Takeshi Haraki

Properties of hydrogen absorption by nano-structured FeTi alloys

Abstract In this study, two different nano-structured samples of the FeTi compound were prepared by mechanical alloying and mechanical grinding. For these samples, kinetics of the initial rate of hydrogen absorption, and the equilibrium hydrogen pressure as a function of hydrogen concentration were measured. Mechanical alloying of Fe and Ti atoms produced the FeTi compound powder samples with microstructures of a mixture of nano-structured FeTi grains and amorphous phases. This sample exhibited a high initial rate of hydrogen absorption even at 298 K, however, a strongly reduced hydrogen storage capacity. Mechanical grinding of the FeTi produced samples of particles with a particular microstructure: surface layers with a mixture of nano-structured FeTi grains and amorphous phases, and a single crystalline phase of FeTi below the surface layers for each particle. This sample exhibited a high initial rate of hydrogen absorption without a significant reduction of the hydrogen storage capacity compared with that of the standard FeTi sample. This mechanical grinding treatment was found to be an effective method of surface modification to improve the initial activation of the FeTi hydrogen storage alloy.

Effect of LaNi5H6 hydride particles size on desorption kinetics

Abstract The H 2 desorption rate of LaNi 5 H 6 hydride was measured as a function of the size distribution of LaNi 5 particles and the number ( N ) of cyclic hydriding and dehydriding reactions by a volumetric method at 298 K. The measured data showed that the rate of H 2 desorption depends on N , i.e. the particle size. The H 2 desorption rate increased as N increased from N =10, to 30 to 100, and as the particle size decreased. The measured desorption curves were found to be diffusion controlled. By fitting a diffusion equation to the measured curves, apparent diffusion coefficients of 1.8×10 −9 –2.2×10 −9 cm 2 /s at 298 K were obtained for LaNi 5 H 6 hydride with a median particle sizes of between N =10 (18 μm) and N =100 (10 μm).

Giant magnetostrictive thin film formation by plasma process

In this study, the effects of geometrical arrangement of substrate on the magnetostriction of the giant magnetostrictive materials film using an ion plating process were discussed. With increasing substrate angle, coercive force remained almost constant value and the in-plane magnetization at 15 kOe was strongly increased. This indicates that the easy direction of magnetization was changed from perpendicular to in-plane by obliquely deposition. The magnetic and magnetostrictive characteristics of the obliquely deposited films were affected by an oblique anisotropy. The oblique anisotropy may be induced by the shape anisotropy connected with columnar grain morphology.

Unidirectional solidification of Tb-Fe giant magnetostrictive alloys under microgravity conditions

The crystal growth of Tb-Fe giant magnetostrictive materials under microgravity (μG) and terrestrial gravity (1G) was investigated. The microgravity conditions were obtained by free fall in drop tower facility at the Japan Microgravity Center (JAMIC). TbFe1.83 alloy with 1 gram and cubic form was prepared for unidirectional solidification under microgravity environment. The samples were melted just before drop and solidified by contact chill against a sample at the period of microgravity after dropping. The microstructure of μG sample was columnar structure and growth direction was aligned in thermal gradient. In 1G sample, the microstructure was weak aligned in thermal gradient. The composition was measured by EDX. The TbFe3 phase was observed in 1G sample, and no TbFe3 phase was observed in μG sample, caused by reduction of thermal convection in microgravity environment. In μG sample, the columnar structure that aligned thermal gradient was oriented orientation. The magnetostriction of parallel direction to the thermal gradient was larger than perpendicular direction in μG and 1G. The magnetostriction of μG sample, the measurement direction was parallel to the thermal gradient, was larger than 1G sample caused by microstructure.

Comparison of different measurement methods of magnetostriction

With increasing interest in micro machine and micro mechatronics, high interest is given to micro devices using magnetostrictive alloys with small sizes. The accurate measurement of magnetostritions ranging from several tens ppm to 1000 ppm of small alloy samples is not easy, and the establishment of measurement methods of magnetostriction of small samples is needed. In this study, four different methods were examined to measure magnetostrictions of small metal samples (cubic samples with sizes smaller than 5 mm x 5 mm x 5 mm ) : Optical interference (OI) method, cantilever (CL) method, strain gauge (SG) method and capacitance (CP) method. The OI method was found to have a high sensitivity, however, too sensitive to measure small deformations of a small sample because of mechanical disturbance induced by surrounding mechanical noises. The CL, SG, and CP methods exhibited similar values of magnetostriction with high stability and reproducibility. This paper reports and discusses results of these measurement methods.