Ohmic Current Density Research Articles

Modeling of ionic charge density

Inclusion of the space charge depolarization in the model to calculate transport properties of a nano-composite material is investigated. Specifically, the effect of diffusion current density; displacement current density; ohmic current density and excess charge current density are included under an open circuit condition. The proposed model takes into account of the excess charge produced during the space charge polarization, and may be applied to any ionic material.

Simulation of the role of vibration on Scanning Vibrating Electrode Technique measurements close to a disc in plane

An elegant and accessible way to account for the local stirring created by the vibration of the SVET tip by adding a new diffusion–like term into the molar flux expression is proposed, in order to avoid solving the fluid flow. This term is maximal in the point of vibration and rapidly decreases with the distance. It is shown that the local mixing leads to a substantial increase of the migration current density in the vicinity of the probe with simultaneous decrease of the diffusion current contribution. This local mixing has no effect on the pH distribution, regardless the applied polarization, and increases under cathodic polarization the oxygen concentration only when the probe is close to the electrode surface which is confirmed by experimental observations. The proposed model is compared with the analytical current density distributions obtained from potential model and experimental data. All this indicates that local mixing might explain why the SVET technique, although based on the measurement of an ohmic current density, measures always the total current density.

Negative differential conductivity in thin ferroelectric films

A phenomenon of negative differential conductivity in ferroelectric thin films is discussed. We proposed that the reason is polarization recovery current arising at current-voltage I(V) measurements as a result of polarization relaxation after pre-polarization of ferroelectric film. Simulation of this current by Weibull distribution provides a good correlation with the experimental data. The obtained values of the recovered polarization Prec and the field strength Erec at which the recovery polarization current reaches maximum do not depend on the voltage sweep rate and are well correlated with the values of polarization relaxation Prel and coercive field strength Ec obtained from dielectric hysteresis loop. It is shown that the current density due to polarization recovery Jrec may exceed by about an order the ohmic current density J Ω in ferroelectric film at Ec .

Modeling of ionic diffusion by space charge depolarization

For the study of transport properties of a nano-composite material, an open circuit condition of current is considered, where the sum of diffusion current density, displacement current density, ohmic current density, and excess charge current density is vanished. Including the effects of drift and trapping of ions, drift and trapping diffusion coefficients are modeled respectively and the significance of the type of ionic motion in the study of ionic diffusion is discussed.

Bifluorene compounds containing carbazole and/or diphenylamine groups and their bipolar charge transport properties in organic light emitting devices

Novel bipolar bifluorene compounds containing carbazole and/or diphenylamine groups were synthesized by C–C and C–N coupling using a palladium catalyst. The ionization potentials of the compounds reflected the electron withdrawing or donating nature of the substituents. The charge transport properties of these compounds were evaluated from time-of-flight transient photocurrent measurements. Large transient currents (10 −4–10 −3 cm 2/Vs) based on holes and electrons were observed. Hole or electron-only devices containing these compounds with p-type doping with MoO 3 and n-type with Cs showed Ohmic current density–voltage characteristics. Organic light emitting devices with homo-junction structures containing the bifluorene compounds (ITO/bifluorene:MoO 3 (50 mol%, 20 nm)/bifluorene (10 nm)/bifluorene:rubrene (60 nm)/bifluorene (10 nm)/bifluorene:Cs (30 wt.%, 20 nm)/Al) exhibited external quantum efficiencies of 1.5–2.0%.

Global plasma oscillations in electron internal transport barriers in TCV

In the Tokamak à Configuration Variable (TCV) (Hofmann F et al1994 Plasma Phys. Control. Fusion 36 B277), global plasma oscillations have been discovered in fully non-inductively driven plasmas featuring electron internal transport barriers (ITB) with strong ECRH/ECCD. These oscillations are linked to the destabilization and stabilization of MHD modes near the foot of the ITB and can lead to large oscillations of the total plasma current and line-averaged density, among others. They are intrinsically related to the fact that ITBs have large pressure gradients in a region of low magnetic shear. Therefore, the ideal MHD limit is relatively low and infernal modes can be unstable. Depending on the proximity to the ideal limit, small crashes or resistive modes can appear which affect the time evolution of the discharge. Being near marginal stability, the modes can self-stabilize due to the modification of the pressure gradient and local q-profile. The plasma recovers good confinement, reverses shear and the ITB builds up, until a new MHD mode is destabilized. TCV results show that this cycling behaviour can be controlled by modifying the current density or the pressure profiles, either with Ohmic current density perturbation or by modifying the ECH/ECCD power. It is demonstrated that many observations such as q ⩾ 2 sawteeth, beta collapses, minor disruptions and oscillation regimes in ITBs can be assigned to the same physics origin: the proximity to the infernal mode stability limit.

Observation of Large-Scale Profile Change of Magnetic Field in a Low-Aspect Ratio Reversed Field Pinch

Observation of Large-Scale Profile Change of Magnetic Field in a Low-Aspect Ratio Reversed Field Pinch

Effects of the ohmic current on collective scattering spectra

We present a numerical and analytical study of the modifications induced in the collective scattering spectra by the ohmic current governing the equilibrium magnetic configuration in toroidal plasmas. The spectral density function is calculated assuming equilibrium distributions for the (bulk and impurity) ion species and a Spitzer-like distribution to describe the response of the electrons to the applied DC electric field. As expected, the spectral asymmetries can be non-negligibly enhanced in the region of the ion-acoustic frequency. They reach their maxima for tangential scattering geometries, where the magnetic effects on the spectra are negligible. This justifies the assumption of the non-magnetized spectra. The role of the spectral asymmetries in affecting the ion temperature measurements is investigated, and the possibility of inferring the local ohmic current density from the analysis of the low-frequency part of the spectra is verified. The additional effects due to the presence of impurities affecting the spectral shapes are also discussed under realistic conditions. Reference is made to the widely complementary FTU and JET tokamak plasmas, taken as examples of high- and medium-current density regimes, respectively. As a general result, mainly due to the absence of a true ion acoustic resonance in tokamak plasmas (related to a low electron to ion temperature ratio), the peculiarities of the high-current density regimes (high density, low impurity content, etc.) must be met for the current-induced asymmetry in the spectra to become measurable. The ion temperature measurements can be made unaffected by the asymmetries using a simple symmetrization procedure which applies in all cases. In conclusion, a theoretically motivated potential exists for a more detailed experimental investigation of the feasibility of current-density measurements in ohmic plasmas, based on collective scattering.

The general solution of the time-dependent Maxwell’s equations in an infinite medium with constant conductivity

We solve the time-dependent Maxwell’s equations in all generality for an infinite medium in which the electric and magnetic permittivities and the conductivity are constant. In addition to the Ohmic current density, we include the effects of the presence of a charge density distribution and an external current density submerged in the medium. The longitudinal component of the magnetic field is shown to be zero, and the longitudinal component of the electric field is the gradient of the Coulomb potential, as in the case of zero conductivity. The longitudinal component of the external current density is given in terms of the charge density, as a generalization of the equation of continuity. The transverse electromagnetic fields are solved in terms of the initial values of the transverse fields and the transverse part of the external current density. Riemann functions are introduced for the problem. A critical wavelength is found such that fields associated with wavelengths shorter than this critical wavelength attenuate more rapidly in time than fields associated with wavelengths longer than the critical wavelength. Moreover, the fields with the shorter wavelengths are superpositions of standing waves that have a damping factor independent of the wavelength, whereas the fields whose wavelengths are longer than the critical wavelength are superpositions of ‘static’ helical fields, except for the damping factor which does depend on the wavelength. Stratton’s travelling and ‘diffusive’ plane wave solutions appear as special cases of these fields. There appear to be implications in biology and naval communications, which are discussed briefly.

Unipolar current transport through metal-insulator-metal (MIM) structures, supplied with barrier contacts, in the diffusion limit

The diffusion-limited unipolar current density-voltage ( j− V) dependencies through semi-insulating materials supplied with two barrier contacts have been computed. Two cases are treated. In the first, the sum of the two quiescent barrier thicknesses is less than the specimen thickness. Here there are three successive j− V regimes: At low voltages ( less than V th= kT e ),j∞V , at medium voltages, j∞√ V, and at high voltages j∞ V L but much less in magnitude than the true ohmic current density. In the second case, the sum of the two quiescent barrier thicknesses exceeds the specimen thickness and the j∞ ρV does not develop; only the two linear regimes are obtained separated by a short transitional interval over which the current is generally sublinear. The analysis is meant to apply to the practical cases of wide bandgap, insulating materials supplied with either rectifying or poorly characterized electrodes and to emphasize that a j∞ V L regime may develop that can easily be misinterpreted as Ohm's Law.