Aluminum–magnesium alloys can potentially be used as hydrogen storage materials. In order to enhance the kinetics of hydrogenation, it is desirable to have agglomerates of fine powders with very small grain size. Electrodeposition is a viable method to produce pure powders with an ultrafine structure. In this paper we report the effect of magnesium content, electrolyte temperature, and current density on the characteristics of Al–Mg deposits produced galvanostatically using an organometallic-based electrolyte. The magnesium content of the deposits improved with increasing Mg concentration in the electrolyte, temperature, and current density. The results indicate that the morphology of the deposits is significantly affected by the magnesium content. Depending on the composition, the deposits consisted of face-centered cubic -Al(+Mg) and hexagonal close-packed -Mg(+Al) phases.

No abstract

Hydrogen uptake at 250 °C, P H2 > 15 bar and release at 320, 350 °C by MgH2 mixed with 10 wt % rGO alleviates the incubation period (slow kinetics) encountered during hydrogen release by pure MgH2. Ball milling establishes Mg–C interactions (∼283 eV) in these nanocomposites through electron-transfer from Mg to π* of C and weakens the C–C π bond. These Mg–C interactions persist in the nanocomposites upon subsequent hydrogen uptake and release. These interactions change the hybridization of C from sp2 to sp3, aiding hydrogen uptake by C (C–H). On hydrogen release, H releases from C–H, and electrons are donated back from C to Mg. This electron back-donation weakens the Mg–H bond and enhances hydrogen release from MgH2. The persistent Mg–C interactions are crucial for alleviating the incubation period. For the present study, X-ray diffraction, Raman, X-ray photoelectron spectroscopy (C-1s core level, valence band), and Fourier transform infrared spectroscopy are used.

Al-Mg alloy powders were produced in the form of nanocrystalline dendrites using the galvanostatic electrodeposition technique. Two distinct morphologies namely, featherlike and globular, were observed under the employed deposition conditions. The featherlike morphology consisted of only the face-centered cubic ͑fcc͒-Al͑Mg͒ phase. However, the globular morphology was composed of the hexagonal close-packed ͑hcp͒-Mg͑Al͒ phase as well as the fcc-Al͑Mg͒ phase. Compositional and TEM studies revealed that both phases were supersaturated and exhibited a nanocrystalline microstructure. Short-time potentiostatic experiments revealed that the formation of these morphologies is dependent on the applied overpotential such that at high overpotentials, only the globular morphology develops, whereas at low overpotentials, only the featherlike morphology is created. These observations are discussed in terms of the deposition rate.Metallic powders can be produced using several techniques such as the solution-based aggregation, 1 the hydrothermal method, 2,3 and the sol-gel technique. 4 Electrodeposition has been gaining importance as a promising technique for fabricating nanocrystalline powders in dendritic forms. 5-7 The process has several advantages such as the ability to produce powders in their pure form and with metastable phases. [8][9][10] Electrodeposition is a versatile technique offering various parameters such as current density, overpotential, electrolyte composition, temperature, substrate agitation, etc., which can be used to control the morphology, microstructure, and composition of the deposits. Among these parameters, the effect of potential ͑or current density͒ has been studied extensively and has substantial effect on the morphology of the deposits. For example, near the equilibrium potential where the deposition rate is low, usually, growth takes place by epitaxial growth. 11 At higher deposition rates but below the diffusion-limiting current density, the formation of faceted crystals has been reported. 12,13 Under these conditions, charge-transfer mechanisms dominate and the crystal growth is mainly decided by the anisotropy in the interfacial energy and the growth rate. As the overpotential is increased, the deposition rate increases until a state is reached where the rate is limited by the diffusion of ions and the morphology tends to become sensitive to the composition gradient ahead of the deposition front and results in the formation of nodules and dendrites. For example, in Cu electrodeposition, at lower current densities, protruding tips have been reported to develop, which eventually grew into carrotlike structures. 14 At higher current densities or overpotentials, branching and dendritic growth was observed. 5,7,15-17 The geometry of the compositional gradient may also influence the morphology of the deposit. During the galvanostatic deposition of Au-Ag alloys, leaflike dendritic precursors with sharp tips and consisting of large well-defined crystallites were observed in the initial stages of the d...

No abstract