Impedance bridge

Let us consider a conductive material with the conductivity a, in which have been machined relatively small discontinuities shaped like dead hole. The surface has been inspected with a transducer with orthogonal coils connected in an impedance bridge [11]. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

The impedance can be measured in two ways. Figure 5.23 shows an impedance bridge adapted for measuring the electrode impedance in a potentiostatic circuit. This device yields results that can be evaluated up to a frequency of 30 kHz. It is also useful for measuring the differential capacity of the electrode (Section 4.4). A phase-sensitive detector provides better results and yields (mostly automatically) the current amplitude and the phase angle directly without compensation. 

However, the Cole-Cole plot usually makes calculations (done automatically within the impedance bridge concerned) of the two components into which any measured impedance can be resolved. The components consist of the real part of the impedance, which is in phase with the applied voltage, and the imaginary" part, which is 90° out of phase. One can plot either of these quantities against log to to obtain mechanism-indicating plots. 

Fig. 16. Modern set-up for an impedance bridge with controlled d.c. mean potential [55]. The stabilizing condensers Cj and C2 do not disturb the a.c. characteristics of the bridge.

Figure 5.3 Classical impedance bridge for a two-electrode cell.

Experimental methods are applicable for a wide range of frequencies. High-frequency measurements employ commercially available dielectric constant meters, Q-meters, and so on the impedance bridge method is widely employed at low frequencies. The levels of the frequencies applied experimentally are very important for data interpretation and comparison. 

Impedance Bridge Version of the Stern-Geary Approach... 

Conductivity Measurements. Cell resistance measurements were made with a General Radio type 1650-A impedance bridge. It is equipped with an internal, 1000-cycle signal source and tuned null detector. For more sensitive balance at high resistances, a Hewlett Packard 400L vacuum tube voltmeter is used as an external null detector. 

Ca(N03)2-KN03 (CKN) is a well-known molten salt that easily vitrifies upon cooling. An attempt to ascertain the fragility of this system was made on a CKN sample with a glass transition temperature of350 K. This sample was heated up to 390 K and its dielectric relaxation time measured by an impedance bridge as 10 s. Classify this ionic liquid. (Xu)... 

This operation determines the values of R and C that, in series, behave as the cell does at the measurement frequency. The impedance is measured as a function of the frequency of the ac source. The technique where the cell or electrode impedance is plotted V5. frequency is called electrochemical impedance spectroscopy (EIS). In modem practice, the impedance is usually measured with lock-in amplifiers or frequency-response analyzers, which are faster and more convenient than impedance bridges. Such approaches are introduced in Section 10.8. The job of theory is to interpret the equivalent resistance and capacitance values in terms of interfacial phenomena. The mean potential of the working electrode (the dc potential ) is simply the equilibrium potential determined by the ratio of oxidized and reduced forms of the couple. Measurements can be made at other potentials by preparing additional solutions with different concentration ratios. The faradaic impedance method, including EIS, is capable of high precision and is frequently used for the evaluation of heterogeneous charge-transfer parameters and for studies of double-layer structure. 

Fig. 27. Desorption spectrometer using 10 ke bridge (6). Rlt Rt, Rt are part of standard impedance bridge C = 30 p/, Ot = 0-100 pp/.

Electrical Measurements. Both ac and dc conductivity measurements were made on elemental sulfur and P4S3. The ac measurements were made with an Electro Scientific Industries model 290A impedance bridge in conjunction with a model 860A generator/detector at frequencies of 0.1, 1.0, and 10.0 kHz. A compensating variable capacitor was used either in series or in parallel with the variable resistance arm of the bridge. The... 

Some physical effects such as moisture absorption [49], temperature [50], protonation level [50], electron localization (polyorthotoluidine) [5la,b], polyaniline [52], crystallinity [53], elongation [54a,b,55] on microwave properties of polyaniline have been found through characterisation with the perturbation cavity method [56], Other methods have been employed and compared with the former one, such as microwave impedance bridge in the X-band (8.2-12.4 GHz) [57] or APC 7 standard (130 MHz-18 GHz) [47] with good agreement. 

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