摘要:

Superconducting films of niobium can be made by asymmetrical ac sputtering. This modification of the conventional sputtering process uses a reverse sputtering current to bombard the film as it deposits, thus dislodging gaseous impurities to obtain pure films. The deposition conditions to yield the purest films have been established using cylindrical geometry, that is, Vycor tubing substrates which are heated internally and surrounded by a large niobium cathode. At substrate temperatures of 500°C and with a sputtering to reverse sputtering current density ratio of 4:1, films with &bgr;=R300°K/R10°Kas large as 4.7 have been obtained from bulk material of &bgr;=14. Films so made have critical temperatures between 8.9° and 9.2°K; the bulk material critical temperature is 9.25°K. The critical temperature of the films is a strong function of &bgr;, falling below 4.2°K for &bgr;<1.25.

ISSN:0021-8979    

DOI:10.1063/1.1729254

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The relative magnetization of an assembly of fine, ferromagnetic, single‐domain particles which show uniaxial magnetic anisotropy has been formulated as a function of &rgr; and &lgr;, the ratios of magnetic energy of particle moment in field and anisotropy energy to thermal energy, respectively, under various situations for particle orientation. Strict formulation for the system where the anisotropic symmetry axes of particles are distributed in thermal equilibrium gives the simple Langevin equation without any influence of anisotropy. In the case, however, of particles with fixed random orientation of symmetry axes the observed decrease of relative magnetization was deduced, while it was found that the Langevin equation is still valid as low and high field approximations. Several simpler cases where the symmetry axes of particles are constrained on a plane parallel or perpendicular to the applied field were also treated and the deviations from the Langevin equation were estimated. These results confirm Bean and Livingston's view about the behavior of the system in low and high field extremes, but show that Knappwost and Rust's formulation, which was developed to explain the anomaly in the magnetization of fine cobalt particles, is not reasonable, suggesting sources of anisotropy other than those of magnetocrystalline origin.

ISSN:0021-8979    

DOI:10.1063/1.1729255

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

A new technique for the measurement of the thermal diffusivity at high temperatures is described. The diffusivity is determined from the time interval separating two radial isotherms in a cylindrical specimen. The observations are made during the transient heating (or cooling) period. Measurements are reported on Ta, ZrC, and TiC in the temperature range 1300° to 1650°C. The data are in good agreement with diffusivities calculated from specific heat and steady‐state thermal conductivity measurements.

ISSN:0021-8979    

DOI:10.1063/1.1729256

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The tunneling current between two conductors through an intermediate thin‐film insulating barrier in the presence of a magnetic field parallel to the direction of tunneling has been calculated. It is shown that the Fowler—Nordheim tunnel current decreases as the magnetic field is increased and, in certain cases, contains a component which oscillates with a period proportional to 1/H. The oscillations should be observable when the electrons in the material from which the electrons tunnel have a small Fermi energy, a small effective mass, and a long relaxation time. Bismuth, lead telluride, and graphite are examples of materials in which the criteria for observing oscillations are realized.

ISSN:0021-8979    

DOI:10.1063/1.1729257

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

When the surface of a body is subjected to transient heating (e.g., by electron bombardment or rf absorption) elastic waves are produced as a result of surface motion due to thermal expansion. This process is analyzed, with particular emphasis on the case of an input heat flux varying harmonically with time, to relate the elastic wave amplitude to the characteristics of the input flux and the thermal and elastic properties of the body. Experiments performed with both electron impact and rf absorption verify the proportionality of the stress wave amplitude and the absorbed power density, and correlate well with the thermal and elastic properties of the heated medium. Comparison of the elastic wave stress amplitude with radiation pressure shows that the former may be much greater than the latter, as experiments have demonstrated. When a barium titanate crystal was used to detect the elastic waves produced, heating by a single 2‐&mgr;sec pulse of electrons or microwave radiation produced easily detectible signals at power levels down to 2 W/cm2, corresponding to a computed peak surface temperature rise of about 0.001°C, and produced piezoelectric crystal voltages ranging from about one to more than 60 mV/kW/cm2absorbed power density.

ISSN:0021-8979    

DOI:10.1063/1.1729258

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The influence of milling upon some magnetic properties of the intermetallic compound MnAlGe has been studied. The Curie temperature of the material seems to be independent of particle size and is constant at 245°C. The saturation magnetization &sgr;s, which is 55 erg/Oe·g in the annealed state, decreases as the grinding time increases. The anisotropy fieldHA(∼43 000 Oe) was found to be more or less independent of the grinding time. Particles obtained by milling in a vibration mill can possess a coercivityIHCof about 3000 Oe, which can be enhanced by annealing to a value of 6400 Oe. The saturation magnetization and the coercivity decrease appreciably after mechanical loading which does not involve a reduction in particle size. This indicates that structural imperfections, rather than changes in particle size, play an important role in the observed phenomena.

ISSN:0021-8979    

DOI:10.1063/1.1729259

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The energy equation governing a diffusive generator of Cartesian geometry is derived for azcomponent of uniform magnetic field only and for currents and driving forces in thex‐yplane only. The kinetic coefficients may depend on temperature in any manner. Only the induced magnetic field is neglected. Steady state is assumed. Heat is forced to flow in theydirection and current is allowed to flow in thexdirection. By assuming no current in the heat flow direction, and no temperature dependence in the current direction, the current is eliminated from the energy equation, which is then the differential equation for the temperature distribution with arbitrary temperature dependence of the coefficients. The exact temperature and current profiles and the efficiency expression are then worked out in analytic form for temperature‐independent coefficients. The magnetic‐field dependence of the coefficients is then calculated assuming classical statistics, and a single carrier. Numerical integrations are done only for scattering by ions. The upper limit of the efficiency is displayed for various magnetic field strengths as a function of driving‐temperature differences.

ISSN:0021-8979    

DOI:10.1063/1.1729260

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The effect of a uniform magnetic field on the mechanism, kinetics, and resulting morphology of a spinodally decomposing alloy is investigated. It is shown that the magnetic field is most effective in forming uniformly spaced rods parallel to the magnetization if the aging is carried out just below the intersection (on the phase diagram) of the Curie temperature and the temperature where the spinodal instability sets in. This condition limits the effective magnetic aging alloys to the composition range near this intersection.

ISSN:0021-8979    

DOI:10.1063/1.1729261

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

The thermoelectric figure of merit of silver selenide powders prepared by a low‐temperature process is examined. By careful attention to the process variables including the compaction, sintering temperature, and cooling rate through the &bgr;2phase it is possible to obtain ann‐type thermoelement having a figure of merit of 3×10−3per degree at room temperature.

ISSN:0021-8979    

DOI:10.1063/1.1729262

出版商:AIP    

年代:1963

数据来源: AIP

摘要:

Comparison of the annealing properties of radiation‐induced conductivity changes in GaAs indicates that about 10% of the damage created by reactor irradiations anneals in a manner quite similar to but not identical with that created by 1‐MeV electrons. The remaining neutron damage requires much higher annealing temperatures and is presumed to result from complicated damage structures characteristic of highly energetic knock‐on atoms (e.g., disordered regions). Heavy neutron irradiation of eitherp‐ orn‐type GaAs results in very high resistivities which appear to be influenced by the presence of slow surface states. Energy levels resulting from neutron irradiation are estimated to lie at approximately 0.1 and 0.5 eV below the conduction band and at 0.6 eV above the valence band. Moderate irradiation of GaAs by fast neutrons gives rise to a continuous optical absorption spectrum for wavelengths beyond the fundamental absorption edge, with the absorption increasing as the inverse square of the wavelength. Similar behavior occurs in CdTe and CdS after neutron irradiation. Although this effect is not well understood, it is suspected of being associated with defect structures characteristic of fast neutron bombardment, since heavy bombardment with electrons does not produce the same behavior.

ISSN:0021-8979    

DOI:10.1063/1.1729263

出版商:AIP    

年代:1963

数据来源: AIP