AC conductivity

Pulse irradiation of a liquid sample produces electrons and positive ions which lead to an increase in conductivity. DC or AC methods can be used to measure this induced conductivity. The case of DC measurements is discussed in detail in Section 2.5.1. The conductivity produced by electrons and positive ions decreases in time as 

The amplitude of the current density at the end of the radiation pulse is proportional to the electron mobility. DC measurements require suitable conductivity cells with the cathode and anode in contact with the liquid (see Section 2.12). Conductivity measurements by microwaves eliminate the necessity of electrodes. 

When microwaves propagate through a weakly conducting liquid, attenuation of their amplitude occurs. Electrons and ions present in the liquid are accelerated by the electric field vector of the wave at the same time they are decelerated by their interaction with the atoms or molecules of the liquid. The following considerations apply to electrons as well as to ions. The equation of motion of a charge carrier is given as 

M is the mass of the charge carrier, v is its velocity in the direction of the electric field Ex, and v is the frequency of collisions which randomize the x-component of the momentum. It is also called momentum relaxation frequency. 

If a constant electric field, E, is suddenly applied, a charge carrier will acquire a velocity v, given as 

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The electrical (AC) conductivity for a single configuration of each system is obtained using the Kubo-Greenwood formula [8] ... 

The electrical DC and AC response of compacts of ligand-stabilized nanoclusters also reflects the electrical behavior of the 3D system [21]. At high temperature, i.e. several tens of Kelvin below room temperature, the temperature dependent DC and AC conductivity follows a simply activated behavior according to the... 

The most important non-faradaic methods are conductometric analysis and (normal) potentiometric analysis in the former we have to deal essentially with the ionics and in the latter mainly with the electrodics. Strictly, one should assign a separate position to high-frequency analysis, where not so much the ionic conductance but rather the dielectric and/or diamagnetic properties of the solution are playing a role. Nevertheless, we shall still consider this techniques as a special form of conductometry, because the capacitive and inductive properties of the solution show up versus high-frequency as a kind of AC resistance (impedance) and, therefore, as far as its reciprocal is concerned, as a kind of AC conductance. 

Figure 30. Frequency dependence of AC conductivities of octa-alkyl Pc derivatives in the films.

The macroscopic dielectric constant of PS is very sensitive to the frequency range investigated. For very high frequencies the dielectric constant can be obtained by optical measurements. In this regime the effective medium approximation has been found to be an adequate description [Th3]. For lower frequencies the dielectric constant e is commonly obtained by capacitance measurements of metal/PS/ Si structures. AC conductivity measurements of such structures indicate a hop-... 

Kilbride et al. (2002 Andriotis et al., 2003) measured the alternating current (ac) and direct current (dc) conductivities of polymer-SWCNT composite thin films such as PMPV and polyvinylalcohol (PVA), the result showed that the ac conductivity... 

The ac conductivity spectrum local motions and long range conduction... 

In materials that have high ionic conductivity, effects such as the above are undoubtedly very important. They show up particularly in materials that have a high concentration of mobile ions and in experimental values of the ac conductivity measured as a function of frequency. In materials with a high carrier concentration, mobile ions are inevitably quite close together, separated by at most a few angstroms. Consequently, ions cannot hop in isolation but are influenced by the distribution of mobile ions in their vicinity. This contrasts with the behaviour of dilute defect systems with low carrier concentrations. In these, the mobile ions are well separated from each other and their conduction can largely be treated in terms of isolated hops. 

A variety of techniques has been developed to measure the condition of a coating so that some evaluation of its protective ability can be made. Many of these are based on electrochemical measurements [2]. The four techniques used in this study are (1) corrosion potential, (2) AC conductance, (3) tensile adhesion, and (4) weight gain. 

The specific AC conductivity values show a generally decreasing value with decreasing substrate corrosion, with one exception the novolac epoxy cured with an aromatic/cycloaliphatic amine. This is one of the coatings that also did not fit into the trend for the corrosion potential values. 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

Fig. 4.1. Ac-conductivity and capacitance as a function of frequency for three different temperatures...

The effects of carbon-based nanofillers of EG, MWCNTs, and CNFs on the AC conductivity and dielectric constant of elastomeric grade EVA (50% vinyl acetate content) at a particular frequency of 12 Hz, are shown in Fig. 29a, b [194]. EVA-EG, EVA-T, and EVA-F represent EVA-based nanocomposites reinforced with EG, MWCNT, and CNF respectively. 

Unfortunately, other experimental factors, such as contact capacitance at the junction of the cell leads and the measurement system, lead capacitance, and capacitance due to the dielectric properties of the thermostatting medium, may contribute substantially to the parallel capacitance. These effects may be minimized by proper choice of cell design and use of oil rather than water in the thermostatting bath. The art of making ac conductance measurements has been refined to a high degree of precision and accuracy, and detailed discussions of the rather elaborate procedures that are often necessary are available [9,10]. 

Janata E (1992b) Instrumentation of kinetic spectroscopy. 10. A modular data acquisition system for laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 40 437-443 Janata E, Lilie J, Martin M (1993) Instrumentation of kinetic spectroscopy. 11. An apparatus for AC-conductivity measurements in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 43 353-356... 

The timescale over which the conductance of the medium changes is a fundamental issue standard conductivity cells are designed for use with alternating current (AC), but the period of this current imposes a limit on rates of reactions that can be followed. To investigate reactions faster than the AC conductivity cell can handle, it is necessary to build and calibrate appropriate direct current (DC) conductivity cells, which is not a routine business. Conductivity meters that record continuously are uncommon. Nowadays, however, it is easy to interface a simple apparatus to a computer and collect the data with ad hoc software. 

The voltage responsivity of the detector shown in Figure 11.1 is simply derived from the pyroelectric current ip and the electrical admittance Y presented to it. Ignoring for the moment the ac conductance of the pyroelectric element ... 

How can an increase in the ac frequency in AC conductivity detection prevent the faradaic reaction of electrolysis from occurring (3 marks) [208]... 

Separation capillary columns are made in fluorinated ethylene- propylene copolymer. Detection is achieved by conductivity cells and an ac. conductivity mode of detection is used for making the separations visible. The driving current is supphed by a unit enabling independent currents to be preselected for the preseparation and final analytical stages. The run of the analyser is controlled by a programmable timing and control unit. The zone lengths from the conductivity detector, evaluated electronically, can be printed on a line printer. 

The separation unit of the capillary isotachophoresis instrument used is shown in Fig. 13.1. A 0.85mm id capillary tube made of fluorinated ethylene propylene copolymer was used in the pre-separation (first) stage and a capillary tube of 0.30mm id made of the same material served for the separation in the second stage. Both tubes were provided with conductivity detection cells [18] and an ac conductivity mode of detection [15] was used for making the separations visible. 

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