Equilibrium and kinetic surface segregations in Cu–Sn thin films

Equilibrium and kinetic surface segregations in Cu–Sn thin films are simulated based on the modified Darken model for a finite sized system. The simulations are carried out for all compositional ranges and film thicknesses. The segregation energy and the interaction parameter for the simulation are determined respectively by the Miedema model and from the thermodynamic data used for the Cu–Sn phase diagram calculation. The strong negative interaction parameter restrains Sn surface segregation in the Cu–Sn system. The size effect on surface segregation depends on the segregation energy, interaction parameter, initial bulk concentration, temperature and film thickness.     

Concentration dependence of Fe segregation near the surface in V-Fe alloys after V+ ion irradiation

The effect of Fe segregation near the free surface of V model alloys containing 2, 5, or 7 at % Fe is investigated by x-ray photoelectron spectroscopy. Segregation is induced by 50-keV V⁺ ion irradiation at a temperature of 30–40°C with fluences ranging from 10¹⁹ to 10²¹. Young’s moduli in these alloys are measured by the torsion pendulum method. The degree of Fe segregation is estimated, and its dependence on the irradiation dose and iron concentration in these alloys is analyzed. Correlation is found between the behavior of Young’s moduli and the degree of iron surface segregation as functions of the Fe concentration in the alloys.     

Analysis of the effect of the energy of interatomic interaction and the structure of Fe-Cr alloys on the processes of radiation-induced segregation after ion etching

We present the results of a comparative analysis of profiles of the radiation-induced segregation of chromium after irradiation of model alloys Fe-(9, 11, 14) at % Cr by He⁺ ions with the energy of 30 keV with fluences 10¹⁹–10²¹ ions/m² at 450°C and data on varying the moduli of normal elasticity and the structure of these alloys depending on the Cr concentration. It is shown that an interstitial migration mechanism is the determining factor in the case of chromium segregation near the surface. A correlation between the character of varying the elasticity moduli, order, and value of surface segregation and swelling of the Fe-Cr alloys depending on the Cr concentration is established.     

Dual mode segregation of Pd to the surface of polycrystalline Fe99Pd1

On a polycrystalline Fe₉₉Pd₁ sample, unusually fast and abundant segregation is observed up to 55 at% Pd. The dependence on temperature and on the bulk concentration are also contradictory to normal segregation behaviour. Once the sample is heated to at least 750–800°C, normal, slower segregation behaviour is observed up to a maximum of ca. 35 at% Pd. The unusual segregation behaviour can be explained by segregation from a two-phase bulk leading to an ordered Fe₅₀Pd₅₀ surface phase.     

Measuring grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy: Further developments

A technique for the quantification of surface and grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy (WDS) in the Scanning Electron Microscope (SEM) is proposed. As an example, the case of sulphur segregation in nickel is considered. The sulphur segregation can be quantified using a single voltage measurement (20 kV) of the sulphur Kα line intensity. The quantification is made from a simple proportionality equation derived from the XPP microanalysis model of correction. The result of segregation quantification by WDS is a surface concentration expressed in g.cm^-2.Special attention was paid to the quantification of grain boundary segregation on fracture surfaces, taking into account the off-normal incidence of the electron beam on the analyzed surface. A simple technique that allows determination of the tilt angle from the specimen absorbed current is proposed. The influence of the azimuth angle of the analyzed surface with respect to WDS spectrometer is also discussed.The results of the WDS technique are shown to be repeatable within 5% with reasonable counting times (a few minutes per facet). As an example, the kinetics of sulphur grain boundary segregation in nickel at 750 °C was measured using WDS to document the quantitative capabilities of the technique.As already pointed out in a previous paper, it is confirmed that WDS is insensitive to surface contamination, which allows the analysis of ex-situ fractured samples.     

He,Au离子辐照AuCu_3致元素表面偏析

采用磁控溅射方法在单晶硅(111)衬底上制备了AuCu_3薄膜,用2 MeV He离子和1 MeV Au离子对薄膜进行辐照,用卢瑟福背散射对He,Au离子辐照前后AuCu_3薄膜近表面的成分变化进行了分析,对不同离子辐照导致的表面元素偏析行为进行了研究.结果表明:当2 MeV He离子辐照时,随着辐照剂量增大,观察到样品近表面Au元素偏析的趋势;当1 MeV Au离子辐照时,随着辐照剂量增大,观察到样品近表面Cu元素偏析的趋势,与He离子辐照相反.通过对He,Au离子在样品中产生的靶原子空位及其分布分析,发现靶原子空位浓度分布的梯度是导致两种不同表面元素偏析趋势的原因,空位扩散是其中的主要机理.     

Element segregation on the surfaces of pure aluminum foils

The surface segregation trend of trace elements in pure aluminum foils was investigated by density functional theory. The model of nine-layer Al(1 0 0) slab substituted partially by trace element atoms was proposed for calculating surface segregation energy. The calculating results show that (i) B, Mg, Si, Ga, Ge, Y, In, Sn, Sb, Pb and Bi exhibit negative segregation energy and possibly move to the surface, while Be, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr exhibit positive segregation energies and migrated into the bulk; (ii) the segregation energy was found to be related with the covalent radius, the relaxed position at the surface of the substituting atom and the surface energy; (iii) the segregation behavior of trace element generates lots of defects and dislocation, which can increase the initial pitting nucleation sites in the surface of aluminum foils; (iv) the impurity atom concentration was tested with Pb-doped surfaces, the calculated negative segregation energies in all coverage increases rapidly with the Pb coverage. These conclusions are helpful for designing of the chemical composition and to advance the tunnel etching of aluminum foils.     

The surface composition of the silver—Gold system by Auger electron spectroscopy

The surface composition of polycrystalline AgAu alloy foils is studied by Auger electron spectroscopy. The intensities of principal Auger peaks over a wide range of electron energies are obtained as a function of alloy bulk composition for the clean equilibrated surfaces. Their ratios are compared with ratios expected for a surface showing no surface segregation, and for a surface obeying the regular solution monolayer model that predicts silver surface segregation. The experimental results give evidence for enrichment of the surface with silver, but to an extent less than predicted by the regular solution model.     

Kinetics of impurity segregation at grain boundaries in polycrystals. II. Concentrated solution

The isothermal impurity segregation from a finite-size grain into an intergrain boundary region or at an external free surface is investigated when the impurity concentration in the boundary region is not small. Simple algebraic equations are obtained that describe the impurity concentration for the case of several competing and interacting impurities. The process of segregation of two impurities is discussed in detail, and it is shown that the concentration of one of them can have a maximum as a function of time. Fiz. Tverd. Tela (St. Petersburg) 40, 251–253 (February 1998)     

Structure and composition of the segregated Cu in V2O5/Cu system

We have investigated segregation of copper at the surface of V₂O₅ films deposited onto Cu substrate by employing surface analysis techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed that the Cu is segregated at the surface and its chemical state is Cu₂O. According to secondary ion mass spectroscopy (SIMS) and glow discharge spectroscopy (GDS), the Cu concentration inside the deposited V₂O₅ layer is low. Ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) revealed the segregation alters the surface local density of states. Surface analysis of deposited samples in ultra high vacuum (UHV) condition verified that the segregation occurs during the deposition. We have extended kinetic tight binding Ising model (KTBIM) to explain the surface segregation during the deposition. Simulation data approve the possibility of surface segregation during room temperature deposition. These results point out that on pure Cu substrate, oxidation occurs during the segregation and low surface energy of Cu₂O is the original cause of the segregation.     

W-In体系溶质晶界偏聚行为的第一性原理计算

基于第一性原理构建了钨基合金体系的溶质偏聚模型,以W-In体系为例研究了不同浓度下溶质的晶界偏聚行为和成键特征,从电子结构层面揭示了W-In体系的键合作用,预测了W-In体系界面稳定性随溶质浓度的变化规律.结合键布居、电荷密度、差分电荷密度和态密度等电子结构分析,发现了W-In体系中溶质原子在偏聚过程中的键性转变特征,阐明了W-In键由晶粒内部的离子键过渡为晶界区域强共价键的微观机理.模型计算首次得到了W-In体系中溶质本征偏聚能随In浓度的非单调变化规律,结合键合作用和能量分析揭示了溶质浓度对本征偏聚能的影响机制.计算预测了W-In体系达到高热稳定性所需的最佳溶质浓度范围和应避开的溶质浓度范围.本研究为具有高温稳定性的钨基合金材料的设计与制备提供了理论基础和定量化指导.     

Surface composition of native gold and Ag/Au alloys

The surface composition of Ag/Au alloys has been studied by X-ray photoelectron spectroscopy (XPS). Surface segregation of Ag is observed, particularly for alloys having low Ag content. The content of Ag in the first surface monolayer is in accordance with data from ion scattering and with a theoretical model for segregation in ordered solutions. The surface of native gold was found to be enriched with silver in the case of mine gold, and the degree of segregation was significantly higher than for an alloy having similar bulk composition. The surface of mine gold taken from an oxidized zone contained less silver than did that of usual mine gold. The surface of placer gold was depleted of silver as compared to the bulk. These data show the surface composition of native gold to differ markedly from that of an alloy having the same bulk Ag content, and to depend on the genesis of the native gold sample.     

On the selective catalytic behaviour of the Cu-NI alloy surface

The selective catalytic behaviour of the Cu_xNi_1?x alloy surface is studied. The concentration dependence of the catalytic activity with respect to chemical reactions involving cracking and non-cracking of C-C bonds is calculated for a simple model for C-C bonds. Surface segregation and local environment effects on chemisorption both calculated from a microscopic electronic theory using tight-binding approximation are taken into account.     

Carbon segregation to single crystal surfaces of Pt,Pd and Co

Results are reported on the surface segregation behaviour of carbon from dilute solid solutions in Pt, Pd and Co. With Pt(100) no preferential surface segregation was observed; this is similar to previous results for Pt(111). For Pd(lOO), Pd(111) and Co(0001) segregation was observed with evidence for a surface phase transition of the type previously reported for Ni(111). These observations suggest that the strong carbon-carbon interactions within a graphite monolayer are of more importance in producing the transition than a good epitaxial fit to the substrate. A comparison of the kinetics of carbon segregation to Co(0001) with those predicted by a simple diffusion model suggest that surface processes such as nucleation or lateral diffusion may play important roles.     

Equilibrium surface segregation of dissolved nonmetal atoms on iron(100) faces

At elevated temperatures equilibria of surface segregation X (dissolved) = X (adsorbed) have been studied for the nonmetal atoms X = C, N and S. Iron single crystals with (100)orientation have been doped with different concentrations of solute atoms (in the range about 10–100 wt ppm). The samples were introduced into the UHV chamber, cleaned and then heated to temperatures in the α-solid solution range. The surface concentration of the segregated nonmetal atoms was observed by AES for different bulk concentrations in dependence of the temperature. The LEED pattern was also observed during segregation equilibrium at temperatures up to about 750° C. The LEED patterns indicate a c(2 × 2) structure for carbon and nitrogen as well as for sulfur. The temperature dependence of the surface concentration for carbon on Fe(100) can be described by a Langmuir-McLean equation, an average segregation enthalpy of ?85 kJ/mol°C is obtained. Since N₂ desorption occurs the nitrogen segregation is in virtual equilibrium only at temperatures <500°C. The equilibrium surface concentration of sulfur on α-iron is virtually independent of the solute concentration and the temperature: there is always a saturated layer of sulfur on the (100) faces, even at small bulk concentrations. Since the thermodynamic activity of the nonmetal atoms is well defined in the segregation studies (except nitrogen at higher temperatures) , the results can be correlated with studies in gas atmospheres at atmospheric pressure. The relations to the kinetics of the carburization and the nitrogenation of iron are discussed and the influence of sulfur on these reactions.     

Calculation of chemisorption and absorption induced surface segregation

Using a pair bond type model for the interatomic interactions, we determine surface segregation on clean, H, O and CO covered surfaces of various alloys. Furthermore, we study surface segregation caused by bulk hydrogen absorption. Numerical results are presented for Pd_x Zr_1?x H_y We find, that strong surface segregation may result from chemisorption of O and CO and from absorption of H in the bulk of an alloy.     

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_xN_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.     

ToF atom-probe fim investigation of surface segregation in dilute alloys

A time-of-flight atom-probe field ion microscope was successfully employed for the first time to investigate surface segregation in dilute alloys. We were able to achieve a single atomic layer resolution and obtained the absolute concentrations of alloy species of each surface layer. Cr atoms are found to segregate to the surface of Stainless Steel 410 whereas no segregation of the minority species was found for Pt-8% W and Pt-5% Ru. The first layer concentration of Cr in the {110} plane of Stainless Steel 410 at 500°C was found to be 38.5 ± 12.5% and 63.4 ± 10.2% respectively for samples with near surface layers Cr average concentration of 6.3 ± 2.1% and 11.9 ± 2.5%. The heat of segregation of Cr to the {110} plane of Stainless 410 was found to be 3.43 and 3.92 kcal/mole respectively from the two sets of data. The data also gives the difference in surface tensions between iron and chromium at this plane to be 269 and 282 erg/cm² respectively. Segregation studies on the {012} plane as well as on a grain boundary of Stainless Steel 410 were also done. In some cases, though the first surface layer is enriched with Cr in Stainless Steel 410, the near surface layers show a depletion of Cr. In Pt-8% W and Pt-5% Ru, our concentration depth profiles with a single atomic layer resolution show no segregation or depletion of the minority species either for the top layers or for the near surface layers.     

Angle resolved electron spectroscopy for measurement of surface segregation phenomena in liquids and solutions

The technique of variable take-off electron Spectroscopy is applied for the first time to the investigation of surface segregation phenomena in solutions. It is shown that this approach yields quantitative information on depth concentration profiles in electrolytic solutions. In particular, the mean segregation depth and the solute concentration at the solution-vacuum interface is obtained from measured relative intensity data. This is exemplified for the system tetrabutylammonium iodide in formamide.     

Yttrium segregation and surface phases of yttria-stabilized zirconia (1 1 1) surface

Surface segregation of yttria-stabilized zirconia (YSZ) was studied via first-principles computations and thermodynamics. For the cubic YSZ (1 1 1) surface, yttrium can segregate only to a subsurface layer, and these segregation phases are terminated at the surface by defective oxygen layers with honeycomb structure. The segregation is independent of the bulk yttrium concentration at high oxygen partial pressures or low temperatures. At very low oxygen partial pressures and high temperatures there is no surface yttrium segregation and the surface is terminated by O–Zr. Our results provide a reasonable explanation for previous experimental work, and also a framework for extending our understanding of cation segregation in oxide surfaces.