摘要:

Numerical computations of scattering cross sections are made for plane compressional and shear waves incident normally upon an infinitely long circular cylindrical cavity in a homogeneous isotropic elastic solid. Dependencies of the cross sections upon the variableskaand &kgr;a(kand &kgr; are the compressional and shear wave vector amplitudes, respectively, andais the cylinder radius) and upon the material parameter &kgr;/kare discussed, along with the relative contributions of the various components of the total cross sections. Computations are made over the range 0⩽ka⩽6 for incident compressional waves, and 0⩽&kgr;a⩽10 for incident shear waves. Both sets of computations are done for a number of values of &kgr;/kcorresponding to various host materials.

ISSN:0021-8979    

DOI:10.1063/1.322894

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

An integral formalism for surface waves in piezoelectric half‐infinite solids valid up to the critical velocity is developed. Various boundary conditions are considered and, in particular, the problem of which boundary conditions allow surface‐wave solutions for velocities below the limiting velocityvLis discussed in detail. It is proved that (a) with a mechanically free surface and zero dielectric constant for adjoining medium, at most one solution is possible forv<vL, (b) with a mechanically clamped surface and zero dielectric constant for adjoining medium, no solution is possible forv<vL, (c) with a mechanically clamped surface and an infinitely conducting adjoining medium, no solution is possible forv<vL, and (d) with a mechanically free surface and an infinitely conducting adjoining medium, at least one and at most two solutions are possible forv<vL. When two solutions are possible, one solution is of the Bluestein‐Gulyaev type.

ISSN:0021-8979    

DOI:10.1063/1.322895

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

Electrical breakdown of tarnish films (Ag2S) on silver has been investigated in the Au‐Ag2S‐Ag system. Dielectric breakdown in the Au‐Ag2S‐Ag system ispolaritysensitiveand depends on the rate of application of voltage to the film. Breakdown is consistent withmultipleelectronavalancheprocesses. Data indicate intrinsic breakdown fields greater than 3×105V/cm (for fast rise time voltages). For slow rise time voltages, breakdown is strongly affected by silver ion mobility and becomes polarity dependent. The conductivity ratio (&sgr;Agrich/&sgr;Srich) has been obtained as a function of time and the average Ag ion mobility (2×10−8cm2/V sec) has been determined.

ISSN:0021-8979    

DOI:10.1063/1.322896

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

The differential equations which govern enhanced diffusion under ion irradiation are solved using recently developed numerical techniques. Accurate time‐dependent profiles for vacancies, interstitials, and diffusing atoms are generated thereby with minimal computer time. The theoretical description is improved further by incorporating refined calculations of the atomic displacement rate by energetic ions. Three representative cases of enhanced diffusion are treated in detail: Al diffusion in Al under 100‐keV Al irradiation, Al diffusion in Al under uniform irradiation, and W diffusion in W under 100‐keV proton irradiation. It is shown that more information may be obtained from the diffused atomic profiles if the irradiation damage rate varies with depth in a known way over the diffusion region. Under this condition, both the shape and the time dependence of the atomic profile are sensitive to the rate coefficient for point‐defect annihilation. When the annihilation coefficient is so determined, the point‐defect creation rate can be uniquely related to the enhanced atomic diffusion coefficient. Under irradiation with a limited range, the atomic profiles typically pass through a complicated form, but ultimately reach a flat shape with a relatively abrupt drop‐off at greater depths. This end condition is qualitatively independent of the detailed structure of the irradiation profile, but the sharpness of the drop‐off is a function of the rate coefficient for point‐defect annihilation.

ISSN:0021-8979    

DOI:10.1063/1.322897

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

Light scattering properties of smectic‐A liquid‐crystal thin‐film waveguides are studied both theoretically and experimentally. A Green’s function analysis of the depolarized light scattering induced by thermal fluctuations in the average molecular alignment is presented. Scattered power distributions and scattering coefficients for mode conversion and for scattering out of the guide are derived. It is shown that the power distributions for scattering from nematic and smectic waveguides differ greatly and that scattering losses in smectic‐A liquid‐crystal waveguides are several orders of magnitude lower than those encountered in nematic guides. Power loss in these waveguides, due to light scattering arising from thermally induced dynamical distortions of the smectic planes, is calculated to be on the order of 2 dB/cm and is experimentally verified.

ISSN:0021-8979    

DOI:10.1063/1.322898

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A new phase transition occurring in NaNO3at about 260 °K in both cooling and heating cycles is reported. The transition is associated with an expansion of the lattice resulting in approximately a 3.6% decrease in the density.

ISSN:0021-8979    

DOI:10.1063/1.322899

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

The tunneling characteristics in thin epitaxial Bi films (thickness: 400–1200 A˚) were investigated. Bi‐SiO2‐Pb tunnel junctions were prepared by vacuum deposition onto freshly cleaved mica substrates in an ultrahigh vacuum system. The dI/dV‐Vcurve has a large peak at the Bi positive voltage range (150–250 mV) which can be attributed to the band structure of Bi. In the low voltage range (‖V‖<25 mV), d2I/dV2‐Vcurves show the peaks or dips reflecting the state density of superconducting lead. The phonon‐assisted tunneling peaks or dips in the normal state were also observed. For medium voltage range (‖V‖≳25 mV), there are several peaks or dips due to phonons of SiO2. The subband edges generated by quantum size effects were not observed, probably because the electron bands and the hole band of Bi overlap each other, located at theLandTpoints in the Brillouin zone, respectively.

ISSN:0021-8979    

DOI:10.1063/1.322900

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

The energies of cracks are considered in terms of the first and second laws of thermodynamics, and it is concluded that the Griffith criterion provides a necessary but not sufficient condition for crack propagation. Fracture occurs only when the crack tip radius is larger than a critical value that is several times the interatomic spacing. Surface energies calculated from fracture experiments probably give only an upper bound to the true surface energies of solids. The radius of the crack tip is a parameter that cannot be ignored in fracture experiments.

ISSN:0021-8979    

DOI:10.1063/1.322901

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

A group of programs are described which will simulate a photograph of arbitrary contrast ratio or exposure. We have used these programs to simulate x‐ray topographs of stacking faults as described by Authier’s spherical wave treatment of a perfect crystal with a stacking fault. Intrinsic and extrinsic faults are examined with a range of different wavelengths, fault orientations, diffraction conditions, specimen thicknesses, and absorption coefficients. In cases where experimental topographs are available, they correspond closely to the simulations, thereby indicating the basic correctness of Authier’s theory. The simulations illustrate the separation of the complex diffraction pattern into three components. Only one component carries information on the nature of the fault. The simulations clearly delineate the conditions for identifying the fault type; namely, the critical value of &mgr;dfor adequate visibility of theI3fringes is in the range 1<&mgr;d<2. Traverse patterns are demonstrated to be an unreliable guide to the nature of the fault.

ISSN:0021-8979    

DOI:10.1063/1.322902

出版商:AIP    

年代:1976

数据来源: AIP

摘要:

An expression for the maximum temperature difference &Dgr;Tmaxproduced between the hot and cold junctions of a thermocouple is derived when the arms are disks of thermoelectric material. In these disks the flow of current is in a direction radially inwards or radially outwards. No assumptions have been made about the distribution of the Joule heat over the hot and cold junctions. The solution as a function of the ratio (ro/ri) of the outer to inner radii of theNandPdisks constituting the couple exhibits an interesting singularity at one particular value of this ratio. Ifro/riapproaches the numerical value 4.5, the maximum temperature difference &Dgr;Tmaxappears to have no limit. However, its growth is finally arrested by the temperature dependence of the figure of meritZ. It is then found experimentally that the current through the junction may be increased to large values without causing &Dgr;Tto pass through a maximum. The coefficient of performance tends to unity at the critical ratioro/ri=4.5. Finally, it is pointed out that junctions whose arms are in the form of rotationally symmetrical bodies with lateral surfaces generated by hyperbolae are a more perfect approach to the mathematical model adopted than are flat disks. Such junctions produced temperatures lower than junctions with arms of any other shape.

ISSN:0021-8979    

DOI:10.1063/1.322903

出版商:AIP    

年代:1976

数据来源: AIP