Shape of the cathode

This inherent feature of ECM, whereby an equiHbriumgap width is obtained, is used widely in ECM for reproducing the shape of the cathode tool on the workpiece. (J) Under short-circuit conditions the gap width goes to zero. If process conditions such as too high a feed rate arise the equiHbrium gap may be so small that contact between the two electrodes ensues. This condition causes a short circuit between the electrodes and hence premature termination of machining. 

Fig. 21-6 The dependence of the passivation process on the shape of the cathodic partial current potential curve (a) Anodic partial current potential curve, (b) cathodic partial current-potential curve without local cathode rest potential (c) cathodic partial current potential curve with local cathode rest potential I7j p.

Shaping of the cathode and hooded anode to minimize focal spot size is a speciahzed art. Apparent focal spots of 1 mm (as viewed from the detectors) are typical for baggage systems. Tube currents are commonly in the range of... 

Figure 16.22—MS detectors, a) Multiple stage electron multipliers (reproduced by permission of ETP Scientific Inc.) b) channeltron the conical shape of the cathode allows the detection of ions with slightly different trajectories c) electron multiplication within a channeltron d) entrance of a multichannel plate detector (microchanneltron).

The above CVs (Figs. 24 and 25) display well-formed reduction peaks independent of the blank solution and the type of active carbon materials. The combined shape of the cathodic peaks indicates that surface species participate in electrochemical processes in different local environments, or with various structures but convergent peak potentials. The effect of anodic polarization is more readily observed in a basic environment than in an acid solution. Similarly, a positive shift of cathodic peak potential with a decrease in anodic sweep potential limit takes place. Similar results were obtained for studies of electrochemical oxidation of graphite [17] and glass-like carbon [222] electrodes. There was considerable enlargement of both anodic and cathodic peaks after anodic polarization in 20% sulfuric acid (Fig. 26) [17]. 

FIGURE 16 The basic elements of a flash lamp. The envelope is typically made from fused silica, the electrodes from a tungsten alloy. The shape of the cathode helps to increase lamp life. 

During cathodic protection the potential and the current density on the protected structure is usually not uniform, but varies as a function of the shape of the cathode and the effective distance from the nearest anode. Figure 12.51 shows schematically the variation of the potential as a function of the distance between anodes placed in a row on a flat surface such as for example a ship hull. 

Observations relevant to ECM can now be made. Firstly, the rate of metal removal from the anode is not affected by the hardness or other mechanical properties of that electrode. Secondly, the shape of the cathode remains unchanged during the electrolysis, since gas evolution is the only reaction that occurs there. 

This inherent feature of ECM, whereby an equilibrium gap width is obtained, is used widely in ECM for reproducing the shape of the cathode- tool on the workpiece. 

In electrodepostion processes metal ions are reduced to metal atoms at the cathode of an electrolysis cell. The shape of the cathode can be adjusted to obtain the precipitated metal in a desired form. 

DMFC cathode. The model was used to fit experimental impedance spectra the contributions due to catalyst poisoning by MOR intermediates and due to parasitic current produced in the MOR have been calculated. However, Du et al. (2007b) assumed that the ORR and MOR are running uniformly over the cathode thickness. In that case, the above effects only weakly distort the shape of the cathode semicircle. 

Conformal anode (electroplating) An anode made to conform to the shape of the cathode to keep the anode-to-cathode spacing constant. 

Figure 19.2 shows that at the current densities of interest in the plating industry, the Wagner number is less than unity, making it difficult to obtain good macrothrowing power. Hence uniform plating will not be attained on complex shapes of the cathode. 

The geometry in this problem is slightly more complicated than that for the previous cell. The bottom segment considered to be the cathode is a cosine curve, whereas the top edge is the anode (at a constant potential). The cell is depicted in Fig. 12. The shape of the cathode is described by (26) ... 

The validity of criterion (5.2) is demonstrated in the table, where Va and Vc have been obtained by extrapolating the respective anodic and cathodic exchange currents (to find the true shape of the cathodic curve for the second process, the limiting cathodic current was subtracted from the total current)... 

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