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 review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH₂based materials. By providing an insight on metal‐hydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH₂stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH₂that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well‐defined concentration of defects (Frankel pairs) at the surface and sub‐surface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball‐milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg‐ and Mg₂Ni‐based alloys: Equal‐Angular‐Channel‐Pressing and Fast‐Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H‐sorption kinetics have been improved satisfactorily without any additive for mass H‐storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast‐Forging was used for a “quasi‐instantaneous” synthesis of Mg/Mg₂Ni‐based composites. Hydrogenation of the as‐received almost bi‐phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg₂Ni on one side to MgH₂/Mg₂NiH₄on the other side via the rather stable a‐Mg₂NiH_0.3, acting as in‐situ catalyser.

The mass storage of hydrogen is a challenge considering large industrial applications and continuous distribution, e.g., for domestic use as a future energy carrier that respects the environment. For a long time, molecular hydrogen was stored and distributed, either as a gas (pressurized up to 75 MPa) or as a cryogenic liquid (20.4 K). Furthermore, the atomic storage of hydrogen in the solid-state form via metallic or covalent compounds is still the subject of intense research and limited to a small scale for some practical developments. In addition, other type H chemical storage routes are being tested, such as ammonia and LOHC (Liquid Organic Hydrogen Carrier), etc. In any case, the main constraint remains security. However, Hydrogen Solid State Storage (HSSS) using MgH₂ bodies has been shown to be feasible in terms of process and safety. Furthermore, its intrinsic volumetric densification was proven to be much better performing with 106:70:45 kgH₂/m³ for solid (RT):LH (20.4 K):gas (75 MPa), respectively. Very early on, fairly reactive MgH₂-based pellets were produced (for max. ~27 tons/year) at McPhy Energy using a series of unique and self-built installations. Thus, the design of large instrumented reservoirs was undertaken thanks to fundamental research first carried out at the CNRS. So, prototypes of storage units from 100 to ~5500 kWh have been produced. However, McPhy took other routes a few years ago (smelting and refueling stations) because the HSSS market was not merging at that time. Today, a new operator, Jomi–Leman, therefore, decided to try the challenge again focusing on applications with on-site production and mass HSSS.

The series of Mn_2−xFe_xP_1−ySi_y types of compounds form one of the most promising families of magnetocaloric materials in term of performances and availability of the elemental components. Potential for large scale application needs to optimize the synthesis process, and an easy and rather fast process here described is based on the use of two main type of precursors, providing the Fe-P and Mn-Si proportions. The series of Mn_2−xFe_xP_1−ySi_y compounds were synthesized and carefully investigated for their crystal structure versus temperature and compared interestingly with earlier results. A strong magnetoelastic effect accompanying the 1st order magnetic transition—as well as the parent phosphide–arsenides—was related to the relative stability of both the Fe magnetic polarization and the Fe–Fe exchange couplings. In order to better understand this effect, we propose a local distortion index of the non-metal tetrahedron hosting Fe atoms. Besides, from Mn-rich (Si-rich) to Fe-rich (P-rich) compositions, it is shown that the magnetocaloric phenomenon can be established on demand below and above room temperature. Excellent performance compounds were realized in terms of magnetic entropy ΔS_m and adiabatic temperature ΔT_ad variations. Since from literature it was seen that the magnetic performances are very sensitive to the synthesis process, correspondingly here a new effective process is proposed. Mössbauer spectroscopy analysis was performed on Mn-rich, equi-atomic Mn-Fe, and Fe-rich compounds, allowing determination of the distribution of hyperfine fields setting on Fe in the tetrahedral and pyramidal sites, respectively. Electronic structure calculations confirmed the scheme of metal and non-metal preferential ordering, respectively. Moreover, the local magnetic moments were derived, in fair agreement with both the experimental magnetization and the Fe contributions, as determined by Mössbauer spectroscopy.