S. R. Hunter

Electron attachment to the perfluoroalkanes n‐CNF2N+2 (N=1–6) using high pressure swarm techniques

The electron attachment rate constants and negative ion formation mechanisms for six perfluoroalkanes [n‐CNF2N+2(N=1–6)] have been studied in a high pressure swarm experiment within the mean electron energy range from thermal energy (≊0.04 eV) to ≊4.9 eV. These experiments were performed over a total gas number density range of 3.2×1019 to 3.9×1020 cm−3 using N2 and argon as buffer gases. Dissociative electron attachment was found to be the only negative ion formation process for CF4 and C2F6. For C3F8, n‐C4F10, and n‐C5F12 the electron attachment rate constant measurements exhibited a large total pressure dependence which was strongest for C3F8 and decreased with increasing size of the perfluoroalkane molecule. These measurements have been interpreted as electron attachment by parent negative ion formation due to three‐body stabilization processes of the initially excited, short‐lived (5×10−11 s <τ<10−8 s) parent anion. The lifetimes of these transient parent anions have been found to depend on the natur...

Superhydrophobic materials and coatings: a review.

Over the past few years, the scientific community, as well as the worlds coatings industry has seen the introduction of oxide/polymer-based superhydrophobic surfaces and coatings with exceptional water repellency. Online videos have caught the publics imagination by showing people walking through mud puddles without getting their tennis shoes wet or muddy, and water literally flying off coated surfaces. This article attempts to explain the basics of this behavior and to discuss and explain the latest superhydrophobic technological breakthroughs. Since superhydrophobic surfaces and coatings can fundamentally change how water interacts with surfaces, and the fact that earth is a water world, it can legitimately be said that this technology has the potential to literally change the world.

Remote infrared radiation detection using piezoresistive microcantilevers

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.

Low energy electron attachment to SF6 in N2, Ar, and Xe buffer gases

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 〈e〉 from near thermal electron energies (≊0.04 eV) to 〈e〉≊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 〈e〉 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. T...

Electron transport studies of gas mixtures for use in e-beam controlled diffuse discharge switches

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 can...

Electron transport measurements in methane using an improved pulsed Townsend technique

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 e...

Gases for possible use in diffuse‐discharge switches

Desirable electron attachment and electron drift characteristics of gases for possible use in diffuse‐discharge switches are indicated. Gas mixtures for possible use in externally sustained (e‐ beam) diffuse‐discharge switches are suggested on the basis of electron attachment rate constants and electron drift velocities measured as a function of the density‐normalized electric field E/N. Of particular promise are mixtures of Ar and C3F8.

Recent Advances in Gaseous Dielectrics at Oak Ridge National Laboratory

Recent developments in gaseous dielectrics are discussed with emphasis on dielectric gas mixtures containing SF6. Recent findings on the isotopic, pressure, and nonuniform field dependence of the dielectric strength of gases are also presented, and ways to enhance the corona-stabilization characteristics of gas mixtures by appropriate additives (e.g. to SF6) are outlined. In addition, results are presented on the spark decomposition products of SF6 and the alleviation of the harmful effects of conducting particles by particle coating in situ. Finally, electron attachment, electron drift velocity, and breakdown properties of gas mixtures (e.g. C2F6 or C3F8 in Ar or CH4) - comprised of electron attaching and electron conductivity enhancing components - which have promise for use in pulsed power technologies are reported.

A study of the isomeric dependence of low‐energy (<10 eV) electron attachment: Perfluoroalkanes

The attachment of low‐energy ( 10−6 s) and dissociative attachment fragment anions were observed for these molecules in the mass spectrometric study. The relative cross sections for negative ion formation and the τa of the long‐lived parent anions were measured as a function of electron energy in the mass spectrometric study. The absolute total electron attachment rate constants as a function of the mean electron energy have been measured in the swarm study and used to calculate the total electron attachment cross sections as a function of electron energy. Both studies have shown a strong dependence of dissociative and nondissociative electron attachment...

Electron attachment and ionization processes in CF4, C2F6, C3F8, and n‐C4F10

Measurements are reported of the electron attachment (ηT/N) and ionization (αT/N) coefficients for the perfluoroalkanes n‐CNF2N+2 (N=1 to 4) over the density‐reduced electric field (E/N) range 5×10−17≤E/N≤400×10−17 V cm2 using pulsed Townsend (PT) experimental techniques. The present ηT/N measurements are the first to be obtained for pure C2F6, C3F8, and n‐C4F10 at low E/N values. The ηT/N measurements in C3F8 and n‐C4F10 are dependent on gas pressure over a wide E/N range in agreement with previous high pressure electron attachment rate constant ka measurements in these gases. The dissociative and nondissociative electron attachment processes for C3F8 and n‐C4F10 have been quantified from the pressure dependence of the measured electron attachment coefficients ηT/N as a function of E/N. The thermal electron attachment rate constants (ka)th and the high voltage limiting electric field strengths (E/N)lim obtained from the present measurements are in good agreement with previous literature values.