S. Orimo

Hydrogen in the mechanically prepared nanostructured graphite

Nanostructured graphite was prepared by mechanical milling under hydrogen atmosphere. Several samples obtained after different milling times were systematically examined to get fundamental information about the structures and hydrogen concentrations. After the expansion of the graphite interlayer, the long-range ordering of the interlayer disappears continuously with increasing milling time. The hydrogen concentration reaches up to 7.4 mass % (CH0.95) after milling for 80 h. Judging from the radial distribution function determined by the neutron diffraction measurement, there are two types of deuterium coordinations: deuterium atoms in the graphite interlayers and that with the CDx covalent bonds, respectively.Nanostructured graphite was prepared by mechanical milling under hydrogen atmosphere. Several samples obtained after different milling times were systematically examined to get fundamental information about the structures and hydrogen concentrations. After the expansion of the graphite interlayer, the long-range ordering of the interlayer disappears continuously with increasing milling time. The hydrogen concentration reaches up to 7.4 mass % (CH0.95) after milling for 80 h. Judging from the radial distribution function determined by the neutron diffraction measurement, there are two types of deuterium coordinations: deuterium atoms in the graphite interlayers and that with the CDx covalent bonds, respectively.

Notable hydriding properties of a nanostructured composite material of the Mg2Ni-H system synthesized by reactive mechanical grinding

The intermetallic compound Mg2Ni 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% (Mg2NiH1.8) without changing the crystal structure (Mg2Ni 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.

Hydrogen desorption property of mechanically prepared nanostructured graphite

Two desorption peaks of hydrogen molecule (mass number=2), starting at about 600 and 950 K, respectively, are observed in thermal desorption mass spectroscopy of nanostructured graphite mechanically milled for 80 h under hydrogen atmosphere. It follows from a combined analysis of thermal desorption mass spectroscopy and thermogravimetry, that ∼6 mass % of hydrogen (corresponding to 80% of the total amount of hydrogen) is desorbed at the first desorption peak as a mixture of pure hydrogen and hydrocarbons. Below the temperature of the second desorption peak, at which recrystallization related desorption occurs, nanostructured graphite is expected to retain its specific defective structures mainly with carbon dangling bonds as suitable trapping sites for hydrogen storage. The formation process of the nanostructures during milling under hydrogen atmosphere is also discussed on the basis of the profile of Raman spectroscopy.

Remarkable hydrogen storage properties in three-layered Pd/Mg/Pd thin films

We have investigated hydrogen storage and structural properties in nano-composite three-layered Pd(50 nm)/Mg(x nm)/Pd(50 nm) films with x=25, 50, 200, 400 and 800 prepared by an RF-associated magnetron sputtering method. After hydrogenation under a hydrogen gas pressure of 0.1 MPa at 373 K for 24 h, the TDS profiles indicated that the Pd layers contain only 0.15–0.30 mass% hydrogen, whereas the Mg film contains ∼5.0 mass% hydrogen for all the films. The most striking feature is that the temperature corresponding to maximum dehydrogenation rate remarkably shifts to low temperature with increasing the thickness of Mg film, which decreased from 465 K at x=25 nm to 360 K at x=800 nm. These improvements could be understood by the concept of cooperative phenomenon which hydrogen shows in nano-scale composite regions.

Structural and hydriding properties of the MgNiH system with nano- and/or amorphous structures

The alloys Mg-x at.%Ni (x = 33, 38, 43 and 50) with different nanometer-scale structures were successively synthesized by mechanical grinding of Mg2Ni mixed with various amounts of additional Ni, and the relations between their structural and hydriding properties were investigated in detail. The total hydrogen contents in these alloys increase from 1.7 mass% for x = 33 to 2.2 mass% for x = 43 and 50. This result is explainable by estimating both the volume fractions and the maximum hydrogen contents of three regions in each alloy; the intra-grain region of nanostructured Mg2Ni, its inter-grain region, and the amorphous MgNi region. The dehydriding temperature lowers down to 373 K in the alloy composed of only the amorphous MgNi region, while the dehydriding kinetics of the alloy composed of the above three regions are faster than those of only the amorphous MgNi region.

In situ study of hydriding–dehydriding properties in some Pd/Mg thin films with different degree of Mg crystallization

Abstract A new in situ system with the functions of thin film formation and analysis of hydrogen absorption–desorption properties has been developed to clarify hydrogen storage properties in nano-scaled composites. In this work, some Pd/Mg films (Pd (25 nm)-coated Mg (200 nm) films) with different degree of crystallization in the Mg layer were prepared in different sputtering conditions by changing RF coil powers and argon pressures. Hydrogenation under hydrogen gas pressure of 0.1 MPa at 373 K for 24 h indicated that MgH2 and non-crystalline Mg hydrides were formed in all the Pd/Mg films and the hydrogen content reached 2.9∼6.6 mass% independent of the degree of Mg crystallization. From the thermal desorption spectrum, it was deduced that the dehydriding temperature decreased with decreasing the degree of crystallization in the Mg layer in Pd/Mg films and the Pd/Mg film with lowest crystallization absorbed 5.6 mass% of hydrogen and all the hydrogen desorbed at a temperature lower than 463 K in vacuum.

Structural and hydriding properties of MgYNi4 : A new intermetallic compound with C15b-type Laves phase structure

A new intermetallic compound MgYNi4 with C15b(AuBe5)-type Laves phase structure was successfully synthesized by both mechanical milling and casting methods. The structural and hydriding properties of the samples were examined by X-ray diffraction measurement, thermal analysis and hydrogen pressure–composition (p–c) isotherm measurement. The lattice parameter of MgYNi4 was estimated to be a=0.701 nm, which is ca. 2% smaller than that of YNi2. A plateau (miscibility-gap) pressure was clearly observed in the p–c isotherm during the dehydriding process of the sample synthesized by casting. The maximum hydrogen content is ∼1.05 mass% (H/M∼0.6) under a hydrogen pressure of 4.0 MPa at 313 K, and the enthalpy of hydride formation was calculated to be −35.8 kJ/mol H2. The study in this paper reveals, for the first time, an application potential of MgNi2-based Laves phase structure for practical use as hydrogen storage and transport materials.

Hydriding properties of the Mg2Ni-H system synthesized by reactive mechanical grinding

Abstract The intermetallic compound Mg 2 Ni was mechanically ground under a hydrogen atmosphere. Its hydrogen content reaches 1.6 wt.% (Mg 2 NiH 1.8 ) without changing the crystal structure of the matrix Mg 2 Ni phase, and the dehydriding reaction starts at around 440 K. Its properties are most likely to originate from the formation of a nano-scale composite material which is composed of the crystalline matrix phase and the disordered interface phase.

Hydriding properties of the MgNi-based systems

In MgNi with an amorphous phase, a miscibility-gap (plateau) pressure of about 3×10−4 MPa at room temperature was clearly detected by electrochemical p–c isotherm measurements.The total radial distribution functions for X-ray and neutron diffraction indicated that deuterium in MgNi occupies an interstitial tetrahedral site composed of nearly 2Mg2Ni. Structural and hydriding properties of elemental substitution systems, Mg(Ni1−xTx) (T=Co and Cu) and (Mg1−xAlx)Ni with x=0–0.5, were also investigated experimentally.

Cooperative hydriding properties in a nanostructured Mg2Ni–H system

A nanostructured Mg2Ni–H system was prepared by reactive mechanical grinding (RMG) of Mg2Ni under a H2 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 Mg2Ni–H prepared by RMG for 60 min occurs at the same temperature as hydrogen leaves the intra-grain region of crystalline Mg2NiH0.3. Elastic interactions in nanostructured materials are important for the understanding of such cooperative dehydriding phenomena.