Interaction between superconductive films and magnetic nanostructures

The interplay between magnetic and superconductive films is studied. The generalized London equation for this system is solved, and the magnetic fields, the energy and the interaction forces are computed. In particular, we focus on how to manipulate vortices using magnetic nanostructures.Comment: 16 pages, 5 figures, submitted to Phys. Rev.

Vortex interactions in presence of a soft magnetic film

We study theoretically the behavior of vortices in a thin film superconductor placed close to a soft magnetic film. It is shown that the field from the vortex induces a magnetization distribution in the soft magnetic film, thus modifying the fields and vortex interactions. We suggest that the interaction between two otherwise identical vortices is attractive at short distances, but repulsive at larger distances. This is in contrast to the case without the soft magnetic film, where the force is always repulsive.Comment: 7 page

Harvesting electrical energy from water drops falling on a vibrating cantilever

In this work a novel thin-film device combining piezoelectric and contact electrification energy harvesting is created with the aim of investigating how it responds to water droplet impact during vibrations. The two energy harvesting principles utilize the same ground electrode, but the electrical signal outputs are independent and show entirely different electrical signal characteristics in presence of external forcing. While piezoelectricity gives rise to a nearly quadratic increase in harvested energy as a function of vibration velocity, the energy due to contact electrification reaches saturation for larger water drop velocities. On the other hand, when the water stream transitions from discrete droplets to a continuous stream the energy gathered from the piezoelectric mechanism exhibits saturation, whereas the energy due to contact electrification decreases. The proposed device may have applications as a self-powered environmental sensor that allow one to distinguish between forced oscillations and water droplet impacts.acceptedVersio

Triboelectric proximity and contact detection using soft planar spiral electrodes

In this study, planar spiral electrodes for triboelectric contact and proximity detection are investigated. The spiral electrodes are created using liquid metal in a soft elastomer in order to make the sensor flexible and stretchable. When the metallic object to be detected comes in contact with the silicon elastomer surrounding the electrodes, charge transfer occurs, and this can be utilized for contact and noncontact sensing of the metallic object in flat or curved geometries. An equivalent electrical circuit model for the noncontact system is proposed and found to be in good agreement with the experimental data. A comparison between triboelectric proximity detection and eddy current resonance frequency proximity detection, using the same electrode geometry, shows that the two methods have comparable sensitivity. An added advantage of the triboelectric system is that it transforms mechanical into electrical energy and thereby allows one to monitor the triboelectrical signal upon impact of a metal object, and the experimental data show that the generated current scales with the impact force.publishedVersio

Ion Concentration Influences the Charge Transfer Due to a Water−Air Contact Line Moving over a Hydrophobic Surface: Charge Measurements and Theoretical Models

A metal electrode covered by an inert, hydrophobic polymer surface is dipped into water, and the charge transfer was measured as a function of ion concentration for different chlorides, sulfates, and nitrates. A generic behavior is observed wherein the charge transfer first increases and then decreases as the ion concentration increases. However, for acids, the charge transfer decreases monotonously with concentration and even reverses polarity. Two different models, both in which the charge transfer is attributed to removal of ions from the electrical double layer as the contact line passes by, are discussed and shown to provide possible explanations of the experimental data.publishedVersio

The nonlinearities in the galvanostatic charging curves of supercapacitors provide insights into charging mechanisms

Supercapacitors are often charged using constant currents. The capacitance can be determined from the slope of the voltage-time curve if the measured voltage over the supercapacitor increases linearly with time. However, the resulting voltage-time curve is often nonlinear, which may lead one to interpret the capacitance as being either time or voltage dependent. In the current work, systematic experimental studies of the nonlinearity of galvanostatic charging curves as a function of applied current and temperature are undertaken for commercial supercapacitors in the range 1–1000 F. A consistent theory is developed to explain the available data. It is demonstrated that the nonlinearity in the voltage-time curve can be attributed to a constant capacitance in parallel with a resistance, the latter which is inversely proportional to the applied current. The influence of faradaic charge transfer reactions or surface charge reorganization on this parallel resistance is analyzed. The proposed theory is also used to analyze galvanostatic charging data available in the research literature, and the different types of nonlinearities observed provide new insight into the mechanisms occurring during charging of various types of supercapacitors.publishedVersio

A water droplet-powered sensor based on charge transfer to a flow-through front surface electrode

A water droplet sensor based on a flow-through front surface electrode is demonstrated. It is shown that a triboelectric nanogenerator based on a metal-dielectric junction can be utilized to construct simple, self-powered water drop counters that require only a minimum of electronical components. Every water drop generates a flash in a light emitting diode, which is detected by a silicon photodetector located some distance away. Different transducer designs are investigated, either in the form of flat tilted surfaces or polymer-covered metal wires, with the former found to provide the largest charge transfer while the latter is less sensitive to positional stability. It is also demonstrated that the sensor could be utilized to monitor the state of the magnetic field-controlled turbidity of a solution without any additional electronics, thus making a minimalistic design feasible.publishedVersio