摘要:

The temperature and frequency dependence of the attenuation of 0.5–5‐GHz acoustic surface waves on LiNbO3has been measured. For propagation in vacuum a frequency‐squared dependence of the total attenuation is obtained with a value at 1 GHz of 0.9 dB/&mgr;sec. Temperature‐dependence measurements using a novel three‐transducer technique show the dominant loss (0.7 dB/&mgr;sec at 1 GHz) mechanism to be the interaction with thermally excited elastic waves. Propagation in air results in an additional loss linearly proportional to frequency with a value of 0.2 dB/&mgr;sec at 1 GHz. The effects of beam steering and diffraction losses are also investigated both theoretically and experimentally. Both misalignment of transducers with respect to pure mode propagation axes and misalignment of the propagation‐plane perpendicular can add significantly to delay line insertion loss. This beam steering loss onY‐cutZ‐propagating LiNbO3is considerably higher than on the 41.5° rotated‐cutX‐propagating orientation. The loss mechanisms measured in this paper are sufficient to completely account for the insertion loss of surface‐wave delay lines.

ISSN:0021-8979    

DOI:10.1063/1.1658471

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The collision model utilized by Giordmaine and Wang to describe the centerline beam intensity from long tubes has been extended to account for molecular‐speed effects. The modification was accomplished by utilizing the velocity‐dependent mean‐free path instead of the Maxwellian averaged value. The analysis shows that the centerline beam intensity (or peaking factor) is practically unaffected by explicitly accounting for the distribution of molecular speeds. The speed distribution on the axis of the beam, however, is depleted of slow‐speed molecules. The perturbation of the Maxwellian spectrum begins at a Knudsen number based on tube length of ∼10. For Knudsen numbers less than ∼0.1, the spectrum is no longer dependent on the Knudsen number. The average translational energy of the centerline beam in the low Knudsen number limit is 5.5% greater than that of a Maxwellian beam.

ISSN:0021-8979    

DOI:10.1063/1.1658472

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A technique for measuring the speed distribution of molecular beams which uses symmetric modulation and phase‐sensitive detection is described. Corrections for system errors on experiments of this type are developed and verified. In addition to the expected 1/vdependence of the ionizer, the extraction efficiency of the quadrupole mass spectrometer detector is shown to depend on the velocity of the molecules before ionization. The prediction of 5.5% energy enhancement of beams from channel sources operated at pressures well above the free‐molecule flow limit has been verified.

ISSN:0021-8979    

DOI:10.1063/1.1658473

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Mathematical analyses previously made for diffusion‐limited growth and dissolution of spherical and planar precipitates have been reviewed. The analyses include the invariant‐field (Laplace), the invariantsize (stationary interface) and the linear‐gradient approximations, and where possible, exact solutions of the differential time‐dependent diffusion equation and of the independent flux balance. The kinetic parameters are calculated and the different solutions compared. For virtually all cases of practical interest it is shown that the stationary‐interface approximation is the best one. By considering a reversed‐growth analysis and showing that it is merely an approximation to the dissolution of a spherical precipitate, it is thereby shown that growth and dissolution cannot be generally considered as simply conjugate processes.

ISSN:0021-8979    

DOI:10.1063/1.1658474

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The prototype of all dislocation cells composed of dislocations with two mutually perpendicular Burgers vectors is the terminated quadrupole. It consists of two similar, mutually perpendicular, rectangular glide dislocation loops which interpenetrate such that their edge dislocation segments are parallel to each other and that each of the two pairs of like‐handed screw dislocation segments form a cross of fourfold symmetry. The stress components of the terminated quadrupole have been computed numerically and have been presented in the form of contour maps in, or parallel to, the midplane of the quadrupoles as well as in a cut parallel to the prism axis of the quadrupole. It is found that the intensity of the stresses falls with distance from the cell as 1/r5for medium short or squat quadrupoles, and as 1/r4for very elongated quadrupoles.

ISSN:0021-8979    

DOI:10.1063/1.1658475

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A theory of mechanical damping based on the model developed by Granato and Lu¨cke was extended to small numbers of pinning points per network length and to stresses high enough to produce complete breakaway. The detailed response of a dislocation array to an applied stress was calculated using computer techniques in the low‐frequency (kilocycle) range and both the viscous and hysteretic damping were computed. The amplitude dependence of the viscous and hysteretic damping was computed as a function of loop‐length distribution, distribution of dislocation orientations, pinning point density, and stress distribution. The computation techniques allowed these calculations to be made without the usual mathematical approximations which are shown to result in a severe limitation on the previous analytic expressions. The results were applied to a discussion of the use of the damping theory to obtain parameters which describe the dislocation array and dislocation behavior. The time dependence of dislocation damping which results from diffusion of pins along the dislocation and in the lattice is discussed.

ISSN:0021-8979    

DOI:10.1063/1.1658476

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The behavior of pinning points at dislocations during internal friction measurements is discussed. Diffusion of pins along the dislocation prior to breakaway of the dislocation from the pinning points and in the lattice subsequent to breakaway is qualitatively considered. Both types of motion lead to time dependence of the internal friction and the variation of this time dependence with temperature and strain amplitude is discussed. It is shown that a study of these effects can lead to an understanding of the contribution of the amplitude‐dependent viscous damping and of the amplitude‐dependent hysteretic damping to the total specimen damping. An experimental technique is described which allows a separation of these two contributions. This technique is applied to Al and Al&sngbnd;Mg alloys at high and low temperatures and the existence of these two damping contributions is demonstrated.

ISSN:0021-8979    

DOI:10.1063/1.1658477

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A model due to Eshelby for computing the drag force upon an abrupt kink moving at low velocity through a phonon gas and its extension to include the effect of the finite width of the kink are analyzed in detail. The temperature dependence of the drag force is derived and is shown to be in reasonable agreement with earlier qualitative predictions made by Lothe. This force is related to the dislocation viscous damping coefficient appearing in the string model of dislocation motion. In this form the theory is compared with recent experimental results. Good agreement is found if the kink width (in units of the appropriate Burgers vector) is ∼4 or 1 in Cu and Al, respectively. An incidental problem in attempting to unify the theory with that of the Bordoni peak is also discussed briefly.

ISSN:0021-8979    

DOI:10.1063/1.1658478

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The problem of the uniform motion of a screw dislocation in a crystal is treated from the point of view of lattice dynamics. A simple model of cubic lattice with piecewise‐linear nearest‐neighbor interactions is considered in detail and the extension to more realistic interatomic forces is outlined. The external stress necessary to maintain the dislocation in uniform motion is computed as a function of dislocation velocity. This stress has a minimum at one‐half of the sound velocity and motion is quite possible at hypersonic velocities. The calculations are at zero temperature, hence the only dissipative mechanism present is the emission of phonons by the core of the moving dislocation. The theory is relevant to high‐velocity dislocation motion, which is not of the usual thermally activated type.

ISSN:0021-8979    

DOI:10.1063/1.1658479

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The continuous distribution of screw dislocations for a single‐layered double‐ended pileup in a soft lamella embedded between two rigid phases is studied. Explicit expressions are obtained for the distribution function, the number of dislocations, the relative displacement, and the strain energy. The stress concentration at the tips is calculated by using Moutier's theorem. When compared with a similar case in a homogeneous medium under the same stress, all these quantities are smaller by about 30%. The present solution is applicable also to the case of a slit crack under an antiplane shear stress.

ISSN:0021-8979    

DOI:10.1063/1.1658480

出版商:AIP    

年代:1970

数据来源: AIP