Surface Segregation Of Sulfur Research Articles

Influence of the thickness and surface composition on the electronic structure of FeS2 layers

Electronic structures of bulk, bilayer and monolayer FeS2 are studied by DFT-GGA method. The band gap of the FeS2 monolayer is found to be of 0.73eV, which is well below the estimated bulk value (0.85eV). As a result, the gap reaches its maximum (1.39eV) for the bilayer FeS2. It is confirmed that the hybridization of Fe d states in the Fe-rich layer can lead to the decrease of the band gap down to the metallization of the surface. However, the Fe enrichment required for metallization must be very large, which makes it unrealistic. In contrast, the S-rich pyrite surface is found to be inherently metallic. This conclusion follows, in particular, from the revealed metallicity of the FeS3/FeS2 bilayer slab, in which the metallic properties stem from d states of Fe and p states of S of the FeS3 layer, while the FeS2 layer remains essentially semiconducting. Thus, the known decrease of the open-circuit voltage of pyrite solar cells can be attributed to the S enrichment, caused, for example, by surface segregation of sulfur.

Surface-energy-induced selective growth of (001) grains in magnetostrictive ternary Fe–Ga-based alloys

Surface-energy-induced selective grain growth has been used to increase the presence of preferred planes in polycrystalline ternary Fe–Ga based alloys and thereby maximize the magnetostrictive performance of these alloys. In this study, alloys were either doped with elemental sulfur or annealed under sulfur atmospheres to control the sulfur concentration on the surface of samples during annealing. We show that the segregation of sulfur, which is known to play an important role in controlling surface energy, can be correlated to the selective growth of {001} grains and an increase in the saturation magnetostriction of the samples. The results show that sulfur atoms are adsorbed (diffused) from the sulfur atmosphere (bulk interior) and segregate on the sample surface. The correlation between surface chemistry, texture development and magnetostriction is presented. The formation of {001} grains occurred under slight surface segregation of sulfur, i.e. at levels of concentration of surface sulfur between 0.5 and 1.35 at.%, for alloys of (Fe0.813Ga0.187)99B1 and (Fe0.813Ga0.187)99.5B0.5 doped with 50 ppm S. In the case of (Fe0.81Ga0.19)99(NbC)1 alloy annealed under a H2S atmosphere, abnormal growth of (001) grains resulted in 88.3% of the sample area being covered with a (001) grain.

Finite element simulation of interfacial segregation in dilute alloys

The finite element software Comsol is used to simulate surface or grain boundary segregation in dilute alloys. The model computes simultaneously the evolution of interfacial concentration and diffusion in the bulk. The solute exchange between bulk and interface is governed by Darken’s equation. The model is able to reproduce thermodynamic and kinetic aspects of the phenomenon, in particular the saturation segregation level and the short-time segregation kinetics expressed by the McLean approximation. It is also able to reproduce experimental trends in the case of surface segregation of sulphur in a Ni superalloy. In the case of the grain boundary segregation of impurities (P or S) in engineering alloys, the present approach provides a practical tool, as it can be coupled to other finite element simulations (heat transfer and/or mechanics). Thus, it becomes possible to predict the risk of synergetic segregation and thermomechanical damage during service or processing (forging, welding,...).

Surface segregation and texture development in rolled Fe–Ga alloy

The texture development of {1 0 0} 〈0 0 1〉 and {1 1 0} 〈001〉 preferred orientations in Fe–Ga sheets annealed under a sulfur atmosphere and the surface segregation of sulfur were investigated. The two fitted S 2p 3/2 peak positions at 161.46 and 163.06 eV by XPS represent the presence of iron sulfide on sheet surfaces. The selective growth of (1 0 0) and (1 1 0) grains was achieved through controlling the segregation of sulfur by annealing in quartz ampoules with varied sulfur content and varied annealing times at 1200 °C.

Reduction kinetics of Goro nickel oxide using hydrogen

Nickel oxide (NiO) granules formed by vapour deposition from a chloride solution, were reduced in hydrogen using thermal gravimetry. In the temperature range from 400 to 600 ∘ C, the rate of reduction increased with increasing temperature and increasing hydrogen pressure. Microscopic analysis showed that in this temperature range the reaction followed the shrinking core model. The activation energy for reduction was found to be 90 kJ/mol in this temperature range. When the temperature was increased above 600 ∘ C, the reduction rate decreased noticeably before suddenly becoming extremely fast above 950 ∘ C. Microscopy showed inner bands of metallization inside the outer metallic rim, indicating partial blockage of the interface between the reacted and un-reacted NiO core. At these temperatures, particle agglomeration and sintering were also found to take place. Surface segregation of sulphur may account for the slow-down in reduction rate above 600 ∘ C.

The role of the slope out region on fatigue crack initiation in electron beam welded waspaloy

Electron beam (EB) welding can be used to produce compressor drum assemblies for gas turbine aero-engines. The compressor disc components are fatigue limited and the weld has been identified as a fatigue susceptible region. Electron beam welds in forged and double-heat-treated WASPALOY have been characterized in terms of microstructure, hardness, and fatigue initiation for the base metal, full penetration, and partial penetration (slope-out) weld metal. It has been found that the grain size increases from the base metal through the full penetration weld to the partial penetration weld metal, being largest at the end of full penetration (EOP). Fatigue initiation was found to occur preferentially at the EOP and was associated with the presence of large sulfides at, or close to, the weld surface. The sulfides arise from free surface segregation of sulfur, from the fused metal and surface contamination (arising from a preweld sulfuric acid etching stage), into the slope-out region during weld power down. The sulfides provide fatigue initiation sites and also modify the local composition, changing the type and number density of grain-boundary carbides and γ′ precipitates, in the slope-out region near EOP, resulting in lower hardness regions. Removal of the sulfuric acid etching stage resulted in a more uniform microstructure.

Application of percolation theory to surface segregation during recovery

Surface segregation of sulphur on heavily cold rolled nickel is examined during recovery using in-situ Auger Electron Spectroscopy. An unexpected incubation period immediately followed by a very quick segregation step is observed. This unexpected behaviour is discussed using a model, which takes the different stages of recovery and especially the role of dislocation walls into account. In as-cold rolled nickel, dislocations appear as cells with thick walls. During recovery, dislocation wall refinement makes the diffusion of sulphur along dislocation walls much faster. The percolation of this structure is responsible for sudden acceleration of surface segregation. The main conclusion of this study is that superficial segregation kinetics remains very slow until a network of subgrain boundaries acting as diffusion short passes is established and used by the sulphur to reach the surface quickly.

Stabilization against initial oxidation using surface segregation of sulfur on Fe(1 0 0)

Surface segregation process of dissolved elements on a Fe(1 0 0) surface and the initial oxidation processes of the sulfur segregated surfaces were studied in situ by AES and RHEED. The surface enrichment of sulfur has formed a c(2×2) superstructure at elevated temperatures. The Fe(1 0 0)c(2×2)-S surface was found to have stabilization effect against initial oxidation. The retardation of oxidation was most enhanced when the sulfur surface concentration reached at the saturation coverage (1/2 ML). The possible physical origins of the stabilization effect are proposed.

Reflection high-energy electron diffraction studies of epitaxial oxide seed-layer growth on rolling-assisted biaxially textured substrate Ni(001): The role of surface structure and chemistry

We present a study of the {100}〈100〉 biaxially textured Ni(001) surface and seed-layer growth using in situ reflection high-energy electron diffraction and Auger electron spectroscopy. Our observations are consistent with formation of a c(2×2) two-dimensional superstructure due to the surface segregation of sulfur contained in the metal. We show that this superstructure can have a dramatic effect on the heteroepitaxial growth of oxide seed layers. In particular, the surface superstructure promotes the (200) epitaxial oxide growth of Y2O3-stabilized ZrO2, which is necessary for the development of high-Jc superconducting films for coated conductors.

Effect of surface segregation of sulfur on recrystallization kinetics in 3% Si–Fe alloy strip

Effects of bulk content of sulfur on sulfur segregation and surface energy induced recrystallization kinetics have been investigated in two alloys containing 6 and 30 ppm sulfur. During final annealing under a high vacuum, the convex profile in sulfur concentration, which is attributed to sulfur segregation and evaporation, corresponded to the trough in magnetic induction, irrespective of bulk sulfur content. During annealing, surface energy induced secondary and tertiary recrystallization were in turn observed. While the grain boundary pinning effect of segregated sulfur was very weak in the alloy containing 6 ppm sulfur, the grain boundaries in the other alloy were strongly pinned by the segregated sulfur. Under the relatively higher sulfur atmosphere appearing in the alloy containing 30 ppm sulfur, the growth rate of {100} grain was absolutely governed by the Zener term related to the segregation concentration of sulfur. Through such a recrystallization process, a complete {110}〈001〉 Goss texture was formed, and magnetic induction higher than 1.9 T was obtained in both alloys after final annealing for 14.4 ks.

Magnetic induction and surface segregation in thin-gauged 3% Si steel

A correlation between magnetic induction and surface phenomena has been investigated in a 3% Si steel 0.1 mm thick. During final annealing at several temperatures, the saturation level in magnetic induction increased with increasing final annealing temperature, and reached 1.93 T after final annealing at 1300 °C for 4.9 ks. This is attributed to the formation of complete (110)[001] Goss texture. During final annealing, the magnetic induction of the thin-gauged 3% Si strip was inversely proportional to the sulfur concentration on the surface. The surface segregation of sulfur occurred after a critical time the silicon concentration on the surface dropped to the level obtained from the α-iron matrix containing 3% Si. The drop in silicon level on the thin-gauged strip surface is due to the volatile silicon monoxide, which arises from the reaction between the silicon dioxide and the silicon segregated, or between the silicon segregated and the oxygen from the high vacuum condition.

Surface characterization of sulfided precipitated-iron Fischer-Tropsch catalysts by X-ray photoelectron spectroscopy

A series of novel sulfur-containing precipitated-iron catalysts were prepared by the addition of 2000–20000 ppm sulfur (Na2S, (NH4)2S, (NH4)2S5) to iron, during the precipitation reaction. XPS characterization of the highly sulfided catalyst (20000 ppm) after calcination at 200°C and 400°C did not reveal the presence of surface sulfur (160–170 eV region). However, low temperature calcination (200°C) followed by reduction (300°C) revealed surface S2− (162.3 eV), while high temperature calcination (400°C) gave rise to SO42− (169.4 eV) species. Use of a higher reduction temperature (400°C) increased the surface sulfide concentration to 15.6 at% S2− as compared to 1.3 at% after reduction at 300°C. At lower S loadings (2000, 5000 ppm) only sulfate species were observed under analogous conditions. The addition of polysulfide ((NH4)2S5) instead of S2−, resulted in the formation of a surface polysulfide and SO42− surface species. A mechanism for the surface segregation of sulfur after reduction in terms of site competition with oxygen is consistent with the data.

Surface segregation of sulphur in pure iron and in FeMo alloys: A comparative AES-radiotracer study

In this paper, we report kinetics of sulphur surface segregation obtained by Auger electron spectroscopy (AES) in pure iron and in FeMo (0.5, 2 and 3.5 at%) alloys over the temperature range 500–630°C. Moreover, we investigate sulphur equilibrium surface adsorption at 800°C using radioactive 35S. We show first that the maximum sulphur amount increases with bulk Mo content at 800°C and second, that sulphur strongly segregates in the temperature range [550–600°C] on Fe and FeMo alloys even if the maximum S segregated quantities are always lower than the maximum sulphur amount obtained in adsorption experiments. At high temperature, the S segregation kinetics observed are governed in a first stage by grain boundary and pipe diffusion. A first equilibrium state corresponding to the S and Mo surface cosegregation on FeMo alloys (comparable with S segregation on iron) has to be considered. 2D (Fe, Mo) sulphides in which the most of Mo atoms are located near the surface (and not in the substitutional sites of the first layer) are formed in place of 2D (Fe) sulphide. The second point concerns nitrogen which is a residual impurity in FeMo alloys. At low temperature (520°C), a superficial 2D Mo nitride (second equilibrium state) is formed. However, as sulphur is the most active species in the FeMo alloys, non equilibrium states and site competition are observed in an intermediate temperature range.

Composition of a Clean and Sulphur Covered CuNi(100) Single Crystal Surface

Surface composition of the CuNi(100) single crystal in the temperature range of 893=1073 K (620=800°C) has been investigated using low energy electron diffraction and Auger electron spectroscopy methods. A clean sample surface at moderate temperatures reveals small surface copper enrichment, with respect to the bulk Cu-rich composition. For example, at 1006 K (733°C) the copper surface concentration q„, was determined from the quantitative Auger electron spectroscopy analysis as 0.91. After prolonged heating at higher temperatures, the copper surface concentration converges to the bulk value, i.e. at 1073 K (800°C), CsCu = Cb u =0.87. Surface segregation of sulphur proceeds from (1 x 1) through p(2 x 2)S/CuNi(100) to c(2 x 2)S/CuNi(100) structures of low energy electron diffraction. In the presence of segregated sulphur the surface concentration of copper is lower.

Effects of hydrogen annealing, sulfur segregation and diffusion on the cyclic oxidation resistance of superalloys: a review

This review is based on the phenomenon of improved oxide scale adhesion for desulfurized superalloys. The proposed adhesion mechanism involves sulfur interfacial segregation and scale-metal bond weakening. Sulfur surface segregation on superalloys is examined as a function of temperature and sulfur content, and is related to the classical behaviour predicted by the McLean isotherm. Effective desulfurization to less than 1 ppmw can be accomplished by hydrogen annealing and is described by sulfur diffusion kinetics in nickel. Hydrogen annealing results in excellent cyclic oxidation resistance for a number of advanced superalloys. The concept of a critical sulfur content is discussed in terms of practical annealing conditions and section thicknesses.

Enhanced sulfur segregation in plastically deformed OFHC Cu and the effect of surface segregated sulfur on electric contact resistance

Plastic deformation of metals has been known to enhance the diffusion of impurities as a result of the increased diffusivity along dislocation cores. Surface segregation of impurities tends to modify the surface properties of metals. In the present work we have examined the effect of sulfur surface segregation on the contact resistance for OFHC Cu. Two copper specimens were prepared by different surface treatments-one was abraded with SiC paper and another was polished with a metal polishing paste. As expected, the kinetics of thermal sulfur segregation for the two specimens were different due to the differing amounts of cold work done on their surfaces. Contact resistances were measured in situ in UHV at dry circuit levels and at different loads (minimum 1 g) as a function of the sulfur content on the specimen surfaces. The surface concentration of sulfur increased from zero by annealing the specimens at increasing temperatures. As the surface concentration of sulfur increased to approximately 10 a/o, the contact resistances decreased for all loads. However, above a critical sulfur concentration of about 10 a/o, the contact resistance increased sharply. Further increases in the sulfur concentration did not cause an appreciable change in the contact resistance and a saturation was observed. No polarity effects were found for the two specimens. Air exposure of the specimens caused the sulfur to disappear from the surfaces, being replaced by oxygen and carbon. Correspondingly, the contact resistance increased approximately linearly with time of exposure. >

Depletion effects in surface segregation of sulfur from polycrystalline high-purity α-iron

Recently, scanning Auger microscopical investigations of repeated surface segregation of non-metallic impurities like sulfur, nitrogen and carbon from high-purity polycrystalline α-iron have revealed significant differences in the segregation kinetics of sulfur at the surfaces of the individual grains. This is a consequence of the different extent of the sulfur from grains which zone which extends to several 10 μm into the bulk. These differences diminish when surface diffusion of sulfur from grains which have nearly reached the equilibrium S coverage leads to an almost homogeneous equilibrium distribution of sulfur. A computer model is presented which combines the advantages of the layer-by-layer approach and the solutions of the continuous diffusion equations and allows the segregation kinetics to be calculated of highly dilute multi-component alloys. Model calculations performed to simulate the segregation kinetics of sulfur from iron are in good alphaeement with experimental results.

Combined analysis of overlayer/S/GaAs interfaces with photoemission spectroscopy and X-ray standing wave

Metal/GaAs or insulator/GaAs interfaces with a sulfur interlayer have been investigated using photoemission spectroscopy and the X-ray standing wave (XSW) method in order to determine the structure and chemical bonding at the interface and further correlate this with the electrical properties. It is found that while a CaF 2 overlayer is not very reactive towards the S interlayer, deposited Al strongly reacts with the surface S-Ga layer to form a thermodynamically stable Al-S interface monolayer, which plays a role in achieving metal-dependent Schottky contact. On the other hand, reactive Pd metal disrupts the S-passivated GaAs surfaces, resulting in surface segregation of sulfur. In this case, the Schottky barrier heights are identical to Pd/GaAs, indicating the importance of S remaining at the interface. XSW analysis reveals that the CaF 2 and Al/S/GaAs systems have well-ordered interfaces. Thus, the stability of the S passivation layer is discussed in terms of the S-Ga binding energy and the reactivity of the deposited overlayer.

Effect of Ce and Hf on the segregation of sulfur and the anodic polarization of nickel

It is known that sulfur segregates to grain boundaries even at low bulk concentration and embrittles nickel and nickel-based alloys or accelerates intergranular corrosion of this alloy. One way to reduce the sulfur segregation is to introduce strong sulfide forming elements into nickel such as, Ce, Cr, and Hf which can tie up sulfur in the nickel matrix. Another possible method would be to replace sulfur by segregating elements which help the cohesion of grain boundaries or improve grain boundary chemistry such as phosphorus or boron. Therefore, in this article, the authors investigated the surface segregation of sulfur in nickel doped with Ce, Hf and Mn by Auger electron spectroscopy, and the anodic polarization behavior of these alloys was examined to compare to that of pure nickel. Among the tested elements, Hf was found to be the most effective element in suppressing the sulfur surface segregation in nickel. However, Ni+470ppm Hf specimen showed pitting corrosion in 0.1 N H[sub 2]SO[sub 4] solution. All the elements (Mn, Ce, and Hf) added to nickel at the concentration of about 500ppm improved the ability to passivate the nickel but deteriorated the properties of the passive film. Ce addition into nickel might be themore » best choice among the examined elements to reduce the sulfur segregation in nickel-based alloys without deteriorating the corrosion characteristics.« less

Solute segregation to grain boundaries and free surfaces in an FeSi multicomponent alloy

Solute segregation was measured at both the {310} symmetrical tilt grain boundary and the (310) free surface of a sample of an Fe-6at%Si alloy containing traces of P, S, N and C at 873 K. Large phosphorus enrichment and silicon depletion characterize the grain boundary segregation in spite of a different bulk concentration of nitrogen. The surface segregation in nitrogen-containing samples is controlled by strong cosegregation of Si and N, resulting in the formation of a stable Si x N y 2D surface compound, whereas pronounced surface segregation of sulphur dominates in denitridized samples. The differences of grain boundary and surface segregation are discussed as a kind of “anisotropy of interfacial segregation” on the basis of Guttmann's theory with different values of free energies of segregation to grain boundary and free surface. They also suggest that the measurements of surface segregation cannot be unambiguously used for predicting the grain boundary segregation. In some non-brittle multicomponent systems, a better way of predicting segregation behavior at grain boundaries would be the measurement of grain boundary segregation in a related system with solute concentrations that cause embrittlement. The findings can then be applied to the required alloy composition on the basis of Guttmann's theory.