摘要:

Three groups of single‐crystal disks approximately 5‐mm thick with surface normals along [100], [110], and [111] crystallographic directions were prepared from 99.99+ at.% pure copper. These specimens were shock loaded to about 50 kbar in a state of uniaxial strain by nitroguanidine explosive plane‐wave generators, and the propagated wave profiles were measured with quartz gauges. Elastic wavefronts for the single crystals exhibited sharp risetimes (of the order of 10 nsec) to dynamic yield points, and subsequent stress relaxations preceding the plastic wavefronts. For the propagation distances of about 5 mm, the measured yield point normal stresses were about 2.0, 1.3, and 1.3 kbar, respectively, for wave propagation in the [100], [110], and [111] directions. Although the principal stress states at the yield points differed, analysis reveals that the shear stresses on {111} ⟨110⟩ slip systems were about the same for all orientations. Single‐crystal disks prestrained by about 3½% exhibited essentially zero yield stresses and ramp‐like elastic waves. Similar behavior observed for polycrystalline specimens indicates the importance of initial dislocation density on dynamic yielding. In all cases the plastic wave velocities were the same. Constitutive relations derived on the basis of dislocation dynamics are given for the three single crystal orientations. From these relations the decay of the respective dynamic yield points with increasing propagation distance can be predicted as a function of the dislocation mobility and the initial mobile dislocation density. Within the framework of this theory it is shown that simple dislocation damping models for the mobility are not consistent with the experimental results.

ISSN:0021-8979    

DOI:10.1063/1.1657314

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A differential mixing technique is described whereby the Doppler shift of light reflected from a shock wave can be measured in the laboratory. Results are given for the application of the technique to shock waves produced in a conventional pressure‐driven shock tube, and it is found that measurements of shock wave velocity can be performed with a spatial resolution ∼10−5m, and of shock wave curvature with a resolution up to 10−1m−1. Deliberate optimization of the various design parameters involved is expected to improve the sensitivity of either of these measurements by about an order‐of‐magnitude.

ISSN:0021-8979    

DOI:10.1063/1.1657315

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Smear camera photographs of the detonation process in nitromethane and in its solutions with acetone often show dark striations and/or larger dark regions. Photographic evidence now presented leads to the conclusion that these two characteristic entities are not the same, although both may indicate local nonreaction sites. New data is presented showing the possibility of detonation buildup in large dark regions as evidenced by brightly luminous interaction figures and strong local pressure differences.

ISSN:0021-8979    

DOI:10.1063/1.1657316

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The work function of ultrahigh‐vacuum‐deposited beryllium films was measured to be 5.08±0.08 eV. Upon exposure of freshly deposited films to pure oxygen at pressures of 5×10−7Torr, the work function was observed to decrease rapidly to a stable value of 3.60±0.05 eV.

ISSN:0021-8979    

DOI:10.1063/1.1657317

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Transmittance and reflectance studies have been made in the 0.83–25 &mgr; range on GeTe films to obtain and compare the optical properties of amorphous and crystalline structures. The refractive index of crystalline GeTe films (typical value 5.5 at 1.5 &mgr;) shows free carrier dispersion yielding optical dielectric constant=36 and plasma wavelength of 3–5.9 &mgr; (for a corresponding carrier concentration range of 1.5–0.18×1021/cm3) from which a corresponding susceptibility mass of 0.35–0.16mis deduced. On the other hand, amorphous GeTe films exhibit no free carrier dispersion. The refractive index (typical value 4.2 at 1.5 &mgr;), however, rises sharply at the onset of the interband transitions. This rise shifts to higher energies at lower temperatures yielding a coefficient ofdEopt/dT∼−4.5×10−4eV/deg (Eopt=absorption edge). The absorption coefficient, &agr;, of amorphous films at all wavelengths studied is smaller than that of the crystalline films.d&agr;/dh&ngr;(h&ngr;=photon energy) shows a maximum (defined here as the absorption edge) for amorphous films at ∼0.85 eV and for crystalline films at 0.73–0.95 eV (depending on carrier concentration). The observed values of the absorption edge and the relation &agr; ∝ (h&ngr;)1/2near the absorption edge for the crystalline state can be understood in terms of direct transitions from the Fermi level (∼0.3–0.5 eV in the valence band) to the correspondingk‐vector in the conduction band (Burstein shift) with a forbidden bandgap ∼0.1–0.2 eV. The observed value of the absorption edge and the relation &agr; ∝ (h&ngr;)2near the absorption edge in amorphous films are proposed to be due to indirect transitions between thenonlocalized(allowed) levels at and beyond the localization limits in the valence and conduction bands. The observed exponential decay of &agr; (i.e., &agr; ∝ exp (h&ngr;/&Dgr;), &Dgr;=0.068=2.6kT) belowh&ngr;=0.85 eV may be attributed to the exponential distribution of the localized levels within the pseudo‐bandgap of the amorphous state.

ISSN:0021-8979    

DOI:10.1063/1.1657318

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

An analysis procedure and data accumulation scheme is described which allows implementation of a fully interactive, computerized ``process control'' for a general class of physical experiments in terms of derivable experimental results. The method is applicable whenever the derivable physical parameters can be expressed in terms of integrals over functions of the measured quantities and reciprocal relationships allow prediction of the measured values in terms of the derived quantities. The analysis may be most useful when the transformations are orthogonal.

ISSN:0021-8979    

DOI:10.1063/1.1657319

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

This article discusses in the form of a table the spurious signals which can arise in a Hall effect measuring apparatus using alternating current and either an ac or a dc magnetic field. It is pointed out which of these spurious signals can simulate a Hall effect. Checking procedures to distinguish between the real Hall effect and the spurious effects are given.

ISSN:0021-8979    

DOI:10.1063/1.1657320

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Polar angles cannot be determined in general from intensity ratios of Mo¨ssbauer hyperfine lines. However, conditions can be found that specify the polar angle as either perpendicular or parallel to the Mo¨ssbauer photon.

ISSN:0021-8979    

DOI:10.1063/1.1657321

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Resonant absorption of high‐frequency sound was tried as a new method for the determination of bubble growth. In the 85‐cm bubble chamber at DESY, filled with a Ne/H2mixture (90/10 mole%) at 29°K, the growth rate of subvisible bubbles was measured with 122‐kHz ultrasonics for time intervals between 10 and 70 &mgr;sec after the passage of a particle. The results were evaluated according to the theory of Trammell and were compared with an extrapolation from optically measured values under the assumption of anR∼t1/2dependence for the bubble radius.

ISSN:0021-8979    

DOI:10.1063/1.1657322

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A new, self‐modulated mode of operation of the marginal oscillator ultrasonic spectrometer, in which no external modulation is required, is described and analyzed. In this mode, the system consisting of an oscillator‐coupling network‐acoustic resonator is induced to oscillate simultaneously at two frequencies. Small changes in acoustic attenuation are reflected as changes in the amplitude or frequency of the detected beat frequency signal resulting from the interference of the two oscillations. Applications to magnetoacoustic phenomena are reported.

ISSN:0021-8979    

DOI:10.1063/1.1657323

出版商:AIP    

年代:1969

数据来源: AIP