摘要:

A power‐flow theorem which clearly brings out the contribution of various mechanisms to power flow, stored energy, and power dissipation has been derived for acoustic waves in piezoelectric semiconductors. A major feature of the theorem is that it holds for any direction of propagation of acoustic wave with respect to the direction of dc drift velocity of the carriers. Therefore, it is expected to be useful in understanding the effect of bulk‐wave radiation on the performance of surface‐wave interdigital transducers on piezoelectric semiconductors, such as in surface‐wave amplifiers and unidirectional delay lines, where the bulk waves launched are at an angle to the drifting carriers.

ISSN:0021-8979    

DOI:10.1063/1.1660724

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The adiabatic elastic moduli of hexagonal close‐packed lutetium single crystals have been determined by the ultrasonic pulse superposition technique over the temperature range 4.2–300°K. The values at 300°K in units of 1011dyn/cm2areC11=8.62,C33=8.09,C44=2.68,C66=2.71, andC13=2.8; the values extrapolated to 0°K areC11=9.10,C33=8.40,C44=2.91,C66=2.96, andC13=2.9. The Debye temperature calculated using the elastic moduli at 0°K is 184.5°K. The bulk elastic properties have also been calculated and they agree favorably with previously predicted polycrystalline values. The linear thermal‐expansion coefficients of the lutetium single crystals have been determined over the same temperature range by using a linear‐variable‐differential‐transformer dilatometer. The room‐temperature coefficients of thermal expansion are &agr;∥=2.0×10−6and &agr;⊥=4.82×10−6°K−1. A maximum in &agr;⊥was found at 130°K, which apparently correlates with the maximum found in the &ggr;⊥/&ggr;∥ratio at 130°K, where &ggr; is the Gru¨neisen constant.

ISSN:0021-8979    

DOI:10.1063/1.1660725

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Thin gold crystals are prepared by vapor deposition in an ion‐pumped vacuum system onto (100) silver surfaces on rocksalt. The continuity of 15‐Å‐thick carbon‐backed crystals is investigated in the electron microscope for different growth conditions. Specimens made at different thicknesses show that the thinnest stable crystals with areas 1000 Å in diameter free of holes are about 9 Å thick. Diffracted beam intensities of the gold crystals are measured and used with kinematical diffraction contrast theory to obtain an independent measurement of crystal thickness.

ISSN:0021-8979    

DOI:10.1063/1.1660726

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Lattice parameters, thermal expansion coefficients, and Gru¨neisen parameters of tungsten are determined by an x‐ray method in the temperature range of 180–40 K without the use of liquid gases. Lattice parameters are expressed as a function of temperature. Thermal‐expansion coefficients decrease with temperature and show no anomaly in contrast to a hypothesis proposed by Featherston and Neighbours. Gru¨neisen parameters &ggr; are decreasing with temperature in accordance with the theoretical predictions.

ISSN:0021-8979    

DOI:10.1063/1.1660727

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An investigation has been made on the initial separation of high‐current contact electrodes. Three distinct voltage‐time characteristics are observed. Two of these correspond to the formation of a molten‐metal bridge and the third corresponds to an abrupt transition from metallic contact to an arc. Two ideal cases are analyzed theoretically from which a model is developed. The model is then related to the three voltage‐time characteristics observed.

ISSN:0021-8979    

DOI:10.1063/1.1660728

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Dislocation velocities have been measured on loops whose Burgers vectors have been determined by x‐ray topography. The new measurements do not agree with earlier work; the reasons for the discrepancy are discussed. The earlier work was also in error because of an incorrect assumption about the Burgers vector of the dislocations. The measured velocity‐stress behavior falls into two distinct stress regimes. At low stresses the dislocation velocity depends more strongly on stress than in the high‐stress region. The activation energy for dislocation motion depends on stress and varies from 2.9 to 1.5 eV over the stress range 0.6–8 kg/mm2.

ISSN:0021-8979    

DOI:10.1063/1.1660729

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

The theory of disclinations contains the equation ∂i&agr;ij+ejmn&thgr;mn=0, where &agr; and &thgr; are the dislocation and disclination density tensors, respectively. This expression is interpreted to mean that dislocations can end on twist disclinations. A concrete example in a hexagonal crystal is discussed to illustrate this concept. It contains a 60° wedge disclination normal to the basal plane. By basic geometrical construction it is shown how a dislocation can be made to end on a jog in the wedge disclination. This jog is a small segment of twist disclination. Several ramifications of this concept are that disclinations can act as sources and sinks of dislocations, that dislocations change their Burgers vectors as they glide around disclinations, that a dislocation which crosses a disclination remains connected to it by a dislocation, that dislocations encircling a disclination must have a node, and that the local Burgers vector is not conserved on following a dislocation around a disclination.

ISSN:0021-8979    

DOI:10.1063/1.1660730

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

An intrinsic disclination (necessarily a wedge disclination) in a surface crystal can act as a source of or sink for dislocations. In so acting the disclination suffers a displacement. An equation is obtained which relates the displacement to the Burgers vectors of dislocations produced or annihilated. Examples involving disclinated square and hexagonal lattices are presented.

ISSN:0021-8979    

DOI:10.1063/1.1660731

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Some stress components in the plane of a twist and a wedge disclination loop and the strain energies of these loops outside the core region are calculated based on isotropic elasticity theory. These results complement previous results for the stress fields at large distances.

ISSN:0021-8979    

DOI:10.1063/1.1660732

出版商:AIP    

年代:1971

数据来源: AIP

摘要:

Various types of prismatic dislocation loops were produced by quenching in silicone oil of aluminum containing nominally 1 and 7 wt% magnesium, respectively. The annealing behavior of the loops was studied by transmission electron microscopy of thin foils of the quenched material. Direct experimental proof is given for the conversion of hexagonal perfect loops to diamond‐shaped loops, as well as for the rotation of perfect loops to different planes. The mechanism responsible for the coalescence of perfect loops is determined, and the nature of perfect loops evolving from climb sources is determined by their annealing behavior.

ISSN:0021-8979    

DOI:10.1063/1.1660733

出版商:AIP    

年代:1971

数据来源: AIP