Ag-Mg Alloys Research Articles

Hydrogen Sorption Behavior of Cast Ag-Mg Alloys

In this paper, the hydrogen sorption properties of casted Ag-Mg alloys were investigated. The obtained alloys were structurally analyzed by X-ray diffraction (XRD) and observed by scanning electron microscopy (SEM). The study was carried out for four alloys from the two-phase region (Mg) + γ′ (AgMg4) with nominal concentrations of 5 wt. %, 10 wt. %, 15 wt. %, and 20 wt. % Ag, four alloys with nominal compositions equivalent to intermetallic phases: AgMg4, AgMg3, AgMg, and Ag3Mg, one alloy from the two-phase region AgMg + Ag3Mg (Ag60Mg40), and one alloy from the two-phase region AgMg + AgMg3 (Ag40Mg60). The hydrogenation process was performed using a Sievert-type sorption analyzer. The hydride decomposition temperature and kinetic properties of the synthesized hydrides were investigated by differential scanning calorimetry (DSC) coupled with thermogravimetric analysis (TGA). Samples with high magnesium content were found to readily absorb significant amounts of hydrogen, while hydrogen absorption was not observed for samples with silver concentrations higher than 50 at. % (AgMg intermetallic phase).

Determination of thermophysical and thermodynamic properties, of Ag-Mg alloys

The main purpose of this work was pioneering dilatometric measurements of alloys and intermetallic phases of the Ag-Mg system and experimental determination of the standard enthalpy of formation of alloys and intermetallic phases of this binary system, including for the first time alloys above 60 at% Mg. The scope of the work included the preparation of two alloys containing 40 and 60 at% of Mg and intermetallic phases (Ag3Mg, AgMg, ε- AgMg3 and AgMg4) from high purity Ag and Mg and their qualitative and quantitative structural analysis using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). The produced and characterized alloys have been subjected to dilatometric studies in the temperature range from room temperature up to 833 K and measurements of change of the standard enthalpy of formation by solution calorimetric method in a liquid tin as a metallic bath. The addition of Mg to Ag generally increased the linear coefficient of thermal expansion compared to pure silver. The standard enthalpies of formation of the prepared alloys were consistent with literature data, and the observed maximum differences were c.a. 1.7 kJ/mol at.

Calorimetric studies and thermodynamic calculations of the Ag-Mg system

The experimentally determined integral enthalpy of mixing of liquid Ag-Mg alloys has been presented. Three separate measurements have been made at 991, 1141, and 1272 K with the use of an MHTC 96 Line Evo line drop calorimeter. The values of integral molar mixing enthalpy are characterized by negative deviations from the ideal solutions in the entire concentration range. Based on the obtained calorimetric results and other thermodynamic data available in the literature, the Ag-Mg system has been re-optimized. Additionally, the mechanical properties of the Ag-Mg intermetallic phases have been calculated based on the ab initio calculations.

Surface Characterization and Secondary Electron Emission Properties of Alumina Containing MgO Film on Ag-Mg-Al Alloy

Ag-Mg alloy is used as a dynode material in electron multiplier tubes due to the high secondary electron yields (δ) of the surface of MgO film. However, MgO film is readily degraded under strong electron or ion bombardment, which results in a decrease in the lifetime of devices. In this study, alumina-containing MgO films of ~50–150 nm were developed on a Ag-2Mg-2Al alloy (silver alloy containing 2 wt % Mg and 2 wt % Al) after a thermal activation process performed at 500–600 °C under low oxygen pressures of 5.0–20.0 Pa. Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the film consists of a thin layer of pure MgO and a relatively thicker layer of alumina-containing MgO located beneath the top MgO layer. The alumina-containing MgO film exhibits high δ value of 7.7 at a primary electron energy of 580 eV and a much better stability under energetic electron bombardment than pure MgO film on Ag-Mg alloy. Alumina has higher bond dissociation energy than MgO, and the presence of alumina in the film contributes to mitigating the dissociation of the MgO film under electron bombardment. The Ag-2Mg-2Al alloy with alumina-containing MgO film is a promising candidate as a dynode material for electron multiplier tubes.

Modulated Rate (Time) – Dependent Strain Hardening of Ag/Bi2223 Composite Wire Flattened in a Low – Strength AgMg Alloy Matrix

Modulated Rate (Time) – Dependent Strain Hardening of Ag/Bi2223 Composite Wire Flattened in a Low – Strength AgMg Alloy Matrix

Influence of activation parameters on the thickness of MgO thin film on Ag-3Mg alloy and its secondary electron emission property

MgO thin films of different thicknesses were prepared on polished Ag-3wt%Mg alloy by an activation process at 500–650°C under oxygen pressures of 0.5–10.0Pa. The influence of the thickness of MgO film on the secondary electron yield of the Ag-3wt%Mg alloy is investigated. The alloy with an MgO film of moderate thickness of 65±1nm shows the highest secondary electron yield of 10. Oxygen pressure below 10.0Pa is crucial for the formation of intact MgO film on the top surface. Computational simulation analysis reveals that the maximum yield occurs when the penetration depth of primary electrons is 13–18nm less than the thickness of MgO film. Electron supply of this extra MgO film is allowed by an electron tunneling mechanism under certain bias voltage. The replenishment of new electrons to the electron deficient surface is crucial for obtaining optimized secondary electron yields for Ag-Mg alloy cathodes.

Experimental Determination of Mg Activities in Fe-Mg Solutions

The thermodynamics of magnesium in liquid iron was determined at 1823 K (1550 °C). For this purpose, liquid iron was equilibrated with Ag-Mg alloys in a semienclosed molybdenum vessel. From the partition of magnesium between iron and silver, the activity coefficient of Mg and the self-interaction parameter \( \varepsilon_{\text{Mg}}^{\text{Mg}} \) were determined.

Mining for Crystals

MATERIALS SCIENCE Predicting the crystal structure of an alloy is challenging, because even small changes in composition can lead to large changes in the way the atoms prefer to coordinate. Fischer et al. have developed a technique that mines the existing crystal database to determine top candidate structures, which are then evaluated using quantum mechanical calculations. The model determines correlations for structural motifs that jointly appear in a single alloy system at different compositions, and thereby assigns probabilities to candidate structures, given those already known in the system. In one test, the authors considered the Ag-Mg alloy with 75% Mg content, for which the exact crystal structure is undetermined. The top candidate highlighted by their model was the Cu2.82P structure, an uncommon motif that nonetheless was computed to have the lowest ground-state energy. They also tested the model by selectively removing specific compositions from the database to see if the remaining data could be successfully used to predict the correct structures; this approach succeeded 90% of the time in placing the true missing structure among the top five candidates. — MSL Nat. Mater. 5 , 641 (2006).

Fatigue behaviour of multifilamentary BSCCO 2223 superconducting tapes

The mechanical properties and the critical current were studied in three commercial multi-filamentary BSCCO 2223 tapes (formed by Bi 2 Sr 2 Ca 2 Cu 3 O 10+x high-temperature ceramic superconductors embedded in a Ag-Mg alloy) subjected to monotonic and fatigue tension at 77 K in the longitudinal direction. It was found that transport properties were not comprised under monotonic tensions if the maximum tensile stress remained below the conventional 0.2% yield strength. This magnitude was reduced by 10% to 20% in the case of fatigue loading, and the service life of the tape was dictated by the mechanical fatigue life.

Preparation of Bi-2223 bulk composed with silver-alloy wire

Abstract We prepared Bi-2223 sintered bulk composed with silver alloy wires to improve simultaneously both the superconductivity and the mechanical properties. When 24 Ag alloy-wires of 0.4 mm in diameter were composed, the critical current density ( J c ) at 77.3 K in the self-field was obtained 700 A/cm 2 . Scanning electron microscope observation results showed that dense structure of Bi-2223 plate-like grains was formed around the interfacial region between the Bi-2223 oxide and metal Ag. The mechanical property was estimated by three point bending test. The bending stress of 80 MPa at the rupture was obtained for Bi-2223 AgMg alloy composite bulk irrespective of the number of the wires. In the AgZr alloy-wires composite bulks, the bending stress decreased to the half value of that in Bi-2223 bulk without the wires. After the maximum bending stress the sample fractured separately in Bi-2223 bulk without alloy-wires, while the composite did not fracture but only fine cracks were induced.

Thermal conductivity of BSCCO(2223) tapes with different metallic sheats

Thermal conductivity and resistivity of BSCCO/Ag, BSCCO/AgAu and AgMg tapes have been measured. The measured results are quite surprising and unexpected. In fact, despite of the alloys resistivities being nearly the same, the thermal conductivity of AgMg alloys is much lower than those of AgAu ones. This result can be understood taking into accoint the phonon contribution to the thermal conductivity.

Development of reinforced Ag alloy sheathed Bi2223 tapes

We developed the Ag-Mg-Sb alloy which had a tensile strength of about 500 MPa and elongation of 5.5%. The tensile strength of 150 MPa was obtained at room temperature in the Bi-2223 tape using this Ag-Mg alloy for the outer sheath. The J/sub c/ value of the Ag-Mg alloy sheathed Bi-2223 tape has come to be above 10/sup 4/ A/cm/sup 2/ at 77 K in self field. And the J/sub c/ value was kept until a tensile stress of 120 MPa was applied. The critical bending strain of the tape was the same as that of the pure Ag sheathed tape. These results indicated that this tape was tolerant of the mechanical stress in the manufacturing process and hoop stress during the magnet operation.

Long-length processing of BSCCO-2223 tapes made by using Ag alloys sheath

To improve the smoothness of BSCCO/Ag interface as well as to increase strength and resistivity, we have fabricated various compositions of Ag-Mg alloy, Ag-Au alloy, Ag-Pd-Mg alloy, and Ag-Au-Mg alloy. These sheath alloys were evaluated for hardness, resistivity, and microstructure to find out alternative candidate to pure Ag for sheath materials in powder-in-tube (PIT) processing of BSCCO-2223 tapes. For a long-length process, these sheath alloys were extruded into commercial size (OD:12.7 mm, ID:9.5 mm, 200 mm long) of hollow tube, and were investigated for sausaging effect, microstructure, and compatibility (Ic measurement) with 2223 BSCCO superconductor during processing. The alloying of Au and Pd increased resistivity but is not very effective in Increasing hardness. Addition of small quantity of Mg to Ag-Au and Ag-Pd increased hardness significantly due to dispersion hardening. Microstructural studies of BSCCO-2223/Ag-Au-Mg tapes revealed a more dense and aligned BSCCO structure.

Oxygen reduction behavior with silver alloy catalyst in alkaline media

The electrochemical characteristics of silver catalyst impregnated on carbon black at the oxygen electrode for an alkaline fuel cell were studied. The electrode containing 30 wt.% silver catalyst showed a current density equivalent to that of Pt catalyst. To improve the catalytic activity and electrode performance, silver was bimetallized with Mg and the alloy state was confirmed by X-ray diffraction. When the atomic content of Ag-Mg alloy was 3:1, it showed the highest current density. A decrease in the corrosion potential at the electrode was found by the catalytic effect of silver. The contact of carbon and silver caused a decrease in the reaction sites of silver and the electrolyte, and hence the dissolution potential of silver was increased. Because the fuel cell was operated at an overpotential of 100–300 mV, the dissolution of silver was not important.

Electron-Probe Quantitative Energy-Dispersive Analysis of Trace Magnesium Concentrations in Ag-Mg Alloys

Internally oxidized Ag-Mg alloys are used as sheaths for high Tc superconductor wires because of their superior mechanical properties. The preparation and characteristics of these materials have been reported. Performance of the sheaths depends on the concentration of the magnesium which generally is less than 0.5 wt. percent. The purpose of this work was to determine whether electron probe microanalysis using energy dispersive spectrometry (EDS) could be used to quantitate three different Ag-Mg alloys. Quantitative EDS analysis can be difficult because the AgL escape peak occurs at the same energy (1.25 keV) as the Mg Kα peak. An EDS spectrum of a Ag-Mg alloy wire is compared to a pure Ag spectrum in Fig. 1.

Electronic structures of disordered Ag-Mg alloys.

We have calculated the electronic structures of the valence bands in a series of \ensuremath{\alpha}-phase (disordered fcc) Ag-Mg alloys over the range 0--30 at. % Mg using the Korringa-Kohn-Rostoker method within the coherent potential approximation (KKR-CPA). We find that the variation of the equilibrium lattice constant with composition is in good agreement with experimental measurements. The bandwidth of the Ag-related d states decreases with the addition of Mg although the position of the bottom of the band remains roughly fixed in energy with respect to the Fermi level; an observation that is consistent with photoemission measurements. The electronic spectral densities are very sharply peaked at the Fermi energy with widths that are 1% of the Brillouin zone dimension and so the Fermi surfaces of the alloys are well defined throughout the zone. The radius of the neck at the L point and the belly radii in the \ensuremath{\Gamma}X and \ensuremath{\Gamma}K direction increases approximately linearly with increasing Mg content. Taking into account some previous work on the origin of the short range order in Ag-rich alloys, we conclude that the local-density approximation KKR-CPA method provides a realistic description of the electronic structure in \ensuremath{\alpha}-phase Ag-Mg alloys.

R&D of Ag-sheathed Bi-2223 superconducting tapes

Three types of Ag sheathed (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tapes were developed single core tape, multifilamentary one and Ag alloy sheathed one. Single core tape showed high Jc of 66,000 A/cm/sup 2/ at 77 K, 0 T. This was achieved by the increasing of density of the oxide core. High density of the oxide core resulted in highly oriented less second phase contained microstructure. 6 to 8,100 multifilamentary wires were successfully fabricated and showed better bending strain tolerance. AgMg alloy sheath instead of pure silver sheath increased 0.2% proof stress to 210 MPa with Jc of 10,000 A/cm/sup 2/ at 77 K, 0 T. >

Electronic structures of ordered Ag-Mg alloys.

We have investigated the electronic structures of the valence bands of \ensuremath{\beta}'-AgMg (CsCl structure) and \ensuremath{\alpha}'-${\mathrm{Ag}}_{3}$Mg (${\mathrm{DO}}_{23}$ structure) by a combination of ultraviolet and x-ray photoelectron spectroscopies and first-principles band-structure and photocurrent calculations. The overall agreement between the measurements and calculated results is good indicating that the one-electron approach provides a realistic description of the electronic structure in Ag-Mg ordered alloys. We also present results for the core-level shifts observed on alloying.

Les mécanismes de l'oxydation interne dans les alliages à base d'argent

Internal oxidation mechanism in silver alloys was studied by residual resistivity, gravimetric and calorimetric measurements, X-Ray diffraction and by transmission electron microscopy. It has been established in Ag-Mg alloys that the oxygen fixation process involves three steps: firstly, substoichiometric species are initially formed; secondly these elementary species evolve toward irregular clusters including an important excess of oxygen; thirdly, a part of this excess oxygen is released and clusters evolve toward a more compact structure. The mechanism of internal oxidation in Ag-Mg alloys can be generalized to the other studied alloys such as Ag-Cu, Ag-Zn and Ag-Sn

Evidence for the formation of substoichiometric species during internal oxidation of Ag-Mg alloys

Lattice parameters of Ag-Mg (0.5 and 1 at.%) were measured. A comparison between these results and data previously obtained by gravimetry shows that (a) a high dilatation is due mainly to excess oxygen by comparison with the amount necessary to form stoichiometric MgO oxide, and (b) the earliest stage of Mg oxidation occurs in a non-expanded layer beneath the expanded one, with the formation of substoichiometric species (O/Mg < 1). The formation of substoichiometric species is explained by taking account of strain fields close to O and Mg atoms in the silver lattice. Indeed, the strong deformation introduced by oxygen in interstitial position delays the addition of oxygen on the MgO* and Mg2O* initial species and favors the formation of substoichiometric species.