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Hydrogen storage alloys prepared by high-energy milling

Staszewski M., Sierczyńska A., Kamińska M., Osadnik M., Czepelak M., Swoboda P.

Journal of Achievements in Materials and Manufacturing Engineering

2011

Vol. 44, nr 2

154--160

Purpose: The aim of this work was to investigate an efficiency of high-energy milling, as a method to obtain hydrogen storage alloys with good properties. Design/methodology/approach: Two classes of the alloys were studied: AB2 type with atomic composition of (Ti0.5 Zr0.5)(V0.68 Mn0.68 Cr0.34 Ni0.7) and AB5 type with atomic composition of (Ce0.63 La0.37)(Ni3.55 Al0.3 Mn0.4 Co0.75).The materials were prepared by arc melting and initially pulverized and afterwards subjected to wet milling process in a planetary mill. Findings Both initially obtained alloys had proper, single phase structure of hexagonal symmetry. However their elemental composition was greatly inhomogeneous. High-energy milling causes both homogenization of the composition and severe fragmentation of the powder particles, which after milling have mean diameter of about 3 μm (AB2 alloy) and below 2 μm (AB5 alloy). The morphology of obtained powders reveals that they tend to form agglomerates consisting of large number of crystallites. Mean crystallite sizes after milling are of about 4.5 nm and of 20 nm, respectively. The specific surface of the powders, measured using BET method, equals 8.74 m2 /g and 2.70 m2 /g, respectively. Research limitations/implications The results provide the information on the possibility of obtaining hydrogen storage alloys by high-energy milling and on the transformations taking place as a result of this process. Practical implications: The obtained powders can be used to produce the elements of hydrogen-nickel batteries and fuel cells, providing improved properties; especially extreme rise of the specific surface of the hydrogen storage material, in compare to the standard methods.

Density functional study of Mg2FeH6 complex hydride

Zhang J., Huang Y. N., Long G. G., Zhou D. W., Liu J. S.

Materials Science Poland

2010

Vol. 28, No. 1

357--366

Mg2FeH6, which has the highest volumetric hydrogen density, is considered a promising hydrogen storage material. Within the framework of the density functional theory, the crystal structure, physical properties, electronic structure and formation capacity of Mg2FeH6 complex hydride have been investigated. The optimized structural parameters correspond closely with the experimental data from X-ray and neutron powder diffraction measurements. A detailed study of the electronic structures, including the energy band, density of states (DOS) and charge density distribution, reveals the orbital hybridization and bonding characteristics within this hydride. It was shown that Mg2FeH6 is a semiconductor with the energy gap of ca. 2.3347 eV, and that a mixed ionic-covalent bond between Fe and H in FeH6 complexes is embedded in the matrix of Mg2+ cations. The calculated formation enthalpies of Mg2FeH6 , based on the possible synthesis routes, indicate that optimum conditions are achieved if this hydride is fabricated from pure elements, and that the preparation of other compounds would lead to inferior synthesis.

The Dehydrogenation Process of Destabilized NaBH4-MgH2 Solid State Hydrie Composites

Czujko T., Varin R. A., Zarański Z., Wroński Z. S.

Archives of Metallurgy and Materials

2010

Vol. 55, iss. 2

539-552

The composite behaviour of sodium borohydride – magnesium hydride mixtures was investigated. Mutual influence of both hydrides on their decomposition process was studied. The (NaBH4+MgH2) composite hydride system was synthesized in a wide range of compositions by controlled mechanical (ball) milling in a magneto-mill. In effect, nanocomposites having nanometric grain sizes of the constituent phases residing within micrometric-sized particles were produced. The dehydrogenation process of obtained composites was investigated by Differential Scanning Calorimetry (DSC) method. It is shown that the hydrogen desorption temperature of the composite constituent with the higher desorption temperature in the (NaBH4+MgH2) system substantially decreases linearly with increasing volume fraction of the constituent having lower desorption temperature which is similar behavior to well-known composite Rule-of-Mixtures (ROM) for structural composites. It is also shown that in the (NaBH4+MgH2) composite the constituents such as MgH2 and NaBH4 decompose separately and destabilization of the composite constituent with a higher desorption temperature is unrelated to the formation of MgB2 intermetallic phase. Therefore, the improved dehydrogenation properties for NaBH4 is likely due to the presence of nanostructured metallic Mg which acts as a catalyst. It is also shown that, most likely, the NaBH4 constituent act as a catalyst for the accelerated decomposition of MgH2.

W pracy przedstawiono wyniki badań zachowań kompozytowch mieszaniny borowodorek sodu – wodorek magnezu, gdzie ocenie poddano wzajemne oddziaływanie obu wodorków na ich proces dekompozycji. Układ kompozytów wodorkowych (NaBH4+MgH2) syntetyzowany był w szerokim zakresie składów, poprzez kontrolowane mielenie mechaniczne (kulowe), w młynku magnetycznym. W efekcie powyższego procesu wytworzono nanokompozyty, których składniki fazowe posiadają ziarna o nanometrycznej wielkości, wystepujące w mikrometrycznych cząstkach. Proces odwodorowania uzyskanych kompozytów badano z wykorzystaniem metody kalorymetrycznej DSC (Differential Scanning Calorimetry). Wykazano, że temperatura desorbcji wodoru składnika kompozytu o wyższej temperaturze dekompozycji w układzie (NaBH4+MgH2) istotnie obniża się liniowo wraz ze wzrostem udziału objętościowego składnika o niższej temperaturze dekompozycji, zachowując się w sposób podobny do obowiazującej dla kompozytów strukturalnych reguły mieszanin ROM (Rule-of-Mixtures). Wykazano ponadto, iż w kompozycie (NaBH4+MgH2) jego składniki, MgH2 i NaBH4, dekomponuja oddzielnie i destabilizacja składnika o wyższej temperaturze desorbcji nie jest związana z powstawaniem fazy międzymetalicznej MgB2. Stąd też poprawa właściwości do odwodorowania NaBH4 jest prawdopodobnie spowodowana obecnoącią nanostrukturalnego, metalicznego Mg, który działa katalitycznie. Dodatkowo wykazano, że NaBH4 najprawdopodobniej działa katalitycznie na przyspieszenie dekompozycji MgH2.