摘要:

Ultrasonic velocity measurements have been made on polycrystalline bismuth as a function of temperature and pressure over the temperature and pressure ranges 23–160°C (at atmospheric pressure) and 0–28.4 kbar (at 23°C), respectively. Special care was taken with the room‐temperature and pressure measurements in an attempt to resolve discrepancies in previously reported sound velocity measurements. The data have been used to determine the pressure‐temperature dependences of various elastic parameters, including the (discontinuous) changes across the I–II and II–III phase boundaries. The pressure data are the first that have been taken under truly hydrostatic conditions, and they are compared to results obtained with solid pressure transmitting media.

ISSN:0021-8979    

DOI:10.1063/1.1663018

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A new method is given for computing effective polycrystalline elastic constants from single‐crystal elastic coefficients. Agreement with observation is good. The method is based on the assumed equivalence of the lattice‐vibrational properties of single crystals and polycrystals of the same material; single‐crystal and polycrystal Debye temperatures are equated. Present predictions of polycrystal elastic moduli differ significantly from those of most other averaging methods by being lower than the familiar Voigt‐Reuss‐Hill results.

ISSN:0021-8979    

DOI:10.1063/1.1663019

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

Mo¨ssbauer absorption of Fe57in single crystals of FeS2(pyrite) has been studied as a function of the orientation of the crystal axes with respect to the unpolarized &ggr;‐ray beam. Using a point charge model, the quadrupole splitting has been calculated to be 0.58 mm/sec, taking only first immediate neighbors into consideration.

ISSN:0021-8979    

DOI:10.1063/1.1663020

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

It is suggested that the recent space‐charge theory of Fatuzzo and Coppo omits consideration of several effects important in general space‐charge situations and probably also important in these authors' experimental studies of silver halides. These effects are discussed in some depth and may explain, at least in part, why Fatuzzo and Coppo's theoretical analysis does not apply well to their experimental results. A detailed discussion is given of the diffusion potential and of surface and electrode charge in blocking and nonblocking electrode situations. Although a careful distinction between electrode and surface (absorbed) charge is usually made in the ideally polarized electrode (completely blocking) electrolyte case, the need for this necessary separation has apparently not been well recognized heretofore for situations where a solid material of interest is assumed to be contained between electrodes which are completely blocking for some or all types of mobile charge carriers in the solid. When this distinction is made for the problem considered by Fatuzzo and Coppo, quite different results than theirs are obtained. Three important improvements over the Fatuzzo‐Coppo work are the present use of more appropriate general expressions for static and differential dielectric constants (or capacitances), the avoidance of their implicit assumption that surface charge is independent of total applied potential difference, and the avoidance of their assumption that the applied potential difference divides equally between the left and right crystal‐electrode regions. The new analysis, appropriate for intrinsic materials with completely blocking electrodes, leads to expressions for the differential capacitance of such a system for a specific situation where surface charge may depend on the applied potential difference.

ISSN:0021-8979    

DOI:10.1063/1.1663021

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The classical ``similarity laws'' for electrical discharges in gases are rederived and shown to apply only to cases where the charge density is low enough so that the only important collisions are with gas molecules and there is no appreciable gas heating. If the charge density is high enough to be well above the transition from free to ambipolar diffusion, then the distribution functions of all the particles, and particularly the charge density, can be scaled by dividing by the neutral gas densityN0at some point in the discharge. The usual similarity laws scale the charge density by dividing by the square of the neutral gas density. These new high‐density plasma scaling laws are shown to apply to the experimental results of the current limit at which striations will appear in argon discharges as reported by two investigators. Here the pressure‐current product was plotted as a function of the pressure‐radius product and found to form one curve over a 4:1 range of tube radii. We believe that these high‐density scaling laws apply to a large group of plasma phenomena. Limitations of the new scaling laws are pointed out in connection with the spontaneous emission of excited states, three‐body collision, magnetic fields produced by the currents in a discharge, the rf shielding effect in a discharge of sufficiently high conductivity, and gravitational effects.

ISSN:0021-8979    

DOI:10.1063/1.1663022

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A distribution function of ion energy parallel to a magnetic field is measured to be deformed along a diverging magnetic field in a single‐endedQmachine. The deformation is well explained by the orbit theory for a collisionless plasma.

ISSN:0021-8979    

DOI:10.1063/1.1663023

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

A model for the positive columns of dc low‐pressure discharges in Cs&sngbnd;Ar mixtures, which includes radial depletion of Cs ground‐state atoms and radiation trapping, is presented. The available experimental data for this discharge system were extended in such a way that comparison could be made between model calculations and experiments for a wide range of discharge currents and Cs vapor densities (0.2 × 1019−3.0 × 1019m−3) and a reasonable agreement is obtained.

ISSN:0021-8979    

DOI:10.1063/1.1663024

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

The effect of a magnetic field upon the properties of a plasma created by CO2laser‐induced gas breakdown in helium has been investigated. Spatially and temporally resolved temperatures in the 7–20‐eV range were observed using the spectral‐line ratio technique, in particular the He II 468.6‐nm line‐to‐continuum ratio. The continuum measurements were also employed to obtain relative values of electron density. The temporal dependences of electron density and temperature indicate that energy is lost from the plasma by an adiabaticlike expansion. The presence of the magnetic field significantly decreases both the energy and density loss rates.

ISSN:0021-8979    

DOI:10.1063/1.1663026

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

When a negative dc voltage is applied to a needle in an electronegative gas such as atmospheric air, highly regular corona current pulses (Trichel pulses) are observed under some conditions. Systematic variation of the macroscopic experimental parameters has yielded the functional dependence of the pulse frequency, charge per pulse, and time‐averaged corona current on the applied voltage, needle‐tip radius, and needle‐to‐plane spacing. In combination with a theoretical analysis of the motion of the charge clouds, these results yield new insight into the physical mechanism of this gas‐discharge phenomenon. For example, above threshold voltage, it appears that several negative‐ion‐charge clouds are simultaneously in transit across the gap.

ISSN:0021-8979    

DOI:10.1063/1.1662943

出版商:AIP    

年代:1974

数据来源: AIP

摘要:

An electron stereoscan microscope is used to examine the surfaces of an aluminum cathode and an aluminium anode which have been subjected to repeated sparkings, under an ultrahigh vacuum (10−9Torr), at voltages of up to 45 kV. The cathode surface is observed to contain large numbers of protrusions which give rise to an enhanced local electric field at the tip of the protrusions. The field enhancement factor, &bgr;, at the microprojections is determined from the scanning electron micrographs. The values of &bgr; are in resonable agreement with those obtained from the Fowler‐Nordheim theory when applied to the current‐voltage measurements in the region prior to breakdown. The anode surface is completely devoid of protrusions and contains large numbers of craters. The diameters and the depths of typical craters are also determined.

ISSN:0021-8979    

DOI:10.1063/1.1662944

出版商:AIP    

年代:1974

数据来源: AIP