Jean-Louis Bobet

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

Abstract We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with oxides Cr 2 O 3 , Al 2 O 3 and CeO 2 . The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH 2 during reactive mechanical grinding. The Mg+10wt.%Cr 2 O 3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . The Mg+10wt.%Cr 2 O 3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . Mg+10wt.%Cr 2 O 3 absorbs 5.87wt.% H at 573 K, 11 bar H 2 during 60 min at the first cycle. The Mg+10wt.%Cr 2 O 3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . It desorbs 4.44 wt.% H at 573 K, 0.5 bar H 2 during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr 2 O 3 in the Mg+10wt.%Cr 2 O 3 powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr 2 O 3 after cycling.

Hydrogen sorption of Mg-based mixtures elaborated by reactive mechanical grinding

The use of mechanical grinding (MG) under H2 of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process that allows suppressing the activation procedure generally requested for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface. A critical milling time exists below which the diffusion process is improved and above which no further improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 h. The addition of Co (catalyst), YNi (hydrogen pump) or oxides (abrasive element and nucleation centre) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of our mixtures are comparable with thus reported for MgH2–metal mixtures.

Effect of reactive mechanical grinding on chemical and hydrogen sorption properties of the Mg+10 wt.% Co mixture☆

Abstract Reactive mechanical grinding (MG under H 2 ) of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process that allows suppressing the activation procedure generally requested for Mg. The addition of Co, which acts as a catalyst for the dissociation of H 2 , also leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). The hydriding is determined to be a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface. A critical milling time exists below which the diffusion process is improved and above which no more improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle which can be decreased by increasing the milling time up to 10 h. However, the sorption properties of Mg–Co mixtures are a little under those reported for MgH 2 –metal mixtures.

The Doniach diagram and hydrogenation of the ternary compounds CePdIn and CePdSn

The process of hydrogenation of the antiferromagnetic compounds CePdIn and CePdSn has been studied. Investigation of the new hydrides CePdInH and CePdSnH by means of x-ray powder diffraction reveals that they adopt the same crystal symmetry as the original intermetallic but the unit cell volume increases during the hydrogenation. Magnetization, electrical resistivity, thermoelectric power and 119Sn Mossbauer spectroscopy measurements reveal that CePdInH and CePdSnH order antiferromagnetically below TN = 3.0(2) and 5.0(2) K respectively. Moreover, the physical properties of the hydrides are less influenced by the Kondo effect. The changes of the Neel temperature induced by H insertion are explained on the basis of Doniachs diagram considering the competition between Kondo and Ruderman–Kittel–Kasuya–Yosida interactions.

Reactive mechanical grinding applied to a (Ti+Ni) mixture and to a TiNi compound

Abstract Reactive mechanical grinding (MG under H 2 ) was applied to a (Ti+Ni) mixture. After only 1 h, all the titanium is transformed into TiH 2 . After 10 h of milling the TiH 2 is nano-structured and it takes approximately 25 h to obtain nano-structured Ni. After 45 h no binary compound is formed between Ti (or TiH 2 ) and Ni and a very homogeneous nano-structured mixture is formed. The electrochemical capacity is approximately 150 mAh g −1 and almost constant upon cycling. Finally the RMG process of TiNi mixture allows one to obtain a partial conversion of ingot into powder.

Modification of the magnetic properties of SmCo5 particles depending on the grinding atmosphere

Abstract Particles of SmCo 5 were synthesized and ground mechanically under two different gaseous atmospheres (Ar or H 2 ). The influence of the gaseous atmosphere on the crystallinity, the morphology of the particles and their magnetic properties has been studied. The major differences in terms of crystallinity and morphology are reached after 20 min of grinding. The sample ground under H 2 is still well crystallized whereas the homologous sample ground under Ar is almost amorphous. For longer grinding times, both the crystallinity and morphology are very close whatever the gaseous atmosphere. The sample is amorphous and consists of aggregates. However, for the sample ground under H 2 during 2 h, formation of SmH 2+δ and Co is observed. Values of the coercivity and saturation magnetization are discussed in regard to these characteristics.

Structure and magnetic properties of the ternary gallides CeMGa (M=Mn, Co and Cu) and their hydrides

The ternary gallides CeMGa present various structural properties since CeMnGa, CeCoGa and CeCuGa crystallize, respectively, in the cubic MgCu2-, monoclinic CeCoAl-, and orthorhombic CeCu2-type structure. These compounds absorb hydrogen at room temperature but hydrogenation induces a change of the crystalline structure: (i) CeCoGaH3.0 and CeCuGaH0.8 adopt the hexagonal AlB2-type; (ii) on the contrary, the cubic MgCu2 structure is preserved for CeMnGaH1.6. Moreover, in some cases hydrogenation leads to a valence change for Ce. For instance, CeCoGa is considered as intermediate valence compound, whereas CeCoGaH3.0 contains Ce3+. This paper demonstrates that hydrogenation strongly influences the Kondo effect in these intermetallics.

Magnetic behaviour of the new hydride CePtAlHx

Abstract CePtAl absorbs hydrogen up to a concentration of 1.1(1)H mol −1 at room temperature. The formation of this hydride CePtAlH 1.1 that loses hydrogen in air corresponds to a plateau pressure of 0.08–0.1 MPa. Its absorption–desorption is practically reversible which can present a potential interest for applications (but only a few due to the high price of the material) around room temperature and normal pressure. Hydrogenation of CePtAl induces both: (i) a structural transition but the structure of the hydride is unknown at present; (ii) an increase of its ferromagnetic character; the hydride has a Curie temperature T c =11.6(2) K twice greater than that determined for the initial intermetallic (i.e. 5.6(2) K).

Magnesium Hydride: From the Laboratory to the Tank

Abstract Magnesium metal is under extensive study for its high hydrogen absorption capacity. The main results obtained by the authors, going from the effects of ‘reactive mechanical grinding’ to the addition of nano oxides or to the deposition of nanoparticles are reported and discussed. The absorption properties are compared with the results of other research groups. An improvement of kinetics has been achieved and the mechanisms of the hydrogenation reaction is almost fully understood, but the effects of catalysts are still subject to hypotheses. Recent developments of MgH2 tanks are also presented.

Fabrication of biomimetic titanium laminated material using flakes powder metallurgy

Inspired from the microstructure of natural biological materials, a laminated titanium material was successfully elaborated using a novel approach of “flakes powder metallurgy.” Ti flakes powders, used as building blocks of the layers microstructure, were prepared by ball milling. They were then assembled into fully dense laminated materials using the spark plasma sintering technique. The results show (1) an anisotropy microstructure of the sintered material prepared from the flakes powder, (2) 15% of contribution of the lamellar architecture to the strength (hardness) of the material, and (3) faster densification of the flakes powder compared to unmilled powder.