Surface Segregation Of Ag Research Articles

Effect of surface modification on de-alloying of Pd 70Ag 30 nanoparticles

Heating of surface modified Pd 70Ag 30 alloy nanoparticle pellets in air was conducted and the effect of surface modification on selective oxidation of Pd and surface segregation of Ag on the surface was examined. For unmodified nanoparticles at 250 °C, free Ag was observed because of preferential oxidation of Pd. Modification with stearic acid reduced the degree of oxidation and the amount of free Ag. With polyethylene glycol, the oxidation of Pd was further suppressed. No free Ag was formed on the pellet surface, instead, more segregation of Ag within the alloy nanoparticles to the surface occurred. The effectiveness of these two surfactants gradually diminished at 350 °C.

Surface segregation and homogenization of Pd 70Ag 30 alloy nanoparticles

Pd 70Ag 30 alloy nanoparticles were prepared using three different reducing agents at various pH values. The composition homogeneity inside the alloy nanoparticles was tuned by the reduction conditions. The surface segregation and homogenization during heating was then studied. Alloy nanoparticles with the smallest size and the highest homogeneity were prepared by the strongest reducing agent NaBH 4, and the change of surface composition after heating was the smallest. The surface segregation of Ag was small and the sintering was retarded by the high concentration of surface Pd or by the residual B from the reducing agent. There was significant surface segregation of Ag and sintering after heating for the alloy nanoparticles prepared by HCHO, where a higher concentration gradient existed inside the nanoparticles. The reducing capability of N 2H 4 was in between those of HCHO and NaBH 4 and the behavior of nanoparticles prepared by N 2H 4 also exhibited a behavior between those of the other two samples.

CrN–Ag self-lubricating hard coatings

CrN–Ag composite films, 2.5–4 μm thick, were deposited by reactive magnetron sputtering on Si and stainless steel substrates in a 0.4 Pa pure nitrogen atmosphere at 500 °C. The layer composition was controlled by the relative power to 7.5-cm-diameter Ag and Cr targets to be 0, 3, 12, and 22 at.% Ag. The layers exhibit a dense columnar microstructure where the Ag is homogeneously distributed in the CrN matrix. The coatings were vacuum annealed for t a = 5–60 min at temperatures ranging from T a = 600–700 °C, to study the Ag-lubricant transport to the surface. Ag surface segregation during annealing is negligible for layers with 3 at.% Ag content. However, Ag diffuses to the surface of layers with ≥ 12 at.% Ag and forms particles with diameters ranging, as a function of T a and t a, from 50 to 1100 nm. Statistical analyses provide average particle sizes of 110 and 375 nm, and surface particle densities of 2 × 10 7 and 1.7 × 10 6 mm − 2 , for the 12 and 22% Ag samples, respectively. The Ag diffusive transport occurs 2–3 times faster for the 22% than for the 12% sample. We attribute the increase in Ag transport as a function of the total Ag concentration to a network of pores that form during deposition due to Ag segregation into nanometer-size pockets. The pores grow in width and connectivity as the total Ag concentration is increased, leading to a faster Ag transport at elevated Ag concentrations. Room-temperature micro-scratch tests show that the average friction coefficients and wear track depths monotonically decrease, by 27 and 56%, respectively, when the Ag concentration is increased from 0 to 22 at.%.

Real-time scanning electron microscopy and auger spectroscopy of wetting in Sn-3.3Ag-4.8Bi solder paste wetted to Ni-Au

Surface and thin film techniques reveal an immediate and intense surface reaction during wetting of Sn-3.3Ag-4.8Bi solder paste to Ni-Au board finishes. Rutherford backscattering spectroscopy (RBS) shows that a typical, vendorsupplied Ni-Au board finish consists of ∼750 A of Au above the Ni. Stylus profilometry indicates that the finish surface is rough with average vertical height variations of ∼1–2 µm. The board finish seen by the solder alloy during wetting contains a large ∼45% atom fraction of adventitious surface C observed by Auger spectroscopy (AES). The AES and energy-dispersive x-ray spectroscopy (EDX) indicates that Ni has fully intermixed throughout the alloy within seconds of wetting. Dissolution of the Au cap layer into the alloy and Bi and Ag surface segregation is observed. Auger point analysis and maps at the leading edge of the wetting front show no evidence of flux advance ahead of the alloy. Bismuth does not move out with the spreading wetting front. A unique feature of this work is the acquisition of real-time, in-situ electron microscopic observations of the spreading Sn-3.3Ag-4.8Bi alloy front across the Ni-Au substrate during wetting. The observations have been recorded on videotape and converted into computer files accessible on the World Wide Web with standard media players (ftp://131.204.44.20).

Cu-Ag (111) polymorphism induced by segregation and advacancies.

Chemical and structural phase transitions induced by Ag surface segregation in the dilute Cu(Ag) (111) system have been investigated by Monte Carlo simulations. The polymorphism observed when depositing Ag on Cu (111) is proven to exist also in equilibrium segregation. If the segregation isotherms are not very sensitive to the superstructures, we show that the superstructure observed in the high part of the isotherm depends strongly on the number of advacancies.

Equilibrium surface segregation of silver to the low-index surfaces of a copper single crystal

The equilibrium surface segregation of Ag to the low-index surfaces of Cu single crystals is presented. It is found that the close-packed surface of the (111) orientation allows a high interaction parameter resulting in a discontinuous transition in the surface concentration against temperature, as well as a step in the segregation kinetics. In contrast, the rougher surfaces of the (110) and (100) orientations yield smaller interaction parameter values and a smooth transition in surface coverage against temperature is observed. Copyright © 1999 John Wiley & Sons, Ltd.

Surface segregation in an Al–4.2 at% Ag alloy

The equilibrium surface composition of a polycrystalline Al–4.2 at% Ag alloy has been measured as a function of temperature using Auger electron spectroscopy. These results indicate that silver segregates to surfaces with an apparent enthalpy of segregation of about −38.5 kJ/mol. The equilibrium average surface composition of the alloy has been calculated as a function of surface orientation using the Lee–Aaronson discrete lattice plane model. A modified regular solution model originally due to Alexander et al. was used to represent the interactions between Al and Ag atoms. While this model successfully described the relevant portion of the Al–Ag phase diagram and the faceting behavior of GP zones in Al rich Al–Ag alloys, it predicted no surface segregation of Ag in the present studies. These results are in contrast to the results obtained experimentally. Possible reasons for the lack of agreement are considered. Finally, the surface segregation of silver measured by AES was compared with that predicted by the Gibbs adsorption isotherm, wherein the latter calculation made use of the results of ultra high vacuum measurements of the bulk composition dependence of the surface tension of α Al–Ag alloys (to be reported elsewhere). Qualitative agreement was found between the two experimental methods, both indicating enrichment of Ag at the free surfaces.

Nucleation of two-dimensional phases on the (111) surface of Cu-Ag alloys

Experiments have been performed on the (111) surface of dilute Cu-Ag alloys under conditions where the surface segregation of Ag leads to a two-dimensional phase transition in which a 2D Cu-rich surface phase undergoes transformation to 2D Ag-rich phase upon cooling, and vice versa. The results show that the formation sites of the Ag-phase upon cooling are different from those at which the Cu-phase forms on heating. Three possible interpretations are provided: nucleation at points of intersection of high diffusivity paths with the surface, nucleation enhancement by sub-surface compositional fluctuations, and nucleation at rough versus smooth regions of the surface. The first interpretation can be ruled out, but it is not possible to distinguish conclusively between the other two interpretations.

Segregation kinetics of Ag on a (100) surface of Al-3at%Ag

Surface segregation of Ag on a (100) surface of Al-3at%Ag at temperatures below the metastable miscibility gap of the bulk is determined as a function of time by means of Auger electron spectroscopy. An activation energy of 106 ± 13 kJ/mol is deduced from the data. This value agrees with results for bulk diffusion. This shows the capability of a surface tool in determining bulk properties of a metal.

Ag segregation and epitaxial Cr growth on Ag(100)

The crystallographic and electronic structure of Cr films deposited on an Ag (100) crystal is studied by angle resolved photoemission. Both valence band and photoelectron forward scattering data show that body-centered-cubic Cr grows epitaxially on Ag (100) held at room temperature during deposition. Yet the Cr film is not uniform and exhibits a poor long range order. As soon as bilayer islands are formed and up to coverages as high as ∼60 layer equivalent (LE), the growth of Cr films is invariably accompanied by surface segregation of Ag. Apparently monolayer islands formed initially are free of segregated Ag. This is confirmed by a study of the flat well-ordered Cr monolayer formed when one LE is deposited on the substrate heated at ∼440 K [C. Krembel, M. C. Hanf, J. C. Peruchetti, D. Bolmont, and G. Gewinner, J. Magn. Magn. Mater. 93, 529 (1991); C. Krembel, M. C. Hanf, J. C. Peruchetti, D. Bolmont, and G. Gewinner, Phys. Rev. B 44, 8407, 11472 (1991)]. In spite of the higher substrate temperature during deposition we could not find any evidence for the presence of segregated Ag on top of the ordered Cr monolayer. Possible reasons for this peculiar behavior, namely that Ag segregation takes place on bi- or multilayers of Cr but not on the monolayer, are discussed.

Structural, electronic, and magnetic properties of a Ni monolayer on Ag(001): Ni adsorption versus Ag surface segregation.

Structural, electronic, and magnetic properties of a Ni monolayer on Ag(001): Ni adsorption versus Ag surface segregation.

Surface segregation of Ag in CdTe induced by photon absorption

The first observation of a new impurity segregation or gettering phenomenon has been demonstrated in CdTe using radioactive Ag. Noble metal dopant impurities have been found to segregate to the surface region of CdTe during exposure to ambient light. This phenomenon occurs while the sample is held at room temperature and is not induced by localized heating or surface chemical reactions. Experimental results suggest that a redistribution of excess carriers produced by photon absorption near the CdTe surface may be responsible for the behavior.

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.

Surface composition and oxygen chemisorption on Ag-Pd alloys

Ag-Pd alloys prepared by room temperature coprecipitation from solutions of the corresponding nitrates have been subjected to surface analysis by Auger electron spectroscopy. Previous work on the adsorption of O 2 on the same alloys showed that except for very high Pd concentration the adsorption heat was independent of alloy composition. On untreated and Ar ion sputtered alloys Pd enrichment in the surface layers was found, while on thermally annealled alloys (<0.1 Ton or 1 atm of H 2 or O 2), surface segregation of Ag took place. This is the expected result from surface tension and/or atomic size considerations. The relation of these findings with the earlier data on the chemisorption heat of oxygen is discussed. The implications of these conclusions upon present models of metallic alloys is pointed out.