Search

Both numerical simulation and hardness measurements were used to determine the mechanical and microstructural behavior of AZ31 bulk samples when submitted to the Equal Channel Angular Pressing (ECAP) technique. Billets of this representative of Mg-rich alloys were submitted to different numbers of passes for various ECAP modes (anisotropic A, isotropic B C ). The strain distribution, the grain size refinement, and the micro-hardness were used as indicators to quantify the effectiveness of the different processing routes. Structural characterizations at different scales were achieved using Scanning Electron Microscopy (SEM), micro-analysis, metallography, Small Angle Neutron Scattering SANS, X-Ray Diffraction (XRD), and texture determination. The grain and crystallite size distribution and orientation as well as defect impacts were determined. Anelastic Spectroscopy (AS) on mechanically deformed samples have shown that the temperature of ECAP differentiate the fragile to ductile regime. MgH₂ consolidated powders were checked for using AS to detect potential hydrogen motions and interaction with host metal atoms. After further optimization, the different mechanically-treated samples were submitted to hydrogenation/dehydrogenation (H/D) cycles, which shows that, for a few passes, the B C mode is better than the A one, as supported by theoretical and experimental microstructure analyses. Accordingly, the hydrogen uptake and (H/D) reactions were correlated with the optimized microstructure peculiarities and interpreted in terms of Johnson-Avrami- Mehl-Kolmogorov (JAMK) and Jander models, successively.

This study analyzes the mechanical behavior of compacted disks made of MgH 2 powders co-milled with vanadium or Ti–V–Cr alloy as an additive, through the evolution of the microstructure and mechanical properties upon hydrogen cycling. The recrystallization of MgH 2 particles results from a dynamic recrystallization phenomenon associated with the reaction of hydrogenation itself. The coalescence of the nanometric particles tends to create large agglomerates, which induces an increase in porosity, and explains the progressive swelling of the composites. A relaxation of the maximum strain is observed after 10 cycles for vanadium whereas for Ti–V–Cr the expansion increases until 200 cycles. This difference of behavior is correlated to the ability of vanadium particles to prevent the recrystallization mechanisms, then to limit the agglomeration of the MgH 2 particles. From the Vickers hardness measured on compacted powders, a hardness of 0.58 GPa was estimated for highly densified magnesium hydride. Nano-indentation tests performed on compacted pellets show an enhancement of about 20% of the Young modulus as the amount of additives raises from 4 to 8 wt. %. The Young modulus markedly improves as the number of hydrogen cycles increases up to 10. The H/E ratio calculated from these data is characteristic of an intermediate state in between elastic and plastic behavior. [ABSTRACT FROM AUTHOR]

Abstract: Among the hydrogen storage materials, BCC structure type materials such as V-based alloys reveal promising characteristics due to there reactivity with hydrogen at ambient temperature. A systematic XRD study was performed in order to obtain a cell parameter-composition map in the BCC domain and attempt a correlation with the sorption properties. However, activation of this type of materials may not be straightforward. The aim of this study was to co-melt a main BCC phase with a minor metallic one acting as activating phase. The synthesis of co-melted materials with a specific microstructure starting from Ti1− x V1− y Cr1+ x + y (x ⩽0.4, y ⩽0.3) and Zr7Ni10 alloys was successfully realized. The hydrogen decrepitation of the compound was studied in correlation with its microstructure. Pressure-composition-isotherms were recorded, the absorption and desorption isotherms measured as a function of time (sorption kinetics measurements) show reversible hydrogen uptake up to 2wt.%. [Copyright &y& Elsevier]

Abstract: Magnesium hydrogenation reaction being exothermic and limited by heat removal, the thermal conductivity of ball-milled magnesium hydride (BM MgH2) powders has to be improved. The compression of BM MgH2 associated to Expanded Natural Graphite (ENG) to form compacted disks has been investigated. Using BM MgH2 without ENG, its compression reduces the porosity and increases its volumetric hydrogen storage capacity. Incorporating ENG before compression drastically improves the thermal conductivity in the direction normal to compression axis. Moreover, the thermal conductivity increases linearly with ENG content, and can be adjusted to fulfill the loading time requirements. The thermodynamic properties and intrinsic sorption kinetics remain unchanged. However, both compression and ENG incorporation reduce the hydrogen permeability, especially in the direction parallel to the compression axis, which imposes a limit to the disk thickness. A small-size instrumented tank has been loaded with either pure BM MgH2 powder or with disks having different ENG contents. The results obtained for both cases are compared. [Copyright &y& Elsevier]

Abstract: The aim of the present work was a systematic investigation on MgH2 co-milled with transition metals M, in order to develop large batches of suitable materials for hydrogen storage. Systematic ball-milling operations were done using a planetary miller, thus leading to combine the transition metal with MgH2 to form successive MH x hydrides. The initiating role of the metal hydride MH x was evidenced by in situ neutron diffraction experiments. The co-milling conditions were transferred to MCP Technologies SA, where highly reactive batches of powders were processed. These powders appear to be very stable on cycling, provided the desorbed powders are not exposed to a temperature higher than 370°C. A small scale MgH2 tank was developed. Absorption and desorption cycles have been operated in different experimental conditions. [Copyright &y& Elsevier]

Abstract: A metastable niobium hydride, believed to trigger rapid H desorption from the MgH2 matrix, was detected in the course of real time synchrotron diffraction studies of the desorption kinetics of hydrogen in MgH2 with about 5% NbH nanoparticles. In order to completely characterize this metastable phase, we performed in situ neutron diffraction experiments during hydrogenation/dehydrogenation of a magnesium–niobium nanocomposite. At 250  C (under 1 bar of H pressure) a hydride phase NbH x appears. Unfortunately, the available low pressure together with the low q-range did not allow to propose a structural arrangement for this phase. We thus switched to standard neutron diffraction (that is at constant pressure and room temperature) of ball-milled Mg–Nb–H nanocomposites for which various NbH hydrides were detected including the solid solution. It turns out that the formation of these niobium hydrides (metastable or not) is closely related to the milling time and depends on the quality of the starting metal. [Copyright &y& Elsevier]

Highly coercive Nd–Fe–B powders for bonded magnets have been successfully produced applying hydrogen processes such as hydrogen decrepitation (HD) or hydrogen–disproportionation–dehydrogenation–recombination (HDDR). XAFS experiments were performed in order to localise the extra elements. Zr addition results in the formation of fine ZrB2 precipitates, whereas Ga substitutes the Fe(4c) site in the Nd2Fe14B structure. Systematic analyses of the hydrogen disproportionation kinetics by thermomagnetic measurements have permitted to point out different regimes of reactivity depending on both the used extra elements and the temperature. It is found that the ZrB2 precipitates act as barriers limiting hydrogen diffusion at the crystallites boundaries and lead to uncompleted disproportionation reactions. Then the Nd2Fe14B remaining particles act as nucleation centres that can be used to induce oriented growth during the recombination step. [Copyright &y& Elsevier]

Abstract: The formation of magnesium hydride under hydrogen pressure is highly exothermic. The enthalpy of reaction represents 31% of the lower heating value (LHV) of the absorbed hydrogen. A new concept of MgH2 tank was developed to store the heat of reaction by using a phase-change material (PCM). The heat of desorption is mainly provided by the latent heat of solidification of the PCM. A metallic alloy based on Mg–Zn eutectic was selected as PCM in order to enhance the thermal exchange with the MgH2 compacts. A pilot tank of about 7000 NL of hydrogen was designed and tested under various experimental conditions. Desorption was achieved in 3 h and the daily storage efficiency was around 70%. [Copyright &y& Elsevier]

Abstract: Magnesium hydrogenation being an exothermic reaction, the loading time of a tank is limited by heat extraction. The compaction of ball-milled MgH2 associated with Expanded Natural Graphite was used to improve the thermal conductivity of the resulting compacts. Taking advantage of these compacts, an intermediate scale tank (1.8 kg MgH2) cooled down by forced air circulation was designed. The absorption is initiated at 90 °C. Since the intrinsic kinetic is not the limiting factor, the hydrogen pressure does not affect the loading process. The loading time is strongly dependent on the cooling efficiency. However, beyond a given air flow rate it doesn’t decrease any more, the heat transfer being limited by the thermal conductivity of the compacted disks. During desorption, the maximum hydrogen flow (25 Nl/mn) is directly proportional to the thermal power of the heating system. The tank absorbs 1170 Nl, has a specific-energy of 270 W h/kg and a system volumetric-density of 42 gr/l. [Copyright &y& Elsevier]

Immediately after the discovery and the crystal structure determination of the new R2Fe14B hard magnet series, it was proven that these materials actively react with hydrogen thus forming stable quaternary hydrides. A successful model for interstitial site occupation was built which allowed understanding the changes observed with structural parameters and magnetic parameters as well. More recently, it was shown that the effect of the hydrogen uptake after substitution of Fe by other elements can also be described using a multi-site occupation scheme owing to the bonding characteristics, e.g. of p-type elements.In addition, other rare-earth transition metal compounds (R(Fe,M)12, RFe17) exhibiting potentially hard magnetic properties were questioned for their ability to absorb significant amounts of interstitial elements, namely hydrogen, but also carbon and nitrogen. Here, again a filling scheme was developed which could well describe the changes observed in the fundamental properties of the series. Finally, the impacts of hydrogen on the mechanical and chemical properties of the magnet compounds and their constituent elements were used with profit to develop typical metallurgy processes leading to gain high extrinsic characteristics in relation with optimised microstructures. [Copyright &y& Elsevier]

A Round Robin Test exercise on magnesium hydride (MgH 2 ) was performed by 14 laboratories with the aim to compare experimental isothermal data such PCI curves, kinetics curves and formation enthalpies together with a basic statistical evaluation of the results. The full hydrogen capacity was found to vary in the range 5.1–6.4 wt.% at 280 °C (553 K) and in the range 5.3–6.6 wt.% at 320 °C (593 K) (value for 1 MPa hydrogen pressure). The relative standard deviations of 6.9% and 7.2%, respectively, were measured for absorption. The absorption plateau pressure of magnesium hydride varies between 0.08 and 0.14 MPa with an average of 0.10 MPa and a relative standard deviation of 17.3% at 280 °C (553 K). At 320 °C (593 K) the absorption plateau pressure results fall in the range 0.26–0.45 MPa, with a relative standard deviation of 17.6%. Kinetics curves were affected by much higher data dispersion than the PCI data. The enthalpy of absorption was −75.7 KJ/moleH 2 , with a relative standard deviation of 4.4%. The results highlight the importance of well defined measuring and reporting protocols as a base for future standard procedures. [ABSTRACT FROM AUTHOR]

Abstract: Hydrogenation and desorption of the bcc compound Ti0.5Cr0.6V1.9 was sequentially followed in the course of a time-resolved in situ neutron diffraction experiment on the D20 high flux diffractometer at the Institut Laue Langevin, Grenoble. From the structural point of view it has been shown that hydrogenation induces a structural type of transformation from bcc (starting alloy) to fcc (hydride). This structural transformation has been analyzed as well as the stability range of the metal-hydrogen solid solution (α-hydride). Calculations allowing to quantify the total energy of the alloys and of their parent hydrides were performed. A fairly good agreement was found between the results of experiments and the theoretical derivations in terms of the relative stability of the different materials. [Copyright &y& Elsevier]

Abstract: In this contribution we report on the results of our recent 1H NMR studies of different metal hydrides of interest for reversible hydrogen storage applications: Ti–V–Cr alloys of various compositions, pure and with Zr7Ni10 or Hf7Ni10 additives, and in additives themselves. The temperature dependences of 1H spin-lattice relaxation have been treated within an exchange model. [Copyright &y& Elsevier]