Bulk conductivity measurements

Fig. 25. (a) Sketch of a possible set-up to perform microcontact measurements using one microelectrode and an extended counter-electrode. The microelectrode is contacted under the microscope by a sharp needle, (b) Set-up for two microelectrodes, (c) Evaporated Au microelectrodes on SrTiC>3 and the needlelike tungsten tip to contact the electrodes. Such a configuration is used to perform spatially resolved bulk conductivity measurements (Sec. 6.2). (d) Ag-coated YBajCujOs+s-microelectrodes on a SrTiC>3 polycrystal contacted by two tungsten tips. The corresponding local grain boundary measurements are discussed in Sec. 6.3. 

A further problem arising with semiconductivity measurements is that the surface conductivity may be of a different type to the bulk conductivity. Measurements of work function or field effect mobility are then necessary to fully define the process. 

The orientation of cellulosic fibers has some effect on the conductivity of the paper. The conductivity in the XY plane of the sheet (surface conductivity parallel to most of the fibers) may be quite different from the conductivity along the Z direction (bulk conductivity perpendicular to the fibers). Comparison of surface and bulk conductivity for a given paper sheet can thus yield information which reflects the anisostropy in the structural morphology due to fiber orientation. Bulk conductivity measurements are also important since many paper sheets used in reprographic processes are composed of a conductive base sheet coated with a dielectric material 16. One important specification for these types of papers is the value of the bulk conductivity of the base paper. 

Figure 9 compares the two methods for making bulk conductivity measurements on conductive base paper the pressurized stainless steel electrode method and the Ga-In eutectic liquid metal method. The conductivity is plotted as a function of voltage at 50% RH. For the stainless steel electrodes, at 2000 psi (13.8 MPa), the measured current is a superlinear function of voltage and a strong function of time 16), up to a voltage of approximately 10V. With the Ga-In electrodes this effect is noticeable only for applied potentials up to approximately 3V. Between 10 and 50V, the bulk conductivity measured by both methods is essentially independent of voltage, and the current traces are reproducible and show no time dependence. 

This equation is a reasonable model of electrokinetic behavior, although for theoretical studies many possible corrections must be considered. Correction must always be made for electrokinetic effects at the wall of the cell, since this wall also carries a double layer. There are corrections for the motion of solvated ions through the medium, surface and bulk conductivity of the particles, nonspherical shape of the particles, etc. The parameter zeta, determined by measuring the particle velocity and substituting in the above equation, is a measure of the potential at the so-called surface of shear, ie, the surface dividing the moving particle and its adherent layer of solution from the stationary bulk of the solution. This surface of shear ties at an indeterrninate distance from the tme particle surface. Thus, the measured zeta potential can be related only semiquantitatively to the curves of Figure 3. 

See 2-3.1. Electrical conduction through solids takes place both through the bulk material and over the surface. In most cases surfaces have different physical and chemical properties than the bulk, for example due to contamination or moisture. Volume and surface resistivity can be separately measured for solid materials such as antistatic plastic sheet. Powders represent a special case since although both surface and bulk conduction occur, their contributions cannot be individually measured and the volume or bulk resistivity of a powder includes surface effects. 

The surface oxidation products dete ted by the decrease in contact angle upon photolysis of PVCa films may dominate the photoconductivity of t. is polymer. Work is underway to confirm this relatio. ship and measure surface conductivity simultaneously with bulk conductivity as a function of photodegradation. 

FIGURE 1.20 Impedance spectra measured at 350°C in air for Gd02Ce08O19 with ( ) 30 ppm, (o) 200 ppm, and ( ) 3000 ppm Si02. Inset is the impedance at high frequencies, showing the effect of Si02 content on the bulk conductivity [94]. 

A considerable volume of literature has accumulated on conductance measurements in mixtures of solvents. Ion mobilities and association constants have been measured over a range of bulk dielectric constants with the aim of correlating bulk solvent properties with mobilities, ion association, and ion size parameters. An example of a widely used solvent mixture is water and 1,4-dioxane, which are miscible over all concentrations, providing a dielectric constant range of 2 to 78. The data obtained in systems containing two or more solvents must be treated with circumspection, as one solvent may interact more strongly with a given species present in solution than the other, and the re-... 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

Let us now concentrate on the specific sample used in this work. For the bulk appKTP crystal (used experimentally in section 7), the commercially-obtained KTP wafer was flux-grown, c-cut, single domain, 1 mm thick, and with the c-faces polished to an optical finish. For practical reasons, the wafers were cut into 10x5 mm pieces after the conductivity measurements. 

The latter authors used anode and cathode symmetrical cells in EIS analysis in order to simplify the complication that often arises from asymmetrical half-cells so that the contributions from anode/ electrolyte and cathode/electrolyte interfaces could be isolated, and consequently, the temperature-dependences of these components could be established. This is an extension of their earlier work, in which the overall impedances of full lithium ion cells were studied and Ret was identified as the controlling factor. As Figure 68 shows, for each of the two interfaces, Ra dominates the overall impedance in the symmetrical cells as in a full lithium ion cell, indicating that, even at room temperature, the electrodic reaction kinetics at both the cathode and anode surfaces dictate the overall lithium ion chemistry. At lower temperature, this determining role of Ra becomes more pronounced, as Figure 69c shows, in which relative resistance , defined as the ratio of a certain resistance at a specific temperature to that at 20 °C, is used to compare the temperature-dependences of bulk resistance (i b), surface layer resistance Rsi), and i ct- For the convenience of comparison, the temperature-dependence of the ion conductivity measured for the bulk electrolyte is also included in Figure 69 as a benchmark. Apparently, both and Rsi vary with temperature at a similar pace to what ion conductivity adopts, as expected, but a significant deviation was observed in the temperature dependence of R below —10 °C. Thus, one... 

There are two main varieties of bulk conductivity detectors contact and contactless. In a contact conductivity detector, the electrodes contact the column effluent directly. The electrodes are usually made of stainless steel, platinum, or gold in order to minimize electrochemical reactions, but they are still subject to fouling over time. In the absence of electrochemical reactions, there is no charge transfer between the solution and the electrodes, so the conductivity measurement is made with an oscillating or alternating voltage. 

The effective bulk polyimide resistivity can be extracted from comparison of the measured curves in Figure 6 with this model, and values at 100°C and 158°C are shown in Figure 7, which is a reproduction of the Arrhenius plots of Figure 4. It is seen that both the values of the resistivity and the temperature dependence are in good agreement with those obtained from the dc conduction measurements. In addition, the saturated value of the measured threshold instability is well predicted by the model. 

Films have been deposited using selenourea and an ammonia-complexed solution at 65°C [96]. Zincblende CdSe was obtained with an optical spectrum corresponding to a bandgap of 1.84 eV (the bulk room-temperature bandgap of zincblende CdSe is ca. 1.8 eV). Analysis of electrical conductivity measurements indicated charge transfer occurred via a variable hopping mechanism through fairly deep states (a level 0.29 eV below the conduction band was found from these measurements). 

The conductivity measurements show that equilibrium (1) sets in rapidly at temperatures as low as 500°C. Since the melting point of zinc oxide is about 2100°C. and accordingly its Tamman temperature about 900°C., the process under consideration cannot possibly involve the bulk of the crystal because defects could not diffuse rapidly enough through the lattice at such low temperatures. Except at very high temperatures, the defect equilibrium is realized only at the surface of the crystal, that is, in a layer of a few unit cells thickness. 

Combining the kinetics of the polymerization and the electric conductivity measurements during the irradiation of bulk styrene (6), the... 

In addition to the microscopic redox processes of bismuth ions and molybdenum ions, the combination of these conductivity measurements leads to the conclusion that the macroscopic, bulk conductivity properties of the bismuth molybdate catalyst affect the catalytic reaction. 

The ion conductivities of a range of TPEOCELL complexes with LiCFsSOs were examined by AC impedance spectroscopy from 293-373 K, so that all the complexes remained in the liquid-crystalline state during the measurements [235]. The inorganic salt concentration, indicated by the molar ratio of lithium ions to oxygens in the polymer chain, [Li]/[0], ranged from 0.038 to 0.125. The bulk conductivities were 10-6 Son-1 at ambient temperatures... 

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