Iron Segregation Research Articles

Proton and temperature-induced competitive segregation of iron on surface and volume sinks of silica

Experimental results are delivered on iron redistribution in silica for proton irradiation followed by thermal annealing. Iron ions are initially implanted into silica at room temperature. Proton irradiation is performed at different temperatures. It is demonstrated, in particular, that radiation-induced migration of iron is more efficient at low temperature. Iron surface segregation and capture of iron by sinks in silica subsurface region during thermal annealing are speculated in terms of diffusion-alternative-sinks problem.

Iron-doped indium oxide by modified RGTO deposition for ozone sensing

Nanostructured thin films based on indium oxide have been prepared by a modified rheotaxial growth and thermal oxidation (RGTO) deposition technique. The layers were additivated with 8–30% iron in order to stabilize the microstructure and to enhance the sensing properties toward ozone. Microstructural characterization highlighted formation of core–shell structures featuring iron segregation in the outer part of the nanosized agglomerates. Such peculiar morphology resulted in improved gas sensing performance. The electrical test of the sensing layers indicated high sensitivity to ozone together with a relatively low cross-sensitivity to interfering gases.

Segregation Effects and Phase Developments during Solidification of Alloy 625

The solidification behaviour of pure Alloy 625, and Alloy 625 enriched respectively in iron and carbon, was investigated in situ by hot-stage light optical microscopy. Using this technique planar front solidification for distances of several hundred microns was facilitated. After solidification, the material that experienced planar front solidification corresponded accurately to that of dendrites tens of microns in width adopting an equivalent dendrite arm width approach. Eventually, the planar solidification front broke down, where after the residual liquid solidified eutectic-like. This material contained γ-phase, Laves phase and, if carbon was dissolved in the liquid, niobium rich carbides formed. Molybdenum and niobium showed strong tendencies to segregate. Their segregation was balanced by inverse segregation of nickel and iron. The chromium concentration remained almost constant in γ in the entire matrix material. Addition of carbon did not cause detectable alterations of the material that experienced planar front solidification. However, it promoted the formation of niobium rich carbides in the material that solidified eutectic-like. Thus, this material differed from that of the pure sample in constitution, and consequently in γ-phase composition. Niobium rich carbides formed prior to Laves phase; in carbon rich volumes only the carbides form. As compared to the pure sample, the sample enriched in iron had decreased global minimal solute concentration in the material that experienced planar front solidification. However, once the concentrations were corrected with respect to the dilution simply caused by the presence of iron, the solidification behaviour in this material was identical to that of the pure sample. The constitution in the material that solidified eutectic-like was γ and Laves phase. As compared to the pure sample, the Laves phase was enriched in iron.

Laser ablated Fe 100− xPd x and Fe 100− xMn x thin films ( x=20 and 50)

Thin films (50 nm) of Fe 100− x Pd x and Fe 100− x Mn x ( x=20 and 50) were deposited by laser ablation on a Si wafer and studied by X-ray diffraction (XRD) and conversion electron Mössbauer spectroscopy (CEMS). In all cases, the XRD patterns of the target surface before and after ablation show a congruent transfer of ablated material. For the Fe 80Pd 20 thin film, XRD spectra show the formation of FePd, iron, Pd oxide and traces of hematite in the ablated film. Complementary information obtained by CEMS indicates the presence of an upper 50-nm thick surface layer of hematite in the deposited film. Fe 50Pd 50 phase was observed by CEMS and a Pd oxide component was identified by XRD, along with traces of hematite. In the case of Fe 80Mn 20, the CEMS spectrum is consistent with the occurrence of the antiferromagnetic FeMn phase and the XRD pattern also shows the presence of Fe in the structure. For the system Fe 50Mn 50, we obtain by CEMS an antiferromagnetic phase of FeMn and by XRD we trace the presence of nanohematite and in-depth iron. For x=20 in both iron-rich systems investigated, we observe the segregation of iron with subsequent conversion to hematite. The study shows that the stoichiometry of the compounds plays a decisive role in determining their structural and magnetic properties.

Growth of Fe-doped GaN by RF plasma-assisted molecular beam epitaxy

Iron-doped GaN films were grown using plasma-assisted molecular beam epitaxy (PA-MBE) for various III/V ratios. Iron concentrations were determined by calibrated secondary-ion mass spectroscopy (SIMS), and films were characterized by high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The iron incorporation strongly depended on the growth conditions, with increasing incorporation with decreasing III/V ratio. The behavior was explained by promotion of surface segregation of iron by the gallium adlayer and droplets. AFM indicated no effect of iron doping on surface morphology. Precipitation of intermetallic FeGa 3 was found for gallium droplet regime growth and high iron fluxes. Nitrogen-rich growth resulted in significantly enhanced iron incorporation and reduced segregation, and nitrogen-rich iron-doped films were grown for iron concentrations of up to 1×10 19 cm −3 with no evidence of iron-related phase precipitation or crystalline degradation. Highly-doped nitrogen-rich films were overgrown with gallium-rich GaN, resulting in specular surface morphologies and sharp turn-on and turn-off of the iron profile in SIMS. Similarities to the reported behavior of manganese doping of GaN suggests that surface segregation of transition metal dopants may be important for PA-MBE growth.

Grain Boundary Decohesion by Sulfur Segregation in Ferromagnetic Iron and Nickel —A First-Principles Study—

Using first-principles calculations, we simulate grain boundary decohesion (embrittlement) in ferromagnetic bcc FeΣ3(111)[1\\bar10] and fcc NiΣ5(012)[100] symmetrical tilt grain boundaries by progressively adding sulfur atoms to the boundaries. We calculate the segregation energy of sulfur atom, tensile strength, and cohesive energy of the grain boundaries. We show that a certain amount of sulfur segregation (two atomic layers, 14.4 atom/nm2) is energetically possible to realize considering the calculated segregation energies. At this concentration, the tensile strength and the cohesive energy of the grain boundaries reduce by one order of magnitude comparing with no segregation case.

Experimental Evidence of Iron Segregation in Copper Grain Boundaries as Deduced from Type B Diffusion Measurements

Grain-boundary heterodiffusion of iron in pure copper and self diffusion of iron in copper–0.091at% iron were measured by the serial sectioning technique in the Harrison B-regime. The penetration profiles corresponding to iron heterodiffusion in pure copper show a strong positive curvature far beyond the (Dvt)1/2 depth . This peculiar shape, which does not exist for self diffusion in the solid solution, proves the presence of a strong non linear grain-boundary segregation of iron in copper in spite of the respective surface energies of these metals. This segregation is linked to the size effect which is, as predicted by numerical simulation, the main driving force for grainboundary segregation.

Olivine as tar removal catalyst for biomass gasifiers: Catalyst characterization

In the present paper olivine is considered as a prospective in-bed tar removal catalyst for fluidized bed biomass gasifiers. The catalytic activity of olivine is investigated via steam reforming reaction of naphthalene as a model biomass tar compound. It is observed that the calcination of olivine improves the performance of the catalyst. Calcination of olivine is done with air at 900 °C for different treatment times. With increasing calcination time, tar conversion increases; more than 80% naphthalene conversion is observed with 10 h of calcination time for olivine, which is found to be an optimum as further increase in time up to 20 h does not improve the naphthalene conversion. The primary focus of this paper is to characterize the olivine catalysts under investigation (specific composition (Mg 1.78, Fe 0.13) SiO 4) with different calcination times. Techniques such as BET-surface area analysis, XPS, Mössbauer spectroscopy, TPR, SEM/EDX analysis are used to understand the basic phenomena of calcination of olivine. Olivine is observed to be non porous. SEM/EDX show iron rich areas at the surface of olivine after calcination. Severe segregation of iron is observed at the surface due to calcination, which is found to be non-uniform. Mössbauer spectroscopy reveals the appearance of iron(III) phase in the olivine after calcination. Besides the catalytic activity towards tar reduction, olivine is also observed to be highly attrition resistant to be used as an in-bed additive for fluid bed and moving bed biomass gasifiers.

Effect of specimen preparation on evaluation of cast iron microstructures

Preparation of cast iron specimens is difficult due to the need to properly retain the graphite phase. Three procedures using automated grinding/polishing processes were used to study the effect on graphite retention of such factors as, the quality of SiC grit paper, the kind of polishing cloth and diamond paste, as well as the number of preparation steps. The best preparation method for different types of graphite consisted of four steps, with the polishing process carried out on a napless cloth with diamond in paste form. A procedure for specimen preparation of white, high-chromium cast iron is also given. Examples of the use of different etchants for revealing unalloyed cast iron microstructures, such as: pearlite, ferrite and cementite, are presented. The phosphorous eutectic (Steadite) in gray iron can be observed after etching with 4% nital, but selective color etching helps to differentiate the revealing constituents. This method is also helpful in phase differentiation of the ADI matrix as well as revealing silicon segregation in ductile iron and microsegregation in the austenitic matrix of mottled iron. Color etching of chromium carbides in alloyed white irons facilitates the use of the color image analyzer for measurements of the amount and morphology of this phase.

The control of grain boundary segregation and segregation-induced brittleness in iron by the application of a magnetic field

Recycling of iron and steel becomes an universally important issue from the viewpoint of energy and resource saving. Impurity elements like Sn and Cu tend to accumulate in steels by repeated recycling and remarkably degrade mechanical properties of recycled iron alloys due to segregation-induced intergranular embrittlement. The goal of this work is to study the potential of magnetic annealing for the control of grain boundary segregation and intergranular embrittlement in iron alloy. This paper reports several important findings regarding the effect of magnetic annealing on segregation-induced brittleness in iron-tin alloy. Of particular importance is the observations that the concentration of tin at grain boundaries in iron is decreased by magnetic annealing and fracture toughness of iron-tin alloy is drastically improved to the level as high as pure iron.

Experimental evidence for the presence of segregation and relaxation gettering of iron in polycrystalline silicon layers on silicon

Experimental evidence is provided that gettering of iron by polycrystalline silicon (polysilicon) is driven by a combination of two gettering mechanisms, segregation and relaxation. The segregation coefficient of iron in polysilicon in samples annealed at temperatures between 1020 and 1175°C varied from approximately 16 to 2. The efficiency of relaxation gettering by polysilicon was characterized using Ham’s model of diffusion-limited gettering. The product nr0 for the 11-μm-thick polysilicon layer was estimated as 106cm−2.

Application of magnetic field to the control of grain boundary segregation in iron

Abstract Magnetic annealing was carried out to examine the effect of a magnetic field on impurity segregation to grain boundaries in an Fe–0.8 at.% Sn alloy at 973 K for 6 h with a superconducting magnetic field up to 6 T. After magnetic annealing, atomic force microscopy was applied to evaluate grain boundary energy, and FEG-TEM/EDS analyses were performed across the grain boundaries whose character was previously determined by orientation imaging microscopy. Of particular interest is the finding that grain boundary energy increased with increase in the strength of a magnetic field applied during annealing. Furthermore, FEG-TEM/EDS analyses revealed that the tin concentration at random boundaries in magnetically (H = 3 T) annealed sample was almost the same as the grain interior, whereas the content in ordinarily (H = 0) annealed sample was approximately 1.5 times higher than that in the grain interior. These findings are the first demonstration that magnetic annealing has a great potential for the control of grain boundary segregation and improvement in grain boundary segregation-induced embrittlement in ferromagnetic polycrystalline materials.

Formation of air stable carbon-skinned iron nanocrystals from FeC2

Charge neutralization reaction in ionic salt of Fe2+C22− is found to produce carbon-skinned Fe nanocrystals. FeC2 is formed as an intermediate product in the reaction of FeCl2 solved in acetonitrile with CaC2 fine powder and also able to be isolated as black nanocrystals. Heating of FeC2 at temperature higher than 250 °C induces segregation of metallic iron. The segregated carbons grow as graphitic sheets parallel to the growing Fe lattice plane. This direct bonding is due to an accidental matching of the Fe–Fe distance (2.866 Å) with that of the C1–C4 distance (2.842 Å) of the hexagonal rings in graphite. The x-ray diffraction pattern indicates that the particles are composed of α-Fe and graphitic carbon. The thickness of the skin is almost constant as thick as 3.5 nm independent of the body size. The particles with an average size of 30 nm exhibit temperature dependence of the magnetic cohesive force as function of T−0.275.

Impact of gettering by helium implantation on boron and iron segregation

The use of gettering techniques that can be precisely localized in the component, such as neutral element implantation, has become crucial for power device manufacturing. High-dose helium implantation in silicon with a subsequent annealing induces defects (cavities and dislocations) which are able to getter metallic impurities and interact with dopants. These interactions can become a drawback of this technique. In this work, Si wafers -oriented, p-doped with a concentration in between 6 × 10 14 and 8 × 10 18 B/cm 3 , were used. Iron contamination was performed by backside implantation (100 keV and 10 12 Fe cm -2 ). Helium implantation was performed at 40 keV and 5 x 10 16 at cm -2 at room temperature and followed by a Furnace Annealing (FA) stage. Secondary Ion Mass Spectrometry (SIMS) was used to study the dopant/metal/cavity layer system. In this paper, we will focus on the cavity layer interactions with both metals and dopant impurities. Our experimental results exhibit a large segregation of both species within the cavity layer that occurs during the annealing stage. Both species segregate simultaneously in helium-implanted samples without interacting one to the other. The number of gettering sites does not limit the trapping reactions.

Caracterização e gênese de perfis plínticos desenvolvidos de arenito do Grupo Bauru: II - mineralogia

Nas paisagens do norte e oeste do estado de São Paulo, plintita e petroplintita constituem feições que se repetem com freqüência sobre os arenitos cretácicos da Formação Adamantina (Grupo Bauru). Com o objetivo de avaliar as características mineralógicas desses materiais e estudar sua gênese, selecionaram-se dois perfis de solos representativos da paisagem local e constituídos por feições plínticas, petroplínticas e mosqueados. O estudo foi realizado na baixa meia encosta de uma vertente situada na Estação Experimental de Agronomia de Pindorama, do Instituto Agronômico (IAC), região norte do estado de São Paulo. Com base nas observações em microscópio de varredura e microanálise pontual realizadas em glébulas selecionadas, bem como nas análises mineralógicas da fração argila desferrificada e dos óxidos de ferro de todos os horizontes dos perfis estudados, constatou-se que caulinita, hematita e goethita são os principais constituintes da fração argila dos nódulos e horizontes estudados. Os minerais mica, gibbsita e anatásio complementam a mineralogia da fração argila das glébulas, assemelhando-se em constituição ao material interglebular e aos demais horizontes dos perfis. Quartzo, feldspatos potássicos, traços de feldspatos sódicos e ilmenita foram identificados como componentes da fração silte e areia dos nódulos. A presença constante de minerais alteráveis nas glébulas petroplínticas é evidência de que a gênese desses materiais está relacionada com a ferruginização do saprolito. Este fato, associado aos baixos teores de Al na estrutura dos óxidos de ferro das glébulas, evidencia sua formação em condições hidromórficas, supostamente relacionadas com a solubilização e mobilização do ferro ferroso, lixiviado da paisagem a montante e reprecipitado na zona de vadosa, onde os maiores potenciais de oxidação favoreceram a segregação e a precipitação do ferro.

Isothermal calorimetric observations of the effect of welding on compatibility of stainless steels with high-test hydrogen peroxide propellant

Welding of stainless steels (CRES 316L and CRES 304L) was found to increase surface roughness and more importantly to decrease their compatibility with high-test hydrogen peroxide (HTP). Compatibility was determined by isothermal microcalorimeter (IMC) measurements. Omission of a purge gas enhances welding effects. Previous literature indicated welding causes segregation of chromium and iron in the heat affected zone (HAZ) and in the weld bead itself. It is likely that these surface modifications are responsible for the diminished compatibility, however a direct correlation was not established.

高純度アルミニウム中の鉄の表面偏析

Surface segregation of iron in high purity aluminum alloys, containing 50 and 280 ppm iron, was investigated by Rutherford back scattering spectroscopy (RBS) and transmission electron microscopy (TEM). The specimens were heat treated at various temperatures ranging from 423 K to 923 K for 72 ks in Ar atmosphere. The surface segregation of iron by heat treatment was detected by RBS measurement. The strongest segregation was observed in the Al–280 ppmFe annealed at 723 K for 72 ks. The surface segregation quantities in Al–50 ppmFe alloy was smaller than those in Al–280 ppmFe alloy, but the degree of segregation (segregation concentration/bulk concentration) in Al–50 ppmFe alloy was larger than that in Al–280 ppmFe alloy. TEM–EDX indicated that almost of iron segregated at the interface between oxide film and the matrix, but a few iron in the oxide films just above aluminum matrix was detected.

Any Old Iron

EARTH SCIENCE The Hf-W system has been used as the basis for determining when iron cores formed in Earth and Mars and when the Moon may have formed. The idea is that 182Hf decays to 182W with a half-life of about 9 million years. Thus, essentially no 182Hf should persist 40 to 60 million years after the formation of the solar system. Chemically, tungsten but not hafnium tends to be incorporated into an iron core, so W crustal abundances are low. Thus, if iron cores formed before the time that all of the 182Hf became extinct—or similarly if the Moon formed because of a giant impact with Earth—an isotopic signal should be present in W isotope data from crustal rocks. An early analysis suggested that there was no difference between the isotopic values of W for Earth and chondritic meteorites, which provide a reference value, implying that Earth's core formed late. Three new and independent analyses by Schoenberg et al ., Yin et al ., and Kleine et al . instead indicate that the value for chondrites needs revision. A difference was observed between crustal values and those for carbonaceous chondrites, and crust samples from Earth and the Moon do contain evidence of the former presence of live 182Hf. Together the data imply that the planets of the solar system accreted rapidly, within a few tens of millions of years, and formed their cores rapidly by internal segregation of iron. This notion is consistent with recent numerical models and other chemical and isotopic data. — BH Geochim. Cosmochim. Acta 66 , 17 (2002); Nature 418 , 949; 952 (2002).

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

Directional solidification of unmodified and strontium modified binary, high-purity, aluminium–7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth interface was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification interface extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified high-purity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy.

Observation of Tritium Distribution in Pure Vanadium and V-5 mol%Fe Alloy by Tritium Radioluminography

ABSTRACTTritium radioluminography was applied to pure vanadium and V-5 mol%Fe alloy to observe the tritium distribution and to evaluate the local tritium concentration in them. It was demonstrated that the tritium distribution at a microscopic area in the specimens was quantitatively and graphically displayed. In the pure vanadium specimen, the local tritium concentration was about three times different depending on the crystal orientation of the grains. The tritium radioactivity of the grains with (001) and (111) orientation are 1 Bq/mm2 and 0.4 Bq/mm2, respectively. These values correspond to the tritium concentration of 15 mol ppb and 6 mol ppb. The difference of the local tritium concentration was attributed to the variety of the morphology of precipitated hydride depending on the crystal orientation of the grains. For the radioactivity recorded in the imaging plate, the component of the X-rays generated from tritium in the specimen was only 2%, i.e., most of the intensity was attributed to the β-rays. In the V-Fe alloy specimen, it was shown that the tritium distribution correlates with iron segregation formed during solidification after the arc melting. The cross sectional observation showed that the local tritium concentration in equilibrium distribution depends on the local iron concentration in the specimen. The local tritium concentration gradually decreases from 115 mol ppb to 70 mol ppb as the iron concentration at the iron segregated region increases from 3 mol% to 4.5 mol%.