The influence of the Co addition and synthesis route on desorption properties of MgH2
were investigated. Ball milling of MgH2-Co blends was performed under Ar using different
milling intensities and different weight ratios. Microstructural and morphological
characterization, performed by XRD and SEM, show a huge correlation with thermal stability
and hydrogen desorption properties investigated by DSC. A complex desorption behaviour is
correlated with the dispersion of the catalytic particles that appears to play a main role in
desorption performances. The optimum catalyst concentration was found to be around 10 wt.%,
while the optimum value of the ball to powder ratio was 10:1.
Abstract. The alloying and phase formation in Ni-Hf samples with 0.2-, 2-, and 5-at.% Hf were studied by X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). Both characterization methods, XRD and SEM, reveal the presence of the HfNi 5 phase (fcc structure) where the excess Ni atoms are present in the form of Ni or Ni-rich segregations in the sample containing 5-at.% Hf. The sample with 2-at.% Hf is characterized by the presence of the two phases present in the 5-at.% sample and by Hf atoms, which occupy substitutional lattice positions in the Ni lattice. Finally, in the third sample with 0.2-at.% Hf, the Hf atoms mainly substitute the Ni atoms in the lattice. This analysis is being complemented with additional information on the local structure around Hf by extended X-ray absorption fine structure spectroscopy (EXAFS).
Mg-based nanostructured hydrides have been synthesized by ball milling using two alternative approaches. The first is based on the reactive milling of Mg powders in H 2 atmosphere, while the second on the milling of commercial MgH 2 powders under inert atmosphere. In both cases 10 wt.% of Fe was added to the powder mixture, with the aim of introducing a catalyst agent. The microstructural characterization was carried out by X-ray diffraction, and both scanning and transmission electron microscopy. Hydrogen desorption behavior was evaluated by differential scanning calorimetry. Almost full hydrogenation of pure Mg powders can be achieved by reactive milling. Catalyst addition strongly accelerates the hydride formation. Both reactive milling of Mg powder and inert gas milling of MgH 2 induce a nanosized microstructure with similar H-desorption behavior. The role played by Fe becomes particularly evident in H-desorption. In fact, a temperature decrease of about 100°C was found in samples having the same crystallite size and similar powder morphology.
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