Tokio Ohta

Influence of hydrogen on the magnetic properties of YCo compounds

Abstract The effect of hydrogen absorption on the magnetic properties of the Y2Co7Hx and the YCo3Hx systems was examined in situ. The two systems show similar β and γ phase formation and magnetic behaviour. The β phase exhibits the anomaly that a slight hydrogen enrichment induces a substantial increase in the cobalt moment which varies from a minimum on the low concentration side (0.13 μB in Y2Co7H2 and 0.15 μB in YCo3H) to a maximum on the high concentration side (1.2 μB in Y2Co7H3 and 0.7 μB in YCo3H2). These maximum values are almost equal to those in the respective uncharged compounds. The cobalt moment in the γ phase decreases with increasing hydrogen concentration in the same way as in ordinary cobalt-based ferromagnets. Both systems tend to lose ferromagnetism in the most hydrogenated compounds Y2Co7H8 and YCo3H4. The anomalous dependence of the cobalt moment on composition is discussed.

Photochemical and photoelectrochemical hydrogen production from water

Abstract The recent results of research and developments in photochemical and photoelectrochemical water decomposition technologies are reviewed. A new system using a semiconductor photoelectrode combined with thermoelectric generation is also introduced. Basic principles as well as the feasibilities of these hydrogen production systems are interpreted laying stress on the hybrid, homogeneous and heterogeneous methods. Unsolved problems in the reaction mechanisms, instabilities, and corrosion are discussed. The present aim of these systems is to attain a greater efficiency than that of the combined system of solar cells and water electrolysis.

Magnetic studies of hydride phase transformations in NdCo5Hx, PrCo5Hx and LaCo5Hx☆

Abstract Hydride-phase transformations in the ferromagnetic RCo 5 H x systems (R  Nd, Pr and La) are studied from the magnetic behaviour of the systems under the influence of hydrogen. In magnetization-temperature isobars stepwise changes in magnetization indicate the transformations between the α, β II , β I and γ phases in NdCo 5 H x and PrCo 5 H x and between the α, β I and γ phases in LaCo 5 H x . The limiting composition of the γ phase at 238 K is x = 4.7 ± 0.1 for NdCo 5 H x and x = 4.8 ± 0.1 for PrCo 5 H x . The change in enthalpy for each transformation is determined from the magnetic behaviour.

Hydrogen production using solar radiation

Abstract Various water-splitting methods using solar energy are reviewed and compared to each other. Direct thermal method has the highest efficiency, however it poses difficulties because of the need for heat-resisting materials. Thermochemical method becomes promising if corrosion-resisting materials are found. Electrolytic method is straightforward and conventional. However, a hybrid system combining electrolytic method with thermochemical and/or photochemical methods looks promising and is believed to result in optimum conversion efficiencies in the near future. Photolysis and biochemical methods are environmentally most acceptable, but are of low conversion efficiencies presently.

High field magnetization of Y2Co7 and YCo3 hydrides

Abstract The magnetization curves for the Y2Co7Hx and the YCo3Hx systems have been measured at 4.2 K in the pulsed high magnetic fields up to 280 kOe. The metamagnetic transition is observed in the β and the γ hydrides except for Y2Co7H3 and YCo3H1, which is interpreted in terms of the itinerant electron metamagnetism.

Mechano-catalytic water-splitting

The simultaneous H2 and O2 evolutions can be achieved by just stirring a powder of a p-type semiconductor, such as Cu2O, NiO, Co3O4 or Fe3O4 in distilled water at room temperature. Frictional rubbing of the stirring rod onto the bottom of the glass vessel is an important condition. The friction between the stirring rod and the glass surface generates an electrostatic field to make a field emission of positive holes from the semiconductor-powder, which is fixed in the micro crack spaces on the glass surface. The emitted positive holes react with water molecules to evolve O2 and protons, which go to the glass surface and combine with the electrons there to evolve H2. The energy conversion mechanism, the generation of frictional electricity, the effect of the water, the carrier-tunnelling emission probability, the hopping conduction in the semiconductor, and the effects of fluid dynamics, are discussed.

Effect of hydrogen absorption on magnetic properties of GdCo5 and YCo5

Abstract The magnetization of hydrides based on GdCo 5 and YCo 5 has been investigated as a function of hydrogen composition and temperature between 4.2 and 400 K under the direct influence of hydrogen. Hydrogen absorption leads to a reduction in the Co moment and weakenings in the Co-Co and the Co-Gd exchange interactions.

Magnetic behavior of metal hydrides as a function of hydrogen pressure and composition

A versatile vibrating‐sample magnetometer has been developed which measures the magnetization of metal hydrides as a function of magnetic field, hydrogen pressure (up to 6 MPa), and composition, temperature, and time. For the ferromagnetic NdCo5Hx system, magnetic properties of well‐defined hydrides and hydride‐phase transformations are studied by this method. The existence of a hydrogen‐rich phase in the NdCo5Hx system, the γ phase, is indicated by the temperature dependence of magnetization under the influence of high pressure hydrogen.

The motion of hydrogen in Mg2NiHx studied by proton magnetic resonance

Abstract Proton magnetic resonance in the α- and β-phases of Mg2NiHx was observed between 210 K and 480 K. The experimental results support that the motional narrowing due to a reorientation process occurs above 250 K in Mg2NiH4.

Effect of nickel, gadolinium and lanthanum substitution on the magnetic properties of Y2Co7 hydride

Abstract High field magnetization has been measured at 4.2 K in pulsed fields of up to 30 T for the Y 2 (Co 1 − y Ni y ) 7 H x , the (Y 1 − y Gd y ) 2 Co 7 H x and the (Y 1 − y La y ) 2 Co 7 H x systems to investigate the influence of the hydrogen on the magnetic moment in the β phase (1.7 ⩽ x ⩽ 3). The magnetic properties of the nickel-substituted system, including the metamagnetic transition, can be explained by using the density-of-states (DOS) curve of Y 2 Co 7 or Y 2 Ni 7 as a first approximation. The gadolinium substitution suppresses the reduction in the moment in the β L hydride ( x = 1.7). The lanthanum substitution causes a decrease in the critical field of the metamagnetic transition. It has been concluded that the hydrogen absorption leads to a weakening in the exchange interaction and an increase in the DOS at the Fermi level. The mechanism of the variation in the moment with the hydrogen composition has been discussed.