graphite single crystals.

ISSN:0021-8979    

DOI:10.1063/1.1658559

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The materials considered in this paper exhibit the following phenomena: (1) a time‐dependent stress approaching a steady value with increasing time in a simple‐tension relaxation test, and (2) a time‐dependent strain rate approaching a steady value with increasing time in a simple‐tension creep test. They are also substantially isotropic and incompressible. Such materials are very common, but, as is shown in this paper, continuum models which will represent them are not. Even very general models such as the Rivilin‐Ericksen and hygrosteric models are shown to be inadequate for this purpose. Models of function type, which do admit the phenomena described above, are considered here within the domain of finite deformations and nonlinear material response. The level of generality of the models is such that the variables appearing in the constitutive equation may be chosen but the form of the function relating them may not. This level forms the groundwork for specialized classification, and leaves open the possibility that a theoretical program of experimentation may be constructed which allows a given material to numerically identify the form of its function within the framework of the general model. As a first step in the search for a continuum model for these materials, the following question arises. Within the level of generality described, what is the simplest model of function type which will represent the material behavior discussed? Re‐expressed: what is the least number of variables which must appear in the constitutive equation, and what are these variables? These questions are answered in this paper.

ISSN:0021-8979    

DOI:10.1063/1.1658560

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

An evaluation of experiments already performed shows that three equations involving the four fourth‐order room‐temperature elastic stiffnesses of fused quartz have been determined. They are:C111−1.4C1112+0.34C1122+0.23C1123=(5.5±3.5)×1012N/m2C1111+4C1112+2C1122+2C1123=(55±10)×1012N/m2C1111+2C1112−3C1123=(−0.9±0.6)×1012N/m2.

ISSN:0021-8979    

DOI:10.1063/1.1658561

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The temperature and the stress dependence of tensile creep has been studied in ferromagnetic &agr; iron (Ferrovac‐E 99.91% pure) in the temperature range 620°–700°C. By using the change‐of‐slope method, activation energies for creep were obtained. The modulus‐compensated activation energy is shown to be essentially independent of the initial stress and of strain up to the necking point. The change‐of‐slope method gave a modulus‐compensated activation energy of 65 kcal/mole while a plot of the logarithm of the modulus‐compensated strain rate versus reciprocal of the absolute temperature gave a straight line yielding a modulus‐compensated activation energy of 62 kcal/mole. These are in essential agreement with the value for self‐diffusion in this temperature range. The logarithm of the temperature‐compensated creep rate versus logarithm of stress gave a straight line with a slope of 6.6, i.e., a power‐law dependence of the strain rate on the stress. The fact that the activation energy for creep is the same as that for self‐diffusion indicates that the creep process is controlled by diffusion and hence by a nonconservative dislocation motion: either the climb of edge dislocations or the dragging of jogs on screw dislocations. It is also found that there is no noticeable drop in the activation energy with increased stress. This means that at the strain rates and temperatures employed here, the average vacancy concentration is but a small perturbation from the equilibrium vacancy concentration in the undeformed specimen. By utilizing the recent data of Ishidaet al.which extends to considerably lower temperatures good estimates of vacancy supersaturations can be obtained from 150° to 700°C (0.23Tm–0.54Tm) at different creep rates. From the available creep data on some fcc metals it is shown to be possible to get a good estimate of energy for motion of vacancies. We obtain by this techniqueEmv=15 500 cal/mole for aluminum which is in essential agreement with the value obtained by other techniques andEmv=22 800 cal/mole for copper.

ISSN:0021-8979    

DOI:10.1063/1.1658562

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

It is shown that a correlation interferometer can be used to obtain directly an image of a coherently or incoherently illuminated object through a changing random diffusing plate. An image of a pair of coherently illuminated point sources was obtained through a rotating ground‐glass plate using a correlation interferometer.

ISSN:0021-8979    

DOI:10.1063/1.1658563

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Surface damage in GaAs has been studied by measuring the room‐temperature reflectance in the 3‐eV spectral region. Controlled levels of damage were created by implanting samples with known doses of 75‐keV argon ions. The damage from doses as low as 1011ions/cm2has been observed by studying the energy derivative of the reflectance (dR/dE)/R. As the damage level is increased, the optical reflectance structure broadens and hence the amplitude of (dR/dE)/Rdecreases. The sensitivity of this technique is comparable to or greater than that obtainable with other damage measurement techniques. The results for three other techniques (ellipsometry, channeling, and photoluminescence) are briefly discussed. Possible applications of the reflectance method are considered.

ISSN:0021-8979    

DOI:10.1063/1.1658564

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The effects of a static electric field on the dispersive free‐carrier magneto‐optic effects in semiconductors are discussed for the conditions of low degeneracy, deformation‐potential acoustic scattering, a simple parabolic band, and (1) moderately high magnetic fields, for whichsH/cE≈1 but &ohgr;c&tgr;0<1, (2) strong, but nonquantizing, fields (&ohgr;c&tgr;0>1), and (3) the extreme quantum limit where the carriers are confined to a single Landau level. In the last case, only the situation where the static electric and magnetic fields are perpendicular is considered. In the first case, only small electric‐field‐induced effects occur. In the second case, only the low‐frequency Voigt effect is changed much by the electric field; it is strongly anisotropic and relatively independent of magnetic field strength. In the quantum limit, an electric field should produce a considerable increase in an already large low‐frequency Voigt effect. Effects of higher‐order terms in relaxation time and distribution function in the quantum limit are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1658565

出版商:AIP    

年代:1970

数据来源: AIP