Resistance-capacitance relaxation

About 30 individual grain boundaries, i.e. 30 different electrode configurations were investigated by microcontact impedance spectroscopy. The resulting histograms of the resistance, capacitance and relaxation frequency obtained from an equivalent circuit fit of the low-frequency arc are shown in Fig. 38. For comparison, a conventional (macroscopic) impedance measurement was performed on an identically prepared sample. The relaxation frequency of the grain boundary semicircle is indicated in Fig. 38c by a solid line. 

The role of the RC (resistive-capacitative) characteristics of the inert support elements in the thin film can be appreciated by the following experiment. Similar oriented BR thin films were deposited on Teflon films of various thicknesses (6.35, 12.7, 25.4 /zm). The relaxation time course varies with the thickness of the Teflon support, but the effect is predictable by the equivalent circuit. Figure 10.3(C) shows that the time course for the thin films of 12.7 and 25.4 fj.m thickness can be predicted by the equivalent-circuit analysis based solely on data obtained at the 6.35 /U,m thin film [10]. As expected, deconvolution of either the... 

The revelation that Cp is indeed a novel capacitance requires the inclusion of two capacitances in the equivalent circuit shown in Figure 2 (36, 37). In essence, with the exception of a genuine short-circuit measurement, the macroscopically measured photosignal is a manifestation of the interaction of two circuits in parallel One circuit represents the photochemical event associated with bacteriorhodopsin, and the other circuit represents the resistance-capacitance (RC) relaxation event of the inert supporting structure —the membrane dielectric. 

The internal cell resistance is approximately the sum of / , Rc i, and Rah- The values of the resistance were determined by using the complex non-linear least squares (CNLS) fitting of the impedance spectra to the equivalent circuit of Figure 10(b). As a matter of fact, whatever models one selects for the impedance spectra in Figure 10(a), the values of total internal cell resistance and relaxation time necessary for charge/discharge of all the capacitive elements remain constant (see below). 

Fig.2 Schematic complex impedance spectrum of the stabilized zirconia. R, C, and x mean the resistance, capacitance and relaxation time constant, respectively. From the low frequency, subscripts ep , gb , and gi mean the electrode polarization, grain boundary and grain interior, respectively.

The impedance analysis of the dry paint revealed two prominent capacitive relaxations—at 10 kHz (with a corresponding capacitance of 50 pF and resistance 20 kohm) and a 300 kohm impedance relaxation at 10 Hz [9]. The... 

The Maxwell and Voigt models of the last two sections have been investigated in all sorts of combinations. For our purposes, it is sufficient that they provide us with a way of thinking about relaxation and creep experiments. Probably one of the reasons that the various combinations of springs and dash-pots have been so popular as a way of representing viscoelastic phenomena is the fact that simple and direct comparison is possible between mechanical and electrical networks, as shown in Table 3.3. In this parallel, the compliance of a spring is equivalent to the capacitance of a condenser and the viscosity of a dashpot is equivalent to the resistance of a resistor. The analogy is complete... 

For given values of double layer capacitance solution resistance Rjj and Warburg coefficient a, plots of -Z versus Z have been made for selected values of charge transfer resistance. Ret (26). It is observed that at smaller values of R t ("10 S2 cm ) relaxation due to Rct dl Warburg diffusion behavior are both clearly seen. 

Direct vasodilators, which reduce pressure by relaxing vascular smooth muscle, thus dilating resistance vessels and—to varying degrees—increasing capacitance as well. 

Blood pressure lowering by clonidine results from reduction of cardiac output due to decreased heart rate and relaxation of capacitance vessels, with a reduction in peripheral vascular resistance. 

Now, it has already been seen in Chapter 7 that one may write an equivalent circuit for a simple electrode/solution interface as given in Fig. 7.52, where Rsoln is the resistance of the solution RF is the Faradaic resistance and DL the double-layer capacitance then the relaxation time6 is given by ... 

Vascular smooth muscle tone is regulated by adrenoceptors consequently, catecholamines are important in controlling peripheral vascular resistance and venous capacitance. Alpha receptors increase arterial resistance, whereas 2 receptors promote smooth muscle relaxation. There are major differences in receptor types in the various vascular beds (Table 9-4). The skin vessels have predominantly receptors and constrict in the presence of epinephrine and norepinephrine, as do the splanchnic vessels. Vessels in skeletal muscle may constrict or dilate depending on whether ffor 13 receptors are activated. Consequently, the overall effects of a sympathomimetic drug on blood vessels depend on the relative activities of that drug at and 8receptors and the anatomic sites of the vessels affected. In addition, Di receptors promote vasodilation of renal, splanchnic, coronary, cerebral, and perhaps other resistance vessels. Activation of the Di receptors in the renal vasculature may play a major role in the natriuresis induced by pharmacologic administration of dopamine. 

The hypotensive action of guanethidine early in the course of therapy is associated with reduced cardiac output, due to bradycardia and relaxation of capacitance vessels. With long-term therapy, peripheral vascular resistance decreases. Compensatory sodium and water retention may be marked... 

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