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  • 1
    Electronic Resource
    Electronic Resource
    Low energy electron attachment to SF6 in N2, Ar, and Xe buffer gases (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4879-4891 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The electron attachment rate constants ka for SF6 have been measured in dilute mixtures of SF6 in high pressure (〉1 atm) N2, Ar, and Xe buffer gases at room temperature (T≈300 K) over a wide E/N range (electric field strength to gas number density ratio), corresponding to mean electron energies 〈ε〉 from near thermal electron energies (≈0.04 eV) to 〈ε〉≈4.3 eV. Particular attention has been paid to the effects of space charge distortion, molecular impurities, and changes in the electron energy distribution function on the measured electron attachment rate constant values at the lower E/N values in these mixtures. The present measured thermal electron attachment rate constants in SF6/N2 and SF6/Xe gas mixtures are in excellent agreement with recent accurate measurements of these parameters in several SF6/buffer gas mixtures. At higher 〈ε〉 values, the present SF6/N2 measurements are in fair agreement with previous measurements, while no previous measurements using Ar and Xe buffer gases have been published. These measurements have been used in numerical two term, spherical harmonic Boltzmann equation analyses of the electron motion in these gas mixtures to obtain the low energy (〈10 eV) nondissociative and dissociative electron attachment cross sections for SF6. The present derived electron attachment cross sections are compared with recently measured and derived nondissociative and dissociative electron attachment cross sections for SF6. The primary value of the present results is in the large and overlapping 〈ε〉 ranges of the present ka measurements for the three buffer gases compared with that for SF6/N2 gas mixtures alone, which in turn, makes these measurements ideal for testing cross-section sets in SF6 for use in many applied studies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456582
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  • 2
    Electronic Resource
    Electronic Resource
    Haemodynamic changes during the puerperium: a Doppler and M-mode echocardiographic study (1987)
    Oxford, UK : Blackwell Publishing Ltd
    BJOG 94 (1987), S. 0 
    Details
    ISSN: 1471-0528
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Summary. Serial haemodynamic investigations were performed in 15 women at 38 weeks gestation and then 2, 6, 12 and 24 weeks after delivery. Cardiac output was measured by Doppler and cross-sectional echocardiography at the aortic, pulmonary and mitral valves. Cardiac chamber size and ventricular function were investigated by M-mode echocardiography. Flow measurements at the three intracardiac sites correlated closely. Cardiac output fell from a mean of 7.421/min at 38 weeks to 4.961/min at 24 weeks after delivery, a fall of 33%. Most of this decrease (28%) had occurred by 2 weeks. This was associated with a 20% reduction in heart rate and an 18% reduction in stroke volume. By 2 weeks after delivery there was a significant decrease in left atrial dimension and left ventricular end-diastolic dimension. Left ventricular wall thickness and mass declined throughout the period of study as did aortic, pulmonary and mitral valve areas. M-mode derived indices of myocardial contractility were all significantly reduced by 2 weeks and thereafter showed no further change. No haemodynamic differences were found between lactating and non-lactating mothers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1471-0528.1987.tb02286.x
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  • 3
    Electronic Resource
    Electronic Resource
    Remote infrared radiation detection using piezoresistive microcantilevers (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 2986-2988 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel micromechanical infrared (IR) radiation sensor has been developed using commercially available piezoresistive microcantilevers. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress between the top layer (coating) and the substrate. The bending causes a change in the piezoresistance and is proportional to the amount of heat absorbed. The microcantilever IR sensor exhibits two distinct thermal responses: a fast one (〈ms) and a slower one (∼10 ms). A noise equivalent power (at a modulation frequency of 30 Hz) was estimated to be ∼70 nW/Hz1/2. This value can be further reduced by designing microcantilevers with better thermal isolation that can allow microcantilevers to be used as uncooled IR radiation detectors. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.117752
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  • 4
    Electronic Resource
    Electronic Resource
    Electron transport measurements in methane using an improved pulsed Townsend technique (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 24-35 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An improved pulsed Townsend technique for the measurement of electron transport parameters in gases is described. The accuracy and sensitivity of the technique have been investigated by performing, respectively, electron attachment coefficient measurements in pure O2 over a wide range of E/N at selected O2 pressures and by determining the electron attachment and ionization coefficients and electron drift velocity in CH4 over a wide E/N range. Good agreement has been obtained between the present and the previously published electron attachment coefficients in O2 and for the drift velocity measurements in CH4. The data on the electron attachment coefficient in CH4 (measured for the first time) showed that with the present improved pulsed Townsend method, electron attachment coefficients up to 10 times smaller than the ionization coefficients at a given E/N value can be accurately measured. Our measurements of the electron attachment and ionization coefficients in CH4 are in good agreement with a Boltzmann equation analysis of the electron gain and loss processes in CH4 using published electron scattering cross sections for this molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337690
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  • 5
    Electronic Resource
    Electronic Resource
    Electron transport studies of gas mixtures for use in e-beam controlled diffuse discharge switches (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 58 (1985), S. 3001-3015 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gas mixtures for possible use in diffuse discharge switching applications require both high dielectric strength and specific electron transport properties in the conducting and the opening stages of the operation of the switch. In the conducting stage, the electron drift velocity must be large, and the electron loss processes (e.g., due to electron attachment and recombination) must be low so as to maximize the current in the discharge while maintaining low-discharge impedance. In the opening stage, strong electron attachment along with high dielectric strength is required of the gas mixtures in order to extinguish the discharge as quickly as possible (and, thus, achieve a fast opening time) and to prevent arcing occurring between the switch electrodes due to the high voltages induced across the switch in the opening phase. Measurements of the electron drift velocity, attachment, ionization and transverse diffusion coefficients, and high-voltage dielectric strengths of several gas mixtures proposed as candidates for use in diffuse discharge switching applications have been made and are reported and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.335849
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  • 6
    Electronic Resource
    Electronic Resource
    Electron attachment and ionization coefficients and electron drift velocities in BF3 and SiF4 (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 65 (1989), S. 1858-1865 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The drift velocity of electrons (w) in SiF4 and BF3 has been measured over the density-reduced electric field (E/N) range 0.5×10−17 V cm2 ≤E/N≤300×10−17 V cm2 using a pulsed Townsend experimental technique. The electron attachment (η/N) and ionization (α/N) coefficients have also been measured in SiF4 and BF3 over the E/N range 30×10−17 V cm2 ≤E/N≤300×10−17 V cm2 in the same experiment. The electron drift velocities in these two gases exhibit regions of negative differential conductivity (NDC) similar to but smaller in magnitude than those in CH4 and CF4 . The effective ionization coefficient (α¯/N) has been obtained from the η/N and α/N measurements in both gases, from which the high-voltage limiting electric field strengths (E/N)lim have been found to be 123×10−17 V cm2 for BF3 and 121×10−17 V cm2 for SiF4. The significance of these results in modeling gas discharges for a number of technological applications is indicated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.342920
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  • 7
    Electronic Resource
    Electronic Resource
    Effect of temperature on the uniform field breakdown strength of electronegative gases (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 52-59 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In general, the electron attachment rate constant ka (〈ε〉, T), as a function of the mean electron energy 〈ε〉 and temperature T for electronegative gases which attach electrons nondissociative ily, decreases greatly with T from room temperature to T(approximately-less-than)600 K, while the ka (〈ε〉, T) of electronegative gases which attach electrons dissociatively increases with increasing T. Based on recent studies at our laboratory on ka (〈ε〉, T), we investigated the variation with T (∼295–575 K) of the uniform field breakdown strength, (E/N)lim, for three classes of electronegative gases: (a) gases such as c-C4F8 (and c-C4F6, 1−C3F6) which attach strongly low-energy ((approximately-less-than)1 eV) electrons nondissociatively and for which ka (〈ε〉, T) decreases precipitously with T above ambient: (b) gases such as C2F6 and CF3Cl which attach electrons exclusively dissociatively and whose ka (〈ε〉, T) increases with T; and (c) gases such as C3F8 and n-C4F10 which attach electrons both nondissociatively and dissociatively over a common low-energy range and whose ka (〈ε〉, T) first decreases and then increases with T above ambient. The (E/N)lim(T) has been found to decrease significantly with T for (a), to decrease slowly with T for (c), and to increase slightly with T for (b). These changes in (E/N)lim follow those in ka (〈ε〉, T). A similar behavior is expected for other electronegative gaseous dielectrics in the respective three groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.340462
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  • 8
    Electronic Resource
    Electronic Resource
    Comment on "Shortening of electron conduction pulses by electron attachers O2, N2O, and CF4'' [J. Appl. Phys. 56, 3169 (1984)] (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 4335-4337 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Negative ion formation in pure CF4 and CF4/N2 gas mixtures is shown to proceed exclusively by dissociative electron attachment mechanisms at electron energies 〉4 eV in a high-pressure electron swarm experiment. These results are in contrast to a recent swarm study where CF4 was reportedly shown to attach electrons by transient parent anion formation processes at low electron energies ε≈0.5 eV. Possible sources of errors in that study are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337483
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  • 9
    Electronic Resource
    Electronic Resource
    Studies of total ionization in gases/mixtures of interest to pulsed power applications (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 499-508 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The energy required to produce an electron-ion pair (i.p.), W, for the α-particle irradiated gases CF4, C2F6, C3F8, and n-C4F10 has been measured and found to be 34.3, 34.5, 34.4, and 34.2 eV/ i.p., respectively. The large values of W for these perfluorocarbons (PFCs) are attributed to their large cross sections for electron impact-induced dissociation. The constancy of W for this group of molecules is discussed with respect to the constancy of the ratio (∼0.68) of the respective (energy integrated) total ionization cross section to the total (energy integrated) inelastic scattering cross section. W values are also reported for the binary gas mixtures Ar/CF4, Ar/C2F6, Ar/C3F8, and CH4/CF4 which have conduction and insulation properties suitable for use in diffuse discharge opening switches. The W values for these binary mixtures (and for C2H2/CF4, C2H2/C2F6, and C2H2/C3F8) increase with the amount of PFCs in the mixture and (contrary to the Ar/C2H2 mixture) exhibit no Jesse effect. The ternary gas mixtures Ar/C2H2/PFC (=CF4, C2F6, C3F8) containing (approximately-less-than)10% PFC were found to exhibit a pronounced Jesse effect which is a function of the content of the PFC in the mixture and the electron scattering cross section of the PFC itself. Substantial increases in ionization can be achieved by adding to the Ar/PFC binaries 1–3% of C2H2. An analysis of the W data yielded information on the partition of the α particle's energy among the gas components and indicated that the rate constant for energy transfer from excited argon atoms (Ar*) to the PFC is smaller than that from Ar* to C2H2; it showed, also, that the rate constant for energy transfer from Ar* to C2H2 is ∼104 larger than that for collisional deexcitation of Ar* by ground-state Ar atoms. The quantitative measurements reported herewith and the parameters deduced from a simple modeling of the data permit the tailoring of ternary gas mixtures with substantial current increases over those achieved in the binary Ar/PFC (=CF4, C2F6, C3F8) mixtures which possess the desirable conduction and insulation characteristics for pulsed power opening switches.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337792
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  • 10
    Electronic Resource
    Electronic Resource
    Penning ionization ternary gas mixtures for diffuse discharge switching applications (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 58 (1985), S. 633-641 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The increase in the total ionization produced by high-energy α particles in Ar/C2F6 mixtures, which have conduction and insulation properties appropriate for use in diffuse discharge switching applications by addition of low ionization energy additives has been quantitatively studied. The energy to produce an electron-positive ion pair (ip) W in C2F6 was found to be 34.7 eV/ip; this rather high value is attributed to the large cross section for electron impact-induced dissociation of C2F6. The W values of Ar/C2F6 mixtures have also been measured and are reported; they increase with increasing C2F6 content. The W values of Ar/C2F6 binary gas mixtures have been found to decrease by addition of C2H2 or 2-C4H8. Quantitative measurements of the W values of the ternary gas mixtures are reported. The amounts of C2H2 or 2-C4H8 in Ar/C2F6, which maximize the increase in total ionization have been estimated; some of these ternary gas mixtures may be useful for diffuse discharge switches sustained by external electron beams. The experimental results on the W values for the binary and ternary gas mixtures studied have been modeled and good agreement has been found between the experimental and the calculated results. From an analysis of the data it has been found that the α-particle energy is partitioned between C2F6 and Ar approximately in the ratio 4.2:1. It has also been found that at the argon pressures employed, the probability of deexcitation of excited argon atoms by C2F6 (or C2H2 or 2-C4H8) molecules is more than three orders of magnitude larger than the probability of radiative deexcitation of the excited argon atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.336201
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