Bulk conductivity cell

In the following section, a new bulk conductivity cell is described that significantly reduces the contact resistance to a level where the measurements of paper bulk conductivity can be made with an accuracy that is limited primarily by the anisotropic structure of the paper itself. A small uncertainty in the measured conductivity arises from compaction ( 10%) of the paper sample in the apparatus caused by the application of 13-8 MPa pressure to the stainless steel electrode system in the cell. This pressure is used to eliminate contact resistance. Despite this uncertainty, measurement errors in the new cell are significantly less than the spread in the conductivity values ( 200)t) determined at different points in a single paper sheet. The variability arises from inhomogeneities in the cellulose fiber network within the sheet. 

Figure 4. A cross-sectional view of the in situ pressure bulk conductivity cell.

Two important requirements must be met by a technique designed to provide accurate measurements of the bulk conductivity of paper. First, the contact resistance between the electrode and the paper should be either known or negligible and, secondly, the paper should not be significantly modified by the technique used. The in situ pressure bulk conductivity cell satisfies these requirements, as will be shown in the following sections. 

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. 

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Chapter 1 by Joachim Maier continues the solid state electrochemistry discussion that he began in Volume 39 of the Modem Aspects of Electrochemistry. He begins by introducing the reader to the major electrochemical parameters needed for the treatment of electrochemical cells. In section 2 he discusses various sensors electrochemical (composition), bulk conductivity, surface conductivity, galvanic. He also discusses electrochemical energy storage and conversion devices such as fuel cells. 

For the purpose of making relative measurements of the bulk condutivity in paper, a pressure of 2000 psi (13.8 MPa) was chosen. At that pressure, most paper samples typically compres 5 /tm, from 70 (im to 65 /W16)- As can be concluded from the discussion below, the results of measurements at 2000 psi (13-8 MPa) are in good agreement with the Ga-In liquid alloy method. Samples were contacted on both sides with Ga-In over an area of 0.8 cm2 and were placed in the pressure conductivity cell. A nominal pressure of 3-2 psi (22 kPa) was applied to the electrodes. 

Surface profiles of both sides of a calendered and an uncalendered sheet were recorded with a Talysurf 5 instrument (Rank Taylor Hobson, England), and are reported in Fig. 13. The surface is different in each case, but the bulk conductivity is not significantly altered. The pressure applied to the electrodes within the cell has not changed the surface profile of the sheet for the calendered sample. Although a difference is observed for the uncalendered sample, the surface is not as smooth as that of the calendered sheet after pressure is applied. 

This article has addressed a number of issues relating to the electrical properties of paper or fibrous structures. It was shown that reliable measurement methods are now available for estimating both the bulk and surface conductivities of paper. In the case of the bulk conductivity, a new in situ pressure conductivity cell was described which significantlyreduces contact resistance. The surface conductivity can be determined by the application of a modified four-point probe method first used on paper by Cronch<15). It was shown that the degree of refining has a small effect on the bulk conductivity of paper. 

During the operation of the cell the conductivity of the KOH solution changes by dissolution of Zn(OH)2- The bulk conductivity of a powdered-zinc gel anode is determined by the metallic structure. In Figure 4 the relationship between weight percent of zinc and the resistivity of an electrolyte/metal-powder paste is shown [26]. Good electronic conductivity is achieved when the mixture contains 35-70 % zinc. The usable cell capacity is exhausted as soon as the amount of metallic zinc in the anode gel decreases to about 30 % of weight [27,28]. The admixture of materials wth good surface conductivity improves the anode quality with respect to efficiency and capacity [29,30]. 

We discuss the similarities and differences between Kquid-state electrochemistry (LSE) and solid-state electrochemistry (SSE). Although based on the same thermodynamic principles, the properties of these cells are quite distinct. Differences exist in the bulk conduction mechanism, partially in electrode reaction and in cell construction and morphology. This also leads to differences in appKcations. 

If the electrical conductance of the electrolyte bridging the galvanic contact is low, either because the bulk conductivity is low or because the electrolyte is present only as a thin film as is the case in atmospheric exposure to humid environments, the effective areas taking part in galvanic cell reactions are small and the total amount of... 

Because of the low bulk conductivity losses, in practical low-temperature fuel cell apphca-tions, a passivation oxide layer on the metal can dominate the bulk electron transfer losses. Metal fuel cell bipolar plates are highly robust and can be less than 0.5 mm in total thickness. Because the current collectors and flow fields are often used for mechanical support, they must have higher electrical conductivity to assure low losses. Remember, each fuel cell has only 1 V to work with, so even millivolts are important. 

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