Oxidation circuits

Pb sulphide circuit Pb oxide circuit Zn sulphide circuit Zn oxide circuit... 

Figure 3.14. Mode of control of the oxidant circuit in a PEMFC...

The liquid from the centrifuge flows into a clarifier from which settled particles are returned to the crystallizer, and clarified liquor (containing 0.7 to 1.0% sulforic acid) is returned to the absorber/oxidizer circuit The flow rate of the return stream is approximately 500 gpm. A small amount of the dilute acid is continuously purged from the system to prevent build-up of chlorides that could cause corrosion of stainless steel equipment. A chloride limit of 200 ppm has been specifred for the plant Hie catalyst, iron sulfate, is not a major... 

Figure 9-18. Stretford process oxidation circuits Zwicky and Mills, 1980i.

As botli processes, reduction and oxidation, take place on tlie same electrode surface (a short-circuited system), it is not possible to directly measure tlie corrosion current. Experimentally, only tlie sum of tlie anodic and catliodic... 

It should be pointed out that external polarization differs from the unbiased (open circuit) case in that after application of, say, an anodic voltage only the oxidation reaction takes place on the metal, whereas the cathodic reaction (H — H2) occurs at the external counter-electrode. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Etching. After a resist is patterned on a wafer, the exposed or unwanted substrate is removed by etching processes. Subsequentiy the resist is removed, leaving a desired pattern in a functional layer of the integrated circuit. Etching is performed to pattern a number of materials in the IC fabrication process, including blanket polysiHcon, metal layers, and oxide and nitride layers. The etch process for each material is different, and adapted to the material requirements of the substrate. 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

Miscellaneous. Ruthenium dioxide-based thick-film resistors have been used as secondary thermometers below I K (92). Ruthenium dioxide-coated anodes ate the most widely used anode for chlorine production (93). Ruthenium(IV) oxide and other compounds ate used in the electronics industry as resistor material in apphcations where thick-film technology is used to print electrical circuits (94) (see Electronic materials). Ruthenium electroplate has similar properties to those of rhodium, but is much less expensive. Electrolytes used for mthenium electroplating (95) include [Ru2Clg(OH2)2N] Na2[Ru(N02)4(N0)0H] [13859-66-0] and (NH 2P uds(NO)] [13820-58-1], Several photocatalytic cycles that generate... 

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