Akito Takasaki

Hydride dissociation and hydrogen evolution behavior of electrochemically charged pure titanium

Abstract A commercially pure α-titanium was electrochemically charged with hydrogen in a 5% H2SO4 solution at a current density of 5 kA m−2 for 14.4 ks (4 h), and the dissociation process of the electrochemically formed hydride and the evolution behavior of hydrogen from the samples were investigated by means of high temperature X-ray diffractometry, thermal desorption spectroscopy (TDS) and differential thermal analysis (DTA). The electrochemical charging produced δ-titanium hydride; this dissociated completely at temperatures around 600 K; (α + β) titanium then appeared, indicating that the hydride formed eutectoidally. The DTA detected the dissociation of the hydride (or (α + δ)−(α + β) boundary in the titanium-hydrogen system) as an endothermic peak. The TDS analysis, however, revealed that the accelerated hydrogen evolution could not be found at the dissociation temperature of the hydride but could be at higher temperatures. It was suggested that the hydride dissociation, (α + δ), into (α + β) two-phase region was not accompanied by hydrogen evolution from the samples, but the free hydrogen owing to the hydride dissociation was diffused into the samples. The peak temperatures of both DTA and TDS analyses shifted to lower temperatures with decreasing heating rate. The Kissinger plots fitted these results fairly well and indicated that the apparent activation energies for δ-hydride dissociation and hydrogen evolution were estimated to be about 106 kJ mol−1 and about 49 kJ mol−1 respectively.

High-pressure Hydrogen Loading in Ti45Zr38Ni17 Amorphous and Quasicrystal Powders Synthesized by Mechanical Alloying

Amorphous and icosahedral phase (i-phase) powders, synthesized directly by mechanical alloying (MA) and after subsequent annealing, respectively, are hydrogenated at a temperature of 573 K and an initial pressure of 3.8 MPa. The i-phase powder contains a Ti2Ni-type phase (fcc structure, lattice parameter, a=1.23 nm) as a minor phase. Hydrogen cycling for the i-phase powder decreases the coherence length and enhances the formation of an fcc hydride phase, namely (Ti, Zr)H2. The amorphous powder, which transforms to the fcc hydride after hydrogenation, is transformed primarily into a Ti2Ni-type crystal phase and a small amount of the i-phase after hydrogen desorption. Hydrogen cycling and mechanical alloying in a hydrogen gas atmosphere dramatically reduces the loading time of hydrogen for both the i-phase and the amorphous powders.

Mechanical Alloying of the Ti–Ni System

There kinds of Ti-Ni elemental powders, Ti 45 Ni 55 , Ti 50 Ni 50 and Ti 55 Ni 45 , were mechanically alloyed by a planetary ball mill for alloying times up to 10 h, and the alloying process and the microstructures after heating at temperature of 1273 K were investigated by powder X-ray diffractometry. The Ti 55 Ni 45 powder formed an amorphous phase after mechanical alloying for 10 h, while the Ti 50 Ni 50 powders formed a disordered b.c.c.-TiNi phase. The Ti 45 Ni 55 powder also formed the disordered b.c.c.-TiNi phase at an intermediate stage of mechanical alloying but turned to an amorphous state with increasing alloying time. After heating at 1273 K, the Ti 2 Ni phase has been formed in all powders, and the ordered B2-TiNi (CsCl structure) phase was observed in the Ti 50 Ni 50 and Ti 45 Ni 55 powders, but a monoclinic TiNi phase in the Ti 55 Ni 45 powder. In the Ti 45 Ni 55 powder also the TiNi 3 phase has been formed. The amounts of these intermetallic phases are dependent on the chemical compositions of the starting powders.

Hydrogen storage in Ti-Zr and Ti-Hf-based quasicrystals

The depletion of the worlds petroleum reserves and the increased environmental impact of conventional combustion-engine-powered automobiles are leading to renewed interest in hydrogen storage materials. In the mid 1990s, Ti/Zr/Hf-based quasicrystals were demonstrated to store more hydrogen than competing crystal intermetallic phases. Unfortunately, recovery of the hydrogen required temperatures in excess of 400°C, severely limiting the technological application of these materials. Here, the Ti/Zr-Hf-based quasicrystals and crystal approximants are reviewed and their hydrogenation properties are discussed. We also report the discovery of a relatively flat pressure plateau for hydrogenated TiZrNi quasicrystals at modest pressures (100–200 psi) that extends to hydrogen concentrations in excess of 4 wt.%. Approximately 2 wt.% of the hydrogen is easily recovered by heating at temperatures as low as 200°C.

Synthesis of amorphous and quasicrystal phases by mechanical alloying of Ti45Zr38Ni17 powder mixtures, and their hydrogenation

Mechanical alloying of Ti 45 Zr 38 Ni 17 powder mixture forms an amorphous phase, but subsequent annealing causes the formation of an icosahedral ( i ) phase. The maximum hydrogen concentration that can be loaded at 573K at a hydrogen pressure of 3.8MPa is the same (\[H]/\[M] 1.5) for the amorphous and i -phase powders. With hydrogenation, the i -phase is almost stable, forming no hydrides, whereas the amorphous phase transforms to a fcc hydride. The activation energy for hydrogen desorption for the i -phase is about 127kJmol -1, which is lower than that for the amorphous phase, suggesting that the i -phase powder may have better properties for hydrogen-storage applications.

Mechanical alloying of the Ti-Al system in atmosphere of hydrogen and argon

Abstract Three kinds of Ti-Al powders, Ti 72 Al 28 , Ti 57 Al 43 and Ti 48 Al 52 , were mechanically alloyed by a planetary ball mill in atmosphere of argon or hydrogen gases (0.1 MPa) with alloying times up to 30 h. The mechanical alloying (MA) process as well as the phase variations of each powder after subsequent heating at 1173 K were investigated. About 5000 wppm hydrogen, which could be easily removed by a heat treatment at 800 K (heating rate was 20 K/min), was occluded in all powders during MA in the hydrogen atmosphere, whereas the mechanically alloyed powders in the argon atmosphere occluded about 1000 wppm hydrogen. In the hydrogen atmosphere, the titanium powder easily crumbled into finer particles, assisting the diffusion of aluminum into titanium (solid-solid reaction) at an early stage of the MA process and accelerating the formation of an amorphous-like phase at a longer MA process. The phase formation after heat treatment of MA powders at 1173 K could be estimated by the Ti-Al binary phase diagram without the effect of the gas atmosphere.

Hydrogen solubility of two-phase (Ti3Al + TiAl) titanium aluminides

In this study, two-phase ({alpha}{sub 2} + {gamma}) titanium aluminides were thermally charged with hydrogen, and the hydrogen solubility and the hydrogen evolution behavior were investigated by means of thermal desorption spectroscopy (TDS). Hydrogen solubility of two-phase (Ti{sub 3}Al + TiAl) titanium aluminides occurred endothermically. A heat of solution for hydrogen dissolution in a Ti-50Al alloy was estimated to be 36.4 kJ/mol and that for a Ti-45 Al alloy was 58.3 kJ/mol in the temperatures range of 723 K to 843 K. At higher temperature, hydrogen solubility was not fitted well with Arrhenius type plots mainly because of oxidation. It was suggested from thermal desorption spectrums that there were three kinds of dissolution states for hydrogen in the alloys.

Hydrogen uptake in titanium aluminides in high pressure hydrogen

Abstract The weight gains of Ti–25Al, Ti–45Al and Ti–53Al alloys, with typical single-phase Ti 3 Al, two-phase Ti 3 Al/TiAl (fully lamellar) and single-phase TiAl microstructures, respectively, were measured at temperatures up to 923 K in high pressure hydrogen, up to 10 MPa. The total hydrogen uptakes during heating to 923 K at constant hydrogen pressures and during increasing the hydrogen pressure to 10 MPa at constant temperatures increased with increasing amounts of the Ti 3 Al in the alloys. The Ti–25Al alloy cracked and then spontaneously disintegrated at high-hydrogen pressures. A ternary (Ti–Al–H) hydride then formed, whose crystal structure is the same as that of the γ hydride (f.c.c.), known in the titanium–hydrogen binary system. No hydride could be detected in the Ti–45Al and Ti–53Al alloys. Most of the hydrogen taken up in the Ti–45Al and Ti–53Al alloys during heating and during pressure increase was released during cooling to room temperature or during pressure decrease to 0 MPa.

Hydride formation and thermal desorption spectra of hydrogen of cathodically charged single-phase gamma titanium aluminide

The authors have previously reported thermal desorption spectra of hydrogen obtained from cathodically charged two-phase (Ti{sub 3}Al ({alpha}{sub 2}) + TiAl ({gamma})) titanium aluminides by means of thermal desorption spectroscopy (TDS), in which hydrogen ion current (H{sub 2}{sup +}) corresponding to hydrogen evolution rate during heating was measured by a quadrupole mass spectrometer in an ultra-high vacuum condition. Several accelerated hydrogen evolutions (TDS peak temperatures) have been observed in a series of TDS measurement, and then the authors have suggested that these peaks were dependent on the microstructures ({alpha}{sub 2} and {gamma} phases) as well as dissociation of the hydride phase which formed during cathodic charging. A comparison with the TDS spectra from other series of titanium aluminides, such as a single-phase {gamma} alloy, might give clearer views of the microstructural dependence on hydrogen evolution kinetics. In this paper, hydride formation, hydrogen uptake and hydrogen evolution kinetic of a cathodically charged single-phase {gamma} titanium aluminide are investigated, and these results are compared with the previous ones obtained in two-phase ({alpha}{sub 2} + {gamma}) titanium aluminides.

Hydride formation in two-phase (Ti3Al + TiAl) titanium aluminides during cathodic charging and its dissociation

Abstract A (TiAl)H x hydride which has a tetragonal crystal structure with lattice parameters a = 0.452 nm and c = 0.326 nm ( c / a = 0.721) has been observed in Ti42Al, Ti45Al and Ti50Al (at.%) two-phase (Ti 3 Al ( α 2 ) + TiAl ( γ )) titanium aluminides by cathodic charging in a 5% H 2 SO 4 solution. Cracks or pits are also observed within the γ phase regions in the two-phase ( α 2 + γ ) coexisting grains (such as lamellar grains) but not within the α 2 phase or single γ grains. Weights of the samples decrease with increasing charging time owing to the crack or pit formation, and this is more drastic in the Ti—50Al alloy than in the Ti—42Al and Ti—45Al alloys. The hydride is thermally stable at temperatures up to about 550 K (277 °C) and dissociates completely at temperatures between 673 K (400 °C) and 723 K (450 °C).