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7 pp. Published in Journal of Alloys and Compounds Copyright © 2007 Elsevier B.V., Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH2 with and without 2 mol% Nb2O5 as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb2O5 additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics., Authors thank the financial support (project nos. HPRN-CT-2002-00208 and MRTN-CT-2006-035366) from the European Commission

Previous comparative studies on the impact of cryogenic mechanical milling (CMM) of polymers showed significantly changed thermal and mechanical properties and—in the case of some polymer blends (e.g., PMMA/PVDF) an improved miscibility. Those changes were mainly attributed to a reduction of the molecular weight average during the milling process. The crystallization process in macromolecular systems is strongly affected by any small change of the initial chain structure like the chain conformation, the presence of branches and of course a decrease of the molecular weight average. Therefore, the aim of this paper was a comparative study of isothermal crystallization of cryogenic mechanically milled and original (unmilled) polymers and polymer blends. The overall crystal growth rates were determined and the development of the spherulitic morphology was studied in detail by means of hot stage polarized light microscopy. Isothermal crystallization of bulk samples was also investigated using differential scanning calorimetry (DSC). The average spherulite growth rates of the milled homo-polymer as well as the blends were significantly higher than in unmilled samples; even when all experiments started from the molten state. Moreover, the increase in crystal growth was almost independent of the chosen crystallization temperature and thus the degree of undercooling.

Nanocrystalline Mg powder without and with 2 mol% Nb 2 O 5 catalyst was studied in a 6 M KOH electrolyte as electrode material for electrochemical hydrogen charging processes. Since the hydrogen overpotential of Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced by the addition of Nb 2 O 5 , it is assumed that the catalyst influences the electrode reactions. Considering this assumption hydrogenation was studied at different current densities. The storage capacity as well as the kinetic of Mg/Nb 2 O 5 electrodes increased significantly up to 1 wt.% H 2 at a charging time of 30 min with decreasing current density. The storage capacity of nanocrystalline Mg powder showed only minor changes to lower hydrogen contents with decreasing current density.

Nanocrystalline Mg87Ni3Al3M7 (M = Ti, Mn, Mm; Mm = Ce, La-rich mischmetal) hydrides were synthesized by reactive mechanical milling (RMM) in hydrogen atmosphere. Different degrees of the alloys hydrogenation were achieved varying the time of milling at a hydrogen pressure of 5 atm. The influence of the alloying elements (Ti, Mn, Mm) on the amount of the hydrides formed during RMM as well as on the temperature and enthalpy of hydrogen desorption was determined. The amount of hydride formed during RMM was found to be almost the same in the three alloys (∼35 wt.%); for the Ti containing alloy it was slightly higher. The Ti containing alloy reveals the lowest temperature of hydrogen desorption (∼210 °C) and the hydride in the alloy with Mn decomposes at the highest temperature (∼240 °C) among the three alloys studied. The enthalpy of H-desorption is highest for the sample alloyed with Mm (Ce, La), 70–72 kJ/mol; the Ti and Mn containing magnesium alloys have similar enthalpies of hydride decomposition, 56–60 kJ/mol. The alloys studied reveal higher equilibrium pressures of hydrogen absorption compared to nanocrystalline magnesium, indicating some thermodynamic destabilization of the hydride as a result of the alloying. Hydrogen absorption kinetics were studied at isothermal conditions at different temperatures and an influence of the alloying element was observed. The hydrogen absorption in the Ti containing alloy is substantially faster than in the alloys with Mn and Mm.

Zr-based metallic glasses and quasicrystals might be excellent materials for hydrogen storage because of their high number of tetrahedrally coordinated sites suitable for interstitial hydrogen and their favorable hydrogen chemistry. However, hydrogen desorption at low temperatures of the amorphous as well as quasicrystalline Zr–Cu–Ni–Al alloys is hindered by a thin ZrO2 barrier and proceeds only at higher temperatures together or even after phase transformations of the metastable alloys. Pd coating was observed to improve the absorption and in particular the desorption kinetics significantly. Hydrogen charging was performed electrochemically in a 2:1 glycerin–phosphoric acid electrolyte. Hydrogen desorption was studied by means of DSC as well as TDA, the microstructure by X-ray diffraction and TEM. In Pd-coated alloys full desorption is observed even below 300 °C, but only after hydrogenation up to about H/M=0.4; higher hydrogen contents led to irreversible microstructural changes, in amorphous as well as quasicrystalline materials. The characteristics of hydrogen desorption and the micromechanism of decomposition during annealing of the hydrogenated Pd-coated metastable alloys at higher temperatures are discussed in detail.

Oxidation of a rapidly solidified Mg–Ni–Y alloy was studied in a wide temperature range in air by means of TG, DSC, XRD, TEM, SEM/EDAX. The alloy was in different microstructural states and with a different phase composition depending on the temperature of oxidation. The oxidation kinetics have been found to obey different laws (linear, logarithmic, parabolic) at the different temperatures. At low temperatures of oxidation (200–350°C) the alloy oxidizes slowly with linear kinetic law. In the temperature range of 350–430°C generally a logarithmic model describes best the experimental kinetic data, as the initial stage of oxidation follows also a linear kinetic equation. At temperatures above 430°C a parabolic law is valid. During further increase of the temperature, again a linear kinetic law was followed due to partial cracking of the oxide film. The rate constants, activation energies and pre-exponential factors were also derived. The products of oxidation, the morphology and microstructure of the oxide film, alloy matrix and oxide/alloy matrix interface were investigated. The phase composition and microstructure of the metal matrix do not differ very much from those resulting after annealing in a protective argon atmosphere. Grain growth proceeds with a low rate even after long term annealing at temperatures of about 400–450°C. It was found that the oxide film consists of MgO and MgNiO2 and traces of yttrium oxides. The outer layer of the scale is nanocrystalline. At the oxide/alloy matrix interface dense coarse crystals with most probable composition Mg24Y5 were observed. The microscopic observations as well as the kinetic results indicate that the oxide film in Mg87Ni12Y1 works successfully as a protective layer at temperatures up to 500–520°C, retarding the oxygen transport in the scale and to the oxide/metal interface.

The combination of local tetrahedral order and favorable chemical composition makes amorphous and quasicrystalline Zr–Cu–Ni–Al alloys good candidates for hydrogen storage applications. Hydrogen evolution from these alloys has been studied by means of thermal desorption spectroscopy (TDS). The characteristics of hydrogen desorption from an initially quasicrystalline phase are analyzed in detail for the first time. Spectra analysis is supported by data from variety of other experimental techniques, such as differential scanning calorimetry (DSC), Auger electron spectroscopy (AES), elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM). Hydrogen desorption from both alloys is found to be a very complex process, being affected by phase transformations that take place during annealing, by hydride formation, and by the formation of a thin zirconium oxide on the surface.

Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline/amorphous Mg 75 Ni 20 Mm 5 (Mm=Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of α-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 160°C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd ( a o =2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300°C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 .

Nanocrystalline Mg powder without and with 2 mol% Nb 2 O 5 catalyst was studied as an electrode material for electrochemical hydrogen charging in a 6 M KOH electrolyte. A strong influence of the compaction parameters, the current density and the catalyst on the hydrogenation behavior was observed. The addition of graphite and PTFE to the Mg/Nb 2 O 5 electrodes improves the charging kinetics as well as the hydrogen content. The hydrogen contents achieved in Mg with Nb 2 O 5 , however, show a broad scatter. It was concluded that the catalyst distribution influences the upper limit of the storage capacity as well as the oxidation process at the surface during preparation. Since the addition of Nb 2 O 5 was observed to reduce the hydrogen overpotential of Mg depending on the catalyst distribution, it is assumed that the catalyst influences the electrode reactions. Therefore, hydrogenation was investigated for different Nb 2 O 5 distributions at different current densities in detail.

Nanocrystalline Mg 2 (Ni,Y) hydrogen storage alloy (with exact composition Mg 63 Ni 30 Y 7 ) was prepared by rapid solidification, using a melt-spinning technique. Thermal stability and phase transition in the as-quenched alloy were studied by TEM, DSC, X-ray and electron diffraction. It was found that the as-cast material consists of mainly hexagonal Mg 2 (Ni,Y) nanocrystals with an average size of 2–3 nm and a significant amount of amorphous phase with similar composition located between them. During heating the alloy crystallizes completely by three dimensional nanocrystal growth, with an activation energy of 140±7 kJ mol −1 . The hydriding properties of the as-quenched nanocrystalline alloy were studied as well. The maximum hydrogen absorption capacity (about 3.0 wt.%) and hydrogenation kinetics of the melt-spun Mg 2 (Ni,Y) were found to exceed those of the conventionally prepared polycrystalline Mg 2 Ni alloys and to be comparable to the hydrogen absorption characteristics of nanocrystalline ball-milled Mg 2 Ni. Hydrogenation of the as-cast alloy causes a change in the crystallization mechanism during annealing, as the microstructure remains nanocrystalline (15–20 nm) even after complete crystallization of the alloy.