Kinetic peculiarities of anodic dissolution of silver and Ag–Au alloys under the conditions of oxide formation

The kinetics of anodic dissolution of silver and Ag–Au alloys (X_Au = 0.1–30 at.% Au) in aqueous alkaline solution under the conditions of the formation of silver oxides has been examined. The techniques of cyclic voltammetry, chronoammetry, and photopotential measurements have been used. It was established that the anodic formation and cathodic reduction of Ag₂O on silver and alloys are controlled by migration in the oxide layer. Ag₂O oxide is an n-type semiconductor with an excess of silver atoms. Oxide layers formed on monocrystalline Ag(1 1 1) and Ag(1 1 0) are more stoichiometric than the layer formed on polycrystalline Ag.     

Preparation of nanostructure silver powders by mechanical decomposing and mechanochemical reduction of silver oxide

The mechanical decomposing and mechanochemical reductions of silver oxide for preparation of nanocrystalline silver powders by high planetary ball mill was investigated. XRD and HRSEM techniques were used to characterize the structural evolution and morphological changes of products. The results show that the nanostructured silver with an average crystallite size of 14 nm and internal strain of 0.75% is synthesized by mechanical decomposing of Ag₂O after 95 h milling. While, the product of mechanochemical reduction of silver oxide using graphite after 22 h milling is nanostructured silver with an average crystallite size of 28 nm and internal strain of 0.44%.     

On the electrochemical behaviour of tungsten: the formation and dissolution of tungsten oxide in sulphuric acid solutions

The formation and dissolution of the anodic oxide film of tungsten were investigated in sulphuric acid solution of different concentrations. The dissolution process was found to follow a first order mechanism under different conditions. It seemed that, in highly diluted solutions, the hydrate formation is favoured, whereas in concentrated solutions, the process takes place via the formation of WO₂²⁺. The rate of dissolution was found to increase by increasing the initial current density of formation.     

Kinetics of titanium dissolution, and the effect of its alloying with yttrium, lanthanum, and cerium

Refined kinetic equations for titanium electrode are derived; the surface coverage with adsorbed hydrogen is taken into account. Experimental data for titanium and its alloys with yttrium, lanthanum and cerium are shown to agree well with the theory. The kinetic effects of the alloying are traced.     

Preparation of silver tin oxide powders by hydrothermal reduction and crystallization

Silver tin oxide composite powders were synthesized by the hydrothermal method with a silver ammine solution and a Na2SnO3 solution as raw materials. H2C2O4 was used as the co-precipitator of silver ions and tin ions. The co-precipitation conditions were investigated. The results show that the co-precipitate of Ag2C2O4 and Sn（OH）4 is available when the pH value of the solution is 4.27-8.36. Using the obtained precipitate as precursor,the reduction of Ag＋ and the crystallization of tin oxide were carried out simultaneously by the hydrothermal method and silver tin oxide composite powders were obtained. The composite powders were characterized by X-ray diffraction （XRD） analysis,scanning electron microscope （SEM）,and energy spectrum analysis. The results show that the silver tin oxide composite powders are small with a diameter of about 2 μm and with homogeneous distribution of tin.     

Kinetic peculiarities of anodic dissolution of copper and its gold alloys accompanied by the formation of insoluble Cu(I) products

The kinetics of anodic dissolution of Cu and Cu-Au alloys (0.1-30 at.% Au) in aqueous chloride-containing universal buffer mixture (pH 1.25-11.90) in the potential range of insoluble Cu(I) products formation has been examined using the technique of multicyclic voltammetry and chronoamperometry of stationary and rotating electrodes. After the formation of anodic film, mass transport controls the dissolution; the phase where the transport is localised depends on the nature of the film, pH of the solution and the alloy composition. The initial stage of CuCl formation on Cu and low-concentration Cu-Au alloys is controlled by 2D-nucleation.     

Anodic dissolution of a metal accompanied by the formation of unstable anionic complexes

Transport processes that take place during the anodic dissolution of a metal accompanied by the formation of negatively charged intermediate particles are theoretically considered. Distributions of the component concentrations and electric potential in the diffusion layer during the quasi-equilibrium decomposition of intermediate particles are calculated. Analysis predicts the existence of the admissible concentration limit of anions depending on the stability constant of intermediate particles. The effect of the stability constant on the potential distribution within the diffusion layer and on the shape of voltammetric curves of the process is found to be complex.     

纯银溅射靶材的制备、微观结构及溅射成膜研究

银薄膜作为高新技术领域极具潜力的新材料,在现代工业中得到了广泛的应用。以银靶为源材料的磁控溅射已成为制备银薄膜的常用方法。本研究比较了冷轧状态和退火状态下Ag靶的溅射性能,探讨了Ag靶与Ag薄膜之间的关系。结果表明:冷轧变形量为83.33%后进行600 ℃退火可以有效提高Ag{110}的织构密度。冷轧态和退火态Ag靶具有相似的沉积速率。两种Ag薄膜的电阻率均随溅射时间的延长而降低。在溅射时间相同的情况下,退火态Ag靶溅射的Ag薄膜电阻率低于冷轧态Ag靶溅射的Ag薄膜。退火态Ag靶组织均匀,溅射后溅射跑道较浅。     

Pitting and alkaline dissolution of an amorphous-nanocrystalline alloy with solute-lean nanocrystals

The corrosion resistance of partially devitrified metallic glasses is a critical concern for the viability of the glasses in many technological applications. Although partial devitrification is detrimental to the corrosion resistance of some metallic glasses, both the pitting and alkaline corrosion behavior of partially nanocrystalline Al₉₀Fe₅Gd₅ is similar to that of its amorphous precursor. This is in spite of the fact that the microstructure of the amorphous-nanocrystalline alloy is effectively a composite consisting of f.c.c. Al crystals embedded in an amorphous matrix. Here the pitting corrosion and alkaline dissolution of the amorphous-nanocrystalline alloy is compared to that of its fully amorphous precursor, pure polycrystalline Al, and to a micrometer scale composite consisting of electrically connected pure Al wires and amorphous ribbons.     

Revision of the enthalpies and gibbs energies of formation of calcium oxide and magnesium oxide

Major revisions in the enthalpies and Gibbs energies of formation of calcium oxide and magnesium oxide are presented. In free-evaporation experiments over the temperature range of 1830 to 2070 K, the kinetics of vaporization of calcium oxide and magnesium oxide have been studied. Conditions of local equilibrium were established. The results lead to new values for the standard enthalpies of formation of CaO and MgO. The Δ_fH₂₉₈ ⁰ of CaO is ≈33 kJ/mol more positive than the currently tabulated value, whereas that for MgO is ≈34 kJ/mol more negative. The currently tabulated values could not account for these observations nor for other recent studies. These significant changes result in a reversal of the relative thermodynamic stabilities of CaO and MgO, with MgO now indicated as the more stable oxide at lower temperatures on Ellingham diagrams. These results are consistent with recent work on the formation of CaC₂, solubility product measurements in liquid iron, and with theoretical models based on ionic radii.     

Chemistry,phase formation,and catalytic activity of thin palladium-containing oxide films synthesized by plasma-assisted physical vapor deposition

The chemistry, microstructure, and catalytic activity of thin films incorporating palladium were studied using scanning and transmission electron microscopies, X-ray diffraction, spectrophotometry, 4-point probe and catalytic tests. The films were synthesized using pulsed filtered cathodic arc and magnetron sputter deposition, i.e. techniques far from thermodynamic equilibrium. Catalytic particles were formed by thermally cycling thin films of the Pd–Pt–O system. The evolution and phase formation in such films as a function of temperature were discussed in terms of the stability of PdO and PtO₂ in air. The catalytic efficiency was found to be strongly affected by the chemical composition, with oxidized palladium definitely playing a major role in the combustion of methane. Reactive sputter deposition of thin films in the Pd–Zr–Y–O system allowed us forming microstructures ranging from nanocrystalline zirconia to palladium nanoparticles embedded in a (Zr,Y)₄Pd₂O matrix. The sequence of phase formation is put in relation to simple thermodynamic considerations.     

Electric insulation of a FeSiBC soft magnetic amorphous powder by a wet chemical method: Identification of the oxide layer and its thickness control

An electrical insulating layer formed on the surface of an Fe_79.3Si_5.7B_13.3C_1.7 soft magnetic amorphous powder was characterized and its layer thickness was evaluated. The insulating layer was prepared by chemical oxidation of the powder in 10% nitric acid–ethanol solution. The insulating layer mainly consisted of magnetite and/or maghemite. Moreover, the insulating layer avoided further oxidation up to T ≈ 940 K, which is above the crystallization temperature of the amorphous powder. A correlation of the layer thickness with density and saturation magnetization is presented and discussed. Layer thicknesses calculated either from density or saturation magnetization of either magnetite or maghemite were in good agreement with each other.     

Formation of CuO nanorods and their bundles by an electrochemical dissolution and deposition process

Copper oxide nanorods (NRs) and their bundles were deposited on glass substrates by an electrochemical dissolution and deposition process. Changes in the electrode separation, the deposition time and the voltage could be used to control the morphologies, the thickness and the ratio of bundles/NRs. The formation of the NRs and their bundles was explained by an aggregation mechanism. A transformation of the Cu phase in the as-deposited sample to a single CuO phase was effected by an annealing treatment at 500 °C. The increasing photoluminescence (PL) intensity of the annealed sample resulted from a grain size growth and an improvement in the crystallinity.     

Electrochemical and quantum chemical studies on the formation of protective films by alkynols on iron

In order to find out the effect of cathodic and anodic polarisation on the formation of inhibitor films we measured the electrochemical impedance of an iron electrode in sulphuric acid and in hydrochloric acid solutions in the presence of oct-1-yn-3-ol or 2-〈2-(1,1-dimethyl-prop-2-ynyloxy)-ethoxy〉-ethanol inhibitors. We found that the anodic polarisation helped the initial binding of the inhibitor to the iron surface. Cathodic polarisation does not influence the initial formation but, if applied at a later stage, promotes the growth of the inhibitor film. The results support the assumption that film formation on iron surfaces from alkynols takes place in a two-step process: first chemisorption, then polymerisation. The binding of alkynols to iron was studied in model electronic structure calculations which indicate that a dative bond is formed between Fe and the triple bond. The calculated vibrational frequencies of the alkynol bound to iron can be used to successfully interpret the IR spectra found in the literature.     

The peculiarities of the kinetics of silver chemical deposition on dielectrics with various technologies of the surface activation

The chemical reduction of single-charged ions of metal (Ar⁺) under the conditions of a hydrodynamic mode on a rotating cylindrical dielectric (D) specimen is first studied. The partial manifestation of the effect of the hydrodynamical rate limitation (HDRL) of silver deposition has been determined. It has been established that the complete manifestation of the HDRL (V _Ag = 0) effect is blocked by the development of the process of the concentration increasing of the electro-active particles in the reactionary zone due to the increase of the amount of collisions of the noncompact silver deposit (growing in the reactor volume) and the oxidation products of the reducer (inverted sugar) with a surface of a compact Ag layer on the D specimen. The results of this research confirm the conceptions concerning the mechanism of the HDRL phenomenondue to the repulsion of electro-active particles of silver from the reactionary layer at high speeds of rotation.     

EQCN study of anodic dissolution and surface oxide film formation at Au in the presence of Cl or Br ions: A model process for corrosion studies

The anodic dissolution of an Au electrode and associated thin-layer oxide film formation in aq. H₂SO₄ in the presence of Cl⁻ or Br⁻ ions at various concentrations provides a model process for metal corrosion. In the present work such processes were investigated using cyclic voltammetry and chronoamperometry, with complementary nanogravimetry measurements using the EQCN. The results clearly indicate that in 0.5 M H₂SO₄ electrolyte, containing 1 mM Br⁻ or Cl⁻, Au dissolves over the potential range 1.0 - 1.45 V(RHE) through a 3e oxidation process involving Au complex-ion formation that can be followed in situ by means of UV spectroscopy. The linear relationship between mass changes and reciprocal square-root of sweep-rate and between anodic currents in cyclic voltammetry at ca. 1.20 V for Br⁻ (1.39 V for Cl⁻) and square-root of sweep-rate/or electrode rotation rate indicated quantitatively that the dissolution process is diffusion-controlled. It was interesting to find that electrode rotation in the presence of Cl⁻ ions has little effect on the anodic formation of surface oxide, while, on the contrary, with Br⁻ ions present, currents for oxide film reduction are not observed at rotation rates > ca. 400 rpm.     

Enhanced corrosion resistance of magnesium and its alloys through the formation of cerium (and aluminium) oxide surface films

Cerium (and aluminium) oxide layers were formed on magnesium and its alloys (AZ91) by chemical surface treatment with or without subsequent annealing. The corrosion behaviour modifications provided by the formation of these surface films were studied by means of different electrochemical and surface analysis techniques. The electrochemical behaviour, studied in sodium sulphate (Na₂SO₄) solution, showed (i) a marked shift of the corrosion potential towards more positive values, (ii) a slight inhibition of the cathodic reaction and (iii) a significant decrease of the anodic dissolution current. X‐ray photoelectron spectroscopy (XPS) was used for the characterisation of the composition of the deposited films and of the changes in the film composition during the electrochemical corrosion tests. The components of some oxide films are cerium dioxide (CeO₂), aluminium oxide (Al₂O₃) and aluminium hydroxide (Al(OH)₃). Other metallic mixed oxide films were obtained as a function of the solution composition. Very little (or no) change in the oxide film composition during the cathodic and anodic polarization experiments was observed from XPS measurements. Chemical treatment provides thick and moderately adherent protective oxide films. Annealing under oxygen further improves the beneficial effect of the chemical treatment.