摘要:

A square‐wave generator current source was used to study the behavior of gold wires exploded by current densities in the range of 0.25×108to 3.26×108amp/cm2. Measurements of wire resistance vs cumulative energy and action at various current densities are compared with a simple theoretical exploding wire model and the results of Kerr cell photographic studies. Experimentally, the total energy input to the time of maximum wire resistance increases with current density and may exceed by threefold the normal vaporization energy. Also, the instantaneous wire resistance at any point is smaller with a larger current density. The resistance depression is first apparent in the region following vaporization. At higher current densities, the effect is observed to occur at progressively earlier phases up to and including the point at which melting occurs.

ISSN:0021-8979    

DOI:10.1063/1.1728259

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Experimental investigation of noise generation in InSb infrared photodiodes is reported. Minimum noise occurs when the diode is biased to zero potential. Noise spectra taken on seven photodiodes determined the white noise level above 1 kc. The experimental value of white noise is compared with the noise induced by random fluctuations in background radiation. The photonoise is calculated from⟨i2⟩/&Dgr;f=2qIPHfor an equivalent noise generator. The photoinduced currentIPHis measurable directly for zero bias potential. The total measured white noise is of the order of one decibel above the calculated photoinduced noise. Thus the photoinduced noise is the primary source observed, and InSb photodiodes closely approach the ideal case of being background‐limited infrared detectors.

ISSN:0021-8979    

DOI:10.1063/1.1728260

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The experimental behavior of isotropic ferrimagnetic insulators in second harmonic generation at microwave frequencies has been investigated and compared with the behavior predicted by the equations of motion with Landau‐Lifshitz and with Bloch‐Bloem‐bergen damping. In general, a close agreement between experiment and theory was found. The experiments were carried out with small samples of single‐crystal manganese ferrite, YIG, and other ferrimagnetic materials. Since harmonic generation results from asymmetries in the precessional orbit, measurements were made with disk‐shaped specimens in which the demagnetizing fields create the asymmetry. The technique employed a cavity simultaneously resonant to 8.5 and 17 kMc. Absolute values of field intensities and magnetizations were computed from absolute power measurements. A figure of meritFfor a ferrimagnet is defined. In a practical 2&ohgr; generator the output powerP2&ohgr; is expected to vary as the productF2Pi2, wherePiis the input &ohgr; power. In comparing materials at ferrimagnetic resonance (FMR),Fshould vary as &khgr;0″. This was verified experimentally, even at high drive levels. At constant frequency and drive levelFshould be nearly independent ofHdc. Only slight deviations from this were found at low drive levels.

ISSN:0021-8979    

DOI:10.1063/1.1728261

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Diffusion coefficients for the diffusion of zinc in 10% Zn — 90% Cu single crystals are determined for both irradiated samples and unirradiated samples. This is accomplished by measuring the zinc lost from the samples when they are heated in a vacuum of 3×10−5mm Hg at temperatures from 207°C to 316°C. The zinc loss is measured by observing the change in optical reflection coefficient at a wavelength of 550 m&mgr;. The diffusion length (Dt)½is then found using the known optical constants of ZnCu alloys and the expected concentration distribution. The diffusion lengths thus obtained range from 20 to 1000 A. Bombardment by a 30‐kev Ne+ion beam of 2.9 &mgr;a/cm2produces defects at a calculated rate of 4.5×10−2atomic fractions/sec over the depth of 300 A penetrated by the beam. For well‐annealed samples the bombardment enhances the diffusion. The radiation‐enhanced diffusion coefficient depends on the square root of the bombarding flux density and has an activation energy of 9300±1700 cal/g mole. The results agree with theoretical predictions for enhanced bulk diffusion for the case of defects annealing by direct recombination plus linear anneal to fixed sinks.

ISSN:0021-8979    

DOI:10.1063/1.1728262

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A quantitative analysis has been made of elastic interactions in three types of pile‐up groups of dislocations in MgO crystals; (i) single pile‐up, (ii) double pile‐up on two orthogonal slip planes, and (iii) double pile‐up on two slip planes intersecting at 120°. The equilibrium positions of dislocations in these groups are given. It is found that the stress concentration factor is highest in type (iii) and lowest in type (i). The number of dislocations in the pile‐up groups sufficient to raise the tip stress to 1000&sgr;0is 1000 in type (i), about 120, determined by extrapolation, in type (ii), and 62 in type (iii).

ISSN:0021-8979    

DOI:10.1063/1.1728263

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A plasma model and a close‐spaced model are used to analyze the detailed variation of the open circuit voltage of the cesium diode. The plasma model open circuit voltage expression is compared to one derived by Lewis and Reitz in which the ion contribution to the net electron current is neglected. For either model the main features of the open circuit voltage at emitter temperatures less than 1800°K arise from the complex ion and electron emission properties as a function of emitter temperature and cesium pressure. Although at higher emitter temperatures the plasma model open circuit voltage becomes independent of the detailed surface characteristics of the emitter, the two models continue to give similar numerical results. Calculations based on these models and incorporating the data of Langmuir agree qualitatively with the sinusoidal and discontinuous open circuit voltage variations observed by Piddet al., as well as the anomalous load characteristic observed by Wilson.

ISSN:0021-8979    

DOI:10.1063/1.1728264

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Results obtained by low‐energy electron‐diffraction and photoelectric work‐function measurements indicate that the following features result from exposure to oxygen at room temperature. Initial adsorption occurs in an amorphous molecular form. Some of the adsorbed molecules diffuse over the surface to lattice‐defect sites where they dissociate. The resulting atoms form in a lattice structure surrounding the defect site. With continued exposure to oxygen, a place exchange between some oxygen and nickel atoms occurs to form a single‐spaced simple‐square structure with a slight increase in the lattice constant. This place exchange immediately precedes, in exposure, the formation of the oxide structure of the rock‐salt type. The work function of clean (100) nickel depends on the type of anneal. &phgr;=5.08 ev after argon ion bombardment, &phgr;=5.12 ev after a small anneal, &phgr;=5.22 ev after a more complete anneal.

ISSN:0021-8979    

DOI:10.1063/1.1728265

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Deposition of molybdenum by vaporization from simple hairpin filaments has been found to reduce pressures in both unbaked and moderately baked stainless steel vacuum systems to 4×10−10mm Hg. A deposit of 300 mg from a single filament, 0.050 in. in diameter and 6 in. long, has maintained an unbaked 85‐liter volume at pressures below 10−9mm Hg for over 40 hr with the aid of a small well‐trapped diffusion pump. For a substrate area of 8×103cm2, the initial pumping speed of a molybdenum deposit was found to be as high as 105liters/sec for hydrogen and 8×104liters/sec for deuterium. The sticking probability for either hydrogen or deuterium on this deposit is estimated at 0.3.A similar system, baked at 200°C for several days while pumped by an oil‐free ion pump, attained 2×10−10mm Hg with molybdenum evaporation. When the ion pump was valved off, the pressure in this 75‐liter system remained at 2×10−10mm Hg for two weeks with no external pumping.

ISSN:0021-8979    

DOI:10.1063/1.1728266

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

In the Borrmann effect, or the anomalous transmission, by diffraction, of x rays through perfect crystals, one state of polarization of the x‐ray beam is preferentially absorbed. Since this happens in the ``transmitted'' as well as in the diffracted beam, a simple polarizer‐monochromator is possible in the sense that insertion and rotation of the polarizer does not sensibly change the line of action of the x‐ray beam. We have used as our ``Borrmann crystal'' a single‐crystal slab of dislocation‐free germanium approximately 30 mils thick and cut for symmetric Laue diffraction from the 220 planes. The details of the rotating crystal holder, an analysis of the polarization, comments on the double‐crystal geometrical effects, and an example of the use of the polarizer‐monochromator to study the polarization term in Bragg diffraction are presented.

ISSN:0021-8979    

DOI:10.1063/1.1728267

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The design criteria of a proposed ac magnetohydrodynamic (mhd) generator are discussed. It is found that electrical losses to the moving fluid are serious unless the duct length‐to‐width ratio is greater than about 30. Two developments of such a generator are suggested. The first would provide a solution to the problem of power output from a segmented‐electrode generator. The second development, that of capacitive power transfer at the electrode‐fluid interface, would have many benefits, but its application is found to be severely restricted by the limited dielectric strength of materials.

ISSN:0021-8979    

DOI:10.1063/1.1728268

出版商:AIP    

年代:1961

数据来源: AIP