Nonlinear Electrical Effects Research Articles

Frequency and current analysis of non-linear electrical effects in mineralised rocks

Frequency and current analysis of non-linear electrical effects in mineralised rocks

Laboratory Confirmation of Non-Linear Electrical Effects in Mineralised Rocks

Induced Polarisation (IP) and Transient Electromagnetism (TEM) are among the most widely used ground electrical geophysics methods in Australia, being specialised in detecting conductive and chargeable targets such as sulphide deposits. A key challenge of IP T often a chargeable body is not a sulphide deposit but a graphite-rich formation of black shale or clays. As these shale deposits are undesirable targets, distinguishing by physical methods such as drilling is exhaustive on exploration resources. Graphite-rich shales, which plague induced polarisation surveys, cannot be distinguished from sulphide mineralisatio by existing electrical methods. The effect of disseminated sulphide grains acting as a diode in the presence of two electrical signals of nonharmonically related frequencies was originally studied in the 1970s. There is some evidence that shales produce a strictly linear response. This contrasts to the non-linear response of sulphide-rich rocks which generates additional signals in the frequency domain. A laboratory-based reinvestigation into non-linear properties of sulphides has been undertaken with a view towards field application. Variable frequencies and signal strengths were used to validate the method, based initially on the processes described in White (1974). The intermodulation mixing terms produced as a response by sulphide mineralised deposits in this laboratory-based investigation may allow for electrical distinction of sulphides from non-mineralised shales and clays. The application of a direct current (DC) bias to either signal increased the detectability of non-linear responses, allowing for a lower current density threshold for measurement. The magnitude of the non-linear signal detected provides a basis for field testing of this previously unutilised electrical property, with the eventual aim to providing a method for greatly reducing the number of drill targets barren of mineralisation.

Inelastic electron-phonon scattering and excess current in superconducting point contacts with a short coherence length

Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics (I-V curves) and their derivatives for ErNi2B2C point contacts (PC) with d ≥ ξ (where d is the diameter of the PC and ξ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in dV/dI coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For “dirty” NbSe2 and Nb point-contacts with d ≥ ξ, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.

The electroviscous force between charged particles: beyond the thin-double-layer approximation

We have investigated the hydrodynamic drag force between charged particles in electrolyte solutions, specifically the electroviscous force that arises from the distortion of the electrical double layers by the flow field. We report an improvement on the thin-double-layer theory (S.G. Bike, D.C. Prieve, J. Colloid Interface Sci. 136 (1990) 95–112), using a more accurate boundary condition for the radial charge current. The differences become important when the double layers start to overlap. We have found that nonlinear hydrodynamic effects are small, whereas nonlinear electric effects can be significant, in some instances leading to qualitatively different behavior. If the ion diffusivities are highly asymmetric, the electroviscous force can be reduced by an order of magnitude when there is an excess of the mobile ions in the double layer. The common supposition that there are substantial differences in the electroviscous force predicted by constant-charge and constant-potential boundary conditions is incorrect; our calculations show that it is an artifact introduced by the Debye–Hückel approximation.

Non-linear electrical effects in epitaxial graded-gap Cd xHg 1− xTe layers

A gradient of electron (hole) effective masses creates in semiconductors an internal electric field of frequency twice that of the applied field. The internal field is parallel to this gradient and is perpendicular to the variable applied field. This “warm carrier” effect has been measured in epitaxial Cd x Hg 1− x Te layers of various molar compositions. Measurements were performed at 77 and 300 K, with the frequency of applied field in the range 20 Hz to 12.5 kHz. A comparison between the simple model and the observed experimental data is discussed.

Studies of nonlinear electrical effects of model membranes.

The structure of lipid bilayer membranes in investigated by analysis of the harmonics generated by electrical ac excitation with dc bias. For the detection of nonlinear effects this method is complementary to current relaxation experiments and may be suitable for gating current measurements at nerve membranes.

Nonlinear Electrical Effects in Lipid Bilayer Membranes: III. The Dissociation Field Effect

In the course of an analysis of nonlinear electrical effects in lipid bilayer membranes, the influence of the dissociation field (or Wien) effect on the membrane conductivity is investigated. It is shown that the theory of Onsager for the Wien effect in a macroscopic phase can be applied to a thin membrane when the proper boundary conditions at the membrane-solution interface are introduced. It is assumed that an activation energy is associated with the passage of the ion across the interface. The mathematical treatment of the model is restricted to the case for which cations and anions have identical properties except for the charge sign. The resulting differential equations for the ion concentration within the membrane are integrated numerically. The analysis shows that the influence of the Wien effect on the membrane conductivity is appreciable only if the energy barrier at the interface is sufficiently high, i.e. if the rate limiting step for the ion transport is the passage of the ion across the interface.

Nonlinear Electrical Effects and Recombination of the Hot Electrons in Compensated Germanium

AbstractSteady state and transient current‐voltage characteristics are studied for Cu‐doped n‐type Ge samples with the upper Cu level (Ec — 0.26) eV partly compensated. The field strength is varied from ∼ 1 V/cm to 104 V/cm. The I‐U curves obtained are sublinear at field strengths above ∼ 100 V/cm. A study is made of the effect of temperature and of the spectrum of the incident light upon both the steady state and the kinetics of the photocurrent. Negative differential conductivity and coherent low frequency oscillations are observed at nitrogen temperatures with high illumination. The nonlinear effects are shown to be due mainly to the reduction of electron concentration in the conduction band. This could be ascribed to the field dependence of α the probability of electron capture by the doubly charged Cu ions. In the strong electric fields the crystals are becoming inhomogeneous due to the formation of strong‐ and weak‐field regions. The possible influence of these inhomogenities on the nonlinear effects is discussed and the change of α in the strong field is estimated.