Direct current measurements

The techniques and apparatus which have been developed to measure electrolytic conductivities in nonaqueous solutions have been adapted from aqueous conductivity measurements with some modifications. Direct current measurements suffer the limitation of requiring reversible electrodes - a serious limitation in nonaqueous solvents. Although this problem can be circumvented U in some instances, virtually all precision conductance data have been taken using the alternating current method. General descriptions of this method are given in several sources. 2>3)... 

The conductance (s) obtained from the resistance was plotted against temperature in Fig. 14. The conductance obtained from the long DNA film was 10 Scm which is consistent with the result obtained by direct current measurements in Fig. 12a, and was independent of temperatures in the range 20 to 60 °C. On the contrary, the conductance of the short DNA film... 

In the 1960s to 1970s, numerous direct current measurements in the Black Sea were performed at 175 mooring stations, of which 86 were 4 to 29 days long. Their results were generalized in [7] in the form of a schematic that... 

In this expression, bd and bc refer to the appropriate anodic and cathodic Tafel constants. Comparison of weight loss data collected as a function of exposure time determined from R , Rf from EIS, and gravimetric measurements of mild steel exposure to 0.5 M H2S04 are often within a factor of two. This suggests that use of Rn in the Stern-Geary equation may be appropriate for the estimation of corrosion rate (147-150). However, Rn measurements may underestimate corrosion rates. / p is often measured at effective frequencies of 1(T2 Hz or less in linear polarization or EIS measurements, while Rn is measured at 1 Hz or greater. An example of this is provided in Fig. 57, which shows the corrosion rate of carbon steel in 3% NaCl solution as a function of exposure time determined by EIS, linear polarization, noise resistance, and direct current measurement with a ZRA. Among these data, the corrosion rates determined by noise resistance are consistently the lowest. 

Conductivity Direct current measurement, Dielectric measurement, Thermally Stimulated Discharge (TSD) method A—>C 46-49... 

The ionic conductivity of amorphous materials can be obtained not only from the conventional direct current measurement but also from the dielectric loss in the dielectric measurement. The ionic conductivity from the dielectric measurement is based on the following concept ... 

The notion of the predesiccation Aral Sea circulation is somewhat speculative, given that extremely few (if any) direct current measurements have been reported. However, already the early researchers [7, 8] perceived the most peculiar feature of the sea s basin-scale circulation, namely, its anticyclonic direction (although there was also a hint of a smaller cyclonic gyre north of Barsakelmes Island [4, 9]). As is well known, the neighboring enclosed seas lying in the same latitude belt, such as the Black Sea, the Sea of Azov, and the Caspian Sea, all exhibit cyclonic... 

However, in the spring of 2004, the entire column was nearly uniform in salinity at about 86 ppt (see Table 3), indicating that winter convection was likely to had been able to destroy the meromictic conditions, and the enhanced stratification was an intermittent rather than permanent feature. By the late summer of 2004, a new type of stratification had arisen, which persisted in aummns until the fall of 2006. This stratification was mainly three-layered, involving two salinity maxima, one at the surface, and the other one at the bottom, separated by a relatively fresh intermediate layer (Fig. 6). Despite the salinity inversion just below the upper mixed layer, the column was maintained in a stable state by a steep thermocline. The deeper salinity maximum was best developed in 2005, but a hint of it was also observed in the fall of 2006, when there was a marked bottom mixed layer, indicating the presence of strong currents in the bottom layer (which was also confirmed by direct current measurements). 

Electrochemical and Electrical Methods. Electrochemical and electrical methods for studying film properties and corrosion phenomena have been extensively reviewed (29-31). Comparisons of corrosion test results with direct current measurements of conductivity suggest that visible corrosion is associated with film resistance less than about 1 Mohm/cm, but this condition may well correspond with the occurrence of virtual pores in the film allowing development of local conductive pathways. In studies of the equivalent alternating current resistance as a function of frequency, Kendig and Leidheiser (44) found that the development of a region of slope -1 on a log permittivity versus log frequency plot... 

The critical volume concentration for the blends used here lies in the region of 8 vol,%. In agreement with direct current measurements by the four-point method, this was determined in the test design used here by analysing the complex dielectric function (DF) (7 = 260(0 at 5 Hz (see Figure 11.110). 

The FC collects the ion charge for direct current measurement of the ion beam. The FC is essentially a metal bucket that collects the ion beam. Figure 17.10 shows a schematic diagram of a Faraday Cup with associated repeller plates and slits. The ion beam enters from the left and is captured in the Faraday cup on the right. The biased plate to the left of the cup has a negative voltage ( 100 V or less) to repel secondary electrons back into the cup. 

It is conceivable that direct-current measurement, which is a perfectly valid method for solid electrolytes associated with non-polarizable electrodes, would only allow us to access qualitative information if the electrode is ideally or partially polarizable. 

Conductance data can be obtained by alternating current (AC) techniques, direct current measurements, or by the electrode-less high-frequency method. In practice, the first method is by far the most widely used its principles were formulated in the last century by Kohlrausch. 

Jones (1971,1972), based on kinetic considerations, postulated the adoption of a criterion for a permissible corrosion rate on the basis of a polarization characteristic of a given corrosion system determined beforehand. In order to determine the corrosion rate at polarisation potential CP> knowledge of the course of the partial curve of the anodic reaction is necessary. The first steps in the analysis of partial currents of corrosion systems were made by Stern and Geary (1957), who described the method of determination of the Tafel coefficient of the anodic reaction on the basis of direct current measurement results of the cathodic polarization curve. Extrapolation of the linear section of this curve to a corrosion potential Ej-orr allows reproduction of a fragment of the anodic polarization partial curve at chosen potential values, as a difference between the polarization current and the partial cathodic current. 

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