A reevaluation of the NIST low-frequency standards for AC-DCdifference in the voltage range 0.6-100 V

A reevaluation of the NIST standards of ac-dc difference was undertaken in an effort to reduce the calibration uncertainty offered by NIST for thermal voltage converters (TVC's) at frequencies below 100 Hz. This paper describes the measurements taken in support of this effort, as well as the devices used for the reevaluation process and the analysis of the uncertainty of the measurements. This reevaluation of the NIST low-frequency standards will permit a significant reduction in uncertainty for ac-dc difference calibrations at 10 Hz in the voltage range from 0.6-100 V     

Studying emission from a surface discharge in dry air

We have studied the emission from an electric discharge developing in dry air over the surface of a ceramic plate at the edge of a thin aluminum electrode under the action of an ac voltage with a frequency of 5–14 kHz. The intensity of emission from surface microdischarges was measured for the second positive (λ= 337.1 nm) and first negative (λ = 391.5 nm) systems of nitrogen using a monochromator and a photoelectron multiplier. Using a lens and a system of slits, the radiation flux was scanned at a spatial resolution of 0.2 and 2 mm with respect to the width and length, respectively, of the discharge channel, which allowed the structure of emission from microdischarges to be analyzed and the position of the discharge channel relative to the barrier surface to be determined.     

Determination of methane in ambient air by multiplex gas chromatography

A multiplex gas chromatographic technique for the determination of methane in ambient air over extended periods is reported. A modest gas chromatograph which uses air as the carrier gas was modified by adding a silver oxide sample modulator for multiplex operation. The modulator selectively catalyzes the decomposition of methane in air. The resulting analytical systems requires no consumables beyond power. A profile of the methane concentration in this laboratory was obtained for an 8-day period. During this period, methane concentration varied with an approximately daily period from a low of 1.53 +/- 0.60 ppm to a high of 4.63 +/- 0.59 ppm over the entire 8 days. Some of the measured concentrations are higher than those reported elsewhere indicating the presence of some local source or sources for methane. This work has demonstrated the utility of a relatively simple multiplex gas chromatograph for the analysis of environmental samples. The technique should be applicable to other trace components in air through use of other selective modulators.     

Generation of subnanosecond electron beams in air at atmospheric pressure

Optimum conditions for the generation of runaway electron beams with maximum current amplitudes and densities in nanosecond pulsed discharges in air at atmospheric pressure are determined. A supershort avalanche electron beam (SAEB) with a current amplitude of ∼30 A, a current density of ∼20 A/cm², and a pulse full width at half maximum (FWHM) of ∼100 ps has been observed behind the output foil of an air-filled diode. It is shown that the position of the SAEB current maximum relative to the voltage pulse front exhibits a time shift that varies when the small-size collector is moved over the foil surface.     

Shock wave propagation in glow-discharge plasma in dry and humid air

We have experimentally studied propagation shock waves in glow-discharge plasma in dry and humid air. It is established that the structure of a pressure pulse behind a shock wave propagating across the positive column of glow discharge in both cases generally remains the same. At the same time, the wave velocity profile across the column exhibits a change in positions of the velocity peaks.     

Mass spectrometer characterization of halogen gases in air at atmospheric pressure

We have developed a new interface for a commercial ion trap mass spectrometer equipped with APCI capable of real-time measurements of gaseous compounds with limits of detection on the order of pptv. The new interface has been tested using the detection of Br2 and Cl2 over synthetic seawater ice at atmospheric pressure as a model system. A mechanical pump is used to draw gaseous mixtures through a glass manifold into the corona discharge area, where the molecules are ionized. Analysis of bromine and chlorine in dry air show that ion intensity is affected by the pumping rate and the position of the glass manifold. The mass spectrometer signals for Br2 are linear in the 0.1-10.6 ppbv range, and the estimated 3sigma detection limit is 20 pptv. The MS signals for Cl2 are linear in the 0.2-25 ppbv range, and the estimated 3sigma detection limit is 1 ppbv. This new interface advances the field of analytical chemistry by introducing a practical modification to a commercially available ion trap mass spectrometer that expands the available methods for performing highly specific and sensitive measurements of gases in air at atmospheric pressure.     

Oxidation of nickel-based alloys in dry and water vapour containing air

Abstract

Ni-based alloys are widely used in power generation and their oxidation behaviour in the uncoated state is of interest, especially the impact of water vapour in the air on the formation of a protective underlying alumina scale. High-temperature X-ray diffraction was applied to investigate in situ the oxide scale formation in the initial state on the alumina formers CM247DS and CMSX4 in comparison to the chromia formers IN792 and SCA425+. Post-oxidation analysis was performed by field emission scanning electron microscopy. The samples were oxidised for 100 h at 950°C in dry air and in air with 20% relative moisture in the high temperature device on the X-ray diffractometer. In dry air, CM247 and CMSX-4 form α-Al₂O₃ from the beginning simultaneously with spinels and nickel oxide. The alumina scale underlies the spinels which spall partially on cooling. When adding water vapour, the same oxides were formed simultaneously resulting in a comparable oxide scale. IN792 forms in dry air mainly NiAl₂O₄ and transitionally CrO₂ under laid by an alumina scale. With water vapour, Cr₂O₃ forms and the underlying alumina scale shows a non coherent dendritic structure. SCA425+ forms in dry air Cr₂O₃ and Cr containing mixed oxides and with water vapour a more coherent alumina scale than IN792.     

Runaway electron beam generation by a plasma cathode in atmospheric air discharge

Electron beam generation in atmospheric air discharge with a flat cathode has been experimentally studied. The discharge was excited by voltage pulses of negative polarity with an amplitude of 220 kV, a full width at half maximum of 2 ns, and a leading front width of 0.7 ns. The electron beam was monitored using luminescence of a layer of ZnS-CdS:Ag phosphor placed behind a 20-μm-thick foil anode. It was found that the intensity and homogeneity of luminescence of the phosphor layer increased when a dielectric tube with a length smaller than half of the interelectrode distance was placed in the near-cathode part of the air gap. It is concluded that a plasma cathode is formed within the volume confined by the tube and electrons emitted from this region are accelerated in the open part of the gap. In addition, the dielectric tube decreases the divergence of the electron beam.     

Role of oceanic air bubbles in atmospheric correction of ocean color imagery

Ocean color is the radiance that emanates from the ocean because of scattering by chlorophyll pigments and particles of organic and inorganic origin. Air bubbles in the ocean also scatter light and thus contribute to the water-leaving radiance. This additional water-leaving radiance that is due to oceanic air bubbles could violate the black pixel assumption at near-infrared wavelengths and be attributed to chlorophyll in the visible. Hence, the accuracy of the atmospheric correction required for the retrieval of ocean color from satellite measurements is impaired. A comprehensive radiative transfer code for the coupled atmosphere--ocean system is employed to assess the effect of oceanic air bubbles on atmospheric correction of ocean color imagery. This effect is found to depend on the wavelength-dependent optical properties of oceanic air bubbles as well as atmospheric aerosols.     

Fatigue of aluminium alloy 2024-T351 in humid and dry air

Reversed bending fatigue tests conducted on specimens of aluminium alloy 2024-T351 in dry and humid air at stress levels of 248, 276, 290, 317 and 359 MPa showed that at low stress amplitude humid air reduces the fatigue life by as much as 21%. Mirco-hardness tests showed that the reduction in fatigue life is primarily attributed to localized hydrogen-induced overageing. SEM analysis and microhardness data were combined with past studies to propose a mechanism for environmentally induced fatigue in aluminium alloy 2024-T351 over a wide range of stress levels.     

Measuring parameters of volume nanosecond pulsed discharge in air at atmospheric pressure

We report on the basic electrical characteristics of the volume nanosecond pulsed discharges in air at atmospheric pressure. The measurements of temperature were performed using spectral methods, whereas the electron density of plasma was measured using a multibeam Fabry-Perot interferometer. It is shown that, subject to maintaining the form of the volume discharge, the maximum electron density is 1.5 × 10¹³ cm^?3, which is comparable to the critical value in streamer discharges.     

X-ray emission from a low-current volume discharge in air at atmospheric pressure

The discharges in air gaps with cathodes of small curvature radius under the action of nanosecond high-voltage pulses have been studied. Low-current volume (diffused) discharge in air at atmospheric pressure is a source of x-ray emission. In the case of discharge with a low-intensity visible glow, x-ray emission takes place both from the gap and from the anode surface.     

7Be in ground level air in Daejeon, Korea

(7)Be concentrations in the ground level air in Daejeon, Korea were determined during the period of January 1998 to December 2009 by gamma-ray spectrometric analysis of particulate samples collected on filter paper with a high-volume air sampler. The monthly concentrations of (7)Be in the ground level air were in the range of 1.3-7.7 mBq m(-3) with strong seasonal trends of low values in the summer and high values in the spring and autumn. The annual mean values of (7)Be concentrations showed weak reverse correlation with the annual average sunspot number.     

Characterization and thermal properties of carbon-coated aluminum nanopowders prepared by laser-induction complex heating in methane

Carbon-coated aluminum (Al) nanopowders were successfully synthesized by laser-induction complex heating in methane. The characterization of the samples was carried out using X-ray diffraction (XRD), transmission electronic microscopy (TEM) and high resolution transmission electronic microscopy (HRTEM). It was found that the nanoparticles show a spherical morphology with the size ranging from 28 to 50 nm and are covered with 3–4 graphitic layers. Thermal analysis using differential scanning calorimeter (DSC) and thermal gravimeter (TG) revealed that the as-prepared powders exhibit a lower oxidation onset and peak temperature, and a higher enthalpy change, as compared with those of Al₂O₃-passivated Al nanopowders. It was also found that the mass gain for the first oxidation stage of carbon-coated Al nanopowders is 1.5 times higher than that of Al₂O₃-passivated Al nanopowders. These demonstrate that the carbon coating is an effective route to increase the reactivity of Al.     

Study of the specific features of pulsed-plasma generation in air at atmospheric pressure

The structural scheme and operational characteristics of a pulse-plasma device generating a pulsed plasma in air at atmospheric pressure are presented. The functional scheme and the operation algorithm for a high-voltage power supply are given. It is shown that the pulsed-plasma device can generate air plasma flows with a frequency of 1–10 Hz and maximum parameters of pressure, temperature, and velocity of 6.5 MPa, 12,000 K, 4 km ⁄ s, and 110 dB_A, respectively. Comparative analysis is presented of these quantities with characteristics of the plasma flow generated by the detonation pulsed-plasma device, which uses an air-propane mixture as the plasma-forming substance. It is shown that replacing the air-propane gas mixture with air provides a 70 dB_A decrease in the noise level of the pulsed-plasma treatment. Recommendations are given as to using the pulsed-plasma device for surface strengthening of the instrument and parts of the machine.     

Experimental investigation of a cooled thick fin in dry and humid air flows

The results of a study are reported in which a cooled, thick vertical fin was tested in a closed loop tunnel with and without condensation from the air flowing over it. In particular, the temperature distributions for the dry and wet fin cases, together with the condensate film thickness in the wet fin case, were investigated. From a flow visualization investigation, it was found that the boundary layer separates at the leading edge, resulting in a higher air heat transfer coefficient. The wet fin test results also indicated that the mode of condensation was dependent on fin surface characteristics and that the wet fin performance was governed by the air flow parameter. Within the laminar air flow range, the condensate film flowed downward under the action of gravity. However, at higher air velocities, both gravity and shear forces affected the condensate flow, a variation in the condensate film in the direction of air flow being noticed.     

下塔液空液位异常的原因及处理

对液空吸附器损坏造成下塔液空液位异常的故障原因进行分析 ,提出了解决措施。     

Subnanosecond pulsed X-ray source based on nanosecond discharge in air at atmospheric pressure

We have studied the characteristics of an X-ray source based on a gas diode filled with air at atmospheric pressure. Driven by a SLEP-150 pulser with a maximum voltage amplitude of ∼140 kV, a pulse full width at half maximum (FWHM) of ∼1 ns, and a leading front width of ∼0.3 ns, a soft X-ray source produces subnanosecond pulses with an FWHM not exceeding 600 ps and an exposure dose of ∼3 mR per pulse. It is shown that the main contribution to the measured exposure dose is due to X-ray quanta with an effective energy of ∼7.5 keV.     

Cyclic oxidation and wear of tungsten rods in contact with glass in atmospheric air

Tungsten tools used in the glass-forming industry undergo complex damage process resulting from cyclic contact with molten glass and atmospheric air and from friction between glass and tungsten. The damage process involving oxide scale growth and wear has been studied on W rods of two different diameters, in contact or not with glass, with different friction velocities. Damage is characterized quantitatively and the contributions of oxidation, borosilicate glass–tungsten reaction, and wear are discussed.