Coating complex capacitance

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. 

Table I shows the details of surface treatment and coating along with the calcxilated values of total resistance R and effective capacitance C. For specimens with Initial mechanical surface preparation, the Nyqulst Impedance plot shows the characteristic semicircular behavior with a resistance of the order of 1800 n cm and a capacitance of about 40 yF cm. As different surface treatments are Incorporated on a sequential basis, the complex plane diagram shows a gradual evolution.

In this book, an explanation of capacitive behaviour in similar and comparable systems is not directly possible with constant-phase elements because such a comparison is only possible if n values are equal, particularly in the study of surfaces covered with polymer coatings where a unification of the envisaged parameters is necessary. The impedances measured match with a relatively large amount of samples, of which the structure can be complex, showing many sources of non-idealities (e.g. variations in thickness of the membrane, pore size and pore density42 7). A good indication if such non-idealities occur can be found in the values of n. If they are not comparable, non-idealities occur. 

It is often found that the double-layer capacitance or a coating capacitance does not behave like an ideal capacitor, experimentally manifested in the complex plane plot by a depressed semicircle whose center lies below the real axis. This behavior is usually attributed to some distribution (or dispersion) in some physical property of the system (e.g., the porous surface of the metal or the varying thickness or composition of a coating) and is modeled by the use of a constant phase element (CPE) [30]. 

Passive oscillator mode Impedance analysis of the forced oscillation of the quartz plate provides valuable information about the coating even if the active mode is not applicable anymore. For impedance analysis, a frequency generator is used to excite the crystal to a constraint vibration near resonance while monitoring the complex electrical impedance and admittance, respectively, dependent on the applied frequency (Figure 2B). For low load situations near resonance, an equivalent circuit with lumped elements - the so-called Butterworth—van-Dyke (BVD) circuit — can be applied to model the impedance data. The BVD circuit combines a parallel and series (motional branch) resonance circuit. The motional branch consists of an inductance Lq, a capacitance Cq, and a resistance Rq. An additional parallel capacitance Co arises primarily from the presence of the dielectric quartz material between the two surface electrodes (parallel plate capacitor) also containing parasitic contributions of the wiring and the crystal holder (Figure 2B). 

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. 

Figure 7-3 (author s unpublished experimental data) demoiKtrates the results of the impedance analysis for a medium composed of insulating-base silicon oil with characteristic resistance 10 ohm and capacitance 5 10" F with added 25% urea-coated barium titanyl oxalate conductive particles with characteristic resistance 10 ohm and capacitance 10" F at 25 °C. The pure base oil shows a single relaxation, and particle-containing samples show relaxations at high frequency due to the conductive particles and at low frequency due to the base fluid in the complex modulus plot. 

In physical sensors, such as pressure sensors and accelerometers, capacitance measurements are a common means of signal transduction. Such methods can also be used with chemical sensors34, however, they have not received extensive attention to date. For sensors applied to the liquid phase, this may be due to the complexities associated with the impedance of the double layer that forms in solution at a charged interface. This tends to make the sensor sensitive to changes in ionic strength, unless the time constant of the selective coating that is relevant to the analytical measurement is considerably different from that of the double layer. Further, many chemical reactions associated with a coating... 

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