Tracking and Erosion of Silicone Rubber Nanocomposites under DC Voltages of both Polarities

In order to improve the tracking and erosion performance of outdoor polymeric silicone rubber (SR) insulators used in HV power transmission lines, micron sized inorganic fillers are usually added to the base SR matrix. In addition, insulators used in high voltage dc transmission lines are designed to have increased creepage distance to mitigate the tracking and erosion problems. ASTM D2303 standard gives a procedure for finding the tracking and erosion resistance of outdoor polymeric insulator weathershed material samples under laboratory conditions for ac voltages. In this paper, inclined plane (IP) tracking and erosion tests similar to ASTM D2303 were conducted under both positive and negative dc voltages for silicone rubber samples filled with micron and nano sized particles to understand the phenomena occurring during such tests. Micron sized Alumina Trihydrate (ATH) and nano sized alumina fillers were added to silicone rubber matrix to improve the resistance to tracking and erosion. The leakage current during the tests and the eroded mass at the end of the tests were monitored. Scanning Electron Microscopy (SEM) and Energy dispersive Xray (EDX) studies were conducted to understand the filler dispersion and the changes in surface morphology in both nanocomposite and microcomposite samples. The results suggest that nanocomposites performed better than microcomposites even for a small filler loading (4%) for both positive and negative dc stresses. It was also seen that the tracking and erosion performance of silicone rubber is better under negative dc as compared to positive dc voltage. EDX studies showed migration of different ions onto the surface of the sample during the IP test under positive dc which has led to an inferior performance as compared to the performance under negative dc.

Modeling of room temperature current-voltage measurements on homo-junction HgCdTe diodes exhibiting nonequilibrium effects

HgCdTe mid wave infrared (MWIR) n(+)/nu/p(+) homo-junction photodiodes with planar architecture are designed, fabricated, and measured at room temperature. An improved analytical I-V model is reported by incorporating trap assisted tunneling and electric field enhanced Shockley-Read-Hall generation recombination process due to dislocations. Tunneling currents are fitted before and after the Auger suppression of carriers with energy level of trap (E-t), trap density (N-t), and the doping concentrations of n(+) and nu regions as fitting parameters. Values of E-t and N-t are determined as 0.79 E-g and similar to 9 x 10(14) cm(-3), respectively, in all cases. Doping concentration of nu region was found to exhibit nonequilibrium depletion from a value of 2 x 10(16) to 4 x 10(15) cm(-3) for n(+) doping of 2 x 10(17) cm(-3). Pronounced negative differential resistance is observed in the homo-junction HgCdTe diodes. (C) 2012 American Institute of Physics. [doi:10.1063/1.3682483]

Vertical Uplift Resistance of a Group of Two Coaxial Anchors in Clay

The vertical uplift resistance for a group of two horizontal coaxial rigid strip anchors embedded in clay under undrained condition has been determined by using the upper bound theorem of limit analysis in combination with finite elements. An increase of undrained shear strength of soil mass with depth has been incorporated. The uplift factor F-c gamma has been computed. As compared to a single isolated anchor, a group of two anchors provides greater magnitude of the uplift resistance. For a given embedment ratio, the group of two anchors generates almost the maximum uplift resistance when the upper anchor is located midway between ground surface and the lower anchor. For a given embedment ratio, F-c gamma increases linearly with an increase in the normalized unit weight of soil mass up to a certain value before attaining a certain maximum magnitude; the maximum value of F-c gamma increases with an increase in embedment ratio. DOI: 10.1061/(ASCE)GT.19435606.0000599. (C) 2012 American Society of Civil Engineers.

Typhoid fever & vaccine development: a partially answered question

Typhoid fever is a systemic disease caused by the human specific Gram-negative pathogen Salmonella enterica serovar Typhi (S Typhi). The extra-intestinal infections caused by Salmonella are very fatal. The incidence of typhoid fever remains very high in impoverished areas and the emergence of multidrug resistance has made the situation worse. To combat and to reduce the morbidity and mortality caused by typhoid fever, many preventive measures and strategies have been employed, the most important being vaccination. In recent years, many Salmonella vaccines have been developed including live attenuated as well as DNA vaccines and their clinical trials have shown encouraging results. But with the increasing antibiotic resistance, the development of potent vaccine candidate for typhoid fever is a need of the hour. This review discusses the latest trends in the typhoid vaccine development and the clinical trials which are underway.

Electrical Resistance and Seebeck Coefficient in PbTe Nanowires

We address a physics-based simplified analytical formulation of the diffusive electrical resistance ( (Omega)) and Seebeck coefficient () in a PbTe nanowire dominated by acoustic phonon scattering under the presence of a low static longitudinal electric field. The use of a second-order nonparabolic electron energy band structure involving a geometry-dependent band gap has been selected in principle to demonstrate that the electron mean free path (MFP) in such a system can reach as low as about 8 nm at room temperature for a 10-nm-wide PbTe nanowire. This is followed by the formulation of the carrier back-scattering coefficient for determination of (Omega) and as functions of wire dimensions, temperature, and the field, respectively. The present analytical formulation agrees well with the available experimental data and may find extensive use in determination of various electrothermal transport phenomena in PbTe-based one-dimensional electron devices.

Effect of ``dipolar-biasing'' on the tunability of tunneling magnetoresistance in transition metal oxide systems

We observe an unusual tunneling magnetoresistance (TMR) phenomenon in a composite of La2/3Sr1/3MnO3 with CoFe2O4 where the TMR versus applied magnetic field loop suggests a ``negative coercive field.'' Tracing its origin back to a ``dipolar-biasing'' of La2/3Sr1/3MnO3 by CoFe2O4, we show that the TMR of even a single composite can be tuned continuously so that the resistance peak or the highest sensitivity of the TMR can be positioned anywhere on the magnetic field axis with a suitable magnetic history of the sample. This phenomenon of an unprecedented tunability of the TMR should be present in general in all such composites. (C) 2012 American Institute of Physics.http://dx.doi.org/10.1063/1.4731206]

Analysis of current voltage characteristics of MWIR homojunction photodiodes for uncooled operation

Dark currents n(+)/v/p(+) Hg0.69Cd0.Te-31 mid wave infrared photodiodes were measured at room temperature. The diodes exhibited negative differential resistance at room-temperature, but with increasing leakage currents as a function of reverse bias. The current-voltage characteristics were simulated and fitted by incorporating trap assisted tunneling via traps and Shockley-Read-Hall generation recombination process due to dislocations in the carrier transport equations. The thermal suppression of carriers was simulated by taking energy level of trap (E-t), trap density (N-t) and the doping concentrations of n(+) and v regions as fitting parameters. Values of E-t and N-t were 0.78E(g) and similar to 6-9 x 10(14) cm(-3) respectively for most of the diodes. Variable temperature current voltage measurements on variable area diode array (VADA) structures confirmed the fact that variation in zero bias resistance area product (R(0)A) is related to g-r processes originating from variation in concentration and kind of defects that intersect a junction area. (C) 2012 Elsevier B.V. All rights reserved.

Crystallographic Study of Novel Transthyretin Ligands Exhibiting Negative-Cooperativity between Two Thyroxine Binding Sites

Background: Transthyretin (TTR) is a homotetrameric serum and cerebrospinal fluid protein that transports thyroxine (T4) and retinol by binding to retinol binding protein. Rate-limiting tetramer dissociation and rapid monomer misfolding and disassembly of TTR lead to amyloid fibril formation in different tissues causing various amyloid diseases. Based on the current understanding of the pathogenesis of TTR amyloidosis, it is considered that the inhibition of amyloid fibril formation by stabilization of TTR in native tetrameric form is a viable approach for the treatment of TTR amyloidosis. Methodology and Principal Findings: We have examined interactions of the wtTTR with a series of compounds containing various substitutions at biphenyl ether skeleton and a novel compound, previously evaluated for binding and inhibiting tetramer dissociation, by x-ray crystallographic approach. High resolution crystal structures of five ligands in complex with wtTTR provided snapshots of negatively cooperative binding of ligands in two T4 binding sites besides characterizing their binding orientations, conformations, and interactions with binding site residues. In all complexes, the ligand has better fit and more potent interactions in first T4 site i.e. (AC site) than the second T4 site (BD site). Together, these results suggest that AC site is a preferred ligand binding site and retention of ordered water molecules between the dimer interfaces further stabilizes the tetramer by bridging a hydrogen bond interaction between Ser117 and its symmetric copy. Conclusion: Novel biphenyl ether based compounds exhibit negative-cooperativity while binding to two T4 sites which suggests that binding of only single ligand molecule is sufficient to inhibit the TTR tetramer dissociation.

Microstructure and dry sliding wear resistance evaluation of plasma nitrided austenitic stainless steel type AISI 316LN against different sliders

In this work, Plasma Nitriding was carried out at a temperature of 570 degrees C on nuclear grade austenitic stainless steel type AISI 316 LN (316LN SS) in a gas mixture of 20% N-2-80% H-2 to improve the surface hardness and thereby sliding wear resistance. The Plasma Nitride (PN) treated surface has been characterized by Vickers microhardness measurements, Scanning Electron Microscopic (SEM) examination, X-ray Diffraction (XRD) and sliding wear assessment. The average thickness of the PN layer was found to be 70 mu m. Microhardness measurements showed a significant increase in the hardness from 210 HV25g (unnitrided sample) to 1040 HV25g (Plasma Nitrided sample). The XRD reveals that PN layer consists of CrN, Fe4N and Fe3N phases along with austenite phase. The tribological parameters such as the friction coefficient and wear mechanism have been evaluated at ambient conditions for PN treated ring (PN ring) vs. ASTM A453 grade 660 pin (ASTM pin), PN ring vs. Nickel based alloy hard faced pin (Colmonoy pin), PN ring vs. 316LN SS pin and 316LN SS ring vs. 316LN SS pin. The wear tracks have been analyzed by SEM, Energy Dispersive X-ray Analysis (EDX) and Optical Profilometry. The untreated 316LN SS ring vs. 316LN SS pin produced severe wear and was characterized by a combination of delamination and adhesion wear mechanism, whereas wear mechanism of the PN rings reveals mild abrasion and a transfer layer from pin materials. (C) 2012 Elsevier B.V. All rights reserved.

Magneto-Transport Study of Pure and Co Doped ZnO Thin Films

A detailed low temperature magneto-transport study is carried out to understand the transport mechanism in pure and Co doped ZnO thin films grown by pulsed laser deposition (PLD) technique. A negative transverse magneto-resistance (MR) (with a value similar to 4% at 4.5 K) which decreases monotonically with the increase in temperature, is observed for the undoped ZnO film. A competition between positive and negative MR is observed for the Co doped ZnO samples. In this case at higher field values negative MR contribution dominates over the positive MR, which gives rise to a slope change in the MR data. Our data for MR shows excellent agreement with the semi-empirical formula given by Khosla et al., which is originally proposed for the degenerate semiconductors. This formula incorporates the third order perturbation expansion of the s-d exchange scattering of the conduction electrons from the localised spins. We have also obtained the Hall mobility, carrier conc. and mean free path as function of temperature for the pure ZnO film.

Time evolution of resistance in response to magnetic field: Evidence of glassy transport in La0.85Sr0.15CoO3

We demonstrate the distinct glassy transport phenomena associated with the phase separated and spin-glass-like phases of La0.85Sr0.15CoO3, prepared under different heat-treatment conditions. The low-temperature annealed (phase-separated) sample, exhibits a small change in resistance, with evolution of time, as compared to the high-temperature annealed (spin glass) one. However, the resistance change as a function of time, in both cases, is well described by a stretched exponential fit, signifying the slow dynamics. Moreover, the ultraviolet spectroscopy study evidences a relatively higher density of states in the vicinity of EF for low-temperature annealed sample and this correctly points to its less semiconducting behavior.

Mechanical and electrical contact resistance characteristics of a cellular assembly of carbon nanotubes

We employ nanoindentation coupled with electrical contact resistance measurements for simultaneous characterization of the electrical and mechanical behaviors of a cellular assembly of carbon nanotubes (CNTs). Experimental results reveal two different responses that correspond to relatively dense and porous regions of the cellular structure. Distinct nonlinear electron transport characteristics are observed, which mainly originate from diffusive conductance in the CNT structure. In the denser region, differential conductance shows asymmetric minima at lower bias, implying that conductivity mainly results from bulk tunneling. However, the porous regions show insignificant differential conduction as opposed to the denser region.

Biocides - resistance, cross-resistance mechanisms and assessment

Importance of the field: Antibiotic resistance in bacterial pathogens has increased worldwide leading to treatment failures. Concerns have been raised about the use of biocides as a contributing factor to the risk of antimicrobial resistance (AMR) development. In vitro studies demonstrating increase in resistance have often been cited as evidence for increased risks. It is therefore important to understand the mechanisms of resistance employed by bacteria toward biocides used in consumer products and their potential to impart cross-resistance to therapeutic antibiotics. Areas covered: In this review, the mechanisms of resistance and cross-resistance reported in the literature toward biocides commonly used in consumer products are summarized. The physiological and molecular techniques used in describing and examining these mechanisms are reviewed and application of these techniques for systematic assessment of biocides for their potential to develop resistance and/or cross-resistance is discussed. Expert opinion: The guidelines in the usage of biocides in household or industrial purpose should be monitored and regulated to avoid the emergence of any MDR strains. The genetic and molecular methods to monitor the resistance development to biocides should be developed and included in preclinical and clinical studies.

Evaluation of Fracture Parameters by Double-G, Double-K Models and Crack Extension Resistance for High Strength and Ultra High Strength Concrete Beams

This paper presents the advanced analytical methodologies such as Double- G and Double - K models for fracture analysis of concrete specimens made up of high strength concrete (HSC, HSC1) and ultra high strength concrete. Brief details about characterization and experimentation of HSC, HSC1 and UHSC have been provided. Double-G model is based on energy concept and couples the Griffith's brittle fracture theory with the bridging softening property of concrete. The double-K fracture model is based on stress intensity factor approach. Various fracture parameters such as cohesive fracture toughness (4), unstable fracture toughness (K-Ic(c)), unstable fracture toughness (K-Ic(un)) and initiation fracture toughness (K-Ic(ini)) have been evaluated based on linear elastic fracture mechanics and nonlinear fracture mechanics principles. Double-G and double-K method uses the secant compliance at the peak point of measured P-CMOD curves for determining the effective crack length. Bi-linear tension softening model has been employed to account for cohesive stresses ahead of the crack tip. From the studies, it is observed that the fracture parameters obtained by using double - G and double - K models are in good agreement with each other. Crack extension resistance has been estimated by using the fracture parameters obtained through double - K model. It is observed that the values of the crack extension resistance at the critical unstable point are almost equal to the values of the unstable fracture toughness K-Ic(un) of the materials. The computed fracture parameters will be useful for crack growth study, remaining life and residual strength evaluation of concrete structural components.