摘要:

At static pressures below a few millimeters of mercury the optical thickness of wind tunnel gas streams becomes insufficient for conventional flow visualization techniques in aerodynamic testing. In the University of California low density wind tunnel, where the free stream static pressure is of the order of one‐tenth of one millimeter of mercury, use has been made of various gaseous afterglow phenomena in the visualization of low density flows. The present paper describes the successful application of afterglows in flows of nitrogen, air, argon, and helium.On the basis of recent tests the range of practicability of the enriched air glow technique was found to extend from Mach 2 to Mach 4. The argon and helium glows were found to be useful in the range Mach 1.3 to Mach 2. Discussion of the ranges of applicability and of other aspects of the observed glows is supported by representative glow photographs.

ISSN:0021-8979    

DOI:10.1063/1.1722870

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

An experimental study of the role of oxidation in inhibiting metallic interaction between metal surfaces sliding at high speeds (up to 66 m/sec) is described. Metallic interaction was detected by microscopic examination of the surface damage after sliding. By using hardened steel, information concerning surface temperatures was deduced from subsequent hardness measurements. At light loads the metal surfaces were severely torn, but at higher loads the tearing became less severe. Above a certain load which depended on the speed tearing was almost eliminated from at least one of the surfaces. Other experiments supported the conclusion that this was due to the increased oxidation associated with the greater frictional heating, and that severe wear at high loads was due to a process of oxide disintegration. Under these conditions the wear rate increased rapidly with increase in load or speed, whereas the wear associated with metallic tearing at light loads showed little increase with load, and decreased with increasing speed. The friction coefficient decreased with increasing load and with increasing speed. The shapes of the underlying bodies influenced the average surface temperatures and therefore the wear and surface damage and create differences even for a pair of mating surfaces.

ISSN:0021-8979    

DOI:10.1063/1.1722871

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Experimental studies of molecular scattering at the gas‐surface interface are described in which molecular beams of N2were scattered at surfaces of steel, aluminum, and glass. Polar surveys of the issuing molecule flux indicate the scattering to be well represented by a cosine distribution in most cases. A moderate deviation from such a distribution was observed in the case of the glass‐N2interaction.It is shown how measurements of scattering at the surface may be used to derive values of the momentum transfer coefficient,f(s), in a rarefied gas flow wheresis defined as the ratio of the mass speed of the flow to the most probable thermal speed of the molecules. In order to complete the calculation where scattering information alone is available from experiment, assumptions must be made relating to the energy exchange at the surface. In the case of the present calculation, where a surface interaction model was formed in part from the glass ‐N2scattering data, two extremal assumptions relating to the energy exchange lead to values off(s) of approximately 0.97 ats=0.1.

ISSN:0021-8979    

DOI:10.1063/1.1722872

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Transfer of radioactive silver has been studied quantitatively in sliding contacts containing graphite and metal. The rate of transfer from silver‐graphite riders to graphite cylinders is initially high but soon approaches a limiting equilibrium state. The amount of silver transfer increases markedly with surface roughness. Further, the transfer is enhanced by the addition of liquid water to the rider track. Back transfer to the rider has been found to consist mostly of loosely clinging dust in the rider face.Silver transfer to a copper cylinder is much greater than to a graphite cylinder, the difference being roughly twenty‐fold. Within the experimental conditions, the transfer/unit time to both graphite and copper cylinders is greater at 1 cm/sec than at 390 cm/sec.

ISSN:0021-8979    

DOI:10.1063/1.1722873

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

A statistical theory of magnetization reversal in thin films for reversing fields less than the anisotropy field is presented in which it is assumed that reversal takes place by domain wall motion. The number of moving domain walls is determined by a simple statistical model which allows for the random nucleation of walls as the reversing field increases. Experimental data are presented in support of this theory.Two equations for the velocity of a domain wall in a thin film are presented; these arise since two alternative forms of the equation of motion of the magnetization vector are considered.

ISSN:0021-8979    

DOI:10.1063/1.1722874

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

When tea is poured from a teapot it often runs along the under side of the spout rather than falling into the cup. Recent experiments have shown that this ``teapot effect'' is not due to surface tension nor adhesion, as many have supposed. Therefore, a new explanation is presented which is based upon certain exact solutions of the hydrodynamic equations and which seems to account for the effect. In connection with the analysis some new fluid flows with free boundaries are obtained. In addition a method more satisfactory than the usual one is used to study the stability of these flows.

ISSN:0021-8979    

DOI:10.1063/1.1722875

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Cadmium telluride was made by combining purified cadmium and tellurium in stoichiometric proportions in an evacuated Vycor tube. Thin films of this material were deposited by condensation across the short gaps between Aquadag electrodes in Dewar‐type tubes. These films have a high resistance at room temperature and show a large photoconductive effect, with a peak response at about 0.8 micron. When the films are deposited in the presence of indium vapor, the dark resistance can be brought down to a value of approximately one to ten megohms for a film of one‐half millimeter in width. The resistance of the films under illumination will then be a few tenths of a megohm. Iodine vapor will also reduce the dark resistance of cadmium telluride films, but in this case, the photoconductive effect almost disappears.

ISSN:0021-8979    

DOI:10.1063/1.1722876

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Some of the effects of bombarding a silicon single crystal surface with 3×104electron volt helium ions have been examined experimentally. The surface recombination velocity was found to be increased by more than two orders of magnitude, but could be restored to its original value by the removal of approximately 1 &mgr; of surface material. This is believed to correspond to the total depth of material affected by the bombardment, and is in fair agreement with a theoretical estimate of the range of the helium ions. Electron diffraction data shows that the effect of bombardment is to convert the surface layer to a quasi‐stable amorphous form, which may be partially restored to the crystalline state upon annealing. The amorphous material is soluble in aqueous hydrofluoric acid, and its thickness has been measured by an interferometric technique. It is found to be approximately one tenth of the total thickness of silicon affected, depending on the bombardment dosage. The effect of annealing upon the diode characteristics of point contact diodes made with bombarded silicon have been studied. The general result is that the characteristics tend to revert to those of an unbombarded diode, as expected from the electron diffraction data. These experiments are discussed in terms of radiation damage theory.

ISSN:0021-8979    

DOI:10.1063/1.1722877

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Single crystals of electrolytic tough pitch copper and of high purity (99.999%) copper were deformed by bending. The substructure in the as‐bent and annealed crystals was studied by means of sensitive x‐ray techniques. It was found that regardless of purity, crystals of certain orientations in the as‐bent condition do not have sub‐boundaries detectable by the methods used. Polygonization took place only upon prolonged annealing at around 97% of the absolute melting point and, at least in one high purity crystal, not even under those conditions. The sluggishness of the polygonization in copper is experimental evidence for the low rate of climb of extended dislocations predicted by Seeger.

ISSN:0021-8979    

DOI:10.1063/1.1722878

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

A relation between minority carrier lifetime and recovery time has been obtained forp‐njunction rectifiers of arbitrary base width. In the limiting cases of base width large or small compared to a diffusion length the recovery time is found to depend on the lifetime or on the transit across the base region, respectively. In intermediate cases, the recovery time is a function of both the lifetime and the transit time. Recovery time measurements on diffused junction silicon rectifiers as a junction of base width were found to be in satisfactory agreement with the theory.

ISSN:0021-8979    

DOI:10.1063/1.1722879

出版商:AIP    

年代:1957

数据来源: AIP