EXAFS and thermal studies on zinc polymeric electrolytes

(PEO) _n :ZnX ₂ (X = I, Br) complexes were formed at room temperature with values ofn ranging from 8 to 30. Differential scanning calorimetry (DSC) results indicate that none of them contained any crystalline phases that can be associated with the formation of a complex (i.e. it can be assumed that all the salt is in the amorphous phase). EXAFS studies carried out on these samples suggest that the zinc cation is co-ordinated to two of the halide ions and, in addition, to 4 oxygen atoms whereX = I and 6 oxygen atoms whereX = Br. The DSC results confirm that the zinc cation is in an environment independent of overall stoichiometry, as the glass transition temperatures of the samples were found to be similar throughout. By comparing these with those of annealed samples (made by heating the above samples to 150°C and cooling to –80°C at 320°C min^–1) it was found that the stoichiometry of the amorphous phase was roughly 61 forX=I and 81 forX = Br.     

Supercapacitor devices using porous silicon electrodes

Electrical double layer (EDL) supercapacitors have been constructed using gold coated porous silicon (PSi) electrodes in a 0.25 M TEABF₄/PC solution. As a comparison with the PSi, graphite paper, carbon cloth and ITO on glass electrodes have also been tested using the same electrolyte. The capacitors have been characterised using a.c. impedance spectroscopy and cyclic voltammetry (normal staircase mode). Devices using PSi electrodes showed a capacitance of approximately 0.2 mF cm⁻² (equivalent to 5 mF g⁻¹). In comparison, devices based on ITO on glass electrodes had a capacitance of 0.76 mF cm⁻². Those based on graphite-paper electrodes gave 10 mF cm⁻² (equivalent to 131 mF g⁻¹) and those using carbon cloth gave 600 mF cm⁻² (equivalent to 35 F g⁻¹). Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Linking Preservice Teachers' Mathematics Self‐Efficacy and Mathematics Teaching Efficacy to Their Mathematical Performance

This study examined preservice teachers' mathematics self‐efficacy and mathematics teaching efficacy and compared them to their mathematical performance. Participants included 89 early childhood preservice teachers at a Midwestern university. Instruments included the Mathematics Self‐Efficacy Scale (MSES), Mathematics Teaching Efficacy Beliefs Instrument (MTEBI), and the Illinois Certification Testing System (ICTS) Basic Skills Test. The results indicate that preservice teachers' mathematics self‐efficacy is positively correlated to their personal mathematics teaching efficacy. In addition, their mathematical performance is related to their mathematics self‐efficacy and mathematics teaching efficacy. In regard to affecting student outcomes, only those preservice teachers who are very confident in their ability to teach believe they can have an effect on their students. Implications on teacher education programs are discussed.     

Density functional theory study of hydrogen bonding in ionic molecular materials

Crystal structures are usually described in geometric terms. However, it is the energetics of intermolecular interactions that determine the chemical and physical properties of molecular materials.(1) In this paper, we use density functional theory (DFT) in combination with numerical basis sets to analyze the hydrogen bonding interactions in a family of novel ionic molecular materials. We find that the calculated binding energies are consistent with those of other ionic hydrogen bonded systems. We also examine electron density distributions for the systems of interest to gain insight into the nature of the hydrogen bonding interaction and investigate the effects of different aspects of the crystal field on the geometry of the hydrogen bond.     

A two‐phase adaptive finite element method for solid–fluid coupling in complex geometries

In this paper we present a method to solve the Navier–Stokes equations in complex geometries, such as porous sands, using a finite‐element solver but without the complexity of meshing the porous space. The method is based on treating the solid boundaries as a second fluid and solving a set of equations similar to those used for multi‐fluid flow. When combined with anisotropic mesh adaptivity, it is possible to resolve complex geometries starting with an arbitrary coarse mesh. The approach is validated by comparing simulation results with available data in three test cases. In the first we simulate the flow past a cylinder. The second test case compares the pressure drop in flow through random packs of spheres with the Ergun equation. In the last case simulation results are compared with experimental data on the flow past a simplified vehicle model (Ahmed body) at high Reynolds number using large‐eddy simulation (LES). Results are in good agreement with all three reference models. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of geometry and polarity on the prebreakdown and flashoverbehavior of conical high-voltage insulators in vacuum

Concurrent optical and electrical measurements have been used to characterize the physical phenomena associated with the prebreakdown and flashover behavior of high-voltage alumina ceramic insulator samples having both conventional and unconventional conical geometries. The critical parameters are the threshold voltage for initiating flashover events, the stable interelectrode current-voltage relationship, the luminescence pattern, and the breakdown rate as a function of time. These measurements have revealed that a well-conditioned sample of unconventional conical geometry can have very low, i.e., approaching zero, breakdown rate. Physical explanations of the findings are based on the current understanding of flashover mechanisms and the prediction from simulations using a field plotting technique     

Electrical and optical DC vacuum emission phenomena from aceramic-insulated planar electrode configuration

Details are given of an experimental investigation into the DC prebreakdown characteristics of a cylindrical ceramic insulator regime. Current-voltage plots are presented for the radial conduction processes in the surface of the insulator, together with images of the associated optical processes. By employing a third transparent electrode, it is possible to study the effect of an orthogonal field on the radial conduction process. The experimental findings are discussed in terms of electron injection processes from the cathode triple junction and the role played by gas absorbed from the insulator     

Polymer electrolyte based solid state redox supercapacitors with poly (3-methyl thiophene) and polypyrrole conducting polymer electrodes

Types I and II solid state redox supercapacitors have been constructed using polypyrrole (pPy) and poly (3-methyl thiophene) (pMeT) conducting polymer electrodes with lithium ion conducting polymer electrolyte poly(ethylene oxide) (PEO)-LiCF₃SO₃ plasticised with poly (ethylene glycol) (PEG). The performance of the capacitors has been characterised by a.c. impedance, linear sweep voltammetry, galvanostatic charge-discharge methods and long term cycling tests. The asymmetric type II capacitors with p-doped pPy and pMeT electrodes give a capacitance value ∼ 2 mF cm⁻² (equivalent to 18 Fg⁻¹ of the total mass of the electrodes) and can be charged up to the voltage of 1.7 V. The symmetric type 1 capacitors of the configuration pPy | polymer electrolyte | pPy and pMeT | polymer electrolyte | pMeT show comparable values of capacitance but they are limited to the working voltage of <1.0 V. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Measurements of velocity ratio in a 90 MW gyroklystron electronbeam

Measurements of the ratio of perpendicular to parallel velocity (α=υ_⊥/υ,) in a 390-420 kV, 160-240 A, small orbit gyroklystron electron beam produced by a magnetron injection gun have been performed using a capacitive probe. The probe employed guard rings to reduce uncertainties in measurement and calibration due to fringing electric fields. These studies represent the first measurements of a in such a powerful rotating beam. Since the current levels in this experiment are two to three times higher than those in previous velocity ratio studies of beams with similar voltage. The effects of changes in voltage, current, and magnetic compression on the measured velocity ratio are investigated. Values of α in the range of 0.8-1.3 are typically observed, which are considerably higher than the values predicted by theoretical simulations. Errors in the modeling of space charge effects are ruled out as the cause of the discrepancy by the agreement between theoretical and experimental space charge current limits. Instead, a significant portion of the discrepancy is attributed to a reduction in cathode magnetic field by the self-axial magnetic fields of the electron beam