Iron oxide , defect

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. 

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

New results of styrene formation over iron oxide single-crystal model catalysts were reported.326 In ultra-high-vacuum experiments with Fe304(lll) and a-Fe203(0001) films combined with batch reaction studies only Fe203 showed catalytic activity. The activity increased with increasing surface defect... 

Another property of the iron-defective molybdate is the presence of Mo=0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related to Mo6 contained in the Mo=0 as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting to note here that pure iron oxides accelerate combustion and that a W03—Fe2(W04)3 catalyst is practically inactive [254], Replacement of iron by chromium is possible but leads to a lower activity [253]. Baussart et al. [46] prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. 

Mill scale (rust) and pitting. The three layers of iron oxide scale formed on steel during rolling vary with the operation performed and the rolling temperature. When mill scale is placed in an electrolyte, any defect in the mill scale surface becomes the anode the remainder of the mill scale, which is usually many times larger than the defect, becomes a very strong cathode. An electric current can easily be produced between the steel and the mill scale. This electrochemical action will corrode the steel without affecting the mill scale. 

The oxidation of iron at high temperatures, where several iron oxide phases form, obeys the parabolic rate law whereas in CO-CO2mixtures above 900°C, it obeys a linear rate law with the exclusive formation of an FeO layer (18). This result is understandable if one considers the high defect concentration in FeO of approximately 10%, which ensures high diffusion velocity. The linear rate constant - exhibits the following de-... 

The iron oxides in natural surface environments are often poorly crystalline. i. e. the crystals are nano-sized (>100 nm), do not clearly exhibit the typical morphology of well-crystalline forms, are rich in defects and contain impurities. All this is most probably the result of their formation at low-temperature and in contaminated environments. Due to their striking colors (ranging from red to yellow) and their high surface area, small... 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

Successful ammonia conversion required discovery of a catalyst, which would promote a sufficiently rapid reaction at 100-300 atm and 400-500°C to utilize the moderately favorable equilibrium obtained under these conditions. Without this, higher temperatures would be required to obtain sufficiently rapid rates, and the less favorable equilibrium at higher temperatures would necessitate higher pressures as well, in order to obtain an economic conversion to ammonia. The original synthesis experiments were conducted with an osmium catalyst. Haber later discovered that reduced magnetic iron oxide (Fe304) was much more effective, and that its activity could be further enhanced by the presence of the promoters alumina (AI2O3 3%) and potassium oxide (K2O 1%), probably from the introduction of iron lattice defects. Iron with various proprietary variations still forms the basis of all ammonia catalyst systems today. 

The first such experiment reported was that of Clarke and Gibson (176,177) in which an iron oxide catalyst for the Fischer-Tropsch process was found to be about 40% more active than usual if the oxide was irradiated with y-rays before being placed on stream. Such catalysts are normally reduced by the synthesis gas (CO - - H2), and the conversion to hydrocarbons rises to a plateau during the first few days of operation. A sample preirradiated with 2 x lO i ev/gm reached a conversion of 50% after about 150 hours on stream at 280°, while an unirradiated sample leveled off somewhat sooner at about 30%. The irradiated sample showed no deterioration up to the end of the run (about 10 days), an interesting contrast to the usual ready annealing of radiation enhancement. The original defects probably annealed fairly rapidly at 280°, but apparently not before modifying the reduction of... 

Flexibility of the bulk/surface structure and reaction media effect. For such systems as manganese oxides, copper oxides, spinel iron oxides Fe304-y-Fe203 [4, 5, 24, 25 ], reaction media effect at enhanced temperatures (up to 400 °C ) and at prolonged (up to 10 h) exposures in reaction mixtures was found to remove all initial differences in the phase composition and defect structure. All extended defects were washed out due to interaction with a flux of point defects created by reaction media. As a result, a constant level of the catalytic activity was achieved for these oxide systems demonstrating apparent structural insensitivity of the reaction of CO oxidation. Hence, in this case, great flexibility of the oxide bulk structure allows to reach the same true steady state of the catalyst. 

No Vein Compound. [DCS Color Supply] Iron oxide compd. used to eliminate expansion defects in castings. 

Irox. [DCS Color Siqq>ly] Iron oxide con used with irfinidic uteduuie foundrys and binders to eliminate subsurface potoshy and oqMnsioa defects. 

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