Electrical impedance properties methods

Theoretical work fnnction on electrical transport (tunneling) in metal-mole-cule-metal jnnctions indicates that junction impedance is significantly affected by the properties of the metal-molecule contacts. Specifically, the presence of barriers to electron (or hole) injection leads to drops in electrostatic potential at the metal-molecule interface, resulting in contact impedances. Various methods are currently available for probing the electrical conductance of discrete molecules or clusters of molecules bridged between metal or semiconductor electrodes to understand how the structure and electroiuc properties of molecules and their associated contacts affect the current-voltage (I V) characteristics observed for the junction [133,136,146-161]. 

For the dynamic lung impedance model to be useable in Finite Difference Method or Finite Element Method impedance signal simulations, the dynamic tissue sample model is discretized into volume data. At first 3D data with 35 x 35 x 35 voxel resolution is prepared from each of the 40 time frames. This allows for easy import into MATLAB or COMSOL based calculation. The volume data includes percentage of blood vessels (blood) for each of the 35 X 35 X 35 X 40 voxels. It can readily be transformed into electric/dielectric properties for each voxel with tissue data available on the internet. But data can also be exported with arbitrary resolution depending on calculation-simulation requirements. The simulations are run separately for each of the 40 time-frames to get full frequency characteristic of impedance measurement across the tissue sample. Finally we can get 40 frequency characteristics—one for each time-frame and to see a dynamic electrical impedance signal on a certain frequency, we just need to plot the impedance value at the chosen frequency from the 40 time-frames. 

The a.c.-impedance method described in the previous chapter is often employed to characterize the electrical properties of solid proton conductors. The method gives a rapid answer to the question does the material under investigation conduct electricity and the method is not critical with respect to size, shape and quality of the sample used. 

For the investigation of the properties of BLMs, electrical methods have been applied at the very beginning. In addition to the CV technique, other methods such as electrical impedance spectroscopy (EIS) have been applied. Shortly after the discovery of the BLM system, Hanai and Hay don reported the thickness measurement of a planar lipid bilayer using the impedance technique [1 - 3]. Their results are in accord with the value obtained on RBC, estimated by Fricke (see Eq. 1). The impedance technique, nowadays also known as EIS, has subsequently used by many others. The basis of the technique is that a small alternating current (AC) of known frequency and amplitude is applied to the system (e.g. a BLM). The resulting amplitude and phase difference that develop across the BLM are monitored. For a BLM of cross-sectional area (A), and thickness the ability of the BLM to conduct and to store electrical charges are described by the following ... 

Modification of the electrical properties of PS by the intfoduction of SWCNTs was described by Wang et al. [199]. The SWCNT/PS nanocomposites with 0-1.0 wt% content of SWCNTs were successfully fabricated by an in-situ suspension polymerization method. DC resistivity and AC impedance measurements performed on the nanocomposites showed that the presence of SWCNTs significantly modifies the electrical impedance of the composites. For instance, for a loading of 1.0 wt% CNT, the resistivity value dropped by over ten orders of magnitude [199]. 

The impedance for the study of materials and electrochemical processes is of major importance. In principle, each property or external parameter that has an influence on the electrical conductivity of an electrochemical system can be studied by measurement of the impedance. The measured data can provide information for a pure phase, such as electrical conductivity, dielectrical constant or mobility of equilibrium concentration of charge carriers. In addition, parameters related to properties of the interface of a system can be studied in this way heterogeneous electron-transfer constants between ion and electron conductors, or capacity of the electrical double layer. In particular, measurement of the impedance is useful in those systems that cannot be studied with DC methods, e.g. because of the presence of a poor conductive surface coating. 

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

The measurement of electrical properties as a function of frequency and their analysis by complex impedance methods (impedance spectroscopy) allow a separation of contributions to impedance from grains, grain boundaries and electrode polarisation (Jonscher, 1983 MacDonald, 1987). This technique therefore permits the separation of the electrical... 

These inmitive concepts stand at the foundation of the electrical SHM methods for composite materials. This approach is deemed self-sensing because it relies entirely on measuring a material property (i.e., electrical characteristic) and does not require an additional transduction sensor the only instrumentation that needs to be installed on the composite structure consists of the electrodes. In the case of composite transport aircraft, the conductive screen skins currently used to mitigate lightning strike could potentially also serve as the measuring electrodes. Electrical SHM methods range from the simple measurement of the electrical resistance measurements up to more sophisticated methods such as electrical potential mapping, dielectric measurement, and electrochemical impedance. 

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