Toshiro Kuji

Hydrogen absorption of nanocrystalline palladium

Abstract Thermodynamic properties of hydrogen in nanocrystalline Pd (Pd-n) were determined by pressure–composition ( P–C ) isotherms. Pd-n was prepared by repeated compression–extrusion cycles in the die cavity of a high speed forging apparatus. It was found that Pd obtained was composed of nano-sized Pd grains, i.e. ∼10 nm, which were strained in 〈111〉 and 〈100〉 by 1.3 and 2.4%, respectively. From P–C isotherm measurements over the temperatures from 298 to 373 K, it was found that the hydrogen solubility in the α phase region for Pd-n was significantly larger than that for coarser grained Pd. On the other hand, Pd-n showed lower plateau pressures than coarser grained Pd. In addition to the above, the maximum hydrogen solubility of Pd-n was almost 50% lower than that for coarser grained Pd. The differences in the hydrogen solubility and plateau pressure between Pd-n and coarser grained Pd will be discussed under the consideration of the structure of nano-sized Pd grains. The drastic reduction in the maximum hydrogen solubility for Pd-n could be related to the viewpoint of strain of the octahedron in f.c.c. Pd.

The development of high performance Nd–Fe–Co–Ga–B die upset magnets

The magnetic properties and the microstructure of Nd–Fe–B and Nd–Fe–Co–Ga–B die upset magnets produced from amorphous materials were studied. The Nd–Fe–B die upset magnets had fine polygonal Nd2Fe14B grains and showed magnetic anisotropy. The compositional modification and optimization of the die upset condition led to the increase in the remanence and coercivity values of the Nd–Fe–B die upset magnets. The optimally deformed Nd–Fe–Co–Ga–B die upset magnets showed the maximum energy product of 54.4 MGOe.

Synthesis of Mg2Ni alloy by bulk mechanical alloying

Abstract Mg 2 Ni was synthesized by a solid-state reaction from the constituent elemental powder mixtures via bulk mechanical alloying. Since homogeneous refining and alloying takes place efficiently by repeated forging, the process time can be reduced to one fiftieth of time duration necessary for conventional mechanical milling and attrition. This MA Mg 2 Ni alloy can be obtained as a high dense powder compact with a relative mass density in the order of 85%; its average powder particle size ranges from 1 to 3 μm. The prepared Mg 2 Ni alloy has a sufficiently high reaction rate to hydrogen even at relatively lower temperatures and a potential to form a ternary hydride on the pressure-composition isotherms from 523 to 623 K. The enthalpy and entropy for ternary hydride formation were estimated to be −28. 7 KJ/mole-H and −46. 8 J/K/mole-H, respectively. The present bulk mechanical alloying was found to be adaptive to fabrication of single phase Mg 2 Ni in high productivity.

Hydrogen absorption and electrochemical properties of Mg2-xNi (x = 0 - 0.5) alloys prepared by bulk mechanical alloying..

Abstract Thermodynamic properties of hydrides of Mg 2− x Ni alloys produced by bulk mechanical alloying were determined from the pressure–composition isotherms for absorption over the temperatures from 623 to 423 K. The van’t Hoff plots for the plateau pressures of the isotherms clearly indicated the existence of high- and low-temperature hydrides with different entropy and enthalpy for hydride formation. The phase transition temperature was 525 K for Mg 2.0 Ni and decreased with increasing value of x . Chemical and electrochemical behaviors of Mg 2− x Ni alloys in an alkaline solution were precisely determined. It was found that hydrogen absorption in Mg 2− x Ni alloys takes place by only immersing the alloys in 6 M KOH solution. On the other hand, during electrical charging hydrogen content increased with the quantity of charged electricity and reached the maximum value. However, the hydrogen content decreased afterwards because the corrosion of the alloys in the alkaline solution is significant.

Competing stabilisation mechanisms in Mg2NiH4

Abstract By using three different Mg 2 Ni alloys to synthesise Mg 2 NiH 4 an additional stabilisation mechanism of this system has been identified. It has been known for some time that the stacking faults at unit cell level in the lattice, microtwinning, stabilise this hydride thermodynamically, but the results in this study also reveal that the addition of free magnesium from the casting of the starting alloys have a similar stabilising effect on the hydride. It is also clear that these two stabilisation mechanisms are connected to each other, i.e. the microtwinning in Mg 2 NiH 4 is dependent not only on the thermal history of the sample but also of the amount of free magnesium added at the casting of the Mg 2 Ni alloy. The extra magnesium acts as a stabilising dopant to the Mg 2 NiH 4 system and in addition to this gives interesting colour effects of the low-temperature phase of Mg 2 NiH 4 .

Synthesis of nano-structured b.c.c. Mg–Tm–V (Tm=Ni, Co, Cu) alloys and their hydrogen solubility

Abstract Mg–Tm–V (Tm=Ni, Co, Cu) alloys were synthesized by mechanical alloying. The structure characterizations by X-ray diffraction and electron diffraction revealed the prepared alloys to consist of nanocrystalline grains with b.c.c. structure, which was stable up to 573 K. It was found that synthesized Mg 1.0 Ni 1.0 V 1.0 , Mg 1.0 Cu 1.0 V 1.0 and Mg 1.0 Co 1.0 V 1.0 alloys were able to absorb hydrogen up to 2.3, 1.44 and 0.95 wt.% at 3 MPa and 298 K, respectively. The plateau pressures of pressure–composition isotherms for these alloys were observed at 0.2–0.8 MPa and 298 K.

Thermodynamic properties of hydrogen in fine Pd powders

Abstract The effect of Pd particle size on the thermodynamic properties of the Pd–H system was examined at 298 to 373 K. In this study, the pressure–composition(PC) isotherms were determined for Pd fine powders with around 0.2 μm in diameter of primary particle and derived thermodynamic properties were compared with values obtained for Pd sheet. In the solid solution region, the hydrogen solubility increased with reducing Pd particle size. This is similar to the case found for LaNi 5 –H system. On the other hand, it was found that the plateau pressures on P–C isotherms increased with decreasing particle size at relatively low temperatures, which is the reverse to the case for LaNi 5 –H system. This research demonstrates the difference in thermodynamic properties of hydrogen in between fine Pd powders and Pd sheet, well reflecting the difference in hydrogen solubility and plateau pressure on PC isotherms.

NdFeB die upset magnets produced from amorphous bulk materials

Abstract Amorphous bulk materials of an Nd14Fe80B6 alloy (25 mm in diameter and 20 mm in thickness) were produced by dynamic compaction of amorphous melt-spun ribbons. The amorphous bulk materials were successfully hot deformed by die upsetting. Die upsetting of the amorphous bulk materials resulted in crystallization, leading to fine Nd2Fe14B particles with polygonal shape. X-ray diffraction and magnetic measurements confirmed that the polygonal Nd2Fe14B grains were crystallographically aligned. The Nd14Fe80B6 die upset magnets showed excellent value of the maximum energy products, exceeding 400 kJ m−3.

Hydrogen solubility in rare earth based hydrogen storage alloys

Abstract This paper reviews significant results of recent studies on the hydrogen storage properties of rare earth based AB 5 (A: rare earth element, B: transition element) alloys. The hydrogen solubility and the hydride formation, typically appeared in pressure-composition isotherms (PCT), are strongly dependent upon alloy composition, structure, morphology and even alloy particle size. Typical experimental results are shown to describe how these factors affect the hydrogen solubility and storage properties.

Study on Microstructure and Magnetic Properties of TM-Mg (TM: Fe, Co) Alloys Synthesized by Mechanical Alloying

Magnesium alloys are extremely attractive candidates for hydrogen storage applications since they can potentially absorb hydrogen between 3 and 8 wt.%. The purpose of this work is to understand the microstructural and the magnetic properties for 3d-transition metal and magnesium alloys with the difficult to alloy by the conventional method due to the positive value of mixing enthalpy. We successfully formed alloys of iron or cobalt, and magnesium powders with a wide range of compositions by mechanical alloying and characterized synthesized alloys with an X-ray diffractometer, a TEM, an SEM-EPMA and a vibration sample magnetometer. The obtained Fe-Mg alloys containing less than 25 at.% Mg were single phase bcc with expanded lattice parameter. The average powder particle size changes with Mg composition. The magnetization of the samples showed a linear dilution with content of Mg. The opposite variations in lattice parameter and the coercive force with Mg content were observed. On the other hand, we observed markedly broadened XRD lines from Co-Mg alloy compounds. The microstructure of these powders implies that the alloy could be partially amorphized or changed into a nanostructure as expected from microscopy and an XRD results.