摘要:

Measurements by different workers have indicated that the friction coefficient &mgr; of a graphite brush contact decreases with rising contact temperatureT. IfP= mechanical load,Ab= load bearing area andF= friction force, the average pressure isp¯=P/Aband the specific friction force &psgr; =F/Ab. Hence &mgr; =F/P= &psgr;/p¯. It will be shown in this paper that the measured &mgr;(T) effect during smooth sliding is caused by the decrease of &psgr; with risingT.The specific friction force is a function of the character of the sliding surface alone, and &mgr; is, in addition, a function of the actual pressure inAb. It is shown that sliding, after a relatively short time, leads to a stage during whichAb, thus alsop¯, remains practically constant for more than 10 hr. The (&mgr;,T) test begins when such an ``initial stage,'' defined by &mgr;=&mgr;0,T=T0and constant contact voltageU, has been reached. It involvesTincreasing aboveT0and then decreasing toT0again, whereby the time of the test is short enough for constancy ofp¯. During a perfect (&mgr;,T) test, the forward and reversed curves agree with each other indicating thatAbhas remained constant.For all sliding stages with constantp¯, the relationship mentioned above can be written: &mgr;(T) = const&psgr;(T), which makes it possible to study &psgr; as a function ofTby studying &mgr; as a function ofT. The decrease of &psgr; with risingTis explained by assuming that adherence bonds inAbare broken or loosened byTaccording to a probability exp(−&phgr;/kT) = exp(−11600 &phgr;/T) with &phgr; in ev. The measured, strong dependence onTresults from the activation energy &phgr;=0.07 to 0.1 ev between the graphite ``platelets'' being of the order ofkT. Hence it is expected that &mgr; and &psgr; are inversely proportional to the probability factor. It was found that at constantAbup to about 500°K (the upper limit measured):&psgr;(T)&psgr;(T0)=&mgr;(T)&mgr;(T0)=exp11600[open pi]1T−1T0gives a very good agreement in the case of graphite members when the binding energy is assumed to be &phgr;=0.09 ev. For these contacts, &psgr; decreases from 0.12 to about 0.065 ton/cm2; simultaneously &mgr; decreases from 0.13 at 115°C to 0.07 at about 200°C. This has been found at atmospheric air pressure with dew point about 279°K and also in vacuum of 10−4mm Hg with dew point about 75°K. With a copper and gold ring at atmospheric air pressure, the (&mgr;,T) curves have a somewhat steeper slope and agreement with theory requires &psgr;=0.12 or 0.10 ev, respectively.At temperatures of about 500°K, &mgr; and &psgr; decrease very slowly and may reach a lower limit at temperatures when the chemisorbed gases on the graphite platelets begin to evaporate.

ISSN:0021-8979    

DOI:10.1063/1.1728476

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The superconducting critical temperature of a large number of tin films on soda lime glass substrates are reported. The critical temperatures depend upon the stress arising in the films when they are constrained to follow the contraction of the glass on cooling to liquid helium temperature. It is shown that the crystalline orientation of the films is significant in determining the magnitude of the stress and the variation in critical temperature relative to bulk tin. For films with the tetrad axis in the plane, the stresses along that axis at helium temperature can reach 4.5×103atm and the critical temperature may increase by 0.23°K. For both diad axes in the plane, the stress and the change in critical temperature is much less. The piezoresistivity constants along the tetrad and diad axes of single crystals were measured, &pgr;33= 9.8±0.5&rgr;3×10−6/atm, &pgr;11= −7.0±0.6&rgr;1×10−6/atm. The temperature dependent part of the resistivity of films at 77°K is increased above the values of bulk metal in accordance with a piezoresistive effect. For films much thinner than 300A, the critical temperature is observed to increase with decreasing thickness. For these thin films the constraint of thermal contraction imposed by the substrate appears to be insufficient to provide the entire increase in critical temperature.

ISSN:0021-8979    

DOI:10.1063/1.1728478

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Plastic bending of InSb is found to result in changes in the electrical properties which are similar to those found in other semiconductors. The total carrier concentration is decreased by the trapping of charge carriers on dislocation acceptor levels and the electron mobility is reduced for conduction perpendicular to the axis of bend. The mobility parallel to the axis of bend is unaltered, suggesting that the space‐charge cylinders associated with edge‐type dislocations are smooth, although the dislocations themselves are thought to be zig‐zagged. Although the electrical properties are in qualitative agreement with Read's theory, it is shown that large disagreements can occur in heavily‐deformed impure specimens. The theory is extended to the case where scattering by ionized impurities is appreciable. A method is also proposed whereby one may determine the dislocation density from the magnetic‐field dependence of the Hall coefficient. The density so determined is in good agreement with that found from etch‐pit counting and from Nye's relation.

ISSN:0021-8979    

DOI:10.1063/1.1728479

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The magnetic field distribution and supercurrent density are calculated exactly by means of the Maxwell‐London equations for the case of an ideal superconducting spherical shell immersed in a steady uniform magnetic field. It is shown that the magnetic field is strongly attenuated by the spherical shell even if the shell thickness is as small as 1000 A.

ISSN:0021-8979    

DOI:10.1063/1.1728480

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Etch pits and x‐ray studies have been made on several ZnS crystals. Etch pit geometries have indicated, in some cases, a mixed cubic‐hexagonal structure. This has been confirmed by x‐ray analysis. A model to explain the x‐ray and etch pit data is presented.

ISSN:0021-8979    

DOI:10.1063/1.1728481

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A general treatment of the input boundary value problem for two‐dimensional space‐charge flow is developed using a theory of electrostatics (Thomson's) which notes that in an electrostatic problem the electric field distribution is such that the electrostatic‐field energy is a minimum. This principle is used to determine the average energy density mismatch and the starting conditions for finite thickness beams inM‐type backward‐wave oscillators. It is shown that the mismatch is greatest for strong space‐charge fields, high‐circuit attenuation, and signal frequencies below the cyclotron frequency. The starting length and beam velocity are greater as calculated by the variational procedure than obtained by the approximate method. Under certain conditions the average energy density mismatch is greater than 20% of the electrostatic field energy in the absence of the beam.

ISSN:0021-8979    

DOI:10.1063/1.1728482

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The dependence of the coercive force of Ni, Fe, and Ni&sngbnd;Fe films upon film thickness and amount of trapped gas was evaluatedin vacuo. The magneto‐optic Kerr effect was utilized for this investigation. Although the penetration depth of light into the film material was determined to be 150±25 A, measurements ofHccould commence for film thicknesses <100 A. It was found that the coercive forceHcis independent of film thickness if the content of trapped gas, as indicated by thep/r(residual gas pressure over deposition rate) ratio is very small, e.g., 5×10−8torr/A/sec for Ni films. At higherp/rratios, the coercive force of Ni films displays initially a steep rise which is followed by a shallow minimum, while in Ni&sngbnd;Fe films a maximum appears at about 750 A. The different behavior of Fe films is ascribed to higher gas absorption on this material rather than a principal change in the mechanism of the coercive force. A hypothesis is proposed as explanation for the observed effects.

ISSN:0021-8979    

DOI:10.1063/1.1728483

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Some details on dislocation etch pits in tungsten are presented. Etch pits are produced electrolytically with 2% NaOH solution and the Millner‐Sass reagent, and chemically with a potassium ferricyanide etchant. All three etchants can reveal dislocations, but only on certain crystallographic planes. In addition to etch pits, pyramids are produced on octahedral, or nearly octahedral planes. These pyramids do not seem to be related to dislocations. There is no guarantee that all pits correspond to dislocations since impurities, such as carbon, markedly affect the size, shape, and density of etch pits. It was observed that the density of etch pits depends also on the current density during electrolytic etching.

ISSN:0021-8979    

DOI:10.1063/1.1728484

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

It is shown by means of a semiquantitative nonlinear analysis that elementary interaction between an inverted electron spin system and a resonant cavity does not give rise to the pulsed mode of operation of the ruby maser oscillator. It is suggested that the additional nonlinearity necessary for the existence of such a mode resides in the ``distant ENDOR,'' the interaction between the chromium electrons and the Al27nuclei.

ISSN:0021-8979    

DOI:10.1063/1.1728486

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

Electrical conductivity measurements were performed with thin (50 &mgr;) single crystals ofp‐terphenyl,p‐quaterphenyl, and anthracene supplied with aqueous electrodes, one of which was an iodine‐iodide solution (acceptor electrode), and the other an iodide solution. The results strongly indicate that the acceptor electrode can form ohmic contact for hole injection into these crystals and that space‐charge‐limited currents can be drawn through them. The crystals were found to contain hole‐trapping states the location‐in‐energy of which can be approximated by a decreasing exponential distribution above the valence band. The measurements showed that the hole mobility inp‐terphenyl is about 3×10−2cm2/v sec, is independent of the field at least up to about 4×104v/cm, and that the hole‐trap concentration is at least 1013cm−3. The acceptor electrode used does not form ohmic contact to crystals of naphthalene and diphenyl; an explanation for this is proposed. Some theoretical aspects of ohmic contact formation to organic crystals and space‐charge‐limited current flow in insulators are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1728487

出版商:AIP    

年代:1962

数据来源: AIP