Transport frequency response measurements

The results from EQCM studies on conducting polymer films can be ambiguous because the measured mass change results from a combination of independent ion transport, coupled ion transport (i.e., salt transport), and solvent transport. In addition, changes in the viscoelasticity of the films can cause apparent mass changes. The latter problem can be minimized by checking the frequency response of the EQCM,174 while the various mass transport components can be separated by careful data analysis.175,176... 

Impedance spectroscopy (IS) is a measurement of the conductive and dielectric properties of electroactive systems over a wide range of frequencies. Its popularity and applicability has increased dramatically over the past 25 years with the advent of fast-response potentiostats and frequency response analyzers. Impedance spectroscopy has been applied extensively in electrochemistry, especially in battery and sensor research, and it has been used to study active transport in biological membranes. Skin impedance has also been investigated with IS, but many of these studies attempted to correlate impedance with hydration and provided no insight into the mechanism of charge transport. More recent studies have used IS to elucidate the pathways of ion transport through skin, with special emphasis on understanding the mechanism... 

It is the purpose of this paper to study which mechanism, particle-to-particle or vapor phase transport, is responsible fa- the mobility of vanadium in the FCC unit The approach used in this work is to measure the rate of vanadium transport to a basic oxide "vanadium trap" in fluid bed experiments. By varying the particle size distribution, the collision frequency can be changed and the rate of transport determined. Also, calculations of the mass transfer of a vapor species in a fluid are performed. 

Photon absorption generally creates an electron-hole pair. If this occurs near a reasonably conductive junction, and an electrical field is present, then the pair will separate before they can recombine and generate a measurable current. The speed and frequency response of a device can be maximized by reducing dimensions and reverse biasing the device —reducing transport times. Primary deviee structures are p-n junctions, p-i-n structures and metal-semiconductor (Schottky barrier) junctions. Figure... 

Data interpretation is the most challenging aspect of impedance analysis and is often misunderstood and incorrectly performed. Interpretation of impedance data requires use of an appropriate model. Development of a proper model requires knowledge of the chemistry and physics of the system, some prior information about it, and a good understanding of the characteristics of the measured values. This is a difficult task that must be carried out very carefully. The most accurate model will combine all recent knowledge about relaxation processes, frequency dispersion, mass-transport, fractals, and redox processes and will have the same frequency response as the analyzed substance. 

Numerical solutions have been presented for the impedance response of semiconducting systems that accoimt for the coupled influence of transport and kinetic phenomena, see, e.g., Bonham and Orazem. Simplified electrical-circuit analogues have been developed to account for deep-level electronic states, and a graphical method has been used to facilitate interpretation of high-frequency measurements of capacitance. The simplified approaches are described in the following sections. 

One current-based approach is referred to as impedancemetric sensing [32]. This is based on impedance spectroscopy, in which a cyclic voltage is applied to the electrode and an analysis of the resultant electrical current is used to determine the electrode impedance. As different processes have different characteristic frequencies, impedance spectroscopy can be used to identify and separate contributions from different processes, such as electron transfer at the interface from solid-state electronic conduction. The frequency range ofthe applied voltage in impedancemetric sensors is selected so that the measured impedance is related to the electrode reaction, rather than to transport in the electrode or electrolyte material. Thus, the response is different from that in resistance-based sensors, which are related to changes in the electrical conductivity of a semiconducting material in response to changes in the gas composition. 

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