Impedance interpretation

An ultrasonic experiment consists of two stages measurement of the ultrasonic properties of the material, e.g., velocity, attenuation or impedance interpretation of these measurements to provide information about the relevant properties of the material. These may either be fundamental physico-chemical properties (such as composition, microstructure or molecular interactions) or functional properties (such as stability, rheology or appearance). 

Figure 4.4.2. Plots of 1/1 and 1// , as measured using SACV for 90 10 Cu Ni in flowing seawater. Flow velocity = 1.62m/s, [O2] = 0.045 mg/1, T = 26°C, exposure time = 50h. (From D. D. Macdonald, An Impedance Interpretation of Small Amplitude Cyclic Voltammetry 1. Theoretical Analysis for a Resistive-Capacitive System, J. Electrochem. Soc., 125, 1443-1449 [1978]. Reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

Despite great success in ES cells, Cre-mediated excision in transgemcs may be incomplete (30). This likely depends on the uniformity of Cre expression and the strength of the promoter used. Although in certain cases a mosaic knockout may not impede interpretation of the results, it is usually desirable to effect complete ablation in a targeted population. For this reason, tissue-specific in vivo knockout experiments using the Cre-/ox system should be approached cautiously. 

The first system called LiSSA has been developed for interpretation of data from eddy-current inspection of heat exchangers. The data that has to be interpreted consists of a complex impedance signal which can be absolute and/or differential and may be acquired in several frequencies. The interpretation of data is done on the basis of the plot of the signal in the impedance plane the type of defect and/or construction is inferred from the signal shape, the depth from the phase, and the volume is roughly proportional to the signal amplitude. 

Interpretation of the impedance signal for set 2 (imaginary part function of the probe position along the slot)... 

Scully J R, Silverman D C and Kendig M W (eds) 1993 Electrochemical Impedance—Analysis and Interpretation (Philadelphia ASTM)... 

Electrochemical impedance. spectro.scopy, AC probe.s. EIS, although around since the 1960s, has primarily been a laboratory technique. Commercially available probes and monitoring systems that measure EIS are becoming more widely used, especially in plants that have on-staff corrosion experts to interpret the data or to train plant personnel to do so. 

Impact of a thin plate on a sample of interest which is, in turn, backed by a lower impedance window material leads to an interaction of waves which will carry an interior planar region into tension. Spall will ensue if tension exceeds the transient strength of the test sample. A velocity or stress history monitored at the interface indicated in Fig. 8.4 may look as indicated in Fig. 8.5. The velocity (stress) pull-back or undershoot carries information concerning the ability of the test material to support transient tensile stress and, with appropriate interpretation, can provide a reasonable measure of the spall strength of the material. 

Richardson, J. A. and Wood, G. C., The Interpretation of Impedance Charges on Oxide-coated Aluminium Produced by Immersion in Inhibitive and Corrosive Aqueous Media , J. Elecirochem. Soc., 120, 193 (1973)... 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

Developments in electrochemical methods since 1976 for measurement of corrosion have been rapid. Research and development has produced several new techniques, e.g. a.c. impedance and electrochemical noise. These methods require corrosion expertise for both operation and interpretation. Industry generally prefers instrumentation that can be operated by process... 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

Normally, the impedance plots are fitted to an often-complex equivalent circuit. Mathematically, this means searching for a global solution in R". However, problems arise if a complicated equivalent circuit is found which does not allow physical interpretation. Therefore, it is preferable to run a wide variety of experiments with different samples rather than trying to fit in detail the results of a single measurement in order to analyze the resulting impedance plots. 

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-... 

Impedance spectroscopy is best suited for the measurement of electronic conductivities in the range 10 -7to 10 2S cm 1.145 In principle, it is perhaps the best method for this range, but it is often difficult to interpret impedance data for conducting polymer films. The charge-transfer resistance can make measurements of bulk film resistances inaccurate,145 and it is often difficult to distinguish between the film s ionic and electronic resistances.144 This is even more of a problem with chronoamperometry146 and chronopotentiometry,147 so that these methods are best avoided. 

Whichever physical interpretation is chosen, the difference between the high-frequency real axis intercept [Z (high) and the low-frequency limiting real impedance [Z (low)] is one-third of the film s ionic resistance (i.e., R[ = 3[Z (low) - Z (high)]). Ideally, the real component of the... 

Figure 8 shows the COj (MW=45) response curve for the standard pump-probe experimental conditions. COj would be generated from either the combustion of 1,2 C2H4 or the product 1,2 C-vinyl acetate. Since the curve has a maximum at 4.5 seconds which is before the evolution of vinyl acetate at 5.5-7.5 seconds, ethylene combustion seems to be the pathway that can be attributed to the formation of this peak. Moreover, on interpretation of Figure 7, it is apparent that both the Pd-Au w/KOAc and the Pd w/KOAc catalysts proceed less CO2 than their Pd-Au and Pd counterparts. Therefore, it can be concluded that KOAc impeded the combustion of ethylene. 

In our opinion, the interesting photoresponses described by Dvorak et al. were incorrectly interpreted by the spurious definition of the photoinduced charge transfer impedance [157]. Formally, the impedance under illumination is determined by the AC admittance under constant illumination associated with a sinusoidal potential perturbation, i.e., under short-circuit conditions. From a simple phenomenological model, the dynamics of photoinduced charge transfer affect the charge distribution across the interface, thus according to the frequency of potential perturbation, the time constants associated with the various rate constants can be obtained [156,159-163]. It can be concluded from the magnitude of the photoeffects observed in the systems studied by Dvorak et al., that the impedance of the system is mostly determined by the time constant. 

The impedance data have been usually interpreted in terms of the Randles-type equivalent circuit, which consists of the parallel combination of the capacitance Zq of the ITIES and the faradaic impedances of the charge transfer reactions, with the solution resistance in series [15], cf. Fig. 6. While this is a convenient model in many cases, its limitations have to be always considered. First, it is necessary to justify the validity of the basic model assumption that the charging and faradaic currents are additive. Second, the conditions have to be analyzed, under which the measured impedance of the electrochemical cell can represent the impedance of the ITIES. 

In particular, the coupling between the ion transfer and ion adsorption process has serious consequences for the evaluation of the differential capacity or the kinetic parameters from the impedance data [55]. This is the case, e.g., of the interface between two immiscible electrolyte solutions each containing a transferable ion, which adsorbs specifically on both sides of the interface. In general, the separation of the real and the imaginary terms in the complex impedance of such an ITIES is not straightforward, and the interpretation of the impedance in terms of the Randles-type equivalent circuit is not appropriate [54]. More transparent expressions are obtained when the effect of either the potential difference or the ion concentration on the specific ion adsorption is negli-... 

In the end, analysis of vs 0 (the "classical" approach) is not quantitative, a problem we associate with surface area variability and ambiguity arising from the interpretation of P . To help determine which adsorption mechanism is operative, we turn to an alternative parameter, the IS impedance "time constant", T, which does not suffer from these drawbacks. 

Even if we forget, for a moment, the overlap problem and assume that we obtained a pure sensitized emission image, interpretation of this image is still ambiguous. That is because first, the intensity of S varies linearly with the excitation intensity and with the detector sensitivity. The exact same preparation will, when measured on a different microscope, yield different s.e. intensities. In fact, as much as renewing the arc lamp would impede comparison of results obtained on the same microscope. Second, the interpretation... 

The successful use of VIRS methods as research tools requires more than anything a published framework that will provide standards for the collection and interpretation of data, and accessible data libraries that can give examples for comparison and contrast. The spectral libraries used for mineral identification are in part public-domain, but much information remains tied to the instrument software and cannot readily be distributed. Differences in the formats used for spectral files in different libraries complicate their use in standard software and impedes information exchange. 

Even though the effect of moisture on the anode kinetics is well known, interpretation of experimental results on the effect of moisture can be tricky. As Nakagawa et al. [52] pointed out, the measurement of the total cell impedance under the OCV condition is not convincing since the reduction of polarization could as well be due to the availability of H20 for the cathodic reaction. In addition, the measurement of cell performance under the constant voltage or constant current conditions may also lead to wrong conclusions about the effect of water, because the addition of H20 will... 

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