I.G. Konstanchuk

Magnesium mechanical alloys for hydrogen storage

Abstract Mechanical alloying is a promising new way to fabricate hydrogen storage materials consisting of unusual pairs of metals. Mechanical alloying can be used more easily for fabricating Mg2Ni, other MgNi and intermetallic systems than melting and sintering techniques. Systems such as MgFe, MgCo, MgNb and MgTi, that cannot be prepared by conventional techniques, can be fabricated by mechanical alloying, in addition to any other pairs or more complex mixtures of metals and non-metals. We have pointed out that intimate contact of metals can be achieved in high energetic ball mills. This contact plays an important role in hydriding and dehydriding processes. In this study we have used two groups of catalytic additives. The first group are “hydrogen pumps” such as CeHx. In the second group the added metal does not form any hydride in the conditions under study, but is known as an H2 → HH reaction catalyst, e.g., cobalt, nickel and probably iron.

The hydriding properties of a mechanical alloy with composition Mg-25%Fe☆

A method of mechanical alloying was used to produce an Mg-25%Fe composite hydrogen storage material. The presence of a pure metallic iron surface in contact with the magnesium leads to the promotion of the dissociative adsorption of hydrogen and the hydrogenation of Mg-25%Fe begins at the maximum rate even in the first cycle of hydriding. The activation of the mechanical alloy is completed by the third cycle of hydriding. The process of formation of Mg2FeHx is also studied.

Hydrogen interaction with mechanically alloyed magnesium-salt composite materials

The effects of the addition of the inorganic salts NaF, NaCl, MgF2 or CrCl3 on the hydriding and dehydriding behavior of Mg and Mg–Ni mechanical alloys were investigated. It has been found that some inorganic salts, being mechanically alloyed with magnesium or magnesium alloys, not only promote metal powdering but also modify the metal surface. This leads to an acceleration of the hydriding and dehydriding reactions of these materials and to quite high hydrogen capacities (∼5.5–6 wt.%). The investigated salts had different influences on the metal surface properties, which was reflected in reaction kinetics, in particular at first hydriding.

Hydriding properties of mechanical alloys MgNi

Abstract The rapid reaction of MgNi mechanical alloys with hydrogen during initial hydriding is accounted for by the formation of a large interface between the metallic phases upon mechanical alloying, the chemical changes of the alloy surface associated with the electron transfer (from magnesium to nickel) and the reaction between magnesium and nickel. The initial period of the reaction of the mechanical alloys with hydrogen is determined by the dissociative chemisorption of H2 on nickel atoms which are dispersed in the subsurface layer of the mechanical alloy. It is shown that the formation of Mg2Ni during the hydriding-dehydriding of the mechanical alloys proceeds mainly according to the reaction 2MgH2 + Ni → Mg2Ni + 2H2.

Mechanochemical synthesis of icosahedral phases in Mg-Zn-Al and Mg-Cu-Al alloys

Mechanical alloying (MA) has been used to obtain the icosahedral phases: i-(Mg-Zn-Al) and i-(Mg-Cu-Al) from metal powders. X-ray diffraction peaks are identical to those of icosahedral obtained by the melt spun technique. SAD patterns from MA samples indicate 2,3,6 and 5-fold symmetry. Annealing of the MA icosahedral phases leads to the formation of related cubic Frank-Kasper phases, that can be transformed into the icosahedral phases by MA. The disclination network which appears in the cubic phase plastically deformed by MA seems to be intrinsic to producing an aperiodic phase with long-range icosahedral order.

The stages of formation of a solid solution during the mechanical alloying of Si and Ge

Abstract The formation of a solid solution during the mechanical alloying (MA) of Si and Ge was studied using electron microscopy and X-ray diffraction. The process occurs via the formation of aggregates in which, under the action of plastic deformation, the mutual diffusion of Si and Ge proceeds to form the solid solution Si 1− x Ge x . According to our data, the observed partial amorphization of the components during MA is due to the formation of amorphous SiO and Si 1− x Ge x O oxides, structurally similar to amorphous Si.

Hydriding properties of mechanically alloyed icosahedral phase Ti45Zr38Ni17

Abstract The interaction with hydrogen was investigated for mechanochemically synthesised icosahedral phase of the composition Ti 45 Zr 38 Ni 17 . It was demonstrated that, unlike icosahedral phases of the same composition formed as a result of rapid quenching from melt or the annealing of alloys, hydriding of mechanochemically synthesized icosahedral phase at 503 K is not preceded by an induction period and starts at a maximal rate even when hydrogen pressure is less than 0.1 MPa. A diagram of the composition of icosahedral phase versus equilibrium hydrogen pressure ( P – T – C diagram) at 503 K was experimentally obtained. Based on published data and the data obtained in our experiments, an assumption was made on the existence of two types of hydrogen positions in the icosahedral structure of the hydrogenated phase. The first type is more energetically bound hydrogen occupying the positions within the structure-forming icosahedral Bergman clusters. The concentration of such hydrogen is approximately corresponding to the relation H/M=1.1. The second type is less bound hydrogen occupying the linkages between these clusters as solid solution.

Stage formation of quasi-crystals during mechanical treatment of the cubic Frank-Kasper phase Mg32(Zn, Al)49

Abstract Using high resolution electron microscopy, the stage formation of the icosahedral phase during the process of mechanochemical treatment of the cubic Mg 3Z (Zn, Al) 49 crystals has been studied. It has been shown that in the initial stages the icosahedral phase forms locally owing to the formation of rotation defects and disorder of the crystal planes due to the action of plastic deformation. The size of the disordered regions in the icosahedral phase increases with increase in the time of mechanical treatment. An orientation correspondence has also been revealed between the initial cubic and icosahedral phases; this is apparently due to the similarity in the structural designs of the cubic and icosahedral phases.

Comparative study of first hydriding of Mg–NaF and Mg–NaCl mechanical alloys

Abstract A kinetic and electron microscopy study of first hydriding of magnesium preliminary mechanically alloyed with the addition of NaF or NaCl salt has been performed. The salts have been found to modify the magnesium particle surfaces in a different way in the course of both mechanical alloying and hydriding. The action of NaCl consists in the local destruction of the oxide layer on the magnesium surface, facilitating the hydride nucleation process, whereas formation of NaMgF 3 has been observed at the very initial stages of hydriding of the Mg–NaF mechanical alloy. This ternary fluoride has been shown to play an active role in the process of first hydriding affecting the reaction kinetics and altering the overall course of the reaction.

Long-range influence of Pt in the process of MoO3 reduction by hydrogen

Abstract Long-range influence of Pt in the process of MoO 3 reduction to MoO 2 by hydrogen consists in the acceleration of the process due to the increase of the nucleation rate of reaction products. Morphology of metallic molibdenum can be changed with the changing of the nucleation process of the MoO 2 formation.