Nanostructure stabilization in electrodeposited Al-Mg dendrites

Electrodeposited Al-Mg dendrites with globular morphology exhibited core-shell (coarse-fine) type microstructure with grain sizes similar to 100 and similar to 16 nm, respectively. The grain boundary and grain compositions of core are similar to 10 and similar to 6 at.% Mg, respectively. Those of shell are similar to 36 and similar to 20 at.% Mg, respectively. The excess Mg ratio at boundaries of shell and core (Gamma(Mgfine)/Gamma(Mgcoarse)) is 1.17:1. This relative grain, boundary segregation of Mg decreases the grain boundary energy from coarse to fine region and can result in nanostructure stabilization of fine shell at similar to 16 nm. (C) 2016 Elsevier B.V. All rights reserved

Anomalous Al-Mg Electrodeposition Using an Organometallic-Based Electrolyte

The Al-Mg electrodeposition is investigated using the organometallic-based electrolyte: Na[AlEt4]+2Na[Et3Al-H-AlEt3]+2.5AlEt(3)+6toluene (Et =-C2H5), employing Mg anode. With an increase of Mg in the deposit the predominant morphology and its composition changes from ground (90-95 atom% Al) to smooth globules (75-80 atom% Al) to rough globules (15-45 atom% Al). The ground and smooth globules possessed face centered cubic (fcc) Al(+Mg) phase and the rough globules possess hexagonal close packed (hcp) Mg(+Al) phase. The overall composition (atom% Al-the more noble metal) of the deposits is less than that of the electrolyte, and increases steeply with a slight change in the latter. This indicates the absence of preferred deposition of the more noble metal suggesting the anomalous electrodeposition of Al-Mg. A critical ratio of Al/Mg exists in the electrolyte beyond which the anomalousness is enhanced. This critical ratio is identified by the onset of hcp Mg(+Al) phase formation in the deposits. A lack of trend in the overall deposit composition with electrolyte agitation affirms the anomalous electrodeposition of Al-Mg system under present experimental conditions. (C) 2016 The Electrochemical Society. All rights reserved

Effect of calcination temperature on the microstructure and electronic properties of TiO2-ZnO nanocomposites and implications on photocatalytic activity

TiO2-ZnO nanocomposites with a constant Ti:Zn molar ratio of 1:0.1 were prepared via sol-gel process followed by calcination at 300, 400, 500, 600, and 700 degrees C. The structural and compositional characterizations of these nanocomposites were performed through XRD, FTIR, SEM, and EDAX. Bandgap was measured using DRS. Photocatalytic performance of the nanocomposites was evaluated by decolorization of methyl orange dye under UV and visible irradiation with and without aeration. The results showed that increase in calcination temperature resulted in nanocomposites with well-defined morphology. Although the particle size increased with increase in calcination temperature, the crystallinity of the particles also increased, resulting in enhanced photocatalytic activity. A temperature-dependent anatase-to-rutile phase transformation was observed in TiO2-ZnO nanocomposite beyond 600 degrees C. The calcination temperature influenced both dye adsorption on the nanocomposites and also dye decolorization by photocatalysis. Even when present at low molar concentration, ZnO in the nanocomposite caused sufficient decrease in bandgap (2.6 eV) at temperatures as low as 400 degrees C, such that visible irradiation could cause dye decolorization. However, the best decolorization performance was observed in the presence of the nanocomposite calcined at 600 degrees C. Aerated systems showed better performance in all cases. Desorption of the dye remaining adsorbed on the nanocomposite at the end of the photocatalytic reaction, confirmed that adsorption accounted for only 6.6 and 3% of dye removal in the nanocomposites calcined at 600 degrees C with UV and visible irradiation, respectively. However, in other systems, ignoring adsorption may cause significant overestimation in photocatalytic loss of dye from the system

Mg-C Interaction Induced Hydrogen Uptake and Enhanced Hydrogen Release Kinetics in MgH2-rGO Nanocomposites

Hydrogen uptake at 250 degrees C, P-H2 > 15 bar and release at 320, 350 degrees C by MgH(2 )mixed with 10 wt % rGO alleviates the incubation period (slow kinetics) encountered during hydrogen release by pure MgH2. Ball milling establishes Mg-C interactions (similar to 283 eV) in these nano-composites through electron-transfer from Mg to pi* of C and weakens the C-C pi bond. These Mg-C interactions persist in the nanocomposites upon subsequent hydrogen uptake and release. These interactions change the hybridization of C from sp(2) to sp(3), 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

On the parameters of Johnson-Mehl-Avrami-Kolmogorov equation for the hydride growth mechanisms: A case of MgH2

Kinetic parameter (k) and growth dimensionality (n) of Johnson-Mehl-Avrami-Kolmogorov equation are sensitive to phenomena controlling magnesium hydrogenation (210 degrees C, P-H2 = 1 MPa). Interfacial movement followed by H-atom diffusion through hydride controls hydride growth. During interfacial growth, k varies negligibly unlike n(> 0.50). Interfacial-to-diffusional transition is characterized by significantly varying k and negligibly varying n(< 0.50). Diffusional growth renders k and n(< 0.50) almost constant. Combined k - n analysis, supported by other kinetic and geometric parameters, can identify hydride growth mechanisms. (c) 2017 Published by Elsevier B.V

Transition from interfacial to diffusional growth during hydrogenation of Mg

The transition from interfacial to diffusional growth during hydrogenation of Mg -> MgH2 (hydride) at 210 degrees C for 300 min is studied using Johnson-Mehl-Avrami-Kolmogorov equation (alpha = 1 - exp(- kt(n))). The growth dimensionality (n) decreases from 0.73 to 0.23. 1D (hydride/metal) interfacial growth occurs when n > 0.50, suggested by constant interface velocity (U). Diffusional growth at n <0.50 is confirmed by the core-shell (Mg-MgH2) structure, drop in U by similar to 2-orders and the diffusion coefficient (D) of H-atom through hydride. The transition from 1D interfacial to diffusional growth occurs at n approximate to 0.50. (C) 2015 Elsevier B.V. All rights reserved

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2

The dehydrogenation mechanism during the incubation period in nanocrystalline MgH2 (low alpha: converted metal fraction and d alpha/dt) and the reasons for the occurrence of the incubation period at 320, 350, and 400 degrees C were investigated. Pre-existing Mg crystallites can enhance Mg nucleation during the incubation period, as suggested by the estimated activation energy for nucleation (12 +/- 2 kJ per mol H). The released H-atoms enter MgH2 as interstitials, as indicated by the MgH2 unit-cell contraction, resulting in increased equatorial Mg-H bond length, decreased charge-density distribution in the interstitial region, as observed from the charge-density maps, and decreased H-H distance in the {001} plane up to the midway of the incubation period. Eventually, hydrogen vacancies are created, as indicated by the red shift in the E-g and A(lg) peaks of Raman spectra. The high estimated activation energy for the growth of Mg (209 +/- 8 kJ per mol H) renders it difficult and explains the reason for the presence of an incubation period

Annealing response of AA5182 deformed in plane strain and equibiaxial strain paths

The annealing response of AA5182 Al-Mg alloy deformed to an effective prestrain of =0.15 via plane strain and equibiaxial strain paths is compared. The comparison is done at two temperatures namely, 623 and 673K. The recovery, recrystallization and grain growth behaviour of this alloy is studied by electron backscatter diffraction and dislocation density estimation using X-ray line broadening analysis. It is found that recrystallization is slower in equibiaxial deformation condition compared to that in plane strain deformation condition during annealing. Significant recrystallization is observed after annealing for 60s at 673K and for 480s at 623K following plane strain deformation. Furthermore, significant recrystallization is associated with lower grain growth at 673K (approximate to 55m) as opposed to that at 623K (approximate to 75m). The results are explained on the basis of differences in both the strain paths

Contributions of multiple phenomena towards hydrogenation: A case of Mg

Heterogeneous hydrogenation involves chemisorption (Chem), nucleation and growth by interfacial movement (NG) and diffusion (Diff). The slowest one of these phenomena is generally considered to control hydrogenation. However, the considered phenomenon cannot explain the hydrogenation in its entirety. Multiple phenomena can contribute to different extents towards hydrogenation. Contributions of multiple phenomena are explained by developing functions of the form xi(t) = Pi(i = Chem, NG, Diff) (j = Chem,) (i not equal j) f(j) (t)center dot(xi(ai(t))(i). xi(t) is the converted fraction of hydride. The indices a(i)(t) represent the extents to which the explicitly considered phenomena act (represented by xi(i)(t) from literature). Constraints Sigma(i)a(i)(t)6 = 1 and the condition f(j)(t)-> 1 ascertain the exhaustiveness of the phenomena considered. Mg-MgH2 is considered as a proof-of-concept system to apply the present approach. The (t) functions are applied to describe the hydrogenation behaviour of Mg (similar to 44 mu m) at 210 degrees C and P-H2 = 1 MPa. Present analysis shows that multiple phenomena can act simultaneously and the dominant one (highest value of index) controls the hydrogenation. This dominant phenomenon can change with time such that chemisorption followed by NG and finally diffusion contribute in controlling hydrogenation. The estimated activation energies for NG (42 kJ/mol H) and diffusion (97 kJ/mol H) compare well with literature. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved