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In this study, powder mixtures of MgH2 + 2 mol.% X, with X = Nb, Nb2O5, NbF5, Fe, Fe2O3, FeF3, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF3) in the MgH2 matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH2 sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH2, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH2 particle size. DSC analysis revealed that fluorides (NbF5, FeF3) are much more effective than oxides (Nb2O5, Fe2O3) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb2O5 is more efficient than Fe2O3. Finally, the best results for desorption kinetics were observed for the fluorides: NbF5 and FeF3 (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb2O5, Fe2O3 and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample. [ABSTRACT FROM AUTHOR]

We report on a comparative study of high performance isotropic cobalt ferrite (CoFe2O4) powder processed by dry and surfactant assisted (wet) ball milling. Milling times as short as 1.5 min (dry) and 6 min (wet) have resulted in a 4-fold increase in coercivity, with a maximum achieved value above 318 kA/m (4 kOe). The use of surfactant is shown to be advantageous in the formation of a more homogeneous structure constituted by non-agglomerated and strained nanoparticles. A record (BH) max value of 18.6 kJ m −3 (2.34 MGOe) has been obtained for isotropic powder after post-processing annealing. This magnetic performance combined with the required short processing times and the unnecessary requirement of oxygen avoidance in the milling process, makes this CoFe2O4 powder a good candidate for permanent magnet applications. [ABSTRACT FROM AUTHOR]

Magnesium borohydride (Mg(BH4)2) is an attractive candidate as a hydrogen storage material due to its high hydrogen content and predicted favorable thermodynamics. In this work we demonstrate reversible hydrogen desorption in partially decomposed Mg(BH4)2 which was ball milled together with 2 mol% Co-based additives. Powder X-ray diffraction and infrared spectroscopy showed that after partial decomposition at 285 °C, amorphous boron-hydride compounds were formed. Rehydrogenation at equivalent temperatures and hydrogen pressures of 120 bar yielded the formation of crystalline Mg(BH4)2 in the first cycle, and amorphous Mg(BH4)2 with other boron–hydrogen compounds upon the third H2 absorption. Reversibility was observed in the samples with and without Co-based additives, although the additives enhanced hydrogen desorption kinetics in the first cycle. X-ray absorption spectroscopy at Co K-edge revealed that all the additives, apart from Co2B, reacted during the first desorption to form new stable species. [ABSTRACT FROM AUTHOR]

Abstract: A new processing route consisting of a short-time high energy ball milling (HEBM) step followed by cold rolling (CR) to produce MgH2-based nanocomposites was investigated. Samples of pure MgH2 and MgH2-mixtures containing 2 mol% of FeF3 were processed under air atmosphere and their effects on the microstructure and the hydrogen storage properties were studied in detail. X-ray Diffraction (XRD) analysis on all samples revealed crystallite sizes in the nanometer range for the beta-MgH2 and FeF3 phases (beta (beta): ∼8–13 nm and FeF3: ∼16–21 nm). No extra crystallite size reduction was observed for the HEBM + CR samples in comparison with those only cold rolled. Scanning electron microscopy (SEM) evaluation together with elemental composition analysis indicated a finer size distribution of additive particles and also a more intimate level of mixing for the HEBM + CR mixture than for the mixture obtained only by CR. These features were associated with the HEBM step applied before the CR. These characteristics led to a lower hydrogen desorption temperatures and enhanced desorption kinetics behavior for the HEBM + CR samples in comparison with the CR samples. [Copyright &y& Elsevier]

Abstract: In this study we have used cold rolling (CR) as a successful method to incorporate additives into MgH2. The mixtures of MgH2 + 2 Mol% X, with X = Fe, Nb, Fe2O3, Nb2O5, FeF3, were processed by CR using a horizontal rolling mill and then followed by a detailed microstructural characterization which includes X-ray diffraction (XRD) with Rietveld refinement, Differential Scanning Calorimetry (DSC) and kinetic measurements. The CR processing resulted in a drastic crystallite size reduction, ∼8–17 nm, strong texture and partial conversion from the β to the γ-MgH2 phase. The orthorhombic metastable NbH x phase was detected in the XRD pattern of the mixture MgH2 + Nb after CR, indicating the high energy involved in the CR processing. The samples containing Fe and FeF3 as additives presented the lowest desorption temperature ranges. All additives presented beneficial effect on the dehydrogenation kinetic measurements (measured at 623 K and 598 K) compared to pure MgH2, in especial for the samples containing Nb2O5 and FeF3. [Copyright &y& Elsevier]

The reversible hydrogen desorption–absorption of γ–Mg(BH 4 ) 2 ball milled with MoO 3 and TiO 2 has been investigated. No mechanochemical interaction between γ–Mg(BH 4 ) 2 and TiO 2 was observed while for the 1:1 M ratio with MoO 3 , MoB 2 was formed. There was little difference in the properties of γ–Mg(BH 4 ) 2 and γ–Mg(BH 4 ) 2 ball–milled with 0.02 mol percent TiO 2 and MoO 3 . However, mixing in the same ratio γ–Mg(BH 4 ) 2 with TiO 2 and MoO 3 that had previously been subjected to high energy milling provided significant effects. For these materials, the DSC peaks appeared broader and were shifted to slightly lower temperatures, while the TPD peaks were decreased by ca. 20 °C, as compared to the pure γ–Mg(BH 4 ) 2 . Faster hydrogen desorption kinetics was observed for the γ–Mg(BH 4 ) 2 –MoO 3 mixture. The reversible hydrogen storage capacity of γ–Mg(BH 4 ) 2 –TiO 2 at 271 °C was found to be 2.4 wt% H 2 in the first cycle. [ABSTRACT FROM AUTHOR]

Abstract: 2 LiNH2–1.1 MgH2–0.1 LiBH4–3 wt.% ZrCoH3 is a solid state hydrogen storage material with a hydrogen storage capacity of up to 5.3 wt.%. As the material shows sufficiently high desorption rates at temperatures below 200 °C, it is used for a prototype solid state hydrogen storage tank with a hydrogen capacity of 2 kWhel that is coupled to a high temperature proton exchange membrane fuel cell. In order to design an appropriate prototype reactor, model equations for the rate of hydrogen sorption reactions are required. Therefore in the present study, several material properties, like bulk density and thermodynamic data, are measured. Furthermore, isothermal absorption and desorption experiments are performed in a temperature and pressure range that is in the focus of the coupling system. Using experimental data, two-step model equations have been fitted for the hydrogen absorption and desorption reactions. These empirical model equations are able to capture the experimentally measured reaction rates and can be used for model validation of the design simulations. [Copyright &y& Elsevier]