The cathode contact

An interesting alternative cathode, ZrC was explored by Sheats et al.. [31] ZrC has a low work function (between Ca and Al) and is unreactive. For example, it is highly stable in air for long times. Like many other transition-metal carbides, it has a good electrical conductivity. These factors make it appear a nearly ideal contact material. Indeed, good electrical results were obtained for ZrC cathode contacts to MEH-PPV. Unfortunately, the devices showed low luminescence efficiency, possibly because of defect formation near the contact in the organic layer. Likewise, the Hfetimes of the ZrC based devices were relatively poor. Overall, this has led to the abandonment of ZrC as a contact material. Further research might find a solution to the observed problems. 

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Alumina electrolysis cells consist of a rectangular steel shell lined with a 25-35 cm layer of baked and rammed dense carbon, which provides both chemical resistance and the cathode contact with the electrolyte via steel bus bars imbedded in the carbon. Normal lining life is 4—6 years, after which it is replaced as large preformed slabs. Once a reduction pot has been started the bulk of the cathode current to the carbon lining is via the pool of newly formed molten aluminum in the bottom of the cell (Fig. 12.2). 

Cathodes have an effective depositions surface area of 1.67 m and there are 33 cathodes in a cell. A knife switch type is used for the cathode contacts. This type allows the cathode header bar to come in contact with both the clip and the equipotential bar, resulting in a wide contact area. With a self-weight contact type, the contact resistance is increased because the weight of the cathode is light at the initial stage of electrolysis. To minimize this disadvantage, the knife switch type has been consistently in use to the present time. 

What is the cell potential of a concentration cell that contains two hydrogen electrodes if the cathode contacts a solution with pH = 7.8 and the anode contacts a solution with [H+] = 0.05 M ... 

Wet Powder Spraying Bilayer cathodes consisting of cathode and cathode current collector layer can be manufactured by wet powder spraying (WPS). The cathode contact layer is also applied on the metallic interconnects by spraying. Limitations of the technology are the overspray (the amount of suspension which is sprayed past the object to be coated) and the formation of a suspension mist which needs to be extracted by suction. The overspray can be recycled, though. 

A common form of closed contact is the reed relay, where the sealed capsule consists of two Ni-Fe blades, coated in the contact area with a precious metal and sealed into a glass envelope filled with, for example, a N2-H2 mixture. The capsule is positioned within, or close to, a coil which induces the blades to contact magnetically when energized. The blades are usually gold-plated using a continuous belt-feed system, the cathode contact area being limited by judicious control of electrolyte level. 

If the cathode contact to an electron transporter layer is a Schottky barrier type contact, why is there no observable depletion region in the organic material ... 

Corrosion problems are particularly important when two metals are in contact. The more reactive metal becomes the cathode of the cell and goes into solution when the cell is activated by an electrolyte. A typical cell is shown in Figure 13.7. When the metal in contact with iron is more reactive than iron itself, the iron is protected from corrosion. This is important when mechanical strength... 

Caustic Soda. Diaphragm cell caustic is commercially purified by the DH process or the ammonia extraction method offered by PPG and OxyTech (see Fig. 38), essentially involving Hquid—Hquid extraction to reduce the salt and sodium chlorate content (86). Thus 50% caustic comes in contact with ammonia in a countercurrent fashion at 60°C and up to 2500 kPa (25 atm) pressure, the Hquid NH absorbing salt, chlorate, carbonate, water, and some caustic. The overflow from the reactor is stripped of NH, which is then concentrated and returned to the extraction process. The product, about 62% NaOH and devoid of impurities, is stripped free of NH, which is concentrated and recirculated. MetaUic impurities can be reduced to low concentrations by electrolysis employing porous cathodes. The caustic is then freed of Fe, Ni, Pb, and Cu ions, which are deposited on the cathode. 

Significant vapor pressure of aluminum monofluoride [13595-82-9], AIF, has been observed when aluminum trifluoride [7784-18-1] is heated in the presence of reducing agents such as aluminum or magnesium metal, or is in contact with the cathode in the electrolysis of fused salt mixtures. AIF disproportionates into AIF. and aluminum at lower temperatures. The heat of formation at 25°C is —264 kJ/mol(—63.1 kcal/mol) and the free energy of formation is —290 kJ/mol(—69.3 kcal/mol) (1). Aluminum difluoride [13569-23-8] h.3.s been detected in the high temperature equihbrium between aluminum and its fluorides (2). 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

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. 

The cell for this process is unlike the cell for the electrolysis of aluminum which is made of carbon and also acts as the cathode. The cell for the fused-salt electrolysis is made of high temperature refractory oxide material because molten manganese readily dissolves carbon. The anode, like that for aluminum, is made of carbon. Cathode contact is made by water-cooled iron bars that are buried in the wall near the hearth of the refractory oxide cell. 

Other commercial cells designed for the electrolysis of fused sodium chloride iaclude the Danneel-Lon2a cell and the Seward cell, both used before World War I. The former had no diaphragm and the sodium was confined to the cathode 2one by salt curtains (ceramic walls) the latter utili2ed the contact-electrode principle, where the cathode was immersed only a few millimeters ia the electrolyte. The Ciba cell was used over a longer period of time. 

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