Time Lag Measurements Research Articles

Mechanisms for High‐Frequency Quasi‐periodic Oscillations in Neutron Star and Black Hole Binaries

We explain the millisecond variability detected by Rossi X-Ray Timing Explorer (RXTE) in the X-ray emission from a number of low-mass X-ray binary systems (Sco X-1, 4U 1728-34, 4U 1608-522, 4U 1636-536, 4U 0614+091, 4U 1735-44, 4U 1820-30, GX 5-1) in terms of dynamics of the centrifugal barrier, a hot boundary region surrounding a neutron star (NS). We demonstrate that this region may experience the relaxation oscillations and that the displacements of a gas element both in radial and vertical directions occur at the same main frequency, of order of the local Keplerian frequency. We show the importance of the effect of a splitting of the main frequency produced by the Coriolis force in a rotating disk for the interpretation of a spacing between the quasi-periodic oscillation (QPO) peaks. We estimate a magnitude of the splitting effect and present a simple formula for the whole spectrum of the split frequencies. It is interesting that the first three lowest order overtones (corresponding to the azimuthal numbers m = 0, -1, and -2) fall in the range of 200-1200 Hz and match the kHz QPO frequencies observed by RXTE. Similar phenomena should also occur in black hole (BH) systems, but, since the QPO frequency is inversely proportional to the mass of a compact object, the frequency of the centrifugal-barrier oscillations in the BH systems should be a factor of 5-10 lower than that for the NS systems. The X-ray spectrum formed in this region is a result of upscattering of a soft radiation (from a disk and an NS surface) off relatively hot electrons in the boundary layer. The typical size of the emission region should be 1-3 km, which is consistent with the time-lag measurements. We also briefly discuss some alternative QPO models, including the possibility of acoustic oscillations in the boundary layer, the proper stellar rotation, and g-mode disk oscillations.

Mass transfer in composite polymer-zeolite catalytic membranes

The incorporation of zeolite-encaged iron-phthalocyanine partial oxidation catalysts into a dense hydrophobic polymer membrane results in a substantial improvement in catalyst performance. The diffusion through these zeolite-filled membranes is described using a two-dimensional model, and it is demonstrated that for a permeability ratio (polymer over catalyst) higher than 10 4, the diffusion through composite polymer-zeolite membranes can be described by a one-dimensional model. Such a one-dimensional mathematical model was developed and validated with the experimental data, obtained from the time lag measurements on zeolite-filled poly(dimethylsiloxane) polymer membranes. Consequently diffusion through composite catalytic membranes can be predicted using the mass transfer coefficients of pure catalyst and polymer material, and a single tortuosity factor, only dependent on the catalyst loading.

Gas transport properties of soluble poly(phenylene sulfone imide)s

Gas transport of helium, hydrogen, carbon dioxide, oxygen, argon, nitrogen, and methane in three soluble poly(phenylene sulfone imide)s based on 2,2-bis(3,4- decarboxyphenyl) hexafluoropropane dianhydride (6FDA) has been investigated. The effects of increasing length of well-defined oligo(phenylene sulfone) units on the gas permeabilities and diffusivities were determined and correlated with chain packing of the polymers. Activation energies of diffusion and permeation were calculated from temperature-dependent time-lag measurements. The influences of the central group in the diamine moiety of 6FDA-based polyimides on physical and gas transport properties are discussed. The incorporation of a long oligo(phenylene sulfone) segment in the polymer backbone decreases gas permeability and permselectivity simultaneously. The decreases in permeability coefficients can be mainly related to decreases in diffusion coefficients. Changing the central group of diamine moiety from {S{ to {SO2{ leads to a 45-50% decrease in CO2 and O2 permeabilities without appreciable increase in the selectivities. This is considered to be due to the formation of charge transfer complexes. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1855-1868, 1997

Characteristics of Fuel Discharge in Multihole VCO Nozzle.

An experimental study was conducted on the behavior of the fuel spray and the nozzle needle in a multihole valve cut orifice (VCO) nozzle for diesel engines. The measurements of time lag for fuel discharge among individual holes and the photographic observation of fuel sprays at an arbitrary instant after the start of fuel discharge were carried out using a video camera and a microflash. The magnified image of the top portion of the nozzle needle illuminated by an argon laser and two He-Ne lasers was observed using a high-speed video camera through a window of diameter 0. 6 mm installed at the nozzle tip. The results showed that the fuel spray was classified into solid cone, hollow cone and mixed types, and that the radial displacement of the nozzle needle was straight and toward the opposite side of the hole from which the earliest fuel spray appeared among the five holes.

Time-lag measurements for diffusion of aromatics through a silicalite membrane

Intracrystalline diffusivities of benzene, toluene, and the three xylene isomers in silicalite were determined by the membrane technique. A zeolite membrane was fabricated by embedding a large silicalite crystal in an epoxy resin and mounted in a permeability cell. The membrane was subjected to a known pressure gradient and the rate of transport and, hence, the intracrystalline diffusivity through the zeolite membrane was determined from the rate of pressure increase on the outflow side. The measured diffusivities of aromatics ranged from 5 × 10 −14 to 10 −13 m 2/s. The technique offers a reasonable alternative to other macroscopic methods such as gravimetric and chromatographic methods for measuring intracrystalline diffusivities. However, the limitations of the technique are that (1) it measures integral diffusivity rather than differential diffusivity and (2) the fabrication of a zeolite membrane represents a formidable task.

Pulsed electrical breakdown in liquid helium in the μs range

Information about breakdown time-lags under pulsed voltage stress is prerequisite to the insulation design of superconducting devices. Experiments have been carried out using a pulse generator which provides a step voltage with 200 ns rise time and a long decay time constant of more than 1 ms. The present experiments may bridge the information gap between time lag measurements in the ns range using extremely high field strengths, and existing data found with usual lightning impulse voltage. The test device is described in the paper, and some typical results are discussed in detail.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Novel organosilicon‐containing polymers for an oxygen permselective membrane

AbstractSynthesis and gas permeation measurements of several types of polymers from methylstyrene derivatives containing mono‐and bis(trialkylsilyl) group(s) were carried out. Upon radical homo‐and co‐polymerization of silicon‐containing monomers high‐molecular‐weight polymers were obtained. Results from gas permeation measurements showed that these types of silicon‐containing polymers exhibit fairly high oxygen permselectivity (ratio of oxygen and nitrogen permeation coefficients P/P = 3,1 − 4,6), keeping the oxygen permeation coefficients (P) in the range of between 1,4 · 10−9 and 4,5·10−9 cm3 (STP) · cm · cm−2 · s−1 · cmHg−1. From time lag measurements, it was found that permeation coefficients are dependent on the solubility of gases in the membranes rather than on diffusivity. Actually, the oxygen solubility coefficients increase with increasing silicon content in the polymers. Permselectivity of oxygen against nitrogen is governed by polymer constitution rather than by silicon content.

Measurement of small diffusional time lags in gas permeation.

The origin of the error in the measurement of small diffusional time-lags (θ) was studied by use of an aluminum foil with an orifice in place of a polymer membrane. The error was caused by the flow of the low-concentration gas, i.e., the pressure gradient in the downstream side. Based on the result, a new apparatus for the measurement of small θ was developed, in which a high-response transducer was directly attached to the membrane to monitor the pressure of the downstream side. A microcomputer was used for the on-line measurement of the pressure and the data analysis. The performance of this new system was tested for low density polyethylene and silicone rubber samples with various thicknesses. By this system, θ as small as ca. 0.2s could be measured. The errors in the measurement of small θ due to the structure of the downstream side of the permeation cell and the setting up of the start time in the time-lag measurement are also discussed.

Statistical time lags in low-pressure SF6 breakdown

Statistical time lags have been measured in a discharge tube containing SF6 at a pressure of 1 Torr, with the aim of determining the source of the electrons which initiate breakdownY″Te,hf cx pressure SF6. Measurements with and without careful shielding of the discharge tube give similar values of mean statistical time lag showing that the initiating electrons are not produced by external radiation. Measurements, in which the time between the 500 successive breakdowns used for a single measurement of mean statistical time lag is increased over the range 1–103 s, provide values of mean statistical time lag which increase over the range 10−5–10−2s: This reveals that the breakdown process is controlled by the time since the discharge produced by a previous breakdown. Data from further such measurements, made for different applied tube voltages, are shown to fit well with the Fowler–Nordheim theory of field emission. The conclusion drawn is that the initiating electrons are produced at the cathode surface by field emission, and that the state of the surface at any time depends primarily on the intensity of the previous discharge and the length of time since it took place. During the time between successive breakdowns, a surface layer on the cathode surface is continually developing and reducing the rate of field emission from the cathode. Although the applied electric fields are well below that required for field emission, microscopic fields exist at protrusions and craters on the cathode surface which are high enough to provide field emission: This is supported by experiments on electron emission made with the discharge tube evacuated below 10−6 Torr.

Time lag measurement of electrical breakdown of C3F6 gas

On etudie la performance de duree (delai) des decharges electriques dans le gaz C3F6. Elles sont comparees a celles obtenues dans le SF6, dans une plage donnee de pression du gaz. On presente la structure du generateur H.T. permettant d'effectuer les mesures de temps de decharge qui evolue entre microsecondes et centaines de secondes

Analysis of the permeation of CO 2 through glassy polymers

Dual-mode sorption and mobility parameters for the transport of CO 2 in glassy poly (ethylene terephthalate) were determined from the pressure dependence of the apparent diffusion coefficients, D a, below 1 atm at various temperatures. Experimental values of these diffusion coefficients were obtained from permeation time-lag measurements. Values of D a calculated by means of the above parameters are in excellent agreement with the experimental data. Moreover, the permeation time-lag curves calculated by the finite-difference technique in conjunction with the dualmode sorption parameters are consistent with the experimental time-lag curves. These results appear to support the validity of the dual-mode sorption model of gas transport in glassy polymers. However, equally good agreement between the calculated and experimental curves was obtained by using the apparent solubility and diffusion coefficient without the dual-mode sorption parameters. Similar results were obtained with poly(vinyl acetate)/CO 2 system at temperatures both above and below the glass transition temperature.

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 1020 m-3 for annealed iron to over 1023 m-3 for heavily deformed iron. For the metallic glass Fe40Ni40P14B6 hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 x 1029 m-3 . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.

Hydrogen in iron

The applicability of advanced permeation techniques to the study of hydrogen and deuterium in iron and iron alloys is described. Time lag measurements lead to detailed information about hydrogen transport processes, including lattice diffusivities and trap binding energies and densities. The experimental technique couples gas phase charging of palladium coated specimens with the sensitive electrochemical detection method. In both annealed and deformed iron the trap binding energy for hydrogen and deuterium is 50 to 58 kJ/mol, while the trap density varies from about 1020 m−3 for annealed iron to over 1023 m−3 for heavily deformed iron. For the metallic glass Fe40Ni40P14B6 hydrogen transport occurs between energetically equivalent sites, with no evidence of trapping. The site density was estimated as about 6 × 1029 m−3 . The hydrogen concentrations studied were several orders of magnitude less. Hydrogen and deuterium in iron differs only in their lattice diffusivities. The diffusivity ratio conforms nearly to the classical inverse square root of mass ratio, but shows a slight temperature dependence. The solubilities, trap binding energies, and partial atomic volumes of the two isotopes in iron are identical.

Inhomogeneous field breakdown in GIS-the prediction of breakdown probabilities and voltages. II. Ion density and statistical time lag

For pt.I see ibid., vol.3, no.3, p.923-30 (1988). An improved model for the calculation of statistical time lags in SF/sub 6/ insulated electrode arrangements for positive impulse voltage is presented. On the basis of physical considerations, the production rate of initial electrons is ascribed to the local density of negative SF/sub 6/ ions and to field dependent electron detachment. Values of negative ion densities are measured directly, while the detachment coefficient is determined by a fitting procedure to measurements of statistical time lags.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hydrogen Trapping in Cold Worked High-Purity Iron

Hydrogen trapping in deformed iron has been studied by time lag measurements based upon a double-step electrochemical permeation technique. Two types of trapping sites, TI and TII, have been confirmed to exist in iron deformed 35% by rolling. TI sites have a binding energy of 63.1 kJ/mol and a density of 1×1023/m3. Most of them annihilate by annealing below 673 K. TII sites have a binding energy of 46.4 kJ/mol and a density of 7×1023/m3. TII sites are also significantly reduced by annealing at temperatures below 573 K, while a further reduction occurs by recrystallization around 730 K. TI and TII sites are believed to be vacancies trapped by interstitial impurity atoms and core sites of dislocations, respectively. The significant reduction of TII by annealing at low temperatures suggests that the segregation of interstitial impurity atoms to dislocation-core sites precludes their occupancy by hydrogen. An another type of traps having a binding energy of about 30 kJ/mol has also been presumed to exist.

Carrier Mobility Measurement

Various techniques will be discussed for the evaluation of the carrier mobility such as a time-of-flight method, a transient ient current observation, a surface potential decay method, galvanomagnetic effects and a breakdown time-lag measurement. Their applications to various materials, polymers, liquids, and liquid crystals will be pointed out. Especially, it has been revealed that the carrier mobility in polymers is strongly y dependent on both the measuring technique and the time range of interest: this is interpreted in terms of non-Gaussian carrier transport and severe influence of morphology and impurities. Effects of elongation, doping, and heattreatment also are discussed.

Structure investigations of homogeneous cellulose acetate membranes by gas permeation measurements

Flat sheet homogeneous CA membranes of different thicknesses and different acetyl contents (43.1, 39.1, 32.0 wt.-%) were cast from solutions of Bayer Cellit T-900 and K-700 as well as Eastman Kodak CA E-320 in mixtures of dioxan with dimethylacetamide (9:1 by volume), in pure methyl glycol acetate, and in mixtures of methyl glycol with either dimethylformamide or dimethylacetamide (9:1 by volume). The permeabilities (P g), diffusion coefficients (D g), and solubilities (S g) of the membranes for He, N 2, CH 4, and CO 2 were then determined as functions of pressure (0.1 atm to 100 atm) and temperature (20°C up to 80°C) by use of time-lag measurements. In the presence of pin-holes the permeabilities increased linearly with increasing mean pressure whereas imperfection-free membranes exhibited both pressure-independent and slightly pressure-dependent permeabilities, diffusivities, and solubilities. The sorption isotherms of He, N 2, and CH 4 obeyed either Henry's law or the dual-sorption model relationship whereas the sorption isotherm of CO 2 followed Henry's law only up to pressures of p′ ⋍ 18 atm and increased nonlinearly with increasing p′ at pressures beyond p′ − 18 atm. The temperature dependence of P g, D g, and S g, measured at some reference pressure, yielded activation energies of permeation, Ep, and diffusion, E D, as well as enthalpies of sorption, ΔH s.

The 1984 J. B. Whitehead Memorial Lecture the Physics of Partial Discharges in Solid Dielectrics

The physical processes occurring during partial discharges in voids in dielectrics have been studied utilizing fast oscilloscopic techniques to observe the buildup and decay of current as a function of time for single discharges. A key parameter is the overvoltage at breakdown, determined for time-dependent voltages by the rate of rise of voltage and the statistical time lag. Two distinct types of discharge have been identified, named "Townsendlike" and "streamerlike", and mechanisms responsible for their development have been proposed. The relationship between current-time pulse shapes as well as discharge magnitudes (charge per discharge) and various parameters such as overvoltage, void size, and dielectric thickness and permittivity have been explored. The conditions governing the transition between the two types of partial discharges have been examined quantitatively. The relationship between streamer propagation and Lichtenberg figure development has been studied. Measurements of statistical time lags for partial discharges have been made and significant paramters examined. The usefulness of these results in connection with future studies of chemical degradation of insulating materials are discussed.

Structure investigations of homogeneous cellulose acetate membranes by gas permeation measurements

Flat sheet, homogeneous CA membranes of different thicknesses (10.1 μm, 89 μm, and 157 μm) were cast from solutions of Bayer Cellit T-900 in a mixture of dioxan and dimethyl acetamide (9:1 by weight). The permeabilities (P g), diffusion coefficients (D g), and solubilities (S g) of He, N 2, and CO 2 through the CA membranes were measured as functions of pressure (1 atm to 100 atm) and temperature (20°C up to 80°C) using time-lag measurements. Imperfection-free membranes exhibited pressure-independent permeabilities, diffusivities and solubilities for He and N 2 at pressures up to 30 atm, with a slight change of the gas permeability with pressure at pressures of 30 atm to 100 atm. On the other hand, the permeability increased linearly with increasing mean pressure if the membrane contained pin-holes. The sorption of He, N 2, and CO 2 follows Renry's law over the entire pressure range. However, the sorption capability of the membranes for CO 2 increased discontinuously at p′ ⋍ 22 atm. Two different Henry's law constants were thus obtained for the two different pressure ranges. The temperature dependence of P g, D g, and S g, measured at p′ = 20 atm, gave activation energies of permeation, E P, and diffusion, E D, and enthalpies of dissolution, ΔH S.

Hydrogen permeation in the metallic glass Fe 40Ni 40P 14B 6

Hydrogen permeation from the gas phase through the metallic glass Fe 40Ni 40P 14B 6 (Metglas 2826) was measured with the electrochemical technique over 313 to 353K and 10 5 to 10 3 Pa. Specimen surfaces were coated with a thin layer of electrodeposited palladium to eleminate surface impedances to hydrogen entry and exit. Sievert's law behavior was observed over the temperature and pressure ranges studied. An excellent fit to the permeation transients as obtained from Fick's diffusion theory. Diffusivities derived from least squares fitting of permeation trasients were in good agreement with those obtained from time lag measurements. Within the concentration range studied, diffusivity was found to be independent of input hydrogen concentration. The expressions for the permeability coefficient, 2.24 × 10 16 exp{[(48.99 ± 2.93) kJ/mol]/ RT} atom H/ms Pa 1 2 , the diffusivity 8.52 × 10 −7 exp{[(−47.07 ± 1.63) kJ/mol]/ RT} m 2/s, and the solubility, 2.62 × 10 22 exp{[(−1.92 ± 4.56) kJ/mol]/ RT} atom H/m 3 Pa 1 2 , were determined. An annealing treatment at 573K for one hour decreases bot the permeability and diffusivity of hydrogen in the metallic glass.