摘要:

Fluid-flow properties of porous media, such as permeabilitykand irreducible water saturationSwi,can be estimated from water1Hnuclear magnetic resonance (NMR) relaxation data, but there are basic questions regarding data processing and interpretation. We found thatSwiandkare better estimated if different forms of “average” relaxation time are used. NMR longitudinal relaxation data for a suite of 106 water-saturated clean sandstones were used. Sandstones represent a specialized class of porous media, where even for small porosity, substantially all pore space is connected. The sandstones exhibit distributions of relaxation times ranging over factors from at least 10 to more than103.We tried several forms of “average” relaxation timeT.One family ofTs is⟨Tp⟩1/p,wherelim p→0gives the geometric mean. The best estimator we found forSwiuses a form of average relaxation timeonly, rather than relaxation time cutoff. The time used can be any of several forms ofT,giving more emphasis toshort timesthan the geometric mean does. On the contrary, the bestTfor estimating permeability without other information is precisely the geometric mean. The best estimates of permeability came from fits ofln (k/&fgr;)usingTs with emphasis atslightly longer times. WhileSwiis better estimated by using all the data points (starting from our minimum 0.4 ms),kis better estimated by starting at a few ms, that is by ignoring anon-negligiblefraction of the signal for some samples. These results can be obtained also by using computations that do not need to invert multiexponential relaxation data, and good results are obtained even with only a few data points. The results are compatible with the reasonable picture, where high surface-to-volume pores, giving signal components with short relaxation times but not contributing to the permeability, are important in determining the fraction of the wetting phase which remains trapped in the solid matrix after displacement with a nonwetting phase. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366222

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The results of simulation of microdischarges in a dielectric-barrier discharge are presented. These simulations have been carried out in two dimensions with an adaptive grid. The results presented are for oxygen at atmospheric pressure. The dielectric has little effect on the propagation of the streamerlike discharge until the very last stage. The dielectric capacitance has been found to have very little effect on the radical production efficiency. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366223

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A 1D plasma-wave model initially developed for describing a plasma sustained by a surface wave in the azimuthally symmetric configuration(m=0)was modified for simulating surface waves in the dipolar mode(m=1).Using this model, it is shown that the maintenance limit for unmagnetized plasmas in the symmetric mode, which, as was previously determined, requires that the product of the wave frequency and the plasma radius be smaller than 2 GHz cm, is no longer valid for magnetized plasmas. Comparison of calculated and measured radial distributions of argon atoms in a radiative excited state shows that changes observed in this distribution when varying some of the operating conditions can be explained by a switching of the discharge from one mode to the other. Conditions for selection of the mode are derived from considerations on wave propagation and power dissipation along the plasma column. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366224

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In a dense collision cascade of energetic ions with a solid, energy is deposited instantaneously in a very small region, leading to a disordered liquid-like state. An activated process for diffusion of energized atoms is assumed with a temperature distributionT(r,t), considering the effects of thermal conduction into a substrate and temperature-dependent thermal conductivity. A phenomenological model is developed that the mixing rateDtis correlated with a heat of mixing (analogous to Darken’s relation), and is linearly dependent with nuclear stopping power, instead of a power-law dependence. This resolves the problem of Seitz and Koehler’s 1956 model, and many succeeding publications. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366225

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this work, we have performed Si and C isoelectronic implantations inn-type andvanadium(V) implantations inp-type 6H–SiC to obtain highly resistive regions. The compensation is achieved by the lattice damage created by the Si and C implantations and the chemically active nature of the V implant. For the Si and C implantations, the as-implanted resistivity initially increased with increasing implant fluence due to the introduction of compensating levels caused by the lattice damage, then decreased at higher fluences due to hopping conduction of the trapped carriers. The resistivity of the Si and C implanted material has been measured after isochronal heat treatments over the temperature range of 400–1000 °C. The maximum resistivity values measured for Si and C implanted and heat treated material were∼1012 &OHgr; cm.For the 700 °C V implantation inp-type SiC, resistivities of>1012 &OHgr; cmwere measured after 1500 or 1600 °C annealing to activate the V implant. Redistribution of the V implant is observed after annealing. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366226

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Single crystaln-GaAs substrates have been implanted at room temperature with120Snions at an energy of 70 MeV. The optical microscope and scanning electron microscope studies of the cross section of the implanted samples have shown the formation of two sharp layers at a depth of 8.7 and 11 &mgr;m from the surface of the substrate. The electrical characteristics of the Schottky diodes fabricated on the implanted substrates at room temperature indicate the presence of high series resistance due to radiation defects. The electrical properties of the implanted samples were investigated after implantation and annealing to 850 °C. Low temperature resistance measurements of these samples indicate that the samples annealed to 450 °C are dominated by a variable range hopping conduction mechanism, whereas for the samples annealed at 550 and 650 °C the electrical conduction is due to hopping between the neighboring defect sites. At annealing temperatures higher than 650 °C, the electrical transport below room temperature seems to be dominated by carriers in the extended states, which are also responsible for the electrical conduction at room temperature and above, for the samples annealed at temperatures higher than 450 °C. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366227

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

It was reported [H. Hosono &etal;, Appl. Phys. Lett.65, 1632 (1994)] that nanometer-sized crystalline (nc) Ge colloid particles were formed by implantation of protons into 0.1GeO2–0.9 SiO2glasses at room temperature. The depth profiles of Ge colloids and the density of Si–OH or Ge–OH created by the implantation were measured and compared with those of energy deposition in order to examine the formation mechanism of Ge colloids by proton implantation. The depth region of nc-Ge particles was found to correspond to the overlapped region between the OH distribution and the peak of electronic energy deposition. Transmission electron microscopic observation revealed that the size of Ge colloid particles created by proton implantation was close to that ofGeO2-rich particles occurring in the substrate glasses. These results indicate thatGeO2-rich particles are converted into Ge particles by a combined effect of the electronic excitation and the chemical reaction of implanted protons. A mechanism was proposed consisting of displacement of bridging oxygen into interstitials by electronic excitation and subsequent trapping of the oxygen interstitials by a formation of OH groups. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366228

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We presentCs2NaYF6:Ce3+orPr3+as new x-ray storage phosphors. The radiation damage centers upon x irradiation were investigated using magneto-optical and magnetic resonance spectroscopy. F centers are the photostimulable electron trap centers. For the activatorPr3+it could be shown that upon room temperature x irradiation only F centers andPr4+hole trap centers are involved in the information storage and readout process. There are indications that the process is the same forCe3+-dopedCs2NaYF6.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366229

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The structural evolution in rhombohedral-type boron nitride (r-BN) pressed up to 7.7 GPa at room temperature is examined before and after pressing using high-resolution transmission electron microscopy. In the starting state,r-BN is in a platelike form with some folding. The plates consist of an almost perfect3Rstructure. After pressing, a large number of stacking faults is formed and causes mixing of3Rand2Hin a few areas and a highly irregular stacking structure in most areas. This means that the coherent slip due to shearing betweensp2sheets is the dominant deformation mechanism inr-BN under high pressure at room temperature. All of these structures cause a broadening of the x-ray diffraction peaks. In some plates, the2Hstructure is found in a relatively large area. It has a hexagonal lattice with the stacking sequence of&ellip;ABAB&ellip;and lattice constants ofa=0.25 nmandc=0.67 nm.The structural evolution inr-BN revealed after pressing is in contrast to that in hexagonal-type BN, in which twinning is the dominant deformation mechanism that introduces the folding of plates under high pressure at high temperature. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366230

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present a study of the infrared absorption and Raman scattering intensity of the local carbon mode inSi1−yCyalloys grown by direct carbon implantation followed by different recrystallization procedures. For the case of laser-induced recrystallization, the integrated infrared absorbances are found to agree with an extrapolation of the calibration curve previously determined for very low substitutional carbon concentrations in Si. We argue that this finding provides strong evidence for the achievement of nearly perfect substitutionality in laser-recrystallized films, even though their carbon concentrations are three orders of magnitude beyond the solubility limit of carbon in Si. This conclusion is found to be consistent with measurements of the intensity of defect-induced Si Raman scattering relative to the Raman intensity of the local carbon mode. The Raman intensity of the local carbon mode at605 cm−1relative to the first-order Si Raman line at521 cm−1provides an ideal spectroscopic tool for the determination of substitutional carbon concentrations. By correlating Raman and infrared measurements, we find that for laser excitation at 488 nm the intensity ratio is given byI605/I521=(3.7±0.2)ysub.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366231

出版商:AIP    

年代:1997

数据来源: AIP