Yasuyuki Kitano

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.

Unusual hydrogen absorption properties in graphite mechanically milled under various hydrogen pressures up to 6 MPa

We examined hydrogen absorption/desorption behaviors in graphite mechanically milled under hydrogen pressures ranging from 0.3 to 6.0 MPa. Experimental results indicated that the chemisorbed hydrogen concentration during 80 h milling decreased from 6.1 to 4.1 wt.% with increasing milling hydrogen pressure, while the physisorption-like hydrogen concentration increased with increasing milling hydrogen pressure and reached more than 0.5 wt.% at 6 MPa. Moreover, higher pressure hydrogen suppressed the fracture rate of the milled graphite more effectively, suggesting that the hydrogen atoms trapped at the edges of the graphene sheets and between the graphene layers near the surface are responsible for preserving the lamellar nanocrystalline structure, which may be the host for physisorption-like hydrogen absorption.

Hydriding properties of a nano-/amorphous-structured Mg–Ni–H system

The alloys Mg−x at.% Ni (x=33, 38, 43 and 50) with nanometer-scale structures were successively synthesized by mechanical grinding of Mg2Ni mixed with various amounts of additional Ni. The total hydrogen contents in these alloys increases from 1.7 mass% for x=33 to 2.2 mass% for x=43 and 50, while the dehydriding temperatures gradually lower from 440 K to 373 K with increasing of the Ni amounts. The value of the total hydrogen content in each alloy is reasonably explained by taking into account the hydrogen contents of the following three regions; the intra-grain region of nanostructured Mg2Ni, its inter-grain region, and the amorphous MgNi region. The maximum hydrogen content in the amorphous MgNi region, 2.2 mass%, might imply a specific interatomic occupation of hydrogen atoms in this region.

Hydriding properties of the heat-treated MgNi alloys with nanostructural designed multiphase

Abstract The structural and hydriding properties have been investigated for the heat-treated MgNi alloys with nanoscale structure which consists of the Mg 2 Ni and MgNi 2 phases. The alloys were fabricated by mechanical alloying (MA) and heat treatment (HT), the so called MA/HT method. For the as-alloyed amorphous MgNi (a-MgNi) alloy, the maximum hydrogen content reaches 1.72 mass% at 473 K. The hydride, however, gradually decomposes into Mg 2 NiH 4 and MgNi 2 phases during measurement of pressure–composition isotherm. In the case of nanoscaled crystalline Mg 2 Ni (c-Mg 2 Ni)+amorphous MgNi 2 (a-MgNi 2 ) multiphase alloy, the maximum hydrogen content reaches only 0.35 mass% at 473 K. No plateau-like behavior is observed in the desorption process. In the case of nanoscaled c-Mg 2 Ni+crystalline MgNi 2 (c-MgNi 2 ) multiphase alloy, the maximum hydrogen content reaches 1.45 mass% at 473 K. A well-defined plateau region is observed of nearly 0.012 MPa at 473 K in the desorption process. Therefore, it is concluded that the hydriding properties of the nanoscaled MgNi multiphase alloy will be strongly affected by not only the structural properties of the matrix surrounding the c-Mg 2 Ni grains precipitated but also by the accumulation/release of internal stress on the precipitation process.

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.

Electron microscopy and hydriding properties of MgYNi4 synthesized by mechanical alloying

Abstract The ternary alloy, MgYNi 4 , was synthesized by mechanical alloying (MA). Its microscopic structures and hydriding properties were studied. The annealing of the alloy at 773 K was also studied. The alloy consists of nanocrystals of less than 10 nm in diameter. The capacity of the hydrogen atoms in the alloy increases with the increasing size of the nanocrystals. The crystal structure was found to be the C15 type Laves phase structure for the MA-treated alloy, while crystal structure of the as annealed alloy was the ordered C15 type structure which is conventionally called C15b type and is formally called AuBe 5 type structure. The hydrogen capacity in the alloy might have a strong relation to the atomic arrangement of this alloy.

New stacking variant of laves phase found in (Ti0.95V0.05) Co2 alloy

Microstructures of the Laves phase alloy, (Ti0.95V0.05) Co2, were studied by high resolution electron microscopy and electron diffraction. In this alloy system, coexistence of several kinds of layered structures was observed. Among these structures, a new stacking variant was found and was analyzed to be ABCAB′A′C′BCA′C′B′. This structure belongs to the trigonal system of the space group P3m1 (no. 164). The lattice parameters presented in the hexagonal system are a = 0.4727 ± 0.0009 nm and c = 4.628 ± 0.008 nm. This structure is called 12T in the Ramsdell notation and 4323 in the Zhdanov symbol, and is classified into hP3m1‐(6i)5(3f)(3e)(2d)12(2c)6using the Wyckoff notation. Microsc. Res. Tech. 40:277–283, 1998.