Admittance Measurements

G Yu, Y Cao, C Zhang, Y Li, J Gao, and AJ Heeger, Complex admittance measurements of polymer light-emitting electrochemical cells ionic and electronic contributions, Appl. Phys. Lett., 73 111-113, 1998. 

This work attempts to model a semiconductor/molten salt electrolyte interphase, in the absence of illumination, in terms of its basic circuit elements. Measurement of the equivalent electrical properties has been achieved using a newly developed technique of automated admittance measurements and some progress has been made toward identification of the frequency dependent device components (1 ). The system chosen for studying the semiconductor/ molten salt interphase has the configuration n-GaAs/AlCl3 1-... 

Figure 1. Computer-controlled network analysis system for photoelectrochemical cell admittance measurements...

Figure 5. Variation of Mott-Schottky intercept with pCl for (100) orientation n-GaAs, 40°C. Circles denote intercept values from automated admittance measurements. Bars signify standard deviation of least-squares straight line.

Linear sweep voltammetry, capacitance-voltage and automated admittance measurements have been applied to characterize the n-GaAs/room temperature molten salt interphase. Semiconductor crystal orientation is shown to be an important factor in the manner in which chemical interactions with the electrolyte can influence the surface potentials. For example, the flat-band shift for (100) orientation was (2.3RT/F)V per pCl" unit compared to 2(2.3RT/F)V per pCl" for (111) orientation. The manner in which these interactions may be used to optimize cell performance is discussed. The equivalent parallel conductance method has been used to identify the circuit elements for the non-illum-inated semi conductor/electrolyte interphase. The utility of this... 

Nonfaradaic admittance, measurement of, see Dynamic capacity, measurement of... 

Another quantity also used in admittance measurements is the dissipation factor D, which is useful because it depends only on the phase of the admittance, not on its magnitude. The dissipation factor of a parallel R — Cx circuit is defined as... 

Microdielectrometry was introduced as a research method in 1981 14 and became commercially available in 1983 20). The microdielectrometry instrumentation combines the pair of field-effect transistors on the sensor chip (see Sect. 2.2.3) with external electronics to measure the transfer function H(co) of Eq. (2-18). Because the transistors on the sensor chip function as the input amplifier to the meter, cable admittance and shielding problems are greatly reduced. In addition, the use of a charge measurement rather than the admittance measurement allows the measurements to be made at arbitrarily low frequencies. As a matter of practice, reaction rates in cure studies limits the lowest useful frequency to about 0.1 Hz however, pre-cure or post-cure studies can be made to as low as 0.005 Hz. Finally, the differential connection used for the two transistors provides first-order cancellation of the effects of temperature and pressure on the transistor operation. The devices can be used for cure measurements to 300 °C, and at pressures to 200 psi. 

Admittance measurements were used extensively prior to the widespread use of impedance spectroscopy in the 1980s. Capacitance bridge methods are typically used, though this limits the lower bound on the measured frequency to several hundred hertz. Corrosion processes, whose time constants are normally measured at or below 1 Hz, cannot be directly interrogated with this method. 

The admittance response at 1 kHz has also been interpreted in terms of the behavior at residual defects in anodic films. This interpretation is based on electron optical characterization, which shows that anodic films contain a distribution of preexisting defects associated with substrate inclusions and mechanical flaws (96,102). In aggressive environments, pits nucleate from these defects and propagate into the metal substrate. In this model, pits are distinct from anodic film flaws, and both can contribute to the measured admittance. Measurements of anodic films exposed to chloride solutions showed that the dissipation factor increased with time, but the capacitance remained nearly constant. Under these conditions, pit propagation at a flaw led to a pit area increase, which increased the resistive component of the admittance, resulting in an increased dissipation factor, but no increase in the capacitance. Measurements at 100 kHz were reflective of the electric double layer and not the components of the oxide film. 

During exposure to chromate solutions, defects in the anodic film heal, and there is some thinning of the barrier layer. Under these conditions, the characteristic response in a 1 kHz admittance measurement is a slight increase in the capacitance and dissipation factor. An anodic film with an intentional defect will show an increase in capacitance due to flaw healing. 

R. S. Brown, F. E. C. Culick, and B. T. Zinn, Experimental Methods for Combustion Admittance Measurements , in Experimental Diagnostics in Combustion of Solids, vol. 63 of Progress in Astronautics and Aeronautics, T. L. Boggs and B. T. Zinn, eds.. New York American Institute of Aeronautics and Astronautics, 1978, 191-200. 

Single- frequency admittance measurements (Mott-Schottky plots)... 

Freshly cleaved mica substrates (Spruce Pine Mica Co., Spruce, NC) were vacuum-coated with an approximately 500 A thick layer of gold at one end this served as the reference/counter electrode contact. In all experiments the tip was laterally positioned approximately 1 mm from the Au contact and the imaging process was operated in the constant current mode. In ac-admittance measurements, a small ac voltage (10 mV peak-to-peak amplitude, 10 kHz) was superimposed on the dc bias and the admittance was measured with a phase-sensitive detector. If not otherwise mentioned, the mica substrate was heated with one small drop (—10 /xL) of a phosphate... 

Data Analysis. Complex admittance determinations were fitted by an admittance function (13, 14, 16) based on the linearized HH equations (I). Admittance measurements were made under steady-state conditions (see Figures 2 and 4). Series resistance (Rs), the access resistance between the two voltage electrodes and up to the inner and outer surfaces of the axon membrane was not removed from measurements. Instead Rs was included and determined in the fit of the steady-state admittance model to the data. The measured complex admittance, therefore, is... 

Rapid Complex Admittance Measurements. The voltage clamp system, chamber, and axial electrode techniques were described previously (10, 12). By superposing a repetitive small-amplitude (1-mV root mean square) Fourier-synthesized signal (8, 16, 19) onto large step clamps, a current response was acquired during voltage clamp pulses. Immediately... 

The advantage of this method is that the measurement can be obtained rapidly by admittance measurements at a single frequency. In principle, the relationship between the coverage and the capacitance is... 

Charbonnier and coworkers [53] and by Scbultze and coworkers [42] on tbe basis of admittance measurements. These methods are interesting as a complement to the open-circuit potential, but nevertheless are only qualitatively related to the extent of the phosphating reaction. 

FIGURE 10.15 Monopolar admittance measuring circuit with three skin surface electrodes. SC thickness is exaggerated. Current flow lines are shown as dashed lines. 

Figure 10.15 shows an example of a skin surface admittance measuring circuit. It is a quasi-mono-polar system with three electrodes M is the measuring electrode, R a potential recording reference electrode, and CC is a current-carrying control electrode. 

Human skin conductance is very dependent on SA. The SA instrumentation uses skin surface electrodes for unipolar admittance measurements (Tronstad et al. 2008). Small, portable, and multichannel loggers... 

EXPERIENCES OF AN ON-LINE FOURIER TRANSFORM FARADAIC ADMITTANCE MEASUREMENT (FT-FAM) SYSTEM BASED ON DIGITAL SIGNAL PROCESSORS... 

The theory and practical implementation of the Fourier Transform Faradaic Admittance Measurement technique was introduced and developed by Smith and coworkers.This elec-troanalytical technique now rests on a sound theoretical basis and has been proven in many experimental situations. In this method a multiple-frequency perturbation waveform is used and the cell admittance is calculated at each of these frequencies using the measured current and potential. Measurements over a substantial range of frequencies, at least over two orders of magnitude, are required before any conclusive interpretations can be made concerning the mechanism and kinetics of an electrochemical system. The main advantage of this Fourier transform approach is that the cell admittance can simultaneously by measured at a large number of frequencies. 

During an experiment it is possible to follow the real admittance (measured admittance... 

FOURIER TRANSFORM FARADAIC ADMITTANCE MEASUREMENTS (FT-FAM) A DESCRIPTION AND SOME APPLICATIONS... 

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