Electroluminescence and electric current response spectroscopy applied to the characterization of polymer light-emitting electrochemical cells

Frequency-dependent electroluminescence and electric current response spectroscopy were applied to polymeric light-emitting electrochemical cells in order to obtain information about the operation mechanism regimes of such devices. Three clearly distinct frequency regimes could be identified: a dielectric regime at high frequencies; an ionic transport regime, characterized by ionic drift and electronic diffusion; and an electrolytic regime, characterized by electronic injection from the electrodes and electrochemical doping of the conjugated polymer. From the analysis of the results, it was possible to evaluate parameters like the diffusion speed of electronic charge carriers in the active layer and the voltage drop necessary for operation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752438]

Consolidation of Bi-2223 superconducting powders by spark plasma sintering

The spark plasma sintering (SPS) technique, by using a compacting pressure of 50 MPa, was used to consolidate pre-reacted powders of Bi1.65Pb0.35Sr2Ca2Cu3O10+delta (Bi-2223). The influence of the consolidation temperature, T-D, on the structural and electrical properties has been investigated and compared with those of a reference sample synthesized by the traditional solid-state reaction method and subjected to the same compacting pressure. From the X-ray diffraction patterns, performed in both powder and pellet samples, we have found that the dominant phase is the Bi-2223 in all samples but traces of the Bi2Sr2CaCu2O8+x (Bi-2212) were identified. Their relative density were similar to 85% of the theoretical density and the temperature dependence of the electrical resistivity, rho(T), indicated that increasing T-D results in samples with low oxygen content because the SPS is performed in vacuum. Features of the rho(T) data, as the occurrence of normal-state semiconductor-like behavior of rho(T) and the double resistive superconducting transition, are consistent with samples comprised of grains with shell-core morphology in which the shell is oxygen deficient. The SPS samples also exhibited superconducting critical current density at 77 K, J(c)(77K), between 2 and 10A/cm(2), values much smaller than similar to 22A/cm(2) measured in the reference sample. Reoxygenation of the SPS samples, post-annealed in air at different temperatures and times, was found to improve their microstructural and transport properties. Besides the suppression of the Bragg peaks belonging to the Bi-2212 phase, the superconducting properties of the post-annealed samples and particularly J(c)(77K) were comparable or better than those corresponding to the reference sample. Post-annealed samples at 750 degrees C for 5min exhibited J(c)(77K) similar to 130A/cm(2) even when uniaxially pressed at only 50 MPa. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768257]

Self-potential signals from an analog biogeobattery mode

A tank experiment was conducted to check if self-potential (SP) signals can be generated when buried organic matter is wire-connected to a near-surface, oxygen-rich, sediment layer. This experiment demonstrated that once wired, there was a flux of electrons (hence an electric current) between the lower and upper layers of the sandbox with the system responding as a large-scale microbial fuel cell (a type of bioelectrochemical system). An electric current was generated by this process in the wire and the SP method was used to monitor the associated electric potential distribution at the top of the tank.. The electric field was controlled by the flux of electrons through the wire, the oxidation of the organic matter, the reduction of oxygen used as a terminal electron acceptor, and the distribution of the DC resistivity in the tank. The current density through the wire was limited by the availability of oxygen and not by the oxidation of the organic matter. This laboratory experiment incorporated key elements of the biogeobattery observed in some organic-rich contaminant plumes. This analogy includes the generation of SP signals associated with a flux of electrons, the capacity of buried organic matter in sustaining anodic reactions, network resistance connecting terminal redox reactions spatially separated in space, and the existence of anodic secondary coupled reactions. A resistivity tomogram of the tank, after almost a year in operation, suggests that oxidative processes triggered by this geobattery can be imaged with this method to determine the radius of influence of the bioelectrochemical system.

Comparative degradation of ZnO- and SnO(2)-based polycrystalline non-ohmic devices by current pulse stress

The degradation behaviour of SnO(2)-based varistors (SCNCr) due to current pulses (8/20 mu s) is reported here for the first time in comparison with the ZnO-based commercial varistors (ZnO). Puncturing and/or cracking failures were observed in ZnO-based varistors possessing inferior thermo-mechanical properties in comparison with that found in a SCNCr system free of failures. Both systems presented electric degradation related to the increase in the leakage current and decrease in the electric breakdown field, non-linear coefficient and average value of the potential barrier height. However, it was found that a more severe degradation occurred in the ZnO-based varistors concerning their non-ohmic behaviour, while in the SCNCr system, a strong non-ohmic behaviour remained after the degradation. These results indicate that the degradation in the metal oxide varistors is controlled by a defect diffusion process whose rate depends on the mobility, the concentration of meta-stable defects and the amount of electrically active interfaces. The improved behaviour of the SCNCr system is then inferred to be associated with the higher amount of electrically active interfaces (85%) and to a higher energy necessary to activate the diffusion of the specific defects.

Dirac fermions in strong electric field and quantum transport in graphene

Our previous results on the nonperturbative calculations of the mean current and of the energy-momentum tensor in QED with the T-constant electric field are generalized to arbitrary dimensions. The renormalized mean values are found, and the vacuum polarization contributions and particle creation contributions to these mean values are isolated in the large T limit; we also relate the vacuum polarization contributions to the one-loop effective Euler-Heisenberg Lagrangian. Peculiarities in odd dimensions are considered in detail. We adapt general results obtained in 2 + 1 dimensions to the conditions which are realized in the Dirac model for graphene. We study the quantum electronic and energy transport in the graphene at low carrier density and low temperatures when quantum interference effects are important. Our description of the quantum transport in the graphene is based on the so-called generalized Furry picture in QED where the strong external field is taken into account nonperturbatively; this approach is not restricted to a semiclassical approximation for carriers and does not use any statistical assumptions inherent in the Boltzmann transport theory. In addition, we consider the evolution of the mean electromagnetic field in the graphene, taking into account the backreaction of the matter field to the applied external field. We find solutions of the corresponding Dirac-Maxwell set of equations and with their help we calculate the effective mean electromagnetic field and effective mean values of the current and the energy-momentum tensor. The nonlinear and linear I-V characteristics experimentally observed in both low-and high-mobility graphene samples are quite well explained in the framework of the proposed approach, their peculiarities being essentially due to the carrier creation from the vacuum by the applied electric field. DOI: 10.1103/PhysRevD.86.125022

HIGH VOLTAGE PULSED CURRENT STIMULATION OF THE SCIATIC NERVE IN RATS: ANALYSIS BY THE SFI

Objective: To analyze the efficiency of high voltage pulsed current (HVPC) with early application in three different sites, in the regeneration of the sciatic nerve in rats submitted to crush injury, the sciatic functional index (SFI) was used to assess the functional recovery. Methods: After crushing of the nerve, 57 animals were submitted to cathodal HVPC at frequency of 50Hz and voltage of 100V, 20 minutes per day, 5 days per week. The rats were divided into five groups: control group; ganglion group; ganglion + muscle group; muscle group; and sham group. The SFI was determined weekly for seven weeks, from the preoperative period to the 6th postoperative week. Results: Compared with the control group, the results showed a significantly better performance of group 2 for the first 3 weeks; group 3 showed significantly better performance in the third week; and group 4 showed a significantly negative performance during the 481 and 6th weeks. Conclusion: Early application of HVPC had a positive effect in the treatment of the spinal cord region and the sciatic nerve root ganglion with a dispersive electrode on the contralateral lumbar region or on the gastrocnemius. However, HVPC had a negative effect in the treatment with an active electrode on the gastrocnemius and a dispersive electrode on the contralateral thigh. Level of evidence II, Prospective comparative study.