Atomic hydrogen diffusion in novel magnesium nanostructures : the impact of incorporated subsurface carbon atoms

Ab initio Density Functional Theory (DFT) calculations are performed to study the diffusion of atomic hydrogen on a Mg(0001) surface and their migration into the subsurface layers. A carbon atom located initially on a Mg(0001) surface can migrate into the sub-surface layer and occupy a fcc site, with charge transfer to the C atom from neighboring Mg atoms. The cluster of postively charged Mg atoms surrounding a sub-surface C is then shown to facilitate the dissociative chemisorption of molecular hydrogen on the Mg(0001) surface, and the surface migration and subsequent diffusion into the subsurface of atomic hydrogen. This helps rationalize the experimentally-observed improvement in absorption kinetics of H2 when graphite or single walled carbon nanotubes (SWCNT) are introduced into the Mg powder during ball milling.

The role of V2O5 on the dehydrogenation and hydrogenation in magnesium hydride : An ab initio study

Ab initio density functional theory calculations are performed to study the experimentally observed catalytic role of V2O5 in the recycling of hydrogen in magnesium hydride. We find that the Mg–H bond length becomes elongated when MgH2 clusters are positioned on single, two, and three coordinated oxygen sites (O1, O2, and O3) on the V2O5(001) surface. Molecular hydrogen is predicted to spontaneously form at the hole site on the V2O5(001) surface. Additionally, the activation barrier for the dissociation of hydrogen on V-doped Mg(0001) surface is 0.20 eV, which is only 1/5 of that on pure Mg(0001) surface. Our results indicate that oxygen sites on the V2O5(001)surface and the V dopant in Mg may be important facilitators for dehydrogenation and rehydrogenation, respectively. The understanding gained here will aid in the rational design and development of Mg-based hydrogen storage materials.

A facile chemical screening method for the detection of stress corrosion cracking in 9 carat gold alloys

Stress corrosion cracking (SCC) is a well known form of environmental attack in low carat gold jewellery. It is desirable to have a quick, easy and cost effective way to detect SCC in alloys and prevent them from being used and later failing in their application. A facile chemical method to investigate SCC of 9 carat gold alloys is demonstrated. It involves a simple application of tensile stress to a wire sample in a corrosive environment such as 1–10 % FeCl3 which induces failure in less than 5 minutes. In this study three quaternary (Au, Ag, Cu and Zn) 9 carat gold alloy compositions were investigated for their resistance to SCC and the relationship between time to failure and processing conditions is studied. It is envisaged that the use of such a rapid and facile screening procedure at the production stage may readily identify alloy treatments that produce jewellery that will be susceptible to SCC in its lifetime.

The electrochemical corrosion behaviour of quaternary gold alloys when exposed to 3.5% NaCl solution

Lower carat gold alloys, specifically 9 carat gold alloys, containing less than 40 % gold, and alloying additions of silver, copper and zinc, are commonly used in many jewellery applications, to offset high costs and poor mechanical properties associated with pure gold. While gold is considered to be chemically inert, the presence of active alloying additions raises concerns about certain forms of corrosion, particularly selective dissolution of these alloys. The purpose of this study was to systematically study the corrosion behaviour of a series of quaternary gold–silver–copper–zinc alloys using dc potentiodynamic scanning in saline (3.5 % NaCl) environment. Full anodic/cathodic scans were conducted to determine the overall corrosion characteristics of the alloy, followed by selective anodic scans and subsequent morphological and compositional analysis of the alloy surface and corroding media to determine the extent of selective dissolution. Varying degrees of selective dissolution and associated corrosion rates were observed after anodic polarisation in 3.5 % NaCl, depending on the alloy composition. The corrosion behaviour of the alloys was determined by the extent of anodic reactions which induce (1) formation of oxide scales on the alloy surface and or (2) dissolution of Zn and Cu species. In general, the improved corrosion characteristics of alloy #3 was attributed to the composition of Zn/Cu in the alloy and thus favourable microstructure promoting the formation of protective oxide/chloride scales and reducing the extent of Cu and Zn dissolution.

Synthesis and spectroscopic characterization of magnesium oxalate nano-crystals

Synthesis of MgC2O4⋅2H2O nano particles was carried out by thermal double decomposition of solutions of oxalic acid dihydrate (C2H2O4⋅2H2O) and Mg(OAc)2⋅4H2O employing CATA-2R microwave reactor. Structural elucidation was carried out by employing X-ray diffraction (XRD), particle size and shape were studied by transmission electron microscopy (TEM) and nature of bonding was investigated by optical absorption and near-infrared (NIR) spectral studies. The powder resulting from this method is pure and possesses distorted rhombic octahedral structure. The synthesized nano rod is 80 nm in diameter and 549 nm in length.

Osteoimmunomodulatory properties of magnesium scaffolds coated with β-tricalcium phosphate

The osteoimmunomodulatory property of bone biomaterials is a vital property determining the in vivo fate of the implants. Endowing bone biomaterials with favorable osteoimmunomodulatory properties is of great importance in triggering desired immune response and thus supports the bone healing process. Magnesium (Mg) has been recognized as a revolutionary metal for applications in orthopedics due to it being biodegradable, biocompatible, and having osteoconductive properties. However, Mg's high rate of degradation leads to an excessive inflammatory response and this has restricted its application in bone tissue engineering. In this study, β-tricalcium phosphate (β-TCP) was used to coat Mg scaffolds in an effort to modulate the detrimental osteoimmunomodulatory properties of Mg scaffolds, due to the reported favorable osteoimmunomodulatory properties of β-TCP. It was noted that macrophages switched to the M2 extreme phenotype in response to the Mg-β-TCP scaffolds, which could be due to the inhibition of the toll like receptor (TLR) signaling pathway. VEGF and BMP2 were significantly upregulated in the macrophages exposed to Mg-β-TCP scaffolds, indicating pro-osteogenic properties of macrophages in β-TCP modified Mg scaffolds. This was further demonstrated by the macrophage-mediated osteogenic differentiation of bone marrow stromal cells (BMSCs). When BMSCs were stimulated by conditioned medium from macrophages cultured on Mg-β-TCP scaffolds, osteogenic differentiation of BMSCs was significantly enhanced; whereas osteoclastogenesis was inhibited, as indicated by the downregualtion of MCSF, TRAP and inhibition of the RANKL/RANK system. These findings suggest that β-TCP coating of Mg scaffolds can modulate the scaffold's osteoimmunomodulatory properties, shift the immune microenvironment towards one that favors osteogenesis over osteoclastogenesis. Endowing bone biomaterials with favorable osteoimmunomodulatory properties can be a highly valuable strategy for the development or modification of advanced bone biomaterials.

Crystal structure of the magnesium salt of the herbicide 2,4-D: pentaaqua[(2,4-dichlorophenoxy)acetato-kO]magnesium (2,4-dichlorophenoxy)acetate hemihydrate

In the structure of the title magnesium complex with the phenoxy herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D), [Mg(H2O)5(C8H5Cl2O3)]+ C8H5Cl2O3)- . 0.5H2O, the discrete cationic MgO6 complex units comprise a carboxyl O-donor from a monodentate 2,4-D cationic ligand and five water molecules in a slightly distorted octahedral coordination. The 2,4-D anions are linked to the complex units through duplex water O-H...O(carboxyl) hydrogen bonds through the coordinated water molecules. In the crystal inter-unit O-H...O hydrogen-bonding interactions involving coordinated water molecules as well as the hemi-hydrate solvate molecule with carboxyl O-atom acceptors, give a two-dimensional layered structure lying parallel (001), in which pi-pi ligand-cation interactions [minimum ring centroid separation, 3.6405(17)A] and a short O-H...Cl interaction [3.345(2)A] are also found.

Abatement of aqueous anionic contaminants by thermo-responsive nanocomposites: (Poly(N-isopropylacrylamide))-co-silylanized Magnesium/Aluminun layered double hydroxides

A series of novel thermo-responsive composite sorbents, were prepared by free-radical co-polymerization of N-isopropylacrylamide (NIPAm) and the silylanized Mg/Al layered double hydroxides (SiLDHs), named as PNIPAm-co-SiLDHs. For keeping the high affinity of Mg/Al layered double hydroxides towards anions, the layered structure of LDHs was assumed to be reserved in PNIPAm-co-SiLDHs by the silanization of the wet LDH plates as evidenced by the X-ray powder diffraction. The sorption capacity of PNIPAm-co-SiLDH (13.5 mg/g) for Orange-II from water was found to be seven times higher than that of PNIPAm (2.0 mg/g), and the sorption capacities of arsenate onto PNIPAm-co-SiLDH are also greater than that onto PNIPAm, for both As(III) and As(V). These sorption results suggest that reserved LDH structure played a significant role in enhancing the sorption capacities. NO3− intercalated LDHs composite showed the stronger sorption capacity for Orange-II than that of CO32−. After sorption, the PNIPAm-co-SiLDH may be removed from water because of its gel-like nature, and may be easily regenerated contributing to the accelerated desorption of anionic contaminants from PNIPAm-co-SiLDHs by the unique phase-transfer feature through slightly heating (to 40 °C). These recyclable and regeneratable properties of thermo-responsive nanocomposites facilitate its potential application in the in-situ remediation of organic and inorganic anions from contaminated water.

Near infrared spectroscopy of selected natural magnesium carbonates: implications for geosequestration

The approach to remove greenhouse gases by pumping liquid CO2 several kilometres below the ground implies that many carbonate containing minerals will be formed. Among these minerals, the formation of hydromagnesite, dypingite and nesquehonite are possible, thus necessitating a study of such minerals. These minerals with a hydrotalcite-related formulae have been characterised by a combination of infrared and near infrared spectroscopy. Layered double hydroxides (also known as anionic clays or hydrotalcites) are a group of layered clay minerals described by the general formula: [M1–x2+Mx3+(OH)2]x+[An–]x/n∙mH2O. The infrared spectra of the minerals are characterised by OH and water stretching vibrations. Both the first and second fundamental overtones of these bands are observed in the NIR spectra in the 7030–7235 cm–1 and 10,490–10,570 cm–1 spectral ranges. Intense (CO3)2– symmetrical and anti-symmetrical stretching vibrations confirm the distortion of the carbonate anion. The position of the water bending vibration indicates water is strongly hydrogen-bonded to the carbonate anion in the mineral structure. NIR spectroscopy offers a method for quickly analysing such materials.

Glass forming ability and stability: Ternary Cu bearing Ti, Zr, Hf alloys

Four Cu bearing alloys of nominal composition Zr25Ti25Cu50, Zr34Ti16Cu50, Zr25Hf25Cu50 and Ti25Hf25Cu50 have been rapidly solidified in order to produce ribbons. All the alloys become amorphous after meltspinning. In the Zr34Ti16Cu50 alloy localized precipitation of cF24 Cu5Zr phase can be observed in the amorphous matrix. The alloys show a tendency of phase separation at the initial stages of crystallization. The difference in crystallization behavior of these alloys with Ni bearing ternary alloys can be explained by atomic size, binary heat of mixing and Mendeleev number. It has been observed that both Laves and Anti-Laves phase forming compositions are suitable for glass formation. The structures of the phases, precipitated during rapid solidification and crystallization can be viewed in terms of Bernal deltahedra and Frank-Kasper polyhedra.

Resolidification behaviour of laser surface-melted metals and alloys

The solidification behaviour is described of two pure metals (Bi and Ni) and two eutectic alloys (A1-Ge and AI-Cu) under nonequilibrium conditions, in particular the microsecond pulsed laser surface melting. The resolidification behaviour of bismuth shows that epitaxial regrowth is the dominant mechanism. For mixed grain size, regrowth of larger grains dominates the microstructure and can result in the development of texture. In the case of nickel, epitaxial growth has been noted. For lower energy pulse-melted pool, grain refinement takes place, indicating nucleation of fresh nickel grains. The A1-Ge eutectic alloy indicates the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density laser irradiation. An equiaxed microstructure containing the same monoclinic phase is obtained at a lower power density laser irradiation. It is shown that the requirement of solution partition acts as a barrier to eutectic regrowth from the substrate. The laser-melted pool of A1-Cu eutectic alloy includes columnar growth of c~-A1 and 0-A12Cu phase followed by the dendritic growth of A12Cu phase with ct-Al forming at the interdendritic space. In addition, a banded microstructure was observed in the resolidified laser-melted pool.

Effect of microstructure on the mechanical behavior of b-modified ti-6al-4v alloys

Small additions of B to Titanium alloys refine the as-cast microstructure significantly and hence improve their mechanical performance. In this work, tensile, fracture and fatigue properties of the as-cast and HIPed Ti-6Al-4V alloy with hypoeutectic wt.% of B additions have been examined, with particular emphasis on identifying the microstructural length scale that controls the mechanical properties of these alloys.

Grain boundary strengthening in strongly textured magnesium produced by hot rolling

The grain size dependence of the yield stress in hot rolled 99.87 pct magnesium sheets and rods was measured in the temperature range 77 K to 420 K. Hot rolling produced strong basal textures and, for a given grain size, the hot rolled material has a higher strength than extruded material. The yield strength-grain size relation in the above temperature range follows the Hall-Petch equation, and the temperature dependencies of the Hall-Petch constants σ0 and k are in support of the theory of Armstrong for hcp metals that the intercept σ0 is related to the critical resolved shear stress (CRSS) for basal slip (easy slip) and the slope k is related to the CRSS for prismatic slip (difficult slip) occurring near the grain boundaries. In the hot rolled magnesium, σ0 is larger and k is smaller than in extruded material, observations which are shown to result from strong unfavorable basal and favorable 1010 textures, respectively. Texture affects the Hall-Petch constants through its effect on the orientation factors relating them to the CRSS for the individual slip systems controlling them.

Pressure transitions and electron transport behaviour of crystalline Se-Te alloys

Pressure transitions of Se-Te alloys have been studied over the entire range of compositions. Conductivities have also been measured as a function of temperature and alloy composition. Transition pressures, activation barriers and isothermal conductivities exhibit distinct changes of slope in their variation as a function of composition at about 8 at % of Te. Transition pressures change slope at not, vert, similar 35% Te also. An attempt has been made to explain these observations on the basis of the size effect of Te which, in turn, affects the electron energy dispersions in the band structure.

Subsurface deformation in dry sliding of hypo-eutectic Al-Si alloys

The subsurface deformation during dry sliding of Al-Si alloys is studied by fragmentation of silicon particles. The size of the fragmented particles does not vary with load. The depth of deformation is found to increase with increase in normal load. This experimental observation agrees with load-deformation depth characteristics obtained by a slip line field model.