Temperature-dependent impedance

From measurement data gained at 90 K, the capacitance of the cluster was determined as 3.9xlO F. Temperature-dependent impedance measurements at the same cluster resulted in a very similar value [23]. 

The capacitance of the cluster was calculated from a fit of the experimental data at 90 K to be 3.9 x 10 F. This value, which is very sensitive toward residual charges and nearby background charges, is close to the value of the microscopic capacitance, which was determined earlier by temperature-dependent impedance measurements [21]. Furthermore these results are found to be in good agreement with the capacitance data obtained on the above-mentioned gold nanoclusters on a XYL-modified Au(l 1 1) surface [13,22]. 

The capacitance of the cluster/substrate-junction was calculated to be 3.9 x 10 F. This value is in agreement with the value of the cluster capacitance previously determined by temperature-dependent impedance measurements. These results are, furthermore, in good agreement with capacitance data obtained from self-assembled gold nanoparticles on a dithiol-modified Au surface, reported by Andres and coworkers.Here tunneling spectroscopy has been performed on 1.8 nm Au particles which were grown in the gas phase and a cluster-substrate capacitance of 1.7 X 10 F was obtained. Thus, the small capacitance enables the observation of Coulomb blockade phenomena at room temperature. 

For capacity measurements, several techniques are applicable. Impedance spectroscopy, lock-in technique or pulse measurements can be used, and the advantages and disadvantages of the various techniques are the same as for room temperature measurements. An important factor is the temperature dependent time constant of the system which shifts e.g. the capacitive branch in an impedance-frequency diagram with decreasing temperature to lower frequencies. Comparable changes with temperature are also observed in the potential transients due to galvanostatic pulses. 

The latter authors used anode and cathode symmetrical cells in EIS analysis in order to simplify the complication that often arises from asymmetrical half-cells so that the contributions from anode/ electrolyte and cathode/electrolyte interfaces could be isolated, and consequently, the temperature-dependences of these components could be established. This is an extension of their earlier work, in which the overall impedances of full lithium ion cells were studied and Ret was identified as the controlling factor. As Figure 68 shows, for each of the two interfaces, Ra dominates the overall impedance in the symmetrical cells as in a full lithium ion cell, indicating that, even at room temperature, the electrodic reaction kinetics at both the cathode and anode surfaces dictate the overall lithium ion chemistry. At lower temperature, this determining role of Ra becomes more pronounced, as Figure 69c shows, in which relative resistance , defined as the ratio of a certain resistance at a specific temperature to that at 20 °C, is used to compare the temperature-dependences of bulk resistance (i b), surface layer resistance Rsi), and i ct- For the convenience of comparison, the temperature-dependence of the ion conductivity measured for the bulk electrolyte is also included in Figure 69 as a benchmark. Apparently, both and Rsi vary with temperature at a similar pace to what ion conductivity adopts, as expected, but a significant deviation was observed in the temperature dependence of R below —10 °C. Thus, one... 

Microbial metabolism results in an increase in both conductance and capacitance causing a decrease in impedance and a consequent increase in admittance. In the Rapid Automated Bacterial Impedance Technique (RABIT) system, the admittance was plotted against time to provide results (Bolton, 1990). The final electrical signal is frequency- and temperature dependent and it has a conductive and a capacitive component. At present, impedance instruments are able to detect 10 —10 bacteria/ml (Ivnitski et ah, 2000). Several commercially available systems are operated... 

In particular, VF2/F3E copolymers have also been the subject of extensive research [6,17,96]. As an example to illustrate the dielectric behavior of these copolymers, the temperature dependence of the real and the imaginary part of the complex permittivity at two different frequencies (1 and 100 kHz) are shown in Figs. 23a and 23b respectively. The measurements correspond to the 60/40 copolymer. The data have been collected by using a sandwich geometry with gold evaporated electrodes [95]. Frequencies of 103 and 106 Hz have been used by employing a 4192 A HP Impedance Analyzer. From inspection of Fig. 23b... 

Dc, ac, impedance, and thermoelectric power of the compounds 33-38 in Fig. 9 have been investigated in detail. The measured temperature dependence of the thermoelectric power of 33-38 in thin film varied approximately exponentially with temperature. Compared to 38, the absolute value of the thermopower for the film of 34 is larger by nearly a factor of 3. The positive sign of Seebeck coefficient confirms that thin films of the compounds behave as a p-type semiconductor [46],... 

The capacitance and the series resistance have values which are not constant over the frequency spectrum. The performances may be determined with an impedance spectrum analyzer [70], To take into account the voltage, the temperature, and the frequency dependencies, a simple equivalent electrical circuit has been developed (Figure 11.10). It is a combination of de Levie frequency model and Zubieta voltage model with the addition of a function to consider the temperature dependency. 

Additional information with respect to the mechanism of the grain boundary resistance can be obtained from temperature- and voltage-dependent impedance measurements. The grain boundary semicircle varies, for example, considerably with the applied dc bias (Fig. 39a). The current-voltage relations calculated from such bias-dependent impedance measurements are thus non-linear. In the logarithmic plot (Fig. 39b) it can be seen that the low bias regime exhibits a non-linearity factor a (= d og(I/A)/d og(U/ V)) of almost one (ohmic behavior), while at a bias value of about 0.35 V this factor changes to a x 2. 

Figure 3.14. Percentage of cell individual voltage drop caused by charger transfer, membrane, and mass transfer resistances at different current densities and 80°C [21]. (Reproduced by permission of ECS—The Electrochemical Society, from Tang Y, Zhang J, Song C, Liu H, Zhang J, Wang H, Mackinnon S, Peckham T, Li J, McDermid S, Kozak P. Temperature dependent performance and in situ AC impedance of high-temperature PEM fuel cells using the Nafion-112 membrane.)...

The temperature dependence of molar conductivity, calculated from ionic conductivity determined from complex impedance measurements and molar concentrations, and the VFT fitting curves are shown Figure 5.8. The VFT equation for molar conductivity is... 

Figure 20.5 shows the temperature dependence of the ionic conductivity for zwitterion 10 with and without an equimolar amount of lithium salts. Neat zwitterion 10 shows low ionic conductivity of about 10 S cm at even 200°C from the ac impedance measurement. This is because there are no mobile ions in the system. However, zwitterion 10, which is mixed with an equimolar amount of lithium salt. 

The electrical properties of such different 3D systems are quite interesting, since they show characteristic differences. They have been studied by dc and ac complex impedance measurements. " At temperatures several tens of Kelvin below room temperature, the temperature dependence of both conductivities follows the Arrhenius relation... 

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