Current Density Measurements

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

An important point is that the concentration gradients at the surface, represented by the dashed lines in Figure 4, are proportional to the current densities measured at the appHed voltage... 

Cathodic protection and associated instruments have developed in-line with the changing monitoring demands of both the onshore and offshore industries. In particular, for potential and current density measurements, far greater quantities of data are sought and are required to be processed into an easily assimilated form. Thus cathodic protection instrumentation has benefited from an increased association with microprocessor-based data handling and storage systems. 

Polarisation Resistance slope of the linear plot of overpotential versus current density measured at potentials close to the corrosion potential, or the tangent of such a curve at the corrosion potential if the plot is not linear. If a small change in potential, A , gives rise to a change in current density. A/, then the polarisation resistance is / p(Q m ) = AE/Ai. 

It is worthwhile to note here that in Figs. 3 and 4 the current densities measured at a methanol and air flow rates of 2 ml/min and 2 L/min, respectively show the inconsistency even at the same operating conditions. The reason is expected to be due to the water produced at the cathode. Comparing Figs. 3 and 4, the measured current density response looks quite unstably scattered at low air flow rates in Fig. 3, while it seems to be stable even at the low methanol flow rates in Fig. 4. This instability of measured current density in Fig. 4 could have caused by the water formed at the cathode during the operation of the stack. In fact, it has been observed from the experiment that the water produced at the cathode looks accumulated in the cathode flow channels for a while and it bursts out intermittently to the cathode outlet. Based on the experimental observations, for the stable operation of the 5W... 

The J-V, L-V, and E V characteristics, measured for the OLEDs made with a commercial ITO anode and an AZO anode are plotted in Figure 6.18a and b, respectively. The current density measured for an OLED with an AZO anode is lower than that obtained for a device made with an ITO anode at the same operating voltage. A slight high turn-on voltage observed in the OLED using an AZO anode is attributed to its lower work function compared... 

Fig. 10.8 aZTT Figure of Merit ( Eb,=CVbr/A, in gC/cmz) and leakage currents (measured at a stored charge of 7. iC/cm2, current density measured in A/cm2) as a function of position on the substrate. 

Upon comparison to data, the models demonstrate the aforementioned value of Uc being equal to 1. Not only are exchange current densities measured, but the effect of temperature can be studied in more detail, as well as the type and thickness of mem-... 

Earlier work was complemented with exchange current density measurements. 51,52 E.m.f. and current density data were inconsistent with only one charge-transfer reaction involving oxygen. Again, two reversible charge-transfer reactions were considered, (3.3) and (3.4). The anodic branch of (3.3) was neglected as was the cathodic branch of (3.4) based on the approximate values of the e.m.f. of the cell compared to the standard reversible potentials (however,... 

Fig. 62. Snapshots and space-time current densities measured with ring-arrays of Fe electrodes in the oscillatory region on the transport-limited plateau of the current-potential curve. (Reproduced with permission from Z. Fei, J. Green and J. L. Hudson, J. Phys. Chem. B103 (1999) 2178, (1999) American Chemical Society.)...

In this expression, i is the pit current density, /measured is the measured current density in solution due to the vibrating probe, and h is the standoff distance, which is equal to the radius of the hemisphere of uniform current density surrounding the pit. A pit current of 3 nA is possible given a current density measurement sensitivity of 2 pA/cm2. 

Here, Z(co) local is the magnitude of the local impedance, V((o)apPiied is the magnitude of the voltage between the working electrode and a distant reference electrode, and VXcoVbe is the AC voltage drop measured by the pseudoreference electrode pair. Again, it is implicitly assumed that the current density measured in solution is equal to the current density at the electrode surface. 

Further examples of positron study of defects in HTSC are studies carried out to understand the nature of flux-pinning defects that lead to an increase in critical-current density on neutron-irradiated Y 1 2 3. Experiments [59] on positron lifetime and critical-current density measurements on various neutron-irradiated samples of Y 1 2 3 indicate that the critical current density is correlated with the micro-void density, as obtained from the analysis of positron lifetime measurements. Investigation of defects in other HTSC superconductors, such as La-Sr-Ca-Cu-0 [60], Bi-Sr-Ca-Cu-0 [49], and Nd-Ce-Cu-O [52], have also been carried out. 

In the classical electrolytic experiments the current density, measured in A/dm2, was the quantity which was controlled, possibly because it was the easiest factor to measure and keep constant. For a long time nearly all electrolytic reactions were performed with a control of the current density, although Haber,42,43 as early as 1898 in his famous papers on the stepwise reduction of nitro compounds, realized that the potential of the working electrode was the proper quantity to control. 

Table 2K The effect of solution resistance on the minimum pulse duration and the maximum value of the exchange current density measurable in a coulostatic experiment.

The polish rate vs. current density measured during abrasion. (Adapted from Ref. (21).)... 

Fig. 7. (a) The limiting anodic excitation current density as a function of the hole injection current density measured at an n-GaAs/Ce + electrode in different aqueous electrolytes. 

In some earlier life tests performed by Wilson and co-workers, PEFCs utilizing thin-film platinum catalyst layers typically experienced a gradual performance loss over the first 500 to 1000 h of operation and then stabilized at about 70% of the original performance (here, performance is described in terms of the current density measured at 0.50 V, i.e., close to the maximum power output of the cell). In such life tests. 

Novel composite carbazole polymer films containing silver and gold nanoparticles and being thiol-stabilized have been fabricated. Optical properties of PEPC films are more sensitive to the nanoparticles incorporation than the properties of PVC films. This can be provided by the formation of additional interaction of particles and -CH2-O-CH2- group presented in the polymer chain. High values of the current density measured and the character of the current-voltage dependence in the composite films can be the consequence of several mechanisms of charge carrier transport in both composite PEPC and PVC films. 

Nb thin films have been deposited on PS substrates, with pores diameter and interpore distance 0 = 10 nm and A = 40 nm, respectively. The samples show well developed superconducting properties. Critical temperature and critical current measurements were performed on unpatterned samples. Future work will deal with critical current density measurements carried out on patterned samples. 

Another study dealing with the cathodic hydrogen evolution reaction was concerned with implanted pure iron The current densities measured for the hydrogen evolution with the implanted iron as cathode in H2SO4 solutions are illustrated in Fig. 33 which shows the current-density potential characteristics of pure iron bombarded with 10 — 10 Au 7cm and Pb /cm. The implantation energy was 400 keV. The iron was irradiated in the form of rotating rods in order to obtain an uniform concentration profile from the surface up to the mean range of the ions. 

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