Materials impedance measurement

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

Fig. 14.21 (a) Polarization resistance (partly determined by the catalytic properties of the electrode) of cathode materials, as measured by impedance spectroscopy of symmetrical cells, (b) The area enclosed by the box (bottom left) represents the target area for low-temperature cathode development. 

Low frequency impedance measurements are very similar to thermoelectric power measurements in that impedance is also sensitive to changes in the electron bands of the steel because the addition of hydrogen results in changes in the material resistivity. Low frequency impedance measurements are resistivity measurements as a function of frequency (depth). Unlike thermoelectric power, low frequency impedance measurements push current (eddy currents) within the specimen. 

Thus, not surprisingly, numerous workers have knowingly (or unknowingly) observed this effect in polarization and impedance measure-ments. ° For materials having a... 

A quantitative analysis of grain boundary impedances measured with macroscopic electrodes can be rather problematic if grain boundary properties vary from boundary to boundary (cf. Sec. 3.2). Hence, additional information on the distribution of grain boundary resistivities is often desired. Microelectrode measurements can yield such additional information (Sec. 4.2) and below a microcontact impedance spectroscopic study of grain boundaries in a polycrystal is exemplarily presented. The material of choice is again SrTiCE (0.2 mol % Fe-doped), which represents a model material for the technologically highly important class of perovskite-type titanates (see also above). 

Like the ultrasonic velocity and attenuation coefficient, the acoustic impedance is a fundamental physical characteristic which depends on the composition and microstructure of the material concerned. Measurements of acoustic impedance can therefore be used to obtain valuable information about the properties of materials. 

We are faced with special problems if the volume of the sample, e.g. of a polymer, is varied by swelling. Sorption on such materials may be investigated by means of a horizontally arranged rotary pendulum [7,8] in combination with gravimetric density determinations [9]. Sophisticated pendulum experiments may be replaced by impedance measurements for the determination of the (frequency-dependent) dielectric constant [10]. 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

CoTi03 is a p-type semiconductor and was introduced by Chu et al. for the detection of ethanol.36 Lanthanum volume doped CoTi03 showed higher conductivity and better ethanol sensitivity of the material as shown in Siemons.29 The use of HT-IS enabled the analysis of surface doping with different amounts of Au, Ce, Pd, Pt, Rh, and Ru in order to find out the highest selectivity and sensitivity of the respective material composition. Measurements of CoTi03/La samples at different temperatures and under different atmospheres will be shown in order to illustrate the high reproducibility of the impedance measurements and sensitivities of identical samples on different positions. 

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