Electrochemical and structural characterization of sputter-deposited Mg–Nb and Mg–Nb–Al–Zn alloy films

Nanocrystalline Mg–Nb and Mg–Nb–Al–Zn alloy films were deposited by dc magnetron sputtering on glass and quartz substrates in a wide range of niobium concentrations from 6 to 80 at.%. Structural, electrochemical and corrosion properties of the films were studied by X-ray diffraction, dc voltammetry, electrochemical impedance spectroscopy and electrochemical quartz crystal microbalance. Development of body-centred cubic Nb structure in the Mg–Nb alloy matrix yielded the effects of lattice contraction, grain refining and electrochemical passivity. The measurements showed high corrosion resistance of the films in alkaline solutions when niobium content was one third or more. An increased corrosion resistance was achieved by introducing minor amounts of Al (ca. 2 at.%). In particular, such Al effect was pronounced at lower Nb concentrations (20 to 30 at.%). Semiconductor properties of spontaneously formed oxide on Mg–Nb alloy were studied by Mott–Schottky plots, which indicated highly doped n-type oxide structures on Mg–Nb surface. The paper fills some gap in understanding of niobium–magnesium systems, which show potential for applications in hydrogen storage, switchable mirrors and corrosion protection.     

DSC analysis of phase transformations during precipitation hardening in Al–Zn–Mg alloy (7020)

The hardening of the Al–Zn–Mg alloys during ageing process is based on very complex phase transformations. In order to contribute to the comprehension of these phenomena, we proceed to study the phase transformations of 7020 alloy using differential scanning calorimetry and X-ray diffraction analysis. The results confirm the formation of hardening phase GP zones, intermediate hardening metastable phase η′ and the equilibrium phase η. The calorimetric and X-ray diffraction results are in good agreement and confirm the successive precipitation/dissolution sequence. The dissolution of the precipitates is accompanied by the increase in the crystallographic lattice parameter due to the increase in solid solution concentration and by the softening of the material. On the contrary, the precipitation produces a lower concentration of the Zn/Mg solutes in the Al matrix, which generates a decrease in the lattice parameter value. These precipitates produce the hardening of the alloy. The sequence of phase formation and dissolution explains the evolution of the 7020 hardness as a function of the ageing temperature.

     

Effects of Zr addition on solidification characteristics of Al–Zn–Mg–Cu alloy using thermal analysis

The effect of Zr as a grain refiner on the solidification behavior, micro- and macrostructure of a new Al–Zn–Mg–Cu aluminum super-high strength alloy containing high Zn content was studied. The addition of 2 mass% Zr reduced the grain size from 1500 to 190 μm. Moreover, the dendritic structure of the alloy altered from a coarse, elongated and non-uniform morphology to a rosette-like shape and more uniform one. The parameters of liquidus region of cooling curve obtained from thermal analysis were in a good correlation with grain size results. The maximum of first derivative in the liquidus region was introduced beside recalescence undercooling which could predict the grain refinement level even after disappearing of recalescence in the cooling curve. Furthermore, the addition of 1 mass% Zr enhanced fraction of solid in dendrite coherency point from 21 to 31% and lessened the amounts of porosity from 2.3 to 1.4%.     

Electrochemical behavior of passive films on Al–17Si–14Mg (wt.%) alloy in near-neutral solutions

The anodic polarization behavior of alloy Al–17Si–14Mg in borate solutions with and without 0.01 M NaCl was compared to that for pure Al. Results showed that, for the alloy, the passive current density increased but the pitting susceptibility decreased. The first effect was ascribed to a significant electrochemical activity of the Mg₂Si intermetallics and the second to improved stability of the oxide film. X-ray photoelectron spectroscopy analysis of potentiostatically formed passive film on the alloy showed that it consisted of aluminum oxyhydroxide with incorporation of silicon in its elemental and two oxidized states (+3 and +4). Mott–Schottky analysis showed that trivalent silicon ion acted as an n-type dopant in the film. The interrelationship between passive film composition, electronic properties, and pitting behavior has been discussed.     

Thermal analysis study on the grain refinement of Al–15Zn–2.5Mg–2.5Cu alloy

Computer-aided cooling curve analysis is a reliable method to characterize the solidification behavior of an alloy. In this study, the effect of Al–5Ti–1B grain refiner on the solidification path, microstructure and macrostructure of a new Al–Zn–Mg–Cu super high-strength aluminum alloy containing high amounts of zinc was investigated using thermal analysis technique. The grain size measurement showed that Al–5Ti–1B reduces the grain size from 1402 to 405 μm. Solidification parameters in the liquidus region were in a good accordance with microstructural results. The addition of 1 mass% of Al–5Ti–1B grain refiner decreased ΔT _N from 9.1 to 7.7 °C. It also diminished recalescence undercooling from 1.42 to 0.32 °C. The grain refinement also altered dendritic structure of the alloy from a coarse, elongated and non-uniform to a rosette and more uniform shape. Moreover, the grain refiner resulted in a more uniform distribution of eutectic structure between dendrite arms. Furthermore, the grain refinement enhanced fraction of solid at dendrite coherency point from 21 % for unrefined alloy to 25 % for the alloy containing 1 mass% Al–5Ti–1B. In the same trend, the addition of 1 mass% Al–5Ti–1B reduced the amounts of porosity from 2.3 to 1.8 %.     

Potentiostatic versus galvanostatic electrodeposition of nanocrystalline Al–Mg alloy powders

Nanocrystalline supersaturated dendritic Al–Mg powders were electrodeposited using potentiostatic and galvanostatic techniques under equal-charge conditions. In potentiostatic deposition morphology depended on applied potential: featherlike at lower and globular at higher potentials. Galvanostatic deposits yielded both morphologies at any current density. Morphological evolution was observed in galvanostatic deposits from featherlike to globular. Independent of deposition technique face-centered cubic Al(+Mg) phase with ∼7 atom% Mg (featherlike) with/without ∼20 atom% Mg (smooth globular) composition formed at lower applied/realized potentials (or deposition rates). Higher applied/realized potentials showed hexagonal close packed Mg(+Al) phase with ∼80 atom% Mg (rough globules) over smooth globules. Potentiostatic and galvanostatic deposits were compared for their morphologies, phases, and compositions.     

Influence of Sr addition on microstructure of the hypereutectic Zn–Al–Cu alloy

Journal of Thermal Analysis and Calorimetry - The purpose of the presented work is to answer the questions: how does the addition of strontium to the Zn–8Al–1Cu alloy crystallisation...     

Electrochemical behavior of NbV–EDTA and NbV–citrate systems in aqueous solutions

The electrochemical reduction of Nb^V using oxalatoniobic acid and ammonium oxooxalatoniobate salts was studied in aqueous solutions of citric acid and sodium ethylenediaminetetraacetate (EDTA), in a wide range of supporting electrolyte concentrations and pH. In EDTA two reduction processes were observed: Nb^V to Nb^IV, E = −1.000 V vs. Ag/AgCl at pH 4.50 and Nb^IV to Nb^IIIE = −1.400 V vs. Ag/AgCl (pH 4.50). In citric acid there was only one reduction process: (Nb^V to Nb^IV), E = −1.260 V vs. Ag/AgCl at pH 4.50. In both electrolytes a linear relationship was found between the diffusion current and the niobium concentration in the 1.0 × 10⁻⁵ to 5.0 × 10⁻³ M range. Using cyclic voltammetry, it waa observed that the charge transfer process in the Nb^V to Nb^IV process is reversible in EDTA and reversible–quasireversible in citric acid.     

Radiation formation of Al–Ni bimetallic nanoparticles in aqueous system

This work concerns the study of Al–Ni bimetallic nanoparticles synthesized by gamma-radiolysis of aqueous solution containing aluminium chloride hexahydrate, nickel chloride hexahydrate, polyvinyl alcohol for capping colloidal nanoparticles, and isopropanol as radical scavenger. While the Al/Ni molar ratio is kept constant, size of the nanoparticles can be well controlled by varying the radiation dose. The products were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction analysis (XRD). Observations of UV–vis absorption spectra and TEM images showed that as the radiation dose increases from 50 to 100 kGy the particle size decreases and the number particles distribution increases. It may be explained due to the competition between nucleation and aggregation processes in the formation of metallic nanoparticles under irradiation. The EDX and XRD analysis confirmed directly the formation of Al–Ni bimetallic nanoparticles in form of alloy nanoparticles.     

Effect of grain refinements on the microstructure and thermal behaviour of Mg–Li–Al alloy

Journal of Thermal Analysis and Calorimetry - The light as-cast Mg–9Li–1.5Al alloys were manufactured and modified by 0.2 mass% Zr, commercial 0.2 mass% TiBor and 0.2 mass% AlSr master...     

Electrochemical formation of a cesium–tin alloy in an amide-type ionic liquid

Electrochemical formation of cesium–tin alloys in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing 0.5 M CsTFSA has been studied. Cathodic decomposition of BMPTFSA on a platinum electrode was suppressed by addition of CsTFSA, suggesting that Cs⁺ accumulated on the electrode surface and hindered the reduction of BMPTFSA. Multiple cathodic current peaks were observed on a tin electrode in the ionic liquid. Energy-dispersive X-ray analysis and X-ray diffraction results suggested formation of cesium–tin alloys after potentiostatic cathodic reduction on the tin electrode at room temperature.     

Electrochemical extraction of cerium and formation of Al–Ce alloy from CeO_2 assisted by AlCl_3 in LiCl–KCl melts

This work presents an electrochemical extraction of cerium and synthesization of Al–Ce alloy in LiCl–KCl melts on Mo and Al electrodes by chlorination of CeO2 using AlCl3 at 873 K. The cyclic voltammogram on Mo electrodes in LiCl–KCl–CeO2 melt showed no obvious reduction wave other than the reduction of Li(I). After the addition of AlCl3, the signals of the reaction of Ce(ⅡI)/Ce(0) and the synthesization of Al–Ce and Al–Li alloys were investigated by cyclic voltammetry, square-wave voltammetry, open-circuit chronopotentiometry and chronopotentiometry. These results indicated that AlCl3 can chloridize CeO2 and that it is possible to extract cerium and form Al–Ce and Al–Li–Ce alloys in LiCl–KCl–CeO2–AlCl3 melts. According to potentiostatic electrolysis, only the Al4 Ce layer coated the Al electrodes. According to galvanostatic electrolysis, Al–Ce(Al4Ce, Al3 Ce, and Al92Ce8), Al2Li3, and Al phases were formed on Mo electrodes, and the content of cerium in the Al–Li–Ce alloys was more than 17 wt%.     

Crystallization behavior of Mg–Cu–Y amorphous alloy

Amorphous Mg₆₁Cu₂₄Y₁₅ ribbons were manufactured by melt-spinning at wheel speeds in the range 5?C20?ms^?1. The crystallization behavior of amorphous ribbons was investigated by a combination of differential scanning calorimetry (DSC) and X-ray diffractometry. DSC measurements showed that the amorphous ribbons exhibit distinct glass transition temperature and wide supercooled liquid region before crystallization. During continuous heating three exothermic peaks and two endothermic peaks were observed. The characteristic thermodynamic parameters such as T _g, T _x , ??T _x , and T _rg are around 432?C439, 478?C485, 46?C54?K, and 0.55?C0.56, respectively. Isothermal annealing DSC traces for this amorphous alloy, the first crystallization peak showed a clear incubation period and Avrami exponent was found to be 2.30?C2.74, which indicate that the transformation reaction involved nucleation and three-dimensional diffusion controlled growth. Mechanical properties of the as-quenched and subsequently annealed ribbons were examined by Vickers microhardness (HV) measurements. Results showed that microhardness of the as-quenched ribbons were about 309?HV. However, the results also showed that microhardness of the rapidly solidified ribbons increases with the increasing temperature.     

Electrochemical performance of Co–Al layered double hydroxide nanosheets mixed with multiwall carbon nanotubes

Regular hexagonal Co–Al layered double hydroxides (Co–Al LDH) were synthesized by urea-induced homogeneous precipitation. This material proved to be nanosheets by scanning electron microscopy and X-ray diffraction measurements. The electrochemical capacitive behavior of the nanosheets in 1 M KOH solution were evaluated by constant current charge/discharge and cyclic voltammetric measurements, showing a large specific capacitance of 192 F·g⁻¹ even at the high current density of 2 A·g⁻¹. When multiwall carbon nanotubes (MWNTs) were mixed with the Co–Al LDH, it was found that the specific capacitance and long-life performance of all composite electrodes at high current density are superior to pure LDH electrode. When the added MWNTs content is 10 wt%, the specific capacitance increases to 342.4 F·g⁻¹ and remains at a value of 304 F·g⁻¹ until the 400th cycle at 2 A·g⁻¹, showing that this is a promising electrode material for supercapacitors working at heavy load. According to the electrochemical impedance spectra, MWNTs greatly increase the electronic conductivity between MWNTs and the surface of Co–Al LDH, which consequently facilitates the access of ions in the electrolyte and electrons to the electrode/electrolyte interface.     

Phase transformations in an annealed Cu–9Al–10Mn–3Gd alloy

The phase transformations in the Cu–9Al–10Mn–3Gd alloy were studied using differential scanning calorimetry, X-ray diffraction patterns, scanning electron microscopy, energy dispersion X-ray spectroscopy and magnetic moment change with applied field and temperature. The results showed that the effects produced by the Mn atoms are dominant on those attributed to the Gd atoms in the annealed Cu–9Al–10Mn–3Gd alloy. For quaternary alloy the results also indicated that the Gd stabilizes a fraction of the paramagnetic β₃ phase at lower temperatures and suppresses its paramagnetic–ferromagnetic ordering; in addition, it increases the Curie temperature of the Cu–9Al–10Mn alloy.     

Effect of morphology and precipitation of Si phase on thermal properties of Al–Si–Mg–Cu foundry alloy

Journal of Thermal Analysis and Calorimetry - In this study, CuAl13?xTax (% mass x?=?1; 1.5; 2; 2.5) shape-memory alloys were produced through arc-melting method. Phase...     

Microwave assisted efficient synthesis of β-keto-sulfones in aqueous medium

Abstract

An efficient synthesis of β-keto-sulfones is described. The reaction of α-haloketones with sodium alkyl/aryl sulphinates in aqueous medium under microwave irradiation afforded the corresponding β-keto-sulfones in excellent yields.