Measurements of dc conductivity

There are four experimental techniques that are commonly used to gain information about the electronic transport in a-Si H dc conductivity, the drift mobility, thermopower, and the Hall effect. Sections 7.1 and 7.2 describe these measurements and how the information about s E) and p( ) can be extracted, and are followed by a discussion of the theory of electronic conduction in Sections 7.3 and 7.4. 

The conductivity of a-Si H is usually thermally activated, at least over a limited temperature range and is described by 

The Meyer-Neldel relation is a statement that the temperature dependence data for different samples all intersect at the same value of conductivity, Og, at the temperature 7 x 600 K. There is still no completely satisfactory explanation of why this occurs. 

The most obvious mechanism by which the prefactor is different from the expected value is when the activation energy is temperature-dependent 

Therefore, within the linear approximation, the conductivity activation energy Eg measures the value of ( tr— )g and the prefactor contains the additional factor exp(y/k). The temperature dependence of the energies accounts for the Meyer-Neldel relation provided that 

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Electrical conduction in DNA has become a highly contentious subfield of its own. The uncertainties here are even greater than those in the chemical measurements of charge transfer in DNA, partly because of the greater strand length generally involved, and partly because of the necessity for making contacts in measurement of DC conduction. 

The apparatus used (16) permitted the measurement of dc conductivity In the plane of the film during pyrolysis. Samples were heated In vacuo at about 280 C for about three hours to drive off the DMF, and then coled to about 200°C. They were then reheated to about 400°C and maintained at that temperature for various lengths of time, cooled and reheated. Sample temperature and conductivity were monitored continuously throughout. 

TA Instninenlc DSC cell modifiecf for stmulUneou measuRments of dc conductance (33. (With the peimittionorElsevter.Amttefdam.)... 

As indicated in Eq. (13), the C2 parameters are determined by the nature of the epoxide oligomer or that of the host oligomer in the measurement of dc conduction, which is caused by the ion movement closely related to the segment movement. Therefore, it appears reasonable that Fig. 11 demonstrates a good agreement between three Cj parameters for the same oligomer. 

Measurement of the conductivity can be carried out to high precision with specially designed cells. In practice, tiiese cells are calibrated by first measuring the conductance of an accurately known standard, and then introducing the sample under study. Conductances are usually measured at about 1 kHz AC rather than with DC voltages in order to avoid complications arismg from electrolysis at anode and cathode [8]. 

The ionic conductivity of a solvent is of critical importance in its selection for an electrochemical application. There are a variety of DC and AC methods available for the measurement of ionic conductivity. In the case of ionic liquids, however, the vast majority of data in the literature have been collected by one of two AC techniques the impedance bridge method or the complex impedance method [40]. Both of these methods employ simple two-electrode cells to measure the impedance of the ionic liquid (Z). This impedance arises from resistive (R) and capacitive contributions (C), and can be described by Equation (3.6-1) ... 

The dynamic viscoelasticity of particulate gels of silicone gel and lightly doped poly-p-phenylene (PPP) particles has been studied under ac excitation [55]. The influence of the dielectric constant of the PPP particles has been investigated in detail. It is well known that the dielectric constant varies with the frequency of the applied field, the content of doping, or the measured temperature. In Fig. 11 is displayed the relationship between an increase in shear modulus induced by ac excitation of 0.4kV/mm and the dielectric constant of PPP particles, which was varied by changing the frequency of the applied field. AG increases with s2 and then reaches a constant value. Although the composite gel of PPP particles has dc conductivity, the viscoelastic behavior of the gel in an electric field is qualitatively explained by the model in Sect. 4.2.1, in which the effect of dc conductivity is neglected. 

We have had problems with this approach to the NanoCell due to troubles getting the nanoparticles anchored in the molehole. In an alternative process, we fabricated a two-dimensional unit of juxtaposed Au electrodes atop a Si/Si02 substrate. A discontinuous Au film was vapor deposited onto the Si02 in the central region. Electrical measurements confirmed the absence of DC conduction paths across the discontinuous Au film between the pairs of 5 pm-spaced electrodes (< 1 picoamp up to 30 V). Each pair can serve as an independent memory bit address system. 

Comparative conductivity measurements of the compounds 24-27 in Fig. 7 showed that the order of dc conductivities observed for these compounds are 26 > 25 > 24 > 27 for all temperatures investigated. All measurements and calculations of those compounds were in agreement with the prediction of the CBH model [44], The measurements of the same but metal-free mono and ball-type compounds 29 and 30 also indicated the same CBH model [45],... 

Using different microwave measurements of the conductivity from 3 up to 150 GHz (0.1 up to 5 cm-1), Donovan et al. [103] reported nearly the same temperature dependence at all frequencies between 300 and 20 K. Consequently, they found no evidence for a narrow dc collective mode implied by the FIR studies. The question of the existence of a strongly conductive mode is still open, but an accurate determination of the temperature dependence of the resistivity which may not have been achieved in all cases is a prerequisite before comparing dc and high-frequency data. 

The metallic state of the conducting polymers has been confirmed by optical measurements and thermoelectric power, but only above 200 C by dc conductivity because of semiconducting transport betw een the metallic regions. The intrinsic anisotropy of such a metallic conduction that could specify dimensionality of the electronic states, is rather difficult to measure by dc conductivity for the same reason. The spin dynamic techniques might be a powerful tool to characterize microscopic conduction of charge carriers with spin. Such studies were recently reported for several systems with ESR and NMR as a function of frequency. This... 

Conductivity. Many workers have attempted to measure the dc conductivity of K2Pt(CN)4Bro.3(H20)x (40,166,246,253,258,270,273,309,310,364a, 435). Conductivity (303, 503) is a transport measurement and, therefore, is extremely sensitive to impurities and defects which may interrupt and/or interconnect chains. The defects may arise from cracks or fissures in the crystal or from solvent or impurity occlusions which affect the electron flow. A qualitative measure of crystalline quality is reflected by the anisotropic conductivity ratio, ct, jaj. Better morphology and lower impurity levels suggest fewer interchain bridges and intrachain breaks, allowing the anisotropic ratio to increase to its intrinsic value. Because the measured conductivity is a function of crystalline perfection, only the better characterized measurements are described, Table IV. 

Science is very much about the use of models, to describe and therefore predict, and to explain and therefore understand. Bioimpedance and Bioelectricity Basics emphasizes model thinking, as we see in Chapter 9. The selected model often dictates the measuring method to be used. The interpretation of the results is dependent on the angle of view and the model used. Models, however, have their shortcomings. Important models for bioimmittance are empirical and can, therefore, only describe. Because tissue behaves predominantly electrolytically, a model s treatment of DC conductivity is important. With high-energy pulses or DC, the principle of superposition often is not valid, and different... 

Among the various interesting and useful properties of the new class of polymers, their switchable electrical conductivity has proven the most attractive to the community of chemists and physicists, and so it is understandable that these polymers are called conducting polymers. Much effort has been spent on measurements of their electrical conductivity and on determinations of the factors that affect its value [44,113,119,124,151,213,314,327,328,344,345,393 17]. The use of the conventional ex situ dc four-point method [44,393,398,399] or the ac impedance technique in a metal [polymer metal sandwich arrangement [119,124,410] for measurements of the conductivity of dry polymer samples is straightforward. However, the conductivities of dry polymers are affected by humidity and any gas present. Indeed, this is the property that is utilized in gas sensors. Conductivity can also be measured in situ, i.e., under controlled electrochemical and chemical conditions [151,394,395,397,400,402,406,408,417]. 

Among the various interesting and useful properties of conducting polymers, their switch-able electrical conductivity has proven the most attractive. The use of the conventional dc four-prove method or the ac impedance technique in a metal-polymer-metal sandwich structure for measurements of the conductivity of dry polymer samples is straightforward. 

A moisture meter measuring the dc conductance in a circuit was developed from the Wheatstone bridge [83]. Because the electrical properties of whole grain are preferentially affected by the moisture content of the surface layers, the kernel is ground before the test. In a test cell, the meal is compressed against... 

Matejec ( ) has measured the DC conductance of thin silver bromide single crystals in contact with aqueous solutions of various silver ion concentrations. He found that the conductance of the crystals varied with the silver ion concentration and concluded the applicability of the Grimley-... 

Application of noncontact AC calorimetry using electromagnetic levitation under a strong DC magnetic field enables direct measurement of thermal conductivity melt fiow is suppressed and heat transfer is controlled only by conduction within a droplet, so that a droplet behaves as if it were solid [35, 36]. Furthermore, measurements are free from the sensor insulation coating. The thermal conductivity can be obtained from the ( )-co relation simultaneously with total hemispherical emissivity st, as described in Section 4.3. 

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. 

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