摘要:

Rate‐dependent constitutive relations for single crystals are derived in terms of dislocation dynamics. Contributions from slip on the individual glide planes are assumed to superpose linearly to give the total plastic strain. As an application of the theory, equations describing elastic precursor decay are developed for longitudinal plane‐wave propagation in fcc, bcc, and rocksalt structures with wave propagation in the [100], [110], and [111] directions. In addition, expressions for precursor decay in zinc (hcp structure) are derived for wave propagation both parallel and perpendicular to thec‐axis. Calculated theoretical results are compared with experimental data on precursor amplitudes for single‐crystal copper (fcc), tungsten (bcc), NaCl (rocksalt), and LiF (rocksalt). Dislocation mobilities determined from direct observation of dislocations are used in these calculations. In general the theory predicts the proper relative order of the precursor amplitudes for different propagation directions. The comparisons show that in order for theoretically determined amplitudes to agree with experimental data, initial mobile dislocation densities must be two or three orders‐of‐magnitude greater than initial total densities which are measured in the material prior to shock loading. Possible explanations for this discrepancy are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1659227

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Half‐cycle mechanical hysteresis loops, at positive stress bias, representing damping frequencies in the range of ≈10−1Hz were performed on copper crystals at room temperature. The hysteretic character and stability of the dislocation structure were explored by repetitive stress cycling at constant stress amplitude. It was found that the decrement decreased with continued stress cycling and eventually reached a saturation value. Changes in the decrement, with cycling, provided an extremely sensitive way of detecting dislocation rearrangements at low stress levels. Sectioning and etch pitting of the crystals was performed in order to reveal quantitatively both the forest and primary dislocation structures associated with the hysteresis behavior. Correlation of the damping and etch‐pit data indicated that low‐frequency mechanical hysteresis could be attributed to ``pinning'' of primary dislocations at forest dislocation junctions.

ISSN:0021-8979    

DOI:10.1063/1.1659228

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The information which can be gained from field‐ion micrographs showing grain boundaries is considered critically. An attempt is made to quantify the factors governing the ``ring'' configuration across both low‐angle and high‐angle grain boundaries. It is shown that spiral configurations can be generated in the absence of grain boundary dislocations. Some potential applications of the grain boundary contrast theory are indicated.

ISSN:0021-8979    

DOI:10.1063/1.1659229

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Epitaxial layers of GaAs grown from the liquid phase have been studied by transmission electron microscopy. The layers were grown on mechanically polished substrates and contain intrinsic stacking faults bounded by Shockley partial dislocations. Near each end of most of these grown‐in faults, there are three overlapping intrinsic faults. The two faults which overlap the grown‐in fault form by vacancy migration during growth and are bounded by Frank partials. Unfaulting of these three‐layer defects was observed on annealing and is accomplished by the nucleation and motion of Shockley partials. These unfaulting Shockley partials have been observed for the first time. The final state is one in which the total faulted structure is eliminated.

ISSN:0021-8979    

DOI:10.1063/1.1659230

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Thermal currents have been obtained from corona‐charged Mylar. The results are interpreted in terms of electron and/or ion trapping and subsequent release upon reheating. A theory is developed for thermal release of these ``near surface'' charged specimens when no external field is applied. A new method is presented to obtain energies for a series of peaks present in complex thermal current spectra. The trapping parameters are also determined using both the initial rise analysis of Garlick and Gibson, and the curve fitting technique of Cowell and Woods. The results are consistent with four discrete traps in Mylar at depths of 0.55, 0.85, 1.4 and 2.2 (all ±0.1) eV. The 0.55‐ and 0.85‐eV traps are electronic, that at 1.4 eV is either an electronic trap with a Coulomb barrier, or ionic. The 2.2‐eV trap is either ionic, interfacial, or involves dissociation of a complex with subsequent release of an electron to the conduction band. Assuming that the traps empty under monomolecular conditions, analysis of the thermal‐current peaks yields trap densities of the order of 1016/cm3, an electron mobility—lifetime product of about 10−10cm2/V, and escape probabilities that agree with those determined from isothermal charge decay.

ISSN:0021-8979    

DOI:10.1063/1.1659231

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Direct current diode sputtering of reduced BaTiO3ceramic has been applied to the preparation of thin BaTiO3films. Sputtering was carried out over a range of apparent sputtering rates of ∼500–1200 mg/Ah in atmospheres of argon and air. As the substrate, Pt sheet and fused quartz were principally used. Grain size of the deposited film was ∼1000 Å for the Pt substrate at 400°C. The lattice constants were in good agreement with those of ordinary BaTiO3when the Pt substrate was kept at a temperature above 1000°C, although they were slightly larger for substrates below 900°C. Measurements of optical density and observations of electron micrographs indicated that the sputter‐deposited films have more favorable properties of transparency, uniformity, and crystallization than vacuum‐evaporated films. The film prepared on a Pt substrate at 1000°C in an air atmosphere had a dielectric constant of 1700 and dissipation factor of 1.8% without post‐deposition treatment. However, the remanent polarization was very small and the temperature dependence of the dielectric constant was slight.

ISSN:0021-8979    

DOI:10.1063/1.1659232

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Electromigration studies of large‐ and fine‐grain aluminum films showed that fine‐grain films were characterized by a mixed orientation and a grain size of ≈2 &mgr;, and that large‐grain films had a highly preferred 98% ⟨111⟩ orientation and a grain size of ≈8 &mgr;. Stripes fabricated from the large‐grained films had a consistently longer life, higher activation energy, and lower standard deviation of the life distribution than stripes fabricated from fine‐grained films. These results are interpreted in terms of the nature of flux divergences in the two films. Calculations give the magnitude of vacancy supersaturations at various structural divergences present in the films studied.

ISSN:0021-8979    

DOI:10.1063/1.1659233

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

While wave propagation in gyromagnetic media has been investigated extensively in the technical literature, little attention has been given to the excitation of such waves by a prescribed source. This problem is treated in the present paper. To render the analysis tractable, the source function is taken as a time‐harmonic line source of electric currents with a rapidly varying phase, embedded in an infinite homogeneous gyromagnetic medium whose axis of magnetization is perpendicular to the source direction. For further simplification, the operating frequency is taken to be near gyromagnetic resonance. The resulting model incorporates relevant anomalies of the radiation process, and also serves as a prototype for more practical thin slab configurations. The analysis is performed for permeability tensors in which spin exchange effects are ignored and included. In the former case, the refractive index surface may have open branches and gives rise to an infinity in the total radiated power (infinity catastrophe); this anomaly is removed when spin exchange is accounted for. Detailed study of the radiation fields and radiated power densities shows that except for certain initial directions, the outward power flow is almost entirely in the electromagnetic waves, and the total radiated power in the electromagnetic waves far exceeds that in the spin exchange waves, even when the source dimensions tend to zero. These results imply that a localized electromagnetic current source does not strongly excite the spin exchange waves.

ISSN:0021-8979    

DOI:10.1063/1.1659234

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Electromagnetic modes in a semiconductor cylinder are described for cases of negative and positive conductivity. These modes, with purely axial electric field orientation, exist only for nonzero conductivity. Formulas for the radiated power, quality factor, and field distributions are presented. The idealized model approximates the LSA mode of a microwave oscillator. For such a cylindrical diode oscillator, a self‐resonance condition determines the frequency×diameter product,f×D=2.4×109cm/sec, and doping concentration/frequency ratio,n/f=6×104sec/cm3. The latter result is coincident with that derived previously from a space‐charge control criterion. The electric field amplitude is uniform to within 16% over the cross section of the self‐resonant diode.

ISSN:0021-8979    

DOI:10.1063/1.1659235

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Wave equations are derived for the scalar and vector potentials of guided electromagnetic waves propagating in a laterally bounded magnetized plasma along the axis of the static magnetic field. Solutions are given in the Coulomb and the Lorentz gauge. These solutions are used in a critical examination of the conceptual basis of the quasistatic theory because they allow convenient partitioning of the high‐frequency electric field into a lamellar and a rotational part. In the quasistatic approximation the electric field is considered to be entirely lamellar, i.e., the gradient of a scalar potential that is not consistent with Maxwell's equations. To avoid inconsistencies the problem is formulated so that the dynamic scalar potential replaces the quasistatic potential. The rotational part of the electric field is readily obtained as the time derivative of the vector potential. Exact quantitative comparison of two parts of the electric field is made possible in this way and the relative importance of each part is evaluated for a few typical cases of interest. There exist some waves with dispersion characteristics that can not be obtained with the quasistatic formula although the condition that the phase velocity be smaller than the velocity of light presumed to be necessary for the applicability of the quasistatic approximation is satisfied. In these waves the rotational part of the electric field is comparable or greater than the lamellar part. Conversely, in modes with dispersion characteristics that can be well approximated with the quasistatic formula even when the phase velocity exceeds the velocity of light the lamellar part of the electric field is much larger than the rotational part. It can be inferred that the essential element of the quasistatic theory is the assumption that the high‐frequency electric field is predominantly lamellar rather than the condition that the phase velocity be smaller than the velocity of light and related criteria.

ISSN:0021-8979    

DOI:10.1063/1.1659236

出版商:AIP    

年代:1970

数据来源: AIP