Reactive Mechanical Grinding Research Articles

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr 2O 3, Al 2O 3 and CeO 2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with oxides Cr 2O 3, Al 2O 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 2O 3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2O 3. The Mg+10wt.%Cr 2O 3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al 2O 3 and Mg+10wt.%CeO 2. Mg+10wt.%Cr 2O 3 absorbs 5.87wt.% H at 573 K, 11 bar H 2 during 60 min at the first cycle. The Mg+10wt.%Cr 2O 3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2O 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 2O 3, Mg+10wt.%Al 2O 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 2O 3, Mg+10wt.%Al 2O 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 2O 3 in the Mg+10wt.%Cr 2O 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 2O 3 after cycling.

Effect of mechanical grinding under Ar and H 2 atmospheres on structural and hydriding properties in LaNi 5

The effects of mechanical grinding (MG) under argon and hydrogen gas atmospheres on the hydrogen storage properties of a LaNi 5 alloys were studied in detail. During MG under Ar atmosphere, a crystallite size reaches a ∼20 nm in grinding time of 60 min and reduces to approximately half this size after 600 min without any dissociation. The pressure–composition isotherm ( P– C) in LaNi 5 at 293 K indicates an increase in hydrogen in zero offset region (trapping site region), a lowering of plateau pressure and a narrowing of the width of the pressure plateau by MG. On the other hand, in reactive MG (RMG)-LaNi 5 under H 2 atmosphere, a nanocrystalline LaNi 5H 0.15 and an amorphous phase coexist when the grinding time is less than 180 min. For much longer RMG times than 180 min, the nanostructured LaNi 5H 0.15 phase disappears and the remaining amorphous phase dissociates into nanocrystalline Ni+amorpous LaNi y H z ( y<5). The P– C isotherm indicates no plateau for the LaNi 5 produced by RMG longer than 60 min and the hydriding properties become worse and worse with increasing RMG times. From the above results, we conclude that the hydriding properties cannot be improved by structural modifications in systems containing metals with a strong affinity for hydrogen like rare earth metals.

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

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.

Effects of Reactive Mechanical Grinding on Both Chemical and Hydrogen Sorption Properties of Mg-M (M=Co, Ni and Fe) Mixtures

Effects of Reactive Mechanical Grinding on Both Chemical and Hydrogen Sorption Properties of Mg-M (M=Co, Ni and Fe) Mixtures

Effects of Reactive Mechanical Grinding on Both Chemical and Hydrogen Sorption Properties of Mg-M (M=Co, Ni and Fe) Mixtures

Effects of Reactive Mechanical Grinding on Both Chemical and Hydrogen Sorption Properties of Mg-M (M=Co, Ni and Fe) Mixtures

メカニカル・グラインディング処理によるLaNi&lt;SUB&gt;5&lt;/SUB&gt;の構造変化と水素化特性

Effect of mechanical grinding (MG) under argon and hydrogen atmospheres on structural and hydriding properties were investigated in detail. In MG-LaNi5 under argon atmosphere, the grain size reaches ∼20 nm for MG times of 60 min and almost remains unchanged for more longer MG times. The pressure-composition isotherm (P-C) in LaNi5 at room temperature indicates an enhancement of hydrogen solubility, a lowering of plateau pressure and a narrowing of width of plateau by MG like FeTi but not Mg2Ni. On the other hand, in reactive MG (RMG)-LaNi5 under hydrogen atmosphere, a nanocrystallized LaNi5H0.15 and amorphous phases coexist within 180 min of grinding time. For RMG times longer than 180 min, the nanostructured LaNi5H0.15 phase disappears and the remaining amorphous-LaNi5Hx dissociates into nano-Ni+LaNiyHz(y<5). The P-C isotherm indicates no-plateau in the LaNi5 produced by RMG longer than 60 min and the hydriding properties become worse and worse with increasing RMG times. From these results, we conclude that the hydriding properties could not be improved by structural modifications in the system containing metals with strong affinity for hydrogen like rare earth metals.

Local structures and hydrogen dynamics in amorphous and nanostructured MgNiH systems as studied by 1H and 2H nuclear magnetic resonance

1H and 2H NMR spectra have been measured for amorphous MgNiH(D) and nanostructured Mg 2NiD systems synthesized by reactive mechanical grinding. 1H MAS NMR spectra indicate that the dominant phase in the amorphous MgNiH system is an amorphous MgNiH phase and that a small amount of an amorphous/disordered α-MgH, phase is contained. Hydrogens are randomly distributed in the amorphous MgNiH phase. 2H NMR spectra demonstrate that hydrogen motion is faster in the order of the nanostructured Mg 2NiH, the crystalline Mg 2NiH 4 and the amorphous MgNiH systems.

1H NMR and magnetization measurements of a nanostructured composite material of the Mg 2NiH system synthesized by reactive mechanical grinding

1H NMR spectra and magnetization have been measured for a nanostructured Mg 2NiH system. 1H MAS NMR spectra indicate the presence of four phases; α-Mg 2NiH, β-Mg 2NiH 4, α-MgH, and β-MgH 2. Most of the hydrogens are contained in the disordered β-Mg 2NiH 4 phase. Magnetization consists of superparamagnetic and paramagnetic ones. The former has its origin in Ni particles formed during the hydrogenation. The latter is greater than the values of crystalline α-Mg 2NiH, and β-Mg 2NiH 4. 1H NMR line width increases with the decrease in temperature linearly, most of which is of dipolar origin. The hydrogen is more mobile in the disordered β-Mg 2NiH 4 phase than in the crystalline counterpart.

Cooperative hydriding properties in a nanostructured Mg 2Ni–H system

A nanostructured Mg 2Ni–H system was prepared by reactive mechanical grinding (RMG) of Mg 2Ni under a H 2 gas pressure of 1 MPa during various time periods from 5 to 4800 min. The structural and hydriding properties of the products obtained were examined by X-ray diffraction, thermal analysis, SEM and TEM. The experimental results showed the existence of cooperative dehydriding phenomena in intra- and inter-grain regions which have about the same volume fractions. Dehydrogenation from distorted inter-grain regions in nanostructured Mg 2Ni–H prepared by RMG for 60 min occurs at the same temperature as hydrogen leaves the intra-grain region of crystalline Mg 2NiH 0.3. Elastic interactions in nanostructured materials are important for the understanding of such cooperative dehydriding phenomena.

Electron microscopy of Mg2Ni-H alloy synthesized by reactive mechanical grinding

Abstract A new material containing 1.6 wt.% of hydrogen (Mg2NiH1.8) was synthesized by reactive mechanical grinding under hydrogen atmosphere by Orimo and Fujii [Orimo, S. and Fujii, H., J. Alloys and Compounds, 1996, 232, L16]. The microstructures of this alloy, Mg2NiH1.8, was investigated by high resolution transmission electron microscopy (HRTEM). It has been found from the HRTEM images that the texture mainly consists of nanocrystals of Mg2NiHx (x ≤ 0.3) with an average size of approximately 15 nm, and also found that heavily distorted regions exist around or between nanocrystals. The width or the thickness of these regions is thought to be approximately 1–2 nm. These results are consistent with the results deduced from the peak broadening of the X-ray diffraction pattern and magnetic analysis [Orimo S., Fujii, H. and Ikeda, K. Acta Metallurgica, accepted]. It is plausible to assume that these regions can contain an unexpected number of hydrogen atoms. © 1997 Elsevier Science Limited. All rights reserved.

Notable hydriding properties of a nanostructured composite material of the Mg 2Ni-H system synthesized by reactive mechanical grinding

The intermetallic compound Mg 2Ni was mechanically ground under a hydrogen atmosphere to synthesize a nanostructured composite material that is composed of nanocrystalline intra-grain and disordered inter-grain regions. Both the thermal and magnetic analyses confirmed that a volume fraction of the latter region increases twenty times as much as that in the initial compound, nearly 30%, by grinding for only 60 min. As a result of this structural modification, notable hydriding properties emerged; the dissolved hydrogen content reaches up to 1.6 wt% (Mg 2NiH 1.8) without changing the crystal structure (Mg 2Ni type) of the nanocrystalline intragrain region, and the cooperative dehydriding reaction between both the regions occurs even at 413 K. The hydriding properties are most likely reversible in the temperature ranges below 473 K, above which the disordered inter-grain region transforms into a crystalline phase.

Small angle neutron scattering measurements of a nanostructured Mg 2NiD system

Small angle neutron scattering (SANS) measurements were carried out to investigate the spatial inhomogeneity of deuterium concentration and their size distribution in a nanostructured Mg 2NiD system prepared by the reactive mechanical grinding under a deuterium atmosphere. The scattering profile attributed to the deuterium inhomogeneity was observed, and diameters of the scatterer in the compound mechanically ground for 60 min were deduced to be 48 and 12 nm from Guinier and Porod approximations, respectively. Their values agree well with the near-maximum and average sizes of crystallites of Mg 2NiH 0.3 in the compound, which was previously observed by the high resolution TEM.

ChemInform Abstract: Hydriding Properties of the Mg2Ni‐H System Synthesized by Reactive Mechanical Grinding.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reactive mechanical grinding of ZrNi under various partial pressures of hydrogen

The intermetallic compound ZrNi was mechanically ground under various partial pressures of hydrogen. At the beginning of the grinding, hydride phases (ZrNiH and/or ZrNiH3) are formed, which lowers the free energy of the compound, and the particle size are effectively reduced. Moreover, with progression of the grinding, an amorphous phase starts to grow. Finally, depending on the partial pressure of hydrogen, a composite particle of the amorphous and crystalline phases is formed. Consequently, such a grinding technique under various partial pressures of hydrogen promotes the synthesis of fine composite particles, each of which consists of amorphous hydride and the crystalline hydride phases. In addition, the grinding enhances the decomposition to form ZrH2 even at room temperature. During the heating process under an argon atmosphere, the dehydrogenation of ZrH2 can progress even at low temperatures of around 750 K, which is caused by preferential crystallization of the amorphous phases in the fine composite particles.