Current-density imaging

Mikac U, Demsar A, Demsar F, et al. A study of tablet dissolution by magnetic resonance electric current density imaging. J Magn Reson 2007 185(1) 103-109. 

Imaging in the presence of electric currents through the object is known as current-density imaging (cf. Section 10.1.4) [Joyl, Scol, Sco2, Sco3, Sco4, Seri], The magnetic field associated with the imposed electric current leads to a frequency shift of the... 

Fig. 7.4.1 [Seri] Pulse sequence for current-density imaging in the laboratory frame. Electric current pulses I are applied with opposite polarity in the de- and rephasing periods of a standard spin-echo imaging sequence.

Fig. 2.9.2 Radiofrequency, field gradient and current distributions requires a three-dimen-ionic current pulse sequences for two-dimen- sional imaging sequence [see Figure 2.9.1(a)] sional current density mapping. TE is the Hahn and multiple experiments with the orientation spin-echo time, Tc is the total application time of the sample relative to the magnetic field of ionic currents through the sample. The 180°- incremented until a full 360°-revolution is pulse combined with the z gradient is slice reached. The polarity of the current pulses...

It should be noted here that the ultra thin-layer cells (UTLC) which result from the close approach of an STM tip to a conducting substrate may have important electroanalytical applications in studies other than STM imaging (64). This is because extremely large current densities should be attainable in such cells, and also because of the fast transit times (e.g., 50 nsec for d - 10 nm) for reactants across the cell. Thus, such UTLC s might facilitate the determination of fast heterogeneous rate constants or the study of reactive electrochemical intermediates (64). 

Lithography With the STM Electrochemical Techniques. The nonuniform current density distribution generated by an STM tip has also been exploited for electrochemical surface modification schemes. These applications are treated in this paper as distinct from true in situ STM imaging because the electrochemical modification of a substrate does not a priori necessitate subsequent imaging with the STM. To date, all electrochemical modification experiments in which the tip has served as the counter electrode, the STM has been operated in a two-electrode mode, with the substrate surface acting as the working electrode. The tip-sample bias is typically adjusted to drive electrochemical reactions at both the sample surface and the STM tip. Because it has as yet been impossible to maintain feedback control of the z-piezo (tip-substrate distance) in the presence of significant faradaic current (vide infra), all electrochemical STM modification experiments to date have been performed in the absence of such feedback control. 

Fig. 4.17 Optical micrographs showing the effect of a lateral increase (from left to right) in anodization current density on sample morphology, (b) Surface and (c) cross-section of a p-type (100) substrate anodized in the dark (4x1014 crrf3, 60s, 10V, 1 1 HF 50% ethanol) using a set-up with a small, local ohmic back contact, as shown in (a). An SEM micrograph of the center of (c) is shown in Fig. 6.11c. SEM images of regions A and B are shown in Figs. 6.11c and 2.4c, respec-...

A local variation in porosity can be produced by an inhomogeneous illumination intensity. However, any image projected on the backside of the wafer generates a smoothed-out current density distribution on the frontside, because of random diffusion of the charge carriers in the bulk. This problem can be reduced if thin wafers or illumination from the frontside is used. However, sharp lateral changes in porosity cannot be achieved. 

One drawback of using the MPL is that the water saturation in the CL increases and causes more flooding [108]. Through neutron radiography imaging, Owejan et al. [147] were able to observe that MEAs that had cathode DLs with MPLs had better distribution of water over the active area at high current densities. DLs without MPLs tended to have more water accumulated in one location of the active area (closer to the outlet). One issue with this work was that the water accumulation observed was for the whole MEA and the water quantities were not separated between the anode and cathode sides. 

Another example of neution imaging is the one presented by Yoshizawa et al. [273], who compared the performance of carbon cloth and carbon fiber paper with a parallel FF design. The CC had a better performance than the CFP at high current densities, but the CFP showed less water content over the whole active area. Thus, it was concluded that the CC diffusion layer was less influenced by the accumulation of water because the transport of oxygen toward the catalyst zones was sufficient while still keeping the membrane humidified. 

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