Conductivity measurements impedance plot

Chapter 2 comprehensively describes the use of impedance spectroscopy (IS) in the characterization of commercial and novel membranes. This useful technique measures impedance plots in relevant eleetrolyte environments to determine electrical charge parameters sueh as resistance and eapacitance of a membrane. From these results, electrical/geometrical parameters for membranes or individual layers, such as conductivity, porosity, thickness, and dieleetric constant, can be obtained. Thus, the influence of modification processes ean be monitored and used to guide further strategies for improving the membrane separation processes. 

Figure 15. Complex plane impedance plots for polypyrrole at (A) 0.1, (B) -0.1, (C) -0.2, (D) -0.3, and (E) -0.4 V vs. Ag/AgCl in NaCl04(aq). The circled points are for a bare Pt electrode. Frequencies of selected points are marked in hertz. (Reprinted from X. Ren and P. O. Pickup, Impedance measurements of ionic conductivity as a probe of structure in electrochemi-cally deposited polypyrrole films, / Electmanal Chem. 396, 359-364, 1995, with kind permission from Elsevier Sciences S.A.)...

The resistivity was then measured for PSSA-PVDF membranes with the four probe technique at room temperature under fully hydrated conditions to more accurately determine the conductivity of the samples, and compare these values to the those reported in the literature. The complex impedance plot of a representative sample of PSSA-PVDF is shown in Fig. 1.67, which corresponds to a conductivity value of 54 mS cm . The conductivity of Nafion 117... 

The second technique involves using a four probe apparatus, similar to that described by Cahan and Wainright. The membrane sample is placed in an PTFE apparatus which is equipped with two platinum strips in contact with the film, as shown in Fig. 1.115. Two platinum electrodes in a fixed geometry (distance of 1.026 cm) were placed on the surface of the film to measure the membrane potential and capacitance. Conductivity measurements could be obtained by utilizing complex impedance plots, which employ a circuit diagram... 

Most studies of the electrical properties of HMFG report conductivity versus temperature measurements, either DC (see, e.g. Almeida and Mackenzie 1982) or AC with use of complex impedance plots extrapolated to zero frequency (see, e.g. Ravaine 1985). Arrhenius behavior was always observed for the electrical conductivity cr below Tg... 

Only the third question can be answered directly on the basis of simple a.c. measurements. When two semicircles are observed in the impedance plots (imaginary part vs. real part), they can often be assigned to bulk and grain boundary conduction paths. If only one semicircle is present... 

In addition to the impedance, other derived quantities, such as the dielectric modulus (M), the complex dielectric constant (e), or susceptibility (x), can be calculated from the IS measurements their interrelations have been tabulated elsewhere (Macdonald 1987). Complementary information on the dielectric response of a given system can be obtained from the different impedance plots and the related magnitudes. It is important to point out the different nature of these magnitudes extensive or sample geometry dependent in the case of impedance or admittance, and intensive or characteristic of homogeneous materials in the case of conductivity and the dielectric constant. 

Chambers and coworkers [72-74] also explored the properties of semiconductive sheets of polypyrrole treated paper and cloth, which they characterize as a parallel RC circuit. They too report that the capacitive part of the complex impedance of the sheets depends on the morphology of the poly pyrrole coating. Measured reflectivity plots for Salisbury screen (narrowband) and Jaumann (broadband) absorbers fabricated from sheets of polypyrrole-treated material are also presented in their article. The measured results agree well with their model calculations, indicating the potential utility of conductive polymer-treated fabrics in radar-absorbing structures. 

The calculated impedance curves for the honeycomb electrodes are shown in Fig. 19.1l(A-C) together with the experimental curves. Fig. 19.5 shows the equivalent circuit employed to simulate the impedance plots for the honeycomb / Pt electrodes. Table 19.8 summarizes the values of the fitting parameters and the average relative errors for the calculated curves. For 60 x 500 nm/Pt, the electrolyte conductivity in the pores of 80 mS cm h measured for methanol oxidation, decreased... 

To better understand the diffusion-limited school of thought mentioned above, it is worth digressing momentarily on another noble -metal electrode system silver on YSZ. Kleitz and co-workers conducted a series of studies of silver point-contact microelectrodes, made by solidifying small (200—2000 //m) silver droplets onto polished YSZ surfaces. Following in-situ fabrication, the impedance of these silver microelectrodes was measured as a function of T (600-800 °C), P02 (0.01-1.0 atm), and droplet radius. As an example. Figure 9a shows a Nyquist plot of the impedance under one set of conditions, which the authors resolve into two primary components, the largest (most resistive) occurring at very low frequency (0.01—0.1 Hz) and the second smaller component at moderately low frequency ( 10 Hz). 

Flash Rusting (Bulk Paint and "Wet" Film Studies). The moderate conductivity (50-100 ohm-cm) of the water borne paint formulations allowed both dc potentiodynamic and ac impedance studies of mild steel in the bulk paints to be measured. (Table I). AC impedance measurements at the potentiostatically controlled corrosion potentials indicated depressed semi-circles with a Warburg diffusion low frequency tail in the Nyquist plots (Figure 2). These measurements at 10, 30 and 60 minute exposure times, showed the presence of a reaction involving both charge transfer and mass transfer controlling processes. The charge transfer impedance 0 was readily obtained from extrapolation of the semi-circle to the real axis at low frequencies. The transfer impedance increased with exposure time in all cases. 

Bulk ionic conductivities of the samples were measured by impedance analysis between room temperature and 545°C. The extrapolated ionic conductivity could be as high as 0.14-0.16 S/cm at 800°C. This value is very promising, however, caution should be exercised in interpreting this extrapolated ionic conductivity value. A change in the slope of log(cjT) versus 1/T plot is suspected in the literature above 550°C. 

So far, the ionic conductivity of most ILs has been measured by the complex impedance method [116], In this method, charge transfer between carrier ions and electrode is not necessary. Therefore platinum and stainless steel are frequently used as blocking electrodes. However, it is often difficult to distinguish the resistance and dielectric properties from Nyquist plots obtained by the impedance measurement. In order to clarify this, additional measurements using non-blocking electrodes or DC polarization measurement are needed. 

Ac impedance measurements of the compound sintered at 700 °C were performed in air in the temperature range of 200-600 °C. The Arrhenius plots of the ionic conductivity are shown in Figure 4. The relationship between conductivity and the reciprocal of the absolute temperature is linear over the temperature range measured. 

Wang et al. (1992) developed a new technique for the measurement of electrical conductivity of fiuoride melts, which employs the principle of a Continuously Varying Cell Constant (CVCC) through a moving platinum disc electrode in a relatively large diameter capillary tube-type conductivity cell. At the same time, the real component of the circuit impedance, R a, at a fixed high-frequency current is measured. Since the Rm versus the cell constant plot is linear, the electrical conductivity of the electrolyte is given by the relation... 

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