摘要:

Asymmetric deformation, the movement of material on single‐crystal surfaces in only one crystallographic direction under net zero strain cycle deformation, has been observed in single crystals of iron, niobium, and molybdenum. The effect leads to rapid local deformation and rapid fatigue failure. A dislocation mechanism involving asymmetric slip on {112} and {123} planes and repeated cross slip is proposed to explain the effect.

ISSN:0021-8979    

DOI:10.1063/1.1662056

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

This paper treats the statistics of thermally activated glide of a dislocation, modelled as a string of constant tension, through an array of randomly distributed identical immobile point obstacles. The parameters governing thermally activated glide are defined, and equations are developed giving the expected value and variance of the time to activate through an obstacle array or sequence of arrays in terms of the properties of the obstacle configurations encountered. The proper statistical definition of the velocity of glide is dicussed. It is shown that the velocity of glide, as determined from the strain rate, is simply proportional to the inverse of the expected time to transit the array (or sequence of arrays). Simplifying approximations are identified for use at low temperature or high stress. We finally discuss how these statistical relations and approximations may best be used in numerical simulation of thermally activated glide.

ISSN:0021-8979    

DOI:10.1063/1.1662057

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

The growth and distribution of voids formed by vacancy agglomeration during electromigration is examined. It is demonstrated for the first time that a critical current density must be exceeded for void production.

ISSN:0021-8979    

DOI:10.1063/1.1662058

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

New measurements of the radioactive tracer diffusion coefficients of selenium‐75 and lead‐210 in PbSe, obtained using a vapor source diffusion technique, are reported. These measurements yield significantly lower values of the diffusion coefficients for both species than those previously reported by others. For selenium‐saturated PbSe,DSe*=8.13×10−4exp(−1.33 eV/k T)andDPb*=9.81×10−4exp(−1.67 eV/k T),while for lead‐saturated PbSe,DSe*=0.553exp(−2.22 eV/k T)andDPb*=3.74×10−2exp(−2.05 eV/k T).An analysis of these results indicates that selenium atoms diffuse by means of interstitial defects in selenium‐rich material and most probably through selenium vacancies in lead‐rich material, and that lead atoms diffuse through lead vacancies in selenium‐rich material and most probably through lead interstitials in lead‐rich material. The tracer data are used in conjunction with a theoretical model of interdiffusion to predict values of thep‐njunction diffusion coefficients in PbSe which are in excellent agreement with published experimental results. A similar comparison of the tracer data withn‐pjunction diffusion data shows good qualitative agreement. Quantitative discrepancies are observed which are explained by the theoretical model.

ISSN:0021-8979    

DOI:10.1063/1.1662059

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

An extension of the well‐known eddy current method for measuring the resistivity &rgr; of low‐resistivity specimens is described. The induced voltage in a pickup coil around a cylindrical sample is given, for a sudden field change, asV(t)∝∑n=1∞exp[−&lgr;n2&agr;(&rgr;)t].The usual procedure requires the measurement ofV(t) for values of the timetsuch that only the first term in the sum need be considered. For low‐resistivity metals, this method requires a long observation time (several seconds) and a low‐noise high‐gain amplifier. By including the firstMterms in this expansion, the amplifier requirements and the observation time may be reduced. Resistivity data with an estimated error of less than 5% have been obtained for several ultrapure samples of Mo and Re at 4.2 K.

ISSN:0021-8979    

DOI:10.1063/1.1662060

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

By superimposing a coaxial conduction current on the beam current of relativistic electrons passing through high‐permeability magnetic materials, the Lorentz field, &bgr; ×B, can be made an appreciable fraction of the stopping electric field,G=e−1dU/dz, of the electron in the material, implying that a state of compression is quite possible. A model of the repulsive fieldE&sgr; for the electron beam, arising from momentum being scattered radially from the axial momentum of the beam, is given. The equation of the resultant field acting on an electron provides the information to determine the condition for a pinch to occur. Experimental means of interpreting the results are indicated.

ISSN:0021-8979    

DOI:10.1063/1.1662061

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Several attempts to transiently heat a dense plasma focus discharge using intense electron beams at voltages of 280 to 300 keV and 3.6 MeV are discussed. The results, although requiring further confirmation, suggest a positive interaction and indicate that low &ngr;/&ggr; beams are required for injection into the plasma focus column.

ISSN:0021-8979    

DOI:10.1063/1.1662062

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

In this paper we describe the direct measurement of the neutral gas density and the determination of the neutral temperature profiles in a low‐pressure weakly ionized argon discharge. The measurements were made using a single‐wavelength coupled cavity laser interferometer to study the neutral density variations in the afterglow of a crowbarred dc (steady‐state) discharge. The temporal and radial variations of the neutral gas density in the afterglow were used to infer the dc steady‐state gas density and temperature profiles, assuming an ideal gas. It is shown that the profiles are strongly current and pressure dependent and that the axial depletion of neutrals can be as high as 25–30%. Long neutral relaxation times (2–3 msec) were encountered in order to reach thermal equilibrium in the afterglow for the pressures and currents studied (1–10 Torr, 50–350 mA). Radial profiles of the neutral gas density for a fixed pressure and current are in agreement with the hypothesis that neutral heating is caused primarily by collisions with the electron gas, resulting in a convective flow of the hot neutrals radially outward towards the cooler walls.

ISSN:0021-8979    

DOI:10.1063/1.1662063

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

A technique for measuring the dielectric constant and loss of isotropic films by mounting the specimen longitudinally at the center of a rectangular waveguide is proposed. Experiments have been performed at 9.375 GHz on the samples of Mylar and Teflon films ranging in thickness from 75 to 250 &mgr;m. The experimental results agree well (within 1–2%) with the accepted values. Analytical numerical results for typical samples are also given.

ISSN:0021-8979    

DOI:10.1063/1.1662064

出版商:AIP    

年代:1973

数据来源: AIP

摘要:

Line ratios and absolute intensities of helium singlet and triplet states are used to determine the electron temperature and number density of an electron cyclotron resonance heated (ECRH) plasma. Excitation rates and line ratios are calculated using the best available cross‐section information. An averaging technique based on absolute intersities is used to determine the temperature. This technique permits an experimental calibration of line ratios. In particular, the ratio of the 4713‐Å intensity to the 4921‐Å intensity is found to be strongly dependent on electron and neutral atom number densities. The ratio of the intensities of the 5048‐and 4713‐Å lines is also studied. These results show that the helium line ratios can be used to obtain accurate information on the temperature and number density of an ECRH plasma.

ISSN:0021-8979    

DOI:10.1063/1.1662065

出版商:AIP    

年代:1973

数据来源: AIP