Tarnishing of Au-Ag-Cu alloy studied by Auger electron spectroscopy and coulometry

Tarnishing of a single phase 14 carats Au-Ag-Cu alloy was studied using the Tuccillo-Nielsen test. Auger Electron Spectroscopy and coulometry were employed for the characterization of the composition and the thickness of the surface layers formed. X-ray diffraction was used for further characterization of thicker layers. The layer formation followed linear kinetics. The tarnished layer at short test durations of 0.5 to 5 hours consisted mainly of Ag₂S, CuS and Cu₂S or compounds near to this stoichiometry. The composition of the top surface of the tarnished layer is constant but its global composition changes with the duration of the tarnishing test.     

Synthesis and photoactivity of the highly efficient Ag species/TiO2 nanoflakes photocatalysts

Ag species/TiO₂ nanoflakes photocatalysts with different relative contents (Ag⁺, Ag²⁺, Ag⁰) have been successfully synthesized by a simple template-free synthetic strategy. X-ray photoelectron spectroscopy, X-ray diffraction, and UV-vis diffuse reflectance spectra indicated that the dopant ions (Ag⁺ or Ag²⁺) were partly incorporated into the titanium dioxide nanoflakes. Meanwhile, part of the silver ions migrated to the surface after the subsequent calcination and aggregated into ultra-small metallic Ag nanoclusters (NCs) (1-2 nm), which are well dispersed on the surface of TiO₂ nanoflakes. The photocatalytic activities of the Ag species/TiO₂ materials obtained were evaluated by testing the photodegradation of the azo dye reactive brilliant X-3B (X-3B) under near UV irradiation. Interestingly, it was found that the maximum photocatalytic efficiency was observed when Ag species coexisted in three valence states (Ag⁺, Ag²⁺, Ag⁰ NCs), which was higher than that of Degussa P25. The high photocatalytic activity of the Ag species/TiO₂ can be attributed to the synergy effect of the three Ag species.     

Novel AgCl/Ag<Subscript>2</Subscript>CO<Subscript>3</Subscript> heterostructured photocatalysts with enhanced photocatalytic performance

A series of novel AgCl/Ag₂CO₃ heterostructured photocatalysts with different AgCl contents (5 wt%, 10 wt%, 20 wt%, and 30 wt%) were prepared by facile coprecipitation method at room temperature. The resulting products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic degradation of methyl orange (MO) under UV light irradiation. With the optimal AgCl content of 20 wt%, the AgCl/Ag₂CO₃ composite exhibits the greatest enhancement in photocatalytic degradation efficiency. Its first-order reaction rate constant (0.67 h^?1) is 5.2 times faster than that of Ag₂CO₃ (0.13 h^?1), and 16.8 times faster than that of AgCl (0.04 h^?1). The formation of AgCl/Ag₂CO₃ heterostructure could effectively suppress the recombination of the photo-generated electron and hole, resulting in an increase in photocatalytic activity.     

One-pot synthesis of highly stable silver nanoparticles-conducting polymer nanocomposite and its catalytic application

Herein, we report the preparation of highly stable Ag_nano–PEDOT nanocomposite by one-pot fashion in acidic condition using 3,4-ethylenedioxythiophene (EDOT) as a reductant and polystyrene sulfonate (PSS^?) as a dopant for PEDOT as well as particle stabilizer for silver nanoparticles (AgNPs). The above nanocomposite denoted as Ag_nano–PEDOT/PSS^? nanocomposite. The formation of AgNPs with concomitant EDOT oxidation was followed by UV–visible (UV–vis) spectroscopy at different time intervals. Ag_nano–PEDOT/PSS^? nanocomposite shows absorption bands at 380 and above 700 nm, which correspond to surface plasmon resonance (SPR) peak of AgNPs and oxidized PEDOT, respectively. Ag_nano–PEDOT/PSS^? nanocomposite was characterized by infrared (IR) spectroscopy, transmission electron microscopy (TEM), and XRD. TEM study reveals that AgNPs are distributed uniformly around PEDOT polymer with an average particle size diameter of 10–15 nm. In addition, Ag_nano–PEDOT/PSS^? nanocomposite was tested for the catalytic reduction of 4-nitrophenol. For comparing stability, we were also synthesized AgNPs in the absence of PSS^? (denoted as Ag_nano–PEDOT) using EDOT as reductant. UV–vis spectrum of Ag_nano–PEDOT nanocomposite revealed that AgNPs prepared in the absence of PSS^? was not stable.     

A study of the Ag(s)/Ag2SO4(l) high temperature reference electrode

Ag(s)/Ag₂SO₄(l) reference electrodes for use in hot corrosion research were fabricated and their properties were studied at 927°C (1200°K). The McDanel^TM MV 30 mullite tube was used as sheath of the reference electrode and concentrations of the tested electrolytes were varied from 0.1 to 10 m/o Ag₂SO₄ (l) + Na₂SO₄ (l). 10 m/o Ag₂SO₄ was always used as electrolyte in the reference electrode. The working electrodes with less than 2 m/o Ag₂SO₄ showed marked potential drift as the Ag₂SO₄ concentration was lowered, showing poor potential stability. However, the initial values of cell voltages obeyed Nernstian behavior for all range of Ag₂SO₄ concentrations in the working electrodes, which showed that Ag(s)/Ag₂SO₄(l) electrodes behaved reversibly. Galvanostatic polarization was performed on a pair of 10 m/o Ag₂SO₄ reference electrodes to test the reversibility. The polarizability was 3.5 mV/10 μA with no hysteresis loop on the polarization curve, showing that the reversibility was satisfactory. The potential of gold electrode in molten Na₂SO₄ was measured under pure oxygen atmosphere with 10 m/o Ag₂SO₄ electrode as a reference potential. The results showed that the potential of gold electrode could be explained by the 2 Na + SO₃ + 1/2 O₂ → Na₂SO₄ cell reaction.     

Thermodynamic Properties of Superionic Phase Ag<Subscript>4</Subscript>HgSe<Subscript>2</Subscript>I<Subscript>2</Subscript> Determined by the EMF Method

Triangulation of the Ag-Hg-Se-I system in the vicinity of quaternary phase Ag₄HgSe₂I₂ was performed by differential thermal analysis, X-ray diffraction and electromotive force (EMF) methods. The spatial position of the phase region Ag₄HgSe₂I₂-Se-HgI₂ regarding the figurative point of silver was used to write the chemical reaction of formation of Ag₄HgSe₂I₂. The EMF measurements were carried out by applying an electrochemical cell: (–) C|Ag|Ag₂GeS₃ glass|Ag₄HgSe₂I₂, HgI₂, Se|C (+), where C is graphite and Ag₂GeS₃ glass is the fast purely Ag⁺ ions conducting electrolyte. The linear dependence of the EMF of the electrochemical cell on temperature was used to determine the standard thermodynamic values of Ag₄HgSe₂I₂ for the first time.     

Internal Oxidation of a Ag-l.3 at.% Se Alloy. Part I: Composition and Appearance of Oxidation Products

The internal oxidation of a two-phase Ag-1.3 at.% Se alloy in pure oxygen was studied at 750, 800 and 830°C. The alloy is composed of a dilute solid solution of selenium in silver) and Ag₂Se intermetallic particles. The internal oxidation of this alloy proceeds through gradual in-situ oxidation of the Ag₂Se particles as well as through diffusive internal oxidation of selenium from solid solution. Gradual in-situ internal oxidation of Ag₂Se particles reflects itself in the appearance of two internal-oxidation fronts, inner and outer, marking the beginning and end of the in-situ oxidation of Ag₂Se particles. The oxide phase formed during in-situ oxidation is the molten double oxide, Ag₂SeO₃. A theoretical treatment of the phase relationships in a hypothetical Ag–Ag₂Se–Ag₂SeO₃ ternary phase diagram is presented to assist the explanation of the mechanism of in-situ oxidation. The kinetics of oxidation are presented in Part II.     

Isothermal section of the Ag2S-PbS-GeS2 system at 300 K and the crystal structure of Ag2PbGeS4

The isothermal section of the Ag₂S-PbS-GeS₂ system at room temperature was investigated by XRD. The existence of two quaternary compounds, Ag₂PbGeS₄ and Ag_0.5Pb_1.75GeS₄, was confirmed, and the phase equilibria between the binary system components and the ternary and the quaternary compounds were determined. The crystal structure of Ag₂PbGeS₄ was studied using the single crystal X-ray diffraction. It was established that Ag₂PbGeS₄ crystallizes in an own structural type in non-centrosymmetric space group Ama2 with the lattice parameters a = 1.02390(4) nm, b = 1.02587(5) nm, c = 0.67701(3) nm.     

Galvanically coupled process for the conversion of Ag2O to AgI

The conversion of Ag₂O, grown-on Ag substrates, to AgI has in been investigated, using open-circuit potential, linear polarization, and potentiostatic control measurements. Three distinct reaction stages were clearly identified from the time-dependent behaviour of the open-circuit potential, E_OC, and the corrosion currents measured by linear polarization. In the early stages, when the silver surface was covered with a coherent Ag₂O film with low porosity, E_OC∼(E^e)_Ag2O/Ag and only chemical conversion of Ag₂O to AgI was observed. As Ag₂O was consumed and the area of Ag exposed to I⁻ solution increased, galvanic coupling of Ag₂O reduction to Ag and the oxidation of the Ag substrate to AgI also occurred. Once all the Ag₂O has been reduced, AgI formation stopped and E_OC fell to (E^e)_AgI/Ag. Chemical conversion was shown to produce fine particulate AgI whereas that formed anodically via galvanic coupling was in the form of large crystals. Since few larger crystals were observed, conversion via galvanic coupling appeared to be a minor process.     

Functionalization of the surface of ceramic membranes with 3-mercaptopropyl groups using the sol-gel method

Tubular ceramic membranes based on α-Al₂O₃ were functionalized with 3-mercaptopropyl groups using the sol-gel method. The formation of the polysiloxane layer on the inner surface of the membranes was confirmed by SEM images and FTIR spectroscopy data. The functionalized membranes (identically to the original one) retain the high performance typical of highly efficient microfiltration membranes (320–690 L/m² h); however, they can be used to remove silver(I) ions from water due to the formation of complexes between thiol groups of the active layer and silver(I) ions. It was also shown that the decreasing concentration of functionalizing sol produces a thinner surface polysiloxane layer with more active sites available for the sorption of Ag⁺ ions. Thus, fourfold dilution of the starting sol results in a tenfold increase in the sorption of Ag⁺ ions calculated per mass unit of the functionalizing layer (from 23 to 203 mg/g).     

Effect of Ag addition on the corrosion properties of Sn‐based solder alloys

Corrosion properties of three different Sn‐Ag lead free solder alloys have been investigated in 0.3 wt% Na₂SO₄ solution as corrosive environment. As cast solder alloy was analyzed by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Volume fractions of the Ag₃Sn in the solders were determined by image analysis technique. Pitting potential and corrosion potential for the alloys were determined by potentiodynamic tests. Electrochemical impedance spectroscopy (EIS) was carried out to measure the film and charge transfer resistance. Alloys with lower Ag content have been found as better corrosion resistance material.     

Mechanism of Ag3Sn grain growth in Ag/Sn transient liquid phase soldering

Transient liquid phase (TLP) bonding is a potential high-temperature (HT) electron packaging technology that is used in the interconnection of wide band-gap semiconductors. This study focused on the mechanism of intermetallic compounds (IMCs) evolution in Ag/Sn TLP soldering at different temperatures. Experimental results indicated that morphologies of Ag₃Sn grains mainly were scallop-type, and some other shapes such as prism, needle, hollow column, sheet and wire of Ag₃Sn grains were also observed, which was resulted from their anisotropic growths. However, the scallop-type Ag₃Sn layer turned into more planar with prolonging soldering time, due to grain coarsening and anisotropic mass flow of Ag atoms from substrate. Furthermore, a great amount of nano-Ag₃Sn particles were found on the surfaces of Ag₃Sn grains, which were formed in Ag-rich areas of the molten Sn and adsorbed by the Ag₃Sn grains during solidification process. Growth kinetics of the Ag₃Sn IMCs in TLP soldering followed a parabolic relationship with soldering time, and the growth rate constants of 250, 280 and 320 °C were calculated as 5.83×10^?15 m²/s, 7.83×10^?15 m²/s and 2.83×10^?14 m²/s, respectively. Accordingly, the activation energy of the reaction was estimated about 58.89 kJ/mol.     

Formation of Ag2S nanowires and Ag2S/CdS heterostructures via simple solvothermal route

In our present work, Ag₂S nanowires and Ag₂S/CdS heterostructures have been successfully prepared in anhydrous ethanol through a simple solvothermal route. From the transmission electron microscopy (TEM) analysis, it is found that the factors influenced the final product are the concentration of Ag⁺, reaction temperature, reaction time, and solvent. Ag₂S nanowires are formed by complete Ag⁺ cation exchange. Because of the selectivity for partial cation exchange, the reaction starts preferentially at the ends of the CdS nanowires to produce novel Ag₂S/CdS heterostructures. In addition, as the two end facets of wurtzite CdS nanowires are crystallographically nonequivalent, the produced Ag₂S/CdS heterostructures are asymmetric.     

Experimental thermodynamic study of intermetallic phases in the binary Ag–Te system by an improved EMF method

A thermodynamic study of the solid two-phase regions of the binary Ag–Te system was made by an improved EMF method, using fast ion conductor RbAg₄I₅ as the solid electrolyte. The EMF measurements were made on three galvanic cells: [Ag | RbAg₄I₅ | Ag₅Te₃ + Te], [Ag | RbAg₄I₅ | Ag₅Te₃ + Ag_1.9Te] and [Ag | RbAg₄I₅ | Ag_1.9Te + Ag₂Te].Based on the results obtained, the EMF for each equilibrium phase assembly was expressed as a function of temperature, in the different regions of thermal stability of the substances. By using the observed EMF and temperature relations, the thermodynamic functions of the stoichiometric equilibrium phase assemblages: Ag₅Te₃–Te, Ag₅Te₃–Ag_1.9Te, Ag₅Te₃–Ag₂Te and Ag₂Te–Ag_1.9Te, in the low-temperature range 22–204 °C, as well as phase transformation temperatures, have been determined. Agreement between the results obtained and the literature values were established.     

Oxidation Mechanism of Silver Selenide

The mechanism of selenium vaporisation from silver selenide, the major carrier of selenium in copper depleted anode slimes, was studied using isothermal oxidation of synthetic, massive Ag₂Se in flowing, dry oxygen and oxygen + 20 % sulfur dioxide mixtures at 450–550 °C. The reaction rates expressed as mass change rate indicate that in pure oxygen at 550 °C the reactions essentially cease within 1 h. Silver selenite scales were identified on the interface by SEM–EDS observations. The melting point of silver selenite is 540 °C, and above it selenite is formed in the molten state, where metallic silver as condensed deselenization product is embedded. Metallic silver as a roasting product of Ag₂Se will be partly sulfated in SO₂-bearing atmospheres. A small fraction of selenium is transformed into silver selenite, as the direct oxidation product of Ag₂Se. The reactions proceed much faster into the selenite matrix along the grain boundaries than the surface reactions. The reaction product layer is composed of very fine Ag₂SeO₃ and Ag₂SO₄ particles on the primary Ag₂Se, and significant porosity is generated next to the reaction zone.     

Internal Oxidation of an Ag–1.3 at.% Te Alloy

The internal oxidation of Ag–1.3 at.% Te was studied at 750, 800, and 830°C in pure oxygen (1 atm). The internal oxidation under such high oxygen pressure resulted in formation of two different types of oxide particles and two different fronts of internal oxidation in the internal oxidation zone. The coarser Ag₂TeO₃ particles were formed through the in situ internal oxidation of Ag₂Te particles and the tiny oxide precipitates (most probably also Ag₂TeO₃) were formed through internal oxidation of tellurium from solid solution. Considering the mechanism of internal oxidation, both diffusionless and diffusive modes were found to be present simultaneously in the oxidation of Ag–1.3 at.% Te alloy. These results were examined with regard to the solubility of tellurium in silver, which was found to be 0.1 at.% Te at 750°C and 0.26 at.% Te at 830°C, as well as the presence and dissolution of Ag₂Te particles.     

Use of the scratching electrode technique to interpret the stress corrosion cracking behaviour of α-brass in ammoniacal copper sulphate solutions

Stress corrosion cracking (s.c.c.) of α-brass in Mattsson solutions [1 M (NH₃ + NH₄⁺); 0.05 M CuSO₄] at a given strain-rate intensifies in pH order: 4.75 < 7.35 < 11.35. There is no cracking at pH 8.9 or when tarnishing at pH 7.35 is prevented by reducing [NH₃ + NH₄⁺] to 0.25 M. A technique of continously scratching a shallow circular track on a rotating electrode has been used to investigate the electrochemical behaviour of bared metal surfaces in these environments with the object of examining the relative contributions of slip-dissolution and tarnish-rupture in the crack propagation mechanism. It is shown that the observed s.c.c. propensity correlates well with: (i) the physico-chemical nature of the reaction products, (ii) the bare surface current density at the applied potential and (ii) oxide growth kinetics on the bare surface. Thus, in non-tarnishing solutions s.c.c. does not occur if either the rate of protective oxide formation is sufficiently rapid [pH 7.35 (0.25 M NH₃)] or the whole surface is dissolving uniformly (pH 8.9); in the intermediate case (pH 4.75) cracking is observed. In tarnishing solutions [pH 7.35 (1 M NH₃) and pH 11.35] the crack propagation rate increases with the tarnishing rate. Qualitative agreement between s.c.c. behaviour and bare surface oxide growth rate, as determined by the scratching electrode method, supports a slip-dissolution model of propagation.     

Scaling effect on Ag3Sn growth behaviours in micro-joints for flip chip assemblies

The scaling effect on Ag₃Sn growth behaviours in Sn–3.0Ag–0.5Cu (SAC305) micro-joints of flip chip assemblies was investigated using thermal shock (TS) tests. After assembly reflow, the large plate-like Ag₃Sn compounds only emerged from the Cu interface of small joints, which was strongly related with a higher local Ag concentration in the remaining solder. During TS cycling, the growth of Ag₃Sn grains exhibited comparatively more pronounced growth in the solder matrix of small joints, due to the stronger strain-enhanced coarsening induced by more cycle stress and strain. Coarsening kinetic models based on TS experiments were employed to predict Ag₃Sn growth, the kinetic constants N were determined to clarify the correlation of the joints scaling and Ag₃Sn coarsening in depth.     

Textured VN coatings with Ag3VO4 solid lubricant reservoirs

Silver oxovanadate powders were produced using a low temperature hydrothermal synthesis method. X-ray diffraction (XRD) and Raman microscopy revealed the formation of the α-Ag₃VO₄ phase, and differential scanning calorimetry (DSC) indicated that the powders underwent phase changes with the increase of temperature (formation of Ag-deficient phases). Ag₃VO₄ precipitates were subsequently investigated as potential high temperature solid lubricant (SL) powders that may be burnished onto textured surfaces. Textured coatings were produced by the sputter deposition of vanadium nitride (VN) thin films, followed by the fabrication of a periodic array of micro-scale dimples with reactive ion etching (RIE). The etched patterns acted as reservoirs to replenish the sliding contact with solid lubricants. The effectiveness of this new design of high temperature tribological coating was tested using a pin-on-desk tribotester. A significant decrease in the wear rate and coefficient of friction (CoF) was achieved at high temperatures (750 °C) and was maintained as a result of the storage of the Ag₃VO₄ phase in the dimples. After wear testing, Raman spectroscopy and XRD were used to identify the phase composition developed as a result of tribotesting. It was found that the burnished powders were more effective in enhancing the tribological properties of textured films compared to burnished/un-burnished monolithic VN coatings.