摘要:

Expressions for the two microstructural parameters that appear in the variational third‐order bounds [G. W. Milton, Phys. Rev. Lett.46, 542 (1981)] for the effective conductivity and elastic moduli of composite media are derived analytically to first order in the sphere concentrationcfor random well‐mixed dispersions of impenetrable spheres with an arbitrary size distribution. These relations lead to rigorous bounds on the effective properties which are exactly valid to orderc2for such models. The apparentlinearbehavior of the microstructural parameters up to moderately highcenables one to apply the bounds beyond second‐order inc, however. Employing these results, the effect of polydispersivity on the effective properties is examined. It is worth noting that, under some conditions, polydispersivity can actually lead to a slight decrease of the shear modulus, whereas, for highly conducting particles, polydispersivity always increases the effective conductivity.

ISSN:0021-8979    

DOI:10.1063/1.345169

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Some designs of liquid‐metal current collectors in homopolar motors and generators are essentially rotating liquid‐metal fluids in cylindrical channels with free surfaces and will, at critical rotational speeds, become unstable. An investigation at David Taylor Research Center is being performed to understand the role of gravity in modifying this ejection instability. Some gravitational effects can be theoretically treated by perturbation techniques on the axisymmetric base flow of the liquid metal. This leads to a modification of previously calculated critical‐current‐collector ejection values neglecting gravity effects. The purpose of this paper is to document the derivation of the mathematical model which determines the perturbation of the liquid‐metal base flow due to gravitational effects. Since gravity is a small force compared with the centrifugal effects, the base flow solutions can be expanded in inverse powers of the Froude number and modified liquid‐flow profiles can be determined as a function of the azimuthal angle. This model will be used in later work to theoretically study the effects of gravity on the ejection point of the current collector.

ISSN:0021-8979    

DOI:10.1063/1.345170

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Observations were made on the expansion of polyatomic (CF4) and diatomic (O2) gases into vacuum from a toroidal puff valve. Fast ionization gauge measurements have been compared with an unsteady adiabatic isentropic model. Observed temporal and spatial gas density profiles closely followed model predictions until the arrival of gas reflected from the upstream wall of the vacuum vessel.

ISSN:0021-8979    

DOI:10.1063/1.345171

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Control of electron energy and electric field in a low‐pressure argon plasma produced by a hybrid (2.45 GHz microwave and 13.56 MHz rf) discharge was studied for thin‐film preparation. The hybrid plasma was found to be useful over a wide range of magnetic field strengths, unlike conventional microwave plasma. A novel probe measurement revealed that the electron temperature and density were effectively controllable by the microwave power and the magnetic field strength, rather than the rf power, and the potential profile describing the electric field was controllable by the magnetic field strength. The control of an ion beam injected from the microwave into the rf plasma is described.

ISSN:0021-8979    

DOI:10.1063/1.345172

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The nonlinear evolution of transient electrical discharges initiated from a small charge spot on dielectric surfaces is analyzed with a transmission line model. The relation between the resistance per unit length,Rˆ, and the current,I, is assumed to be given by a local Arc Welder’s Ansatz:Rˆ‖I‖=E*, whereE* is a positive constant. Comparison is made with a similar study of discharges initiated from a large charge spot. While both studies predict conditions under which charge is, or is not, transported to a dielectric edge, significant differences in the two cases are revealed. For example, if charge is not transported to an edge, current reversal is possible if the charge spot is small, but can only be unidirectional if the spot is large.

ISSN:0021-8979    

DOI:10.1063/1.345173

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The problem of radiation from a sheet current source in a moving compressible plasma is treated. In order to know the interaction between the sheet source and the moving plasma, first the sign of the radial component of the propagation constant is determined accurately on the basis of the radiation condition. Then the radiation power is investigated numerically for various sets of the direction of the source and that of the moving plasma. In particular, it is found that the source is given an energy by the surrounding plasma medium under certain conditions.

ISSN:0021-8979    

DOI:10.1063/1.345174

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The electron energy distribution function in a mercury‐argon low‐pressure positive column was determined from the second derivative of voltage‐current characteristics of a single probe. The probe signals were analyzed with a digital signal processing technique. The high‐energy tail of the distribution functions ( f∼10−5) was obtained at mercury coldest spot temperature of 20, 42, and 60 °C (mercury density ratio of 4.8×10−4, 2.8×10−3, and 1.0×10−2), and discharge current of 0.2, 0.4, and 0.6 A for a 24‐mm‐diam tube. The electron density and electric field strength were measured at the same time, and used as input to a Boltzmann equation analysis code. General agreement was found between the experimental and theoretical results for the distribution function.

ISSN:0021-8979    

DOI:10.1063/1.346083

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Annealing of silicon implanted with boron through a surface oxide results in an enhanced diffusion of boron. This enhanced diffusion is suppressed during an initial incubation period. An activation energy of 2 eV is associated with the enhanced diffusion, indicating excess silicon interstitials may be involved. On the other hand, the process leading to the onset of enhanced diffusion possesses an apparent activation energy of 3.7 eV. Two‐step annealing reduces the latter value to 2.6 eV, the activation energy for interstitial oxygen diffusion. The different activation energies evaluated for the saturation process will be discussed. Transmission electron microscopy shows that the coalescence of dislocations, as well as the growth of faulted loops, proceeds rapidly after the incubation period for enhance diffusion. Precipitates along small dislocation loops are also observed after the incubation period. It is proposed that oxygen precipitation, with emission of silicon interstitials, predominates for annealing beyond the incubation period and is therefore responsible for the enhanced diffusion of boron. The enhanced diffusion sequence is initially incubated by trapping oxygen at dislocations. The real onset of the enhanced diffusion occurs when the dislocations are saturated and the oxide precipitation at the dislocations commences.

ISSN:0021-8979    

DOI:10.1063/1.345175

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We describe the concept of the spin‐polarized scanning tunneling microscope. It consists of a ferromagnetic tip with saturated magnetization from which spin‐polarized electrons tunnel into a ferromagnetic sample which has its axis of magnetization aligned with that of the tip. When the magnetization of the sample is alternated periodically from parallel to antiparallel, a portion of the tunnel current is predicted to oscillate at the same frequency, with an amplitude linearly proportional to the average tunnel current. The construction of a prototype microscope, operated in air, is described. When the sample magnetization is alternated, a modulation of the tunnel current is observed at the same frequency. This signal satisfies criteria developed to characterize spin‐polarized tunneling. Spurious signals are also observed and their probable origins identified. A number of improvements are suggested that should eliminate the spurious effects.

ISSN:0021-8979    

DOI:10.1063/1.345176

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The depth distributions of vacancy‐type defects in Si+‐implanted and thermally activated GaAs were studied by a slow positron beam technique and were compared with the results observed with a transmission electron microscope. In as‐implanted GaAs, the concentration of vacancy‐type defects decreased monotonically with increasing depth below the surface and the generation of point defects was demonstrated by the lattice image of transmission electron microscopy. The vacancy concentration is not dependent upon activation conditions; however, the electrical activation coefficiency obtained from Hall measurements is enhanced with increasing activation annealing time. This indicates that the electrical activation of Si+‐implanted GaAs is proceeding by the exchange of interstitial Si with substitutional Ga rather than the recombination of interstitial Si into Ga‐related vacancies. The maximum number of extrinsic‐type stacking faults was observed at 70–80 nm below the surface after the activation annealing, which is compared with that of vacancy‐type defects, at 25–35 nm, obtained by the slow positron beam technique. This discrepancy in both of the damage distributions could originate in different types of defects existing along the depth below the surface, which was discussed with the high‐energy recoil theory.

ISSN:0021-8979    

DOI:10.1063/1.345177

出版商:AIP    

年代:1990

数据来源: AIP