Papers by Author: Daniel Fruchart

Abstract: In the most recent years, MgH₂ has attracted considerable attention for reversible hydrogen storage purposes because of a large 7.6 w% H-uptake, single plateau reaction at low pressure and abundance of metal. If the Mg ↔ H reactions take place at rather high temperature (> 300°C), the kinetic remains very low. However, early transition metal based additives (Ti, V, Nb...) improve dramatically the kinetics of hydrogen absorption/desorption, while having no essential impact on the reversible sorption capacity. Systematic analysis of many experimental data led to question chemical, physical, mechanical... parameters contributing significantly to improve the kinetics of absorption/desorption. Besides, results of theoretical and numerical computation enlighten the impact of structural and mechanical parameters owing to the local bonds of Mg/MgH₂ with of TM elements, in terms of total energy and electronic structure. More specifically, we found highly relevant to consider 1 - the impact of the crystallite sizes of Mg and the TM-phase, 2 - the role of internal and external stresses, as well as 3 - the role of texture on the kinetics of hydrogen absorption/desorption. Apart the previous considerations, we like to underline the role of specific TM in trapping intermediately hydrogen thus forming TMH_x prior initiating the Mg ↔ MgH₂ nucleation process.

Abstract: Magnesium is light, abundant and it can store up to 7.6 wt. % of hydrogen forming MgH₂ and accordingly it is a promising material for hydrogen storage. Processing of Mg-based mixtures by high-energy ball milling (HEBM) can produce materials with high level H-sorption properties. In the present report, we display and compare the effects of different nanocrystalline additives (MgF₂, Fe, NbH_0,89, FeF₃, VF₃) on the formation of MgH₂ by reactive milling. The H-desorption behavior of the as-prepared nanocomposites is also evaluated. A combined catalytic effect is observed due to the synergic action of MgF₂ and Fe (or NbH_0,89) on the hydrogenation rate during processing. The transition metal fluorides promote as well the MgH₂ synthesis. By using more energy-intensive milling conditions and adequate additives in given proportions (e.g. 5 mol. % FeF₃), is shown to be very effective for a full and fast synthesis (4 h) of MgH₂ by reactive milling.

Abstract: According to ¹H NMR study in fcc hydrides of disordered Ti-V alloys hydrogen atoms are distributed over both octahedral and tetrahedral sites. The occupancy of octahedral sites increases with the vanadium concentration. In hydrides of ternary Ti-V-Cr alloys hydrogen atoms occupy tetrahedral sites only.

Abstract: Zr-based AB₂-Laves phase type alloys containing the same type of A and B metals, have been prepared from pure elements by melting and subsequent re-melting under argon atmosphere by using a HF-induction levitation furnace. Characterization of the alloys has resulted from powder X-Ray Diffraction (XRD) measurements and SEM/EDX analyses. Systematic PCI (Pressure-Composition-Isotherms) measurements have been recorded at 20 and 90 °C, using a high-pressure Sievert's type apparatus. The purpose of this study is to find a series of alloys promptly forming metal hydrides (MH) with suitable properties in order to build a MH-based hydrogen compressor, working in the same way between 20 and ~100 °C.

Abstract: Occurrence of phase transitions in TixVyCrz-H systems was analyzed versus temperature. Hydrogen evacuates from the hydrogenated compounds via two main steps. The 1st step corresponds to the destabilization of the fcc hydrogenated lattice leading to a fcc → bcc martensitic type transformation. The 2nd step occurs according to the decomposition of the resulting hydride via evacuation of hydrogen from the bcc lattice. Both the kinetics and temperature of 1st and 2nd step transformations depend on the composition of the starting TixVyCrz alloys.

Abstract: Mg/TM thin films are objects of interest as hydrogen storage materials due to their expected interesting thermodynamic and hydrogen sorption kinetic properties. Here we report on the results of ab initio calculation on the stability of Mgn/Ti and Mgn/Nbm thin films.

Abstract: Perturbed Angular Correlation (PAC) measurements were performed at room temperature on samples of Hf7Ni10 and of Hf7Ni10 combined with TiV0.8Cr1.2, and at temperatures in the range from 17 K to 350 K in a concentrated hydride of the combined alloy. The PAC results show that the Hf7Ni10 intergranular phase becomes amorphous in the combination process. The results for the hydride composite show in the Hf7Ni10 phase the formation of a stable interstitial hydrogen state below 100 K and a smooth rearrangement of the hydrogen atoms position above 100 K,corresponding to a slow diffusion movemnet of a short-range nature. A second phase transition is observed at around 300 K, presumably to aphase precursor of the hydrogen desorption of the hydride.

Abstract: Magnesium complex hydrides as Mg2FeH6 are interesting phases for hydrogen storage in the solid state, mainly due to its high gravimetric and volumetric densities of H2. However, the synthesis of this hydride is not trivial because the intermetallic phase Mg2Fe does not exist and Mg and Fe are virtually immiscible under equilibrium conditions. In this study, we have systematically studied the influence of the most important processing parameters in reactive milling under hydrogen (RM) for Mg2FeH6 synthesis: milling time, ball-to-powder weight ratio (BPR), hydrogen pressure and type of mill. Low cost 2Mg-Fe mixtures were used as raw materials. An important control of the Mg2FeH6 direct synthesis by RM was attained. In optimized combinations of the processing parameters, very high proportions of the complex hydride could be obtained.

Abstract: Laves phases of C15 type of the system Mg2Si1-xSnx with x = 0, 0.4, 0.6 and 1 were synthesized in polycrystalline state in tantalum ampoule heated by high frequency coupling. The as-cast materials were analysed first at room temperature by X-ray diffraction (XRD). Systematically, XRD patterns were recorded up to 700 °C and Differential Scanning Calorimetry analysis was performed up to 1200°C to control the chemical and structural phase transformations. From these experiments, a C15 to C36 structure transformation was pointed out around 600°C in Mg2Si1-xSnx.solid solutions.

Abstract: In this paper we investigate the performances of two bulk magnetocaloric refrigerants based on La(Fe,Co)13-xSix and prepared by powder metallurgy. Both materials were developed especially for a magnetic cooling machine. We have determined the magnetocaloric effect in term of temperature change under magnetic field using a test-bench with practical running conditions. ΔT was measured under 2 T and close to room temperature range. The obtained results will be compared with those of some reference materials reported in the literature. In addition, a composite material based on La(Fe,Co)13-xSix is proposed for magnetic systems using Ericsson and AMR cycles for refrigeration close to room temperature.