Pulsed Dc Fields Research Articles

Platinum-Iridium Alloy Nanoparticle Coatings Produced by Electrophoretic Deposition Reduce Impedance in 3D Neural Electrodes.

Platinum-based neural electrodes, frequently alloyed with Ir or W, are routinely used to treat neurological disorders. However, their performance is impaired by an increase in impedance that compromises long-term implant functionality. Though there are multiple coating techniques available to address this issue, electrode, and base material often exhibit a compositional mismatch, which impairs mechanical stability and may lead to toxicological side effects. In this work, we coated Pt wire electrodes with ligand-free electrostatically stabilized colloidal Pt90Ir10, Pt90W10, and Pt50W50 alloy nanoparticles (NPs) matching electrode compositions using electrophoretic deposition (EPD) with direct-current (DC) and pulsed-DC fields in aqueous medium. The generated alloy NPs exhibit a solid solution structure as evidenced by HR-TEM-EDX and XRD, though additional WOx phases were identified in the Pt50W50 samples. Consequently, coating efficiency was also impaired in the presence of high W mass fractions in the alloy NPs. Characterization of the NP coatings by cyclic voltammetry and impedance spectroscopy yielded a significant reduction of the impedance in the Pt90Ir10 sample in comparison to the control coated with Pt NPs. The electrochemical surface area (ECSA) of the PtW alloy coatings, on the other hand, was significantly reduced.

Electrocoalescence of water drop trains in oil under constant and pulsatile electric fields

This study addresses the effectiveness of constant and pulsed DC fields in promoting coalescence of dispersed water drops in an oil-continuous phase. For this purpose, a train of drops of relatively uniform size is injected into a stream of flowing sunflower oil. This stream is then admitted to a coalescing section, where an electric field is applied between a pair of ladder-shape bare electrodes. The capability of this device to enhance coalescence of droplets in a chain is investigated at different field intensities, frequencies and waveforms. The effect of the initial inter-droplet separation distance on the process performance is also addressed under constant DC fields. The dominant coalescence mechanism is found to be due to dipole–dipole interaction at low field strength, whereas electrophoresis becomes predominant at higher field strength. Experiments reveal the existence of an optimal frequency, where the average droplet size enlargement is maximized, especially at low field strengths. The droplet size at the outlet of the coalescer is also found to be dependent on the field waveform.

Pulsed direct current field induced phase transformation in graphene nanoplatelets

Graphene nanoplatelets were consolidated at various conditions using spark plasma sintering utilizing ON-OFF DC pulse energizing. The phase transformation from graphene nanoplatelets to diamond can occur at pressure of 80 MPa and at temperatures of 1100 °C to 1500 °C without any catalysts being involved. However, no diamond formation could be observed in sintering experiments at the same pressure-temperature conditions with steady AC heating. Our results indicate that the phase formation of diamond induced by the pulsed DC field was determined by the presence of plasmas in areas of locally increased densities of stacking faults in the layered graphene nanoplatelets under sintering-pressures.

Electric field induced pearling instability in cylindrical vesicles

A theoretical and experimental study on the effect of an electric field on a cylindrical vesicle is presented. Experiments show that a cylindrical vesicle when subjected to an axial electric field, displays an axisymmetric pearling instability (the Rayleigh–Plateau instability) beyond a threshold electric field. Unlike fluid jets, in which electric fields are known to stabilize the Rayleigh–Plateau instability, cylindrical vesicles exhibit dual effects of electric field. The tension required to induce the instability is produced by the electric field. At higher values of field strength however, a stabilizing action of the electric field is seen. This renders the fastest growing wavenumber, km, independent of the electric field at high electric field strength. Theoretical results predict, albeit qualitatively, a weak dependence of km on the electric field. It also explains the substantially higher experimental values of km (around 0.64–1.0) observed in vesicles as compared to km = 0.56 for fluid cylinders. Theory and experiments show that the instability is induced using a much lower value of pulsed DC field as compared to an AC field. At long times, a pearled state is observed, with the pearls separated by short cylindrical nanotubules. The pearled structure is converted into crowded, taut spherical globules, which are connected by very thin short bilayer cylinders when the field is switched off, before they eventually disappear and the original cylindrical vesicle is recovered. All of the morphological changes in cylindrical vesicles in electric fields are completely reversible. This study demonstrates the dominance of tension and stretching at short and long timescales respectively.

Power consumption measurements for ac and pulsed dc for electrostatic coalescence of water-in-oil emulsions

True root mean square power consumption measurements for coalescence of well-defined water-in-oil (W/O) emulsions [reverse osmosis water dispersed in Isopar M and stabilized with Paranox 100 surfactant] are presented for a 2-l Teflon insulated electrostatic coalescer. Estimated electricity costs for processing 1000 barrels (42 gal basis) of the emulsion were as low as $0.37 and $0.39 under 60 Hz ac and pulsed dc (i.e., half-wave rectified ac) fields, respectively. These data compare favorably with actual operating costs for an equivalent volume of crude oil for NATCO designed full-scale electrostatic coalescer systems with non-insulated electrodes. Contrary to observations by other investigators including past tests in our own laboratory, ac fields were more effective in increasing coalescence rate than pulsed dc fields. Normalized power consumption data show that the optimized frequency for coalescence with electricity costs as the important factor was at 60 Hz (i.e., the lowest frequency tested) in spite of dramatic increases in throughputs at higher frequencies of 120, 400 and 600 Hz. Comparative tests using identically prepared W/O emulsions showed that the coalescence rate with the present system was up to 11-fold higher as compared to a geometrically similar coalescer using a patented plastic-sealed electrode under identical 600 Hz ac fields.

Deformation and break-up of aqueous drops in dielectric liquids in high electric fields

In the petroleum industries, high electric fields are used in the separation of aqueous drops from an oil phase. However, high electric fields can also deform and break up the drops. Under a high uniform electric field, a drop is usually a prolate spheroid, but it can also be of different forms such as a lamella penetrating into the continuous phase, or a drop with a flat surface on one side and a cone on the other side. The deformed drop shape depends greatly on the electrical conductivity, viscosity, surface tension and density of both liquid phases. Droplets can be produced from a prolate drop head, while the tail has a lamella penetrating into the continuous phase. The onset of drop break-up occurs at an electric field strength of between 300 and 350kV/m, corresponding to an electric Weber number of between 0.06 and 0.13. The current Weber number needs some modifications before it can be fully applied to characterise the onset of drop break-up. However, the form of the applied electric field is important. With pulsed dc fields, drop deformation and detachment rates are influenced by the field strength and the pulsing frequency.

Pulsed magnetic fields enhance nitric oxide synthase activity in rat cerebellum

The effect of pulsed magnetic fields on nitric oxide synthase (NOS) activity in the rat brain was investigated. Sprague–Dawley rats (male, 200–250 g body weight) brain were dissected regionally, and the crude enzyme solutions were treated with pulsed DC, AC or static DC magnetic fields at 0°C for 1 h. After exposure, NOS activity was measured as nitrite and nitrate levels generated from incubation with arginine, CaCl 2 and β-nicotinamide adenine dinucleotide phosphate. Under these experimental conditions, neither AC nor static DC field treatment showed any significant change in NOS activity. A significant increase in NOS activity was observed in the cerebellum (111.2±2.0%, P<0.05, five separate experiments) for a 1 Gauss (0.1 mT) pulsed DC field. Under the same experimental condition, only a slight change or no effect was observed in the hippocampus, cortex, medulla oblongata, hypothalamus, striatum and midbrain. These studies suggest that pulsed magnetic fields result in a different effect on NOS activity in the cerebellum of the rats.

An Experimental Investigation into the Motion of a Single Drop in a Pulsed DC Electric Field

It has been shown that the separation of high volume fraction water-in-oil dispersions is improved by the application of a pulsed DC electric field. It is thought that the improvement in the rate of separation could be due to the formation of dipoles in the aqueous drops, caused by the action of the electric field, but it is also possible that the drops acquire a charge from the continuous phase. At present there is a lack of fundamental experimental results to support these theories. By studying the motion of a single drop in a uniform pulsed DC field, it is possible to make some deductions about the charging mechanism of the drop. A de-ionized water drop, with a diameter of 100 μ to 400 μ, was positioned in n-dodecane between a pair of parallel plate electrodes, the positive electrode being insulated. The drop motion was recorded using a high-speed video camera. Image analysis enabled the x, y co-ordinates of the drop centre in a sequence of images to be obtained and hence the drop trajectory. The pulsed DC voltage applied across the electrodes was also recorded and could be superimposed onto the drop trajectory to enable any correlation between drop motion and the field applied to be observed. The relationship between the drop motion and the continuous phase conductivity was studied and it was found that as the conductivity was increased, the amplitude of the drop motion decreased. This decrease in drop motion was due to a reduction of the field strength in the n-dodecane as its conductivity was increased, relative to that of the insulation coating. Experiments without the insulation coating are shown to confirm this explanation.

Dimensional Analysis of Continuous High-capacity Electrodispersion of Aqueous Based Liquids in an Organic Continuous Phase

Abstract High-intensity electric fields can be used to disperse aqueous-based solutions in a relatively nonconducting immiscible organic phase. Dimensional analysis of electrical dispersion performance from a single grounded nozzle between two charged electrodes can be characterised in terms of five dimensionless groups: an electrode height; nozzle-electrode distance; a nozzle Reynolds number; an electrical Bond number, which relates electrical to surface forces; and a Taylor number, which relates electrical to viscous forces. Experimental results on the electrodispersion of water in 2-ethyl-1-hexanol indicate that pulsed DC fields can accomplish electrodispersion utilizing a lower rms-voltage than steady DC fields. In addition, the pulsed-field-behavior varies with pulse frequency with 200 Hz fields being more effective for higher continuous-phase viscosities while 2000 Hz fields are more effective when the viscosity is lower. A steady DC field displays invariant behavior with changing viscosity. In the ...

Zero-field NMR of nematic liquid crystals with positive and negative magnetic susceptibility anisotropies

Nematic liquid crystal systems with positive and negative magnetic susceptibility anisotropies are studied by NMR in high and zero magnetic fields. The behavior of the system in zero field is dictated by the form of the zero-field Hamiltonian, the symmetry of the liquid crystal phase, and the initial state of the magnetization. Zero-field evolution is initiated both with and without the use of dc pulsed fields in the field cycle. Pulsed dc fields are also used to remove the effects of residual field inhomogeneities by zero-field spin echoes. The order parameters measured in an applied field and in the absence of a field are found to be the same within experimental error for both types of liquid crystal.

Zero-field nuclear magnetic resonance of a nematic liquid crystal

The molecular order parameter of CH/sub 2/Cl/sub 2/ in a nematic liquid crystal was measured by using a version of zero-field NMR employing pulsed dc magnetic fields. Spectral frequencies and intensities are shown to reflect the ordering on a molecular and macroscopic scale, respectively. Samples oriented in high magnetic field did not significantly change their state of alignment during the time scale of the field cycle. Zero-field measurements of the order parameter yielded values within experimental error of those measured in high field. A zero-field echo experiment was performed to decrease the effect of residual fields on line width. Dipolar order was created in zero field by using a pulsed dc field analogue of the Jeener-Brokaert experiment. 26 references, 8 figures.

Zero field NMR and NQR with selective pulses and indirect detection

Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo- and heteronuclear systems are presented as an illustration of these techniques.