Electronics iron contamination

Pozio et al. (2003) reported that iron contamination from end plates accelerates the rate of membrane degradation. It was recently reported that the use of a Pt-Fe alloy catalyst accelerates membrane degradation under OCV conditions, whereas the use of a Pt-Ni alloy catalyst elongates the life of the membrane (Sulek et al. 2008). Electron spin resonance (ESR) signals were detected from deteriorated MEAs and the signals were attributed to carbon radicals (Endoh 2006 Endoh et al. 

The advent of a large international trade in methanol as a chemical feedstock has prompted additional purchase specifications, depending on the end user. Chlorides, which would be potential contaminants from seawater during ocean transport, are common downstream catalyst poisons likely to be excluded. Limitations on iron and sulfur can similarly be expected. Some users are sensitive to specific by-products for a variety of reasons. Eor example, alkaline compounds neutralize MTBE catalysts, and ethanol causes objectionable propionic acid formation in the carbonylation of methanol to acetic acid. Very high purity methanol is available from reagent vendors for small-scale electronic and pharmaceutical appHcations. 

Fig. 9-15. Carbides (light areas) in an iron-nickel alloy. The dark line is the contamination mark made by translating the specimen slowly past the electron beam. (Courtesy of Birks and Brooks, Rev. Sci. Instr., 28, 709.)...

Where small quantities of high-purity steam is required for electronic chip, pharmaceutical, sterilization, food preparation, and similar process applications, a small risk of steam contamination may exist. This may be caused directly by the use of amine treatments or indirectly through process contaminants or the transport of iron oxides. Consequently, alternative arrangements for steam generation are made. 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

Singlet molecular oxygen, Oj, is also thought to arise, perhaps in reaction 4 or 5 (in place of dioxygen). The addition of 02 completes the cast of characters comprising molecules which may mediate the effects of O , since whenever O is formed, H2O2, OH and O2 all may exist in aqueous solutions in the presence of micromolar concentrations of ionic iron which contaminate many buffers. Armed with the capacity to form species with such a variety of affinities for electrons, the PMN is endowed with the ability to initiate a formidable array of chemical reactions, not merely between the various species themselves but also with its own constitutents and those of ingested microbes. 

A similar situation applies for reduced forms of iron (35, 36, 39, 65, 75, 76). As with oxidations, some of the best opportunities for reliably estimating rates of redox transformations are afforded by engineered systems where a reductant of known composition and quantity is added to achieve contaminant remediation. In addition to zero-valent iron, other chemical methods for reduction of contaminants involve dithionite (77-79) and electrolysis (where, in effect, electrons are added directly, e.g., 80, 81). 

Reduction of nitro aromatic compounds often appears to be a two-step process, in which a mediator is required for facile transfer of electrons from a bulk reductant to the contaminant. A well documented example is the coupling of organic matter oxidation by iron reducing bacteria to "abiotic" nitro reduction by biogenic Fe(II) that is adsorbed to mineral surfaces in a column containing aquifer material (36, 39, 76). 

The first surface state was detected as an unexpected bump at — 0.35eV in the energy distribution of the electrons field-emitted by a W(100) tip by Swanson and Crouser in 1966 [35]. Its surface character was claimed on the basis of the sensitivity of the bump to contaminants. It is somewhat ironic that surface resonances in sharp W tips have been recently found to jeopardize local electron spectroscopy of the surface states performed with the STM [36]. 

However, significant barriers to the application of SET might still exist. Typically, oxygen and iron will be present in soils and other hazardous wastes. Both Fe3+ and 02 catalyze the reaction of solvated electrons with NH3 to produce hydrogen and amide [22,23], as shown in Eq. (3) [13,22,23], Also, if solvated electrons must diffuse into soil particles, these electrons could be consumed in a variety of reactions in competition with diffusion and mass transfer. However, extensive work has shown that this need not be the case. For example, slurrying the soil in NH3 first has several benefits. It reduces particle size, swells clay layers, and preextracts contaminants. Thus,... 

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