Capacitive element

Interior of capacitive element containing tantalum, tantalum pentoxide (dielectric), manganese dioxide (solid electroyte)... 

Electrode surfaces in elec trolytes generally possess a surface charge that is balanced by an ion accumulation in the adjacent solution, thus making the system electrically neutral. The first component is a double layer created by a charge difference between the electrode surface and the adjacent molecular layer in the flmd. Electrode surfaces may behave at any given frequency as a network of resistive and capacitive elements from which an elec trical impedance may be measured and analyzed. 

It is convenient to consider thermal systems as being analogous to electrical systems so that they contain both resistive and capacitive elements. 

Noting these uncertainties, we have evaluated the equivalent circuit and present the results in Fig. 3. The potential dependence of the three capacitive elements is shown in Figs. 

We have extended the technique of Relaxation Spectrum Analysis to cover the seven orders of magnitude of the experimentally available frequency range. This frequency range is required for a complete description of the equivalent circuit for our CdSe-polysulfide electrolyte cells. The fastest relaxing capacitive element is due to the fully ionized donor states. On the basis of their potential dependence exhibited in the cell data and their indicated absence in the preliminary measurements of the Au Schottky barriers on CdSe single crystals, the slower relaxing capacitive elements are tentatively associated with charge accumulation at the solid-liquid interface. 

The Relaxation Spectrum Analysis was carried out for a cell consisting of n-CdSe in a liquid junction configuration with NaOH/S=/S 1 1 1M as the electrolyte. Three parallel RC elements were identified for the equivalent circuit of this cell, and the fastest relaxing capacitive element obeys the Mott-Schottky relation. 

As already noted, the present study of dynamic fuel cell behavior involves the analysis of systems with capacitive elements. These elements control the rate at which process parameters change due to net forces imposed by other coupled process parameters. A general dynamic equation showing capacitance behavior is ... 

The goal of this chapter was to open the door to dynamic model analysis of fuel cells. Many of the key known capacitance elements in fuel cell operation were identified and model equations derived. Through the use of these models the following key issues for dynamic modeling have been identified ... 

This additional capacitive element (Cl)315 represents a chemical capacitance in the sense of Part I,2 Section VI.7 (see Figure 37). 

Most processes include some form of capacitance or storage capability. These capacitance elements can provide storage for materials (gas, liquid, or solids) or storage for energy (thermal, chemical, etc.). Thermal capacitance is directly analogous to electric capacitance and can be calculated by multiplying the mass of the object (W) with the specific heat of the material it is made of (Cp). The gas capacitance of a tank is constant and is analogous to electric capacitance. The liquid capacitance equals the cross-sectional area of the tank at the liquid surface, and if the cross-sectional area is constant, the capacitance of the process is also constant at any head. 

The tank circuit may be in the form of discrete capacitors or inductance coils but more usually it is of aluminium cavity construction, with the valve enclosed by inherent inductive and capacitative elements. The last-mentioned arrangement has the advantages of being partly self-screening and of minimizing the possibility of parasitic oscillations. 

Adsorption impedance — The current flowing in an electrochemical system splits into two parts at an interface the charge either transfers across, (-> faradaic current) or gets accumulated at the two sides of the boundary (- non-faradaic or - charging current) the related impedance elements are called - Faraday impedance and non-Faraday impedances, respectively. The latter element is an essentially capacitive element its lossy character is related to the slow kinetics of - adsorption- related processes involved. 

The resistor function is mostly nonlinear and approaches a constant value only in the vicinity of equilibrium. Combining Eqs. (14.5) and (14.38), the constitutive relation for the capacitative element C, is... 

Alternatively, an equally powerful visualization of impedance data involves Bode analysis. In this case, the magnitude of the impedance and the phase shift are plotted separately as functions of the frequency of the perturbation. This approach was developed to analyze electric circuits in terms of critical resistive and capacitive elements. A similar approach is taken in impedance spectroscopy, and impedance responses of materials are interpreted in terms of equivalent electric circuits. The individual components of the equivalent circuit are further interpreted in terms of phemonenological responses such as ionic conductivity, dielectric behavior, relaxation times, mobility, and diffusion. 

Impedance-frequency relationship to yield information on the resistive and capacitive elements of the current path " ... 

The EIS response depends on the flhn thickness and morphology, applied potential, and, obviously, the nature of the components of the hybrid system. The hydro-phobic nature of the polymer, the level of doping within the film, and the size of ions in contact with the polymer surface are factors to be considered for studying the response of such materials. In short, the kinetics of the overall charge transfer process should take into account (1) electron hopping between adjacent redox sites (Andrieux et al., 1986) usually described in terms of a Warburg diffusion impedance element (Nieto and Tucceri, 1996) and (2) double-layer charging at the metal-flhn interface, represented in terms of a double-layer capacitance element. 

Most silicon accelerometers are based on a micromachined variable capacitance element (g-cell) that is converted to a voltage using a C-V converter and then amplified, filtered, and buffered to provide an analog output as shown in Fig. 7.1.4. To date, open-loop implementations for capacitive read-out circuits are more widely employed than closed-loop systems, primarily as a result of the stability of such systems [16]. Interface electronics for micromachined sensors depend not only upon the transduction technique (input specification) and the product requirements (output specification) but also on the packaging approach, as parasit-ics are introduced when a multiple-die packaging technique is used. 

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