In-Situ DC Conductivity

On some occasions it becomes imperative to determine the in-situ conductivity of a CP while it is actively undergoing a chemical or electrochemical transformation, for instance the conductivity as a ftmction of applied potential. Several, varied methods have been used by workers in the field for this purpose [391], all of which however incorporate the principles underlying Eq. (11.1). The Contractor group [392] used an indirect, 2-probe AC method to measure the DC conductivity of a P(ANi) film. 

They forced a 5 nA peak-to-peak AC current at 1.33 KHz across the two points, while simultaneously monitoring the in-phase voltage across the same two points using a two-phase lock in analyzer. 

The techniques used for measurement of AC conductivity are varied, and depend on the frequencies of interest. For v ca. 10 Hz, a capacitance-conductance bridge can be used [396]. EIS, which yields both real and imaginary components of the complex resistance, i.e. impedance, can be used up to ca. 1 GHz. The cavity perturbation and related techniques in microwave measurements have been used for frequencies up to ca. 30 GHz [397]. For higher frequencies, mathematical (Kramers-Kronig) transformation of IR Reflectance data must be used [398]. 

It cannot be proposed in a book such as the present to discuss common techniques such as EIS in detail reference is made to excellent monognq hs on the subject [399]. To summarize briefly, however, in a typical AC technique, a constant-amplitude (sine wave) current of a given frequency is applied across two electrodes connected via the CP alone, or the CP in an electrochemical cell or device. The voltage that results is monitored via a lock-in amplifier which detects the in-phase and 90 -out-of-phase impedance components, which correspond respectively to the resistance and capacitance of the CP sample. If an ElS-type equivalent circuit analysis is not required, this information, as a function of several frequencies, is usually all that is sought for AC conductivity. Microwave and IR measurements are discussed separately within this chapter or elsewhere in this book. 

We may cite representative EIS studies of CPs. Firstly, for reference. Fig. 11-6 shows a Cole-Cole (real vs. imaginary impedance, also called Nyquist) plot of an idealized, planar CP film electrode, also showing the equivalent circuit in a situation where the charge-transfer component of the impedance is negligibly small, the 

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Fig. 7.27. Two band electrode as used for in situ DC conductivity measurements with polymer films deposited on the gap between the two electrodes A cross section after finished preparation B top view after preparation C after embedding, before grinding [262]...

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