Kinetics Of Surface Segregation Research Articles

On determining the surface segregation kinetics of Sb in binary and ternary Fe alloys during continuous annealing

In the present research, Sb surface segregation kinetics for Fe-(0.01/0.03)Sb and Fe-2Mn-(0.01/0.03)Sb at.% alloys were determined based on the modified Darken model and linear heating followed by isothermal annealing. The modified Darken model was used to evaluate the measured Sb segregation profile during continuous galvanizing heat treatments. Based on the kinetic analysis, increasing temperatures and isothermal holding times led to an increase in Sb surface segregation. Sb segregation was observed at the surface for both the Fe-xSb and Fe-2Mn-xSb alloys after annealing. It was found that Sb surface diffusion in the Fe-Sb alloys was faster than in the Fe-2Mn-Sb alloys, which was attributed to the crystal structure and the defect density in the matrix. The segregation rate was determined from the Darken curves and a higher segregation rate was observed in Fe-xSb alloys. The activation energy of Sb diffusion for the Fe-Sb and Fe-2Mn-xSb alloys was determined to be ∼ 193±18 kJ/mol, which is close to the activation energy of bulk Sb diffusion in α-Fe.

Understanding the kinetics of Fe subsurface dissolution and surface segregation upon annealing Fe on Pd(111) in vacuum/oxygen environment

Understanding the kinetics of Fe subsurface dissolution and surface segregation upon annealing Fe on Pd(111) in vacuum/oxygen environment

Kinetics of surface and interface segregation in stressed nano-films on inert substrate

A general model is developed for describing the kinetic segregation in stressed ultrathin alloy films deposited on inert substrates. The essential role of the diffusion-induced stress and the intrinsic stress in ultrathin films upon the kinetic surface and interface segregation is addressed. By the developed model, it is demonstrated that the equilibrium segregation is the natural consequence of the kinetic segregation. The influences of temperature, film thickness, and average concentration on the surface and interface segregation are evaluated. As an example, the model is applied to simulate the kinetic surface/interface segregation in ultrathin Ni(111)(Au) alloy films deposited on the sapphire substrate and the simulation results are in agreement with the experimental observations.

Kinetics of Arsenic Surface Segregation in Scrap-Based Silicon Electrical Steel

The segregation kinetics of surface-active, residual elements are investigated in an in situ study of annealing scrap-based silicon electrical steel sheet where the arsenic (As) surface segregation is highlighted. During annealing in the temperature range of 300–950 °C, different kinds of interactions between the segregated residual elements were observed. Attractive interactions between the segregands produced co-segregation, e.g., between Sn and Sb, whereas repulsive interactions resulted in site competition, e.g., between Sn and As. These competing interactions are strongly time dependent. In spite of there being twice as much Sn compared to As in the bulk material, the As prevailed in the surface enrichments of the polycrystalline silicon steel at 950 °C. The intensity of the As surface segregation in the temperature range 800–950 °C is proportional to the calculated amount of γ-austenite phase in the (α + γ) steel matrix. The detected phenomenon of the As versus Sn site competition could be valuable for the texture design and surface engineering of silicon steels with a thermodynamically stable two-phase (α + γ) region.

Surface segregation modeling in the case of evaporation

A kinetic model is proposed to characterize surface segregation in the case of evaporation. The kinetics is formulated using the difference between the diffusion rate of solute atom in the bulk and the evaporation rate of solute atom from the surface layer. The first-order reaction equation is applied to quantify evaporation flux. A balanced state is quantified when the vapor and ambient pressure is equal and thus a complete quantification of the kinetics forms. The kinetic curve is convex prior to critical time and concave after. The maximum surface concentration decreases as evaporation parameter decreases. The kinetics of Mg surface segregation of Al-0.8 wt%Mg alloy is fitted. An enrichment ratio of 16.484 and evaporation parameter of 0.57 are the values that fit the experimental results the best. The balanced surface concentration of Mg is calculated and implies further evaporation with prolonged time. The proposed model can adequately describe surface segregation in the case of evaporation, and the formulated kinetics can satisfactorily quantify this phenomenon.

Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles.

Surface segregation in bimetallic nanoparticles (NPs) is critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes - much longer than can be modeled with molecular dynamics. Here we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales, and reveal the origin of kinetic stability of the core@shell and random alloy NPs at the atomic level. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, indicating that one of these structures must be metastable and kinetically trapped. Our simulations show that both the Au@Pd and the PdAu random alloy NPs are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the diffusion mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-mediated and a concerted mechanism. The system becomes kinetically trapped when all corner sites in the core of the NP are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the random alloy PdAu NP is suppressed because the random alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for exchange of Pd and Au and between the surface and subsurface.

Surface and grain boundary segregation in 16MND5 steel

An adequate model of quantification when there are many segregating elements is required for industry and research. Hence, for the first time, surface segregation kinetics on industrial 16MND5 steel was studied by XPS spectroscopy at temperatures ranging from 500 to 600 oC. From measurements that highlight the competitive segregation of P, S, Sn, Sb, As, and Cu impurities at the surface, a quantification model was developed and successfully used to deduce the surface concentrations during segregation kinetics as well as derive the corresponding diffusion coefficients. We observed that phosphorus and sulfur are the first elements covering the surface, then they are supplanted by others' impurities. This result may reflect impurities segregation behavior at the grain boundaries that impacts mechanical behavior of the material. Indeed, to further the research, 16MND5 samples were aged in the same range of temperatures. Then, Auger spectroscopy measurements at grain boundaries were conducted on broken samples exhibiting intergranular cracking. Results show that phosphorus is the only segregating element present at grain boundaries after 2 months of aging. Importantly, it appears that phosphorus grain boundary segregation kinetics is significantly lower than at surface. Copyright © 2017 John Wiley & Sons, Ltd.

Effect of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3−δ thin films

The effects of atmospheric CO2 on surface segregation and phase formation in La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF-6428) were investigated. (001)-oriented LSCF-6428 thin films were deposited on lattice matched (110)-oriented NdGaO3 (NGO) substrates by pulsed laser deposition (PLD). Using the synchrotron technique of total reflection X-ray fluorescence (TXRF), it was found that the kinetics of Sr surface segregation was enhanced when annealing at 800°C in a high-CO2 partial pressure, as compared to a similar anneal in a CO2-free atmosphere, with the oxygen partial pressure being constant in both cases. Hard X-ray photoelectron spectroscopy (HAXPES) measurements showed that the contribution of the surface carbonate to surface oxide phases increased significantly for the sample annealed in the high-CO2 atmosphere. Atomic force microscopy (AFM) studies showed enhanced surface phase formation during the high-CO2 partial pressure anneal. Density functional theory (DFT) calculations provide a thermodynamic basis for the enhanced kinetics of surface segregation in the presence of atmospheric CO2.

Electronic Structure Measurements of Heteroepitaxial Solid Oxide Fuel Cell Cathode Thin Films

Synchrotron based total reflection x-ray fluorescence (TXRF) and hard x-ray photoelectron spectroscopy (HAXPES) technique have been utilized to measure the compositional changes and evolution of surface chemistry of strontium doped lanthanum cobalt iron oxide (LSCF) annealed at 800 °C, as a function of strontium dopant concentration and gas composition. TXRF is a useful way to depth-profile the surface composition of material at elevated temperatures in real time. The capability of HAXPES to overcome surface contamination issues helps better understand the segregation phenomena. This study demonstrates that the formation of Sr carbonate results in enhanced kinetics of Sr surface segregation in LSCF thin films with presence of CO2 when annealing at 800 °C and lowering the Sr dopant concentration results in a decrease Sr surface segregation.

Modeling of Surface Segregation Effects Observed on Renewed Electrodes of Eutectic Alloys

Experimental data on the transients of the electric double layer (EDL) capacitance measured after the mechanical renewal of the surface of electrodes of eutectic Sn-Pb and Ag-Bi alloys with small concentrations of second components at the interface with sodium fluoride solutions are analyzed and generalized. The observed transients are associated with the surface segregation of second components of alloys at the boundary with the solution. Phenomenological models of the electrode/solution interface which make it possible to calculate the surface coverage of the metal phase with different alloy components in different times after the renewal are proposed and substantiated. It is shown that the kinetics of surface segregation of lead and bismuth atoms at the alloy electrode interfaces with electrolyte solutions can be described in the first approximation by a model of nonsteady-state surface diffusion.

Kinetics of surface segregation of bismuth atoms at the interface of a mechanically renewable Ag-Bi alloy electrode with a surface-inactive electrolyte solution

The time effects observed on mechanically renewed electrodes of eutectic-type Ag-Bi alloys in NaF solutions in the potential range of ideal polarizability are studied by the impedance method and cyclic voltammetry. The changes in the electric double layer (EDL) capacitance observed with the increase in the time of the electrode-solution contact from the moment of electrode renewal indicate that the process of surface enrichment with Bi atoms occurs. The analysis of obtained data leads to a conclusion that the surface segregation of Bi proceeds by the mechanism of surface diffusion, which provides the anomalously high diffusion rates as compared with the processes of volume diffusion in the solid phase. The results of studies of the surface segregation of bismuth on the Ag-Bi alloy/electrolyte interface are compared with the Auger-spectroscopic data for the interface of the same alloy with vacuum. Assumptions are put forward and substantiated that allow the differences in the kinetics of surface segregation processes observed on different interfaces to be consistently interpreted.

Orientation dependence of the surface segregation kinetics in Cu-6 at % Ge alloy

The surface segregation kinetics of germanium on the (110), (111), and (100) faces of Cu-6 at % Ge alloy has been investigated in the temperature range of 525–700 K by electron spectroscopy. In the initial annealing stage, Ge is accumulated on the surface, forming a phase similar to a supersaturated solid solution. When the Ge concentration reaches 25–40 at % (depending on the face), it forms well-ordered 2D phases, whose structure is determined by the face orientation. In the surface region the bulk diffusion parameters exhibit anisotropy; the Ge diffusion activation energy is minimum near the (110) face.

CO Oxidation over AuPd(100) from Ultrahigh Vacuum to Near-Atmospheric Pressures: CO Adsorption-Induced Surface Segregation and Reaction Kinetics

Polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) is used to study CO adsorption on AuPd(100) surfaces, one well-annealed such that contiguous Pd sites do not initially exist and a second ion sputtered such that contiguous Pd sites are exposed. At CO pressures ≤1 × 10−3 Torr, Pd segregation to the surface is found; however, it is not sufficient to form contiguous Pd sites on the well-annealed sample. At CO pressures higher than ∼0.1 Torr, Pd segregation becomes greatly enhanced such that contiguous Pd sites form. Isosteric heats of adsorption of CO on Au sites and isolated Pd sites are measured to be 69 and 84 kJ/mol, respectively. The CO heat of adsorption on contiguous Pd sites is found to lie between these values. The CO oxidation reaction, monitored using both PM-IRAS and reaction kinetic measurements, occurs only on surfaces with contiguous Pd sites, due to the fact that O2 dissociation takes place exclusively on these sites. The surface composition and distribution can be...

Kinetics of surface atoms during phase transition of Sc–O/W(1 0 0) system at high temperature studied by Auger electron spectroscopy

The kinetics of surface atoms during the phase transition of a Sc–O/W(1 0 0) system at high temperature was investigated by Auger electron spectroscopy (AES). The simulation of the surface coverage was also performed by modeling the kinetics of surface phenomena, i.e., the oxidation, oxygen desorption, diffusion and surface segregation of Sc–O complexes, during phase transition. The results revealed that the stoichiometry of Sc–O complexes on the (1 × 1)-Sc–O/W(1 0 0) surface, the work function of which is low, is close to ScO, and its coverage is approximately 0.6. These findings suggest that the work function of the (1 × 1) surface is decreased by the formation of Sc–O electric dipoles. The increase in the work function during the transition from the (1 × 1) surface to the (2 × 1)-(1 × 2)-Sc–O/W(1 0 0) surface is due to the diffusion of Sc–O complexes into bulk W(1 0 0) and oxygen adsorption on the W(1 0 0) surface. By heating the (2 × 1)-(1 × 2) surface at 1700 K in an ultrahigh vacuum, Sc–O complexes segregate to the surface and excess oxygen adsorbing on the W(1 0 0) surface desorbs, resulting in the formation of the (1 × 1) surface. The present results confirmed that the work function of the Sc–O/W(1 0 0) system strongly correlates with the coverage of ScO on the surface, and the present kinetics models is effective for furthering investigation of the kinetics of surface atoms during the phase transition.

Interface Shape Change and Shift Kinetics on the Nanoscale

In nanostructured materials, where the density of grain- and interphase-boundaries is high, the diffusion and kinetics of surface segregation, i.e. the effective material flow is always influenced by the contributions of these boundaries [1]. Diffusion on the nano/atomic scales in multilayers, thin films has many challenging features even if the role of structural defects can be neglected and ‘only’ the effects related to the nano/atomic scale arise. Different examples for diffusional nanoscale effects discovered recently by the authors will be given in this paper. We show that the continuum descriptions of diffusion cannot be applied automatically on such short distances, the classical continuum approximations (Fick's laws) cannot describe correctly the atomic movements. [2-4] They predict faster kinetics than the atomistic models and the interface shift is always proportional to the square-root of time (x ∝ t1/2 ⇒ x2 ∝ t: parabolic or Fickian kinetics). As we will show, however, the kinetics can be even linear (x ∝ t) on the nano/atomic scale. [3, 4] Furthermore, the continuum descriptions foretell infinitely fast kinetics as the time goes to zero (v=dx/dt∝1/t1/2), which is a long standing paradox of diffusion theory. We will show a possible resolution of this paradox. [5] Moreover, we will show that an initially diffused interface can sharpen even in completely miscible systems. [6, 7]

On the Local Equilibrium during Dissolution of a Thin Film

Thirty years ago, Lagües and Domange [1] studied surface segregation kinetics and proposed a method by which it is possible to determine the equilibrium surface segregation isotherm from one measurement, supposing that the local equilibrium hypothesis (LEH) is obeyed. This method has been using widely in the literature (e.g. [2-4]) since its born. It was used e.g. to study of dissolution of thin films into different substrates. In this case additionally it was assumed that the kinetics can be described by the classical Fick I equation (hybrid method). In this paper, using computer simulations, we investigate that under which conditions the LEH is obeyed during the dissolution of a thin film. Moreover, we show that even when the LEH is fulfilled the proposed ‘hybrid’ method cannot be always applied. We make also some considerations on the effect of stress fields developed by the redistribution of the solute elements during the segregation kinetics.

Atmosphere change of vacuum to hydrogen and sharpness of {110}〈001〉 texture in thin-gaged 3 Pct Si-Fe alloy strips

Heating in vacuum finally resulted in a main {100}〈uvw〉 texture and a trace of {111}〈uvw〉 texture. However, the texture was changed to the {110}〈001〉 texture when the strip was heated in hydrogen or when the vaccuum was changed to hydrogen at a temperature Tc. As Tc increased, the sharpness of the {110}〈001〉 texture increased, resulting in the high magnetic induction. This can be understood in the light of surface segregation kinetics of sulfur.

Role of cold rolling texture and heating rate on final texture and magnetic induction in electrical steels

With increasing heating rate, the final main texture of inhibitor-free 3% Si–Fe alloy strips was roughly changed from the {110}〈001〉 texture to the {110}〈 uvw〉 + the {110}〈 uvw〉 texture and subsequently to the {100}〈 uvw〉 + the {111}〈 uvw〉 texture, while magnetic induction decreased from 2.03 to 1.40 T. Such a dependence on heating rate is deeply related to the change in surface segregation kinetics of sulfur that governs the surface-energy-induced selective growth. The strips cold-rolled without any lubricant showed mainly the {110}〈001〉 and the {110}〈 uvw〉 grains after final annealing, irrespective of heating rate. The final texture of the strips cold-rolled with a lubricant was very sensitive to the heating rate. This may be due to the nucleation behavior of the {110} nuclei that are formed from two kinds of cold rolling textures.

Importance of proper choice of transition rates in kinetic simulations of dynamic processes

Proper transition rates in kinetic mean-field and Monte Carlo simulations of dynamic processes are very important to obtain realistic results. We show that by proper choice of the transition rates one can unify the advantages and eliminate the disadvantages of different models used in the literature [see, e.g., Senhaji et al., Surf. Sci. 274, 297 (1992 )( kinetic tight binding Ising model), Martin, Phys. Rev. B 41, 2279 (1990) and Cserhati et al., Surf. Sci. 290, 345 (1993)]. Furthermore, we also show that this choice cannot be considered simply as an extension of the previous ones. It contains an additional parameter controlling the ratio of the transition rates in the bulk and close to the surface sGsurf / Gbulkd, which influences the kinetics of surface segregation. We show that some results obtained previously need reconsideration. We illustrate, e.g., how the composition dependence of the transition rates (diffusion coefficient) influences the ‘surfactant formation and dissolution mode’. For example, the dissolution kinetics can deviate from the parabolic law on the nanoscale in accordance with our recent results [Erdelyi et al., Phys. Rev. B 69, 113407 (2004)]. Furthermore, we also present how the sGsurf / Gbulkd ratio affects the kinetic segregation isotherm.

Surface segregation kinetics of sulfur influenced by various conditions in thin-gauged 3% Si–Fe alloy strips

Abstract The surface segregation kinetics of sulfur has been investigated in detail in 3% Si–Fe alloy strips. The segregation profile of sulfur mostly shows a maximum during annealing at 1200 °C. The maximum surface concentration and the annealing time required to observe a certain concentration of segregated sulfur depend on hydrogen flow rate, heating rate and strip thickness.