Contact impedance

C. Kleesattel, "The Ultrasonic Contact Impedance Testing Method," The ECHO, Vol. 27, Krautkramer GmbH. 

Rs (Figure 1.22a). The double layer capacitance is represented by the capacitance C, and Rs is the series resistance of the EDLC, also named the equivalent series resistance (ESR). This series resistance shows the nonideal behavior of the system. This resistance is the sum of various ohmic contributions that can be found in the system, such as the electrolyte resistance (ionic contribution), the contact resistance (between the carbon particles, at the current collector/carbon film interface), and the intrinsic resistance of the components (current collectors and carbon). Since the resistivity of the current collectors is low when A1 foils or grids are used, it is generally admitted that the main important contribution to the ESR is the electrolyte resistance (in the bulk and in the porosity of the electrode) and to a smaller extent the current collector/active film contact impedance [25,26], The Nyquist plot related to this simple RC circuit presented in Figure 1.22b shows a vertical line parallel to the imaginary axis. 

Figure 6. Cross-section (left) and schematic diagram illustrating charge transport through a chain of photoconductive pigment particles (A, B, C) dispersed in dielectric binder (right). Contact impedance and trapping at interfaces are critical to performance. (Reproduced, with permission, from Ref. 28. Copyright 1972,...

Theoretical work fnnction on electrical transport (tunneling) in metal-mole-cule-metal jnnctions indicates that junction impedance is significantly affected by the properties of the metal-molecule contacts. Specifically, the presence of barriers to electron (or hole) injection leads to drops in electrostatic potential at the metal-molecule interface, resulting in contact impedances. Various methods are currently available for probing the electrical conductance of discrete molecules or clusters of molecules bridged between metal or semiconductor electrodes to understand how the structure and electroiuc properties of molecules and their associated contacts affect the current-voltage (I V) characteristics observed for the junction [133,136,146-161]. 

From a manufacturer s data sheet the contact impedance data for low voltage MCCBs are -... 

Figure 1 Four-terminal equivalent circuit where / is the current source, internal impedance Z, i V. voltmeter, internal impedance Z, Zi , lead and contact impedances Z.simple impedances outside test region and Z. desired impedance.

Skin electrodes have the largest commercial product volume, most of them are pregelled ready-to-use nonsterile products. Some of them have a snap-action wire contact others are prewired, for instance, adapted for babies. There is a contact electrolyte between the skin and the electrode metal. Dry SC is a poor conductor and this easily results in poor (high impedance) contact and noise. The contact area with the skin should be as large as practically possible, and reducing the SCs thickness by sandpaper abrasion is useful. Hydrating the skin with contact electrolyte or by the covering effect of the electrode will usually reduce the contact impedance with time (minutes to hours). 

Ultrasonic Pulse Echo Contact Impedance Testing... 

The presence of a new feature may be qualitatively explained in terms of the three types of clusters mentioned above. At low frequency only clusters of type (i) and (ii) are active, while, at high frequency, the double layer capacitance reduces the magnitude of the contact impedance and type (iii) clusters progressively switch in , producing a second arc. The degree of overlap of the arcs depends on the volume fraction of the electrode phase, an effect for which Sunde provides a tentative explanation at low volume fractions, where the overall impedance is dominated by the electrolyte, type (iii) clusters have a significant effect and generate a well-... 

Figure 4.1.16. Impedance spectra of an electrode with two volume fractions, , of the electrode phase, for the case where the contact impedances contain only charge transfer resistance and double layer capacitance (a) (j) = 0 (single phase electrode) and (b) (j) = 0.3 (composite electrode). For details of the simulation see Sunde [2000]. The labels show the logarithm of the normalized frequency.

Although the gap model provides only a slight improvement over the continuum model, it still overestimates the resistivities. It is important to note that both the continuum and the gap models ignore not only the shunting effect of the electrodes and but also the contact impedances [1,59] which are produced by the electrochemical effect of the electrodeelectrolyte interface [1]. 

Unfortunately, the shunt model underestimates the resistivities since the contact impedances problems are still ignored [59]. 

To obtain better EIT results in a real subject, the complete electrode model is proposed. The complete electrode model works by taking into account both the shunting effect of the electrodes and the contact impedances for the electrode-electrolyte interface [59]. The complete electrode model consists of Equation 30.1 and the boundary conditions as follows ... 

A major problem is the anchoring of skin-surface electrodes to maintain reasonably constant contact-impedance and to prevent motion artifacts. Usual techniques involve elastic bands and adhesive tape. Conventional metal electrodes generally require the use of an electrode paste to maintain contact and reduce motional problems. Pasteless electrodes as described in Section 8.3 avoid most of the problems associated with conventional metallic electrodes, but they still must be held in place by a strap or belt. 

CEM the specific boundary conditions are imposed in the electrode areas (Fig. le) in contact with the scalp and, modifying the FEM linear system of equations. The CEM boundary conditions take into account the electrode to skin contact impedances (50KQ in this study). The details of the CEM can be found elsewhere [5],... 

The main project requirements are the frequency control and the eurrent amplitude to be injected into tissue, which is standardized by the IEC-60601 standard but it depends on the frequeney. However, the AD5933 inject a sinusoidal voltage with constant anplitude in a bipolar configuration, which is not a recommended method for BIA human applications. Therefore, it was developed two different circuits in a four-electrode configuration. This approach reduces the influence of the contact impedance of the electrodes and is not load dependent. Both circuits use a VCCS circuit, where the injected current has constant anplitude and it is supposed to be load independent. The first Analog Front-End (AFE-1) circuit uses a Load-in-the-loop VCCS and the seeond one (AFE-2) uses a mirrored modified Howland VCCS. 

Pt wires or Pt strips, designated as inner probes) are laid between the two outer Pt probes that serve as voltage sensors, as shown in Fig. 5.13 the two outer Pt strips serve as AC current injectors. In this configuration, the current is passed between the two outer Pt strips, and the conductance of the membrane is calculated from the AC potential difference between the two inner probes. This method is relatively insensitive to contact impedance at the current-carrying electrodes and is therefore well suited to membrane conductivity measurements. 

It is not unreasonable to expect that a normal force of attraction arises when an electrical potential is applied across the contacting surfaces which are separated by a dielectric. Some of the aforementioned results suggest that the contact impedance has a capacitive character. It is interesting to know the details of the model which give rise to an increase in the normal force. 

In order to illustrate the principal elements of the model, a simple electrical scheme is shown in Figure 6. The bulk of the specimen is taken to be purely resistive. The bulk resistance Rg is in series with a contact impedance. The latter is depicted as a resistance R parallel with a contact capacitance C. A further elaboration is necessary before undertaking the task of identifying... 

If the bulk impedance is ignored, the phase angle corresponding to the contact impedance is given by... 

The cell was placed in a thermo-controUed humidity chamber to measure the temperature and humidity dependence of proton conductivity. In this method, a fixed AC current is passed between two outer electrodes, and the conductance of the material is calculated from the ac potential difference observed between the two inner electrodes. The method is relatively insensitive to the contact impedance at the current-carrying electrode and is therefore weU suited for measuring proton conductivity. This open cell is also suited for studying the humidity dependence of conductivity for proton exchange membrane because of the low interfacial resistance. 

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