Impedance pores

Laminae of clay and clay drapes act as vertical or horizontal baffles or barriers to fluid flow and pressure communication. Dispersed days occupy pore space-which in a clean sand would be available for hydrocarbons. They may also obstruct pore throats, thus impeding fluid flow. Reservoir evaluation, is often complicated by the presence of clays. This is particularly true for the estimation of hydrocarbon saturation. 

Figure 5 presents the capacitance-frequency dependence from impedance spectroscopy measurements for CS48 and CS15 in acidic and organic medium. In the low frequency region (from ImHz to lOOmHz) nearly a complete penetration of the ions into the pores is allowed and the quite stable values indicate the domination of the capacitive behavior at the electro 1 ytc/carbon interface. All the curves show a typical drop of... 

FIGURE 2.22 (a) Impedance spectra for symmetrical cells prepared without (square) and with (circle) 40 vol% corn starch as pore former in 3% H20/H2 at 850°C. (From Primdahl, S. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 99(19) 793-802, 1999. Reproduced by permission of ECS-The Electrochemical Society.) (b) Influence of anode support porosity on the performance of cells at 800°C. (From Zhao, F. and Virkar, A.V., J. Power Sources, 141 79-95, 2005. Copyright by Elsevier, reproduced with permission.)... 

Pore-size-dependent conductances are assigned to individual pores and channels. Three possible types of bonds befween pores exist. The corresponding bond conductances—(T), and o X)—can be established straightforwardly. The model was extended toward calculation of the complex impedance of the membrane by assigning capacitances in parallel to conductances to individual pores. The probability distribution of bonds to have conductivify cr b, <7br/ or O, is... 

The impedance factor is strictly empirical, accounting primarily for the geometry of the soil pore network bnt also for ion exclusion by negative adsorption from narrow pores, and for the increased viscosity of water near charged surfaces. It is similar for all simple ions and molecules. It can be measured by following the self diffusion of a nonadsorbed ion, such as Cl , for which C = 0lCl and hence D =... 

It usually takes place close to the melting temperature of the polymer when the pores collapse turning the porous ionically conductive polymer film into a nonporous insulating layer between the electrodes. At this temperature there is a significant increase in cell impedance and passage of current through the cell is restricted. This prevents further electrochemical activity in the cell, thereby shutting the cell down before an explosion can occur. 

The shutdown property of separators is measured by measuring the impedance of a separator while the temperature is linearly increased. Figure 7 shows the actual measurement for Celgard 2325 membrane. The heating rate was around 60 °C/min, and the impedance was measured at 1 kHz. The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. A 1000-fold increase in impedance is necessary for the separator to stop thermal runaway in the battery. The drop in impedance corresponds to opening of the separator due to coalescence of the polymer and/or to penetration of the separator by the electrodes this phenomenon is... 

The lithium-ion cells have demonstrated power loss when aged and/or cycled at high temperatures. Norin et al." demonstrated that the separator is at least partly responsible for the power loss due to the intrinsic increase in its ionic resistance. They showed that impedance increased significantly upon cycling and/or aging of lithium-ion cells at elevated temperatures and that separators accounts for 15% of the total cell impedance rise. They later reported that the loss in ionic conductivity of the separator was due to blocking of the separator pores with the products formed due to electrolyte decomposition, which was significantly accelerated at elevated temperatures. 

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