Critical current results

In the case of the stainless steels, or other readily passivated metals, the rapid reduction of dissolved oxygen on the freely exposed surface will be sufficient to exceed the critical current density so that the metal will become passive with a potential greater than whereas the metal within the crevice will be active with a potential less than. The passivation of the freely exposed surface will be facilitated by the rise in pH resulting from oxygen reduction, whilst passivation within the crevice will be impeded by the high concentration of Cl ions (which increases the critical current density for passivation) and by the H ions (which increases the passivation potential E, see Section 1.4). 

Active-passive transition It has been shown that /p, the current required to maintain a passive film, increases with temperature at a much greater rate than the critical current for passivation as a result of an activation-controlled process. At some temperature /p will exceed /pri,. and no active-passive transition will be observed, and more important no protection by a passive film is possible because of the high rate of dissolution. At this stage the slow process becomes the diffusion of reactants and control of the rate is... 

In reality, this behavior is only observed in the limit of small jg. At currents o 1 A cm-2 that are relevant for fuel cell operation, the electro-osmotic coupling between proton and water fluxes causes nonuniform water distributions in PEMs, which lead to nonlinear effects in r/p M- These deviations result in a critical current density, p at which the increase in r/pp j causes the cell voltage to decrease dramatically. It is thus crucial to develop membrane models that can predicton the basis of experimental data on structure and transport properties. 

In the case of Bi and T1 phases only very short reaction times (of the order of minutes) are possible because of the evaporation of Bi and Tl. This may be compensated by using excess Bi and T1 in the solution or by modification of the solvent (mixture of water and glycerol) that enhances the solubility of Bi and allows the use of less nitric acid in the solution. A more homogeneous deposition results in a better product after heat treatment. Figure 9 shows a typical resistivity measurement for a thallium film that has, however, a very weak critical current (50 A/cm2 at 77K). 

Interesting methods have demonstrated that thin films can be deposited and patterned using microlithography. However, there are still difficulties in thermal processing and the resulting films obtained by most of the solution techniques have wide transitions and low critical currents. 

For example, a low critical current density has been shown useful in using BaPb B Og as a microwave switch. The transmission of 2.8 GHz microwaves through a polycrystalline thin film was switched on in less than 30 ns via a high current pulse (107), resulting in short microwave pulses. 

We obtain AM(He) from the data in Figure 8 and calculate Jcni(He) using the rectangular slab Eq. 13 where a = 0.62 mm and b = 0.040 mm. The result is shown in Figure 9 where the magnetization critical current density Jcm falls monotonically with increasing field strength. Actually, Eq. 13 must be modified for... 

The critical current in the absence of spin-orbit interaction jia 0) = Devp/AL coincides with known results (see e.g. Ref. [11]). 

In this work, we have measured the critical current density of YBa2Cu307 thin film in strong magnetic field H up to 10 T at various angle 6 between H and the crystallographic c-axis. We have compared our results to the intrinsic pinning model proposed by Tachiki and Takahachi [7],... 

Here we consider results on short stacks with typical lateral size 1 pm x 1 pm and containing N = 30-70 elementary junctions. The high quality of the mesas has been approved by the Fraunhofer patterns of critical current Ic on parallel magnetic field with periodicity of one flux per elementary junction [11, 12], Fig 2 shows the I-V characteristics of the short stacked junctions in large and small voltage scales. 

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

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