摘要:

Experiments in microwave scattering from the positive column of a low‐pressure (1.2 mTorr) discharge in mercury under steady‐state conditions are described. A free‐space environment is simulated with an anechoic chamber and a unique cw cancellation method. Both amplitude and phase of the scattered electric field for both polarizations in the 2.6‐ to 5.85‐GHz frequency range are measured, so that diffraction characteristics, radar cross sections, frequency ratio, and phase shifts are determined. A calibration procedure using silver‐plated cylinders provides a higher degree of accuracy than has been attainable heretofore. Diffraction measurements show that, for plane‐wave incidence, the scattering is purely dipolar with a 95% confidence interval, except for overdense scattering where a significant monopole term characteristic of conductors is observed. Theoretical comparisons yield an estimate of the electron density distribution function and the damping of the Tonks‐Dattner resonances, as well as providing a test of the various theories. The results of some auxiliary experiments to find the average electron density in the column and the effects of wall and mercury reservoir temperatures upon the observed results are also given.

ISSN:0021-8979    

DOI:10.1063/1.1657355

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Second‐ and fourth‐order statistical properties of radiation scattered by particles in a turbulent fluid are theoretically investigated. They turn out to be of a non‐Gaussian type so that the measurements of the statistics of the scattered field can furnish information not obtainable by means of a spectrum analysis. In particular, our results are related with the possibility of measuring a Lagrangian type of velocity correlation function, generalized to different fluid elements.

ISSN:0021-8979    

DOI:10.1063/1.1657356

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

When heated, rhenium, like many other metals emits positive ions of the alkali metals. However the emission properties of these ions from rhenium are found to be very different from those observed for tungsten. In addition, the ion emission from rhenium can be greatly reduced relative to that of tungsten by short term vacuum annealing at sufficiently high temperatures. Lanthanum hexaboride coated rhenium filaments also emit positive alkali ions in a manner very similar to that of bare rhenium. The implications, both practical and theoretical of these results are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1657357

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

There is some ambiguity in the interpretation of internal photoemission experiments because of the possible contribution of hole emission from the anode in addition to the cathode electron emission. Furthermore, correction of the photocurrent spectra for optical intensity variations is essential to determination of photoelectric yields. Optical interference is useful as a tool in ascertaining current contributions from the two electrodes since wavelengths of intensity maxima at one electrode correspond approximately to minima at the other. In this work an analysis technique is presented whereby the absorbed intensities are accurately calculated, the absolute quantum yields are determined assuming no hole photocurrent contribution, and the resulting spectra are examined for evidence of such currents. Internal photoemission measurements have been made on Au&sngbnd;SiO2&sngbnd;Si structures, and the technique has been applied to obtain quantum yields and look for evidence of hole emission. It is shown that hole photocurrents (if present) are negligible compared to electron currents and in particular for photoemission from Si into SiO2the quantum yield for holes must be at least two orders‐of‐magnitude smaller than the electron quantum yield. The first determination ofabsolutequantum yields for Si&sngbnd;SiO2and Au&sngbnd;SiO2emission is presented, and the barrier energies are found to be 4.30 and 4.15 eV respectively. The optical problem has been solved for a range of oxide thicknesses and it is shown that intensity variations are not negligible even for very thin (∼500 Å) films.

ISSN:0021-8979    

DOI:10.1063/1.1657358

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

An electrohydrodynamic (EHD) technique is used to generate ions from liquid metals. Liquid metal is fed to the tip of a capillary needle emitter with a voltage difference applied between the emitter and an extractor electrode to produce an intense electric field at the liquid surface. Electrostatic forces overcome surface tension forces to produce ions by field emission. When using liquid cesium, time‐of‐flight mass analysis showed the ion current to be primarily Cs+with a small percentage of Cs2+and Cs3+. Electron currents of over 1 mA have been produced by operating the emitter at 2 kV negative. Besides cesium, alkali ion beams have been generated using NaK/cesium alloy and sodium. Calculations show that liquid metals of low work functions appear more suitable for production of atomic ions while higher work functions metals may produce multiatomic ions and charged droplets in addition to atomic ions.

ISSN:0021-8979    

DOI:10.1063/1.1657359

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

If, in a positive corona discharge in air, the voltage is raised at constant gas pressure over a range of pressures beyond atmospheric, the sparkover voltage is found to exhibit a maximum and then decline, eventually to coincide with the corona‐starting voltage. The pressure at this point of intersection, beyond which sparkover alone obtains, without antecedent corona, is termed the critical pressure, and the associated gas density the critical density. The critical characteristics of dry air at room temperature are here determined in terms of electrode dimensions for a coaxial wire‐tube system. The critical relative density &dgr;cr(air density relative to atmospheric conditions) is shown to fit the empirical relation&dgr;cr=k[(1/r)+c],whereris the wire radius,kan increasing function of tube radius, andcis a constant. The sparkover voltageVcrcorresponding to the critical density is given empirically byVcr=ln(R/r){Ar+B[k(1+cr)]1/2},whereRis the tube radius andAandBare constants of the gas. The unattainability of positive corona above the critical density is shown not to be absolute as hitherto supposed. Substantial corona currents are possible behond &dgr;crprovided that a suitable path in voltage‐current‐pressure space is followed in circumventing the critical density.

ISSN:0021-8979    

DOI:10.1063/1.1657360

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The dynamics of high‐speed impact between a compressible liquid drop and a solid surface are reviewed. Previous estimates for the maximum impact pressure have been based on one‐dimensional approximations. This paper presents a two‐dimensional approximation, adapted from a closely related analysis of the oblique impact between two solid plates. This is valid only for the ``initial'' phase of the impact during which the expanding shock front generated by the impact still remains attached to the target surface, and no lateral outflow takes place. The derivations assume a linear relationship between shock velocity and particle velocity change across the shock front. Numerical results are presented for water and sodium, and can be generalized as follows: The contact pressure remains substantially equal to the one‐dimensional pressure until the contact angle &phgr; at the edge has reached about half of its critical value, at which the assumed model beaks down and lateral outflow must initiate. As this critical condition is further approached, the contact edge pressure increases progressively, and its critical valuePcis taken as the maximum impact pressure. The ratioPc/&rgr;0C0V0always exceeds about 2.75 exhibiting a minimum in the vicinity ofV0/C0=0.2, where &rgr;0andC0are the density and acoustic velocity of the liquid, andV0is the impact velocity. These pressures are considerably higher than have been heretofore supposed, but circumstantial experimental evidence supports the present results.

ISSN:0021-8979    

DOI:10.1063/1.1657361

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

A time‐dependent relaxation model for the heat flux in metals is derived from the quantum mechanical form of the Boltzmann transport equation. In the derivation, the manipulation of the nonlinear integral term of the Boltzmann equation is simplified by assuming that the phonons are in thermal equilibrium at all times and by using the Lorentz approximation to treat free electron‐phonon interactions. The relaxation model for the heat flux is found to yield a damped wave equation for the temperature and as a result, the speed of propagation for heat is shown to be finite instead of infinite as implied by the Fourier model for the heat flux. Approximate expressions for the thermal conductivity and the isothermal electrical conductivity are derived in the appendices in order to estimate the magnitude of the relaxation times and to obtain an expression for the Lorentz number. In general it is found that the thermal and electrical relaxation times are not equal although they are estimated to be at the same order‐of‐magnitude, 10−14sec for the common monovalent metals. The Lorentz number is found to be a function of the ratio of the relaxation times and as a consequence the difference in the relaxation times may account, at least in part, for the derivation of the experimental Lorentz number from the usual theoretical value based upon equal relaxation times.

ISSN:0021-8979    

DOI:10.1063/1.1657362

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

The strain energy and stress concentrations in filled polyethylene are analyzed and the effect of fillers on the ultimate tensile strength of these composites is discussed. It is shown that the usual two‐phase model is inadequate to describe filler‐polyethylene composites when the filler has a high surface energy. Since a high surface energy substrate is known to generate a transcrystalline region, at the polymer matrix‐filler interface, which differs mechanically from the bulk polymer, a three‐phase model is proposed and analyzed. The agreement between theory and experiment lends credence to the three‐phase model.

ISSN:0021-8979    

DOI:10.1063/1.1657363

出版商:AIP    

年代:1969

数据来源: AIP

摘要:

Side light measurements were made to determine the density distribution of cadmium in a discharge tube similar to that used in the helium‐cadmium laser. The tube had an electrode at either end and a single cadmium source near the center. The discharge causes the cadmium vapor to flow toward the cathode, the flow being greater at higher currents. At currents greater than 20 mA almost no cadmium is found a few cm from the cadmium source in the discharge region between it and the anode. In contrast to this, large amounts of cadmium are found in the discharge region between the source and the cathode. It is concluded that an approximately uniform density of metal vapor may be maintained in a helium‐cadmium laser by placing a single cadmium source near the anode and passing currents greater than 50 mA through the tube. A very simple theory gives a qualitative explanation of the gross features of the density distribution curves.

ISSN:0021-8979    

DOI:10.1063/1.1657364

出版商:AIP    

年代:1969

数据来源: AIP