Takahiro Kuriiwa

Ti-V-Cr b.c.c. alloys with high protium content

Abstract The effects of composition and heat-treatment on the protium absorption–desorption properties of Ti–V–Cr alloys were investigated. It was found that the Ti–35V–40Cr alloy shows a 2.6 mass% protium capacity. The plateau pressure of the Ti–35V–xCr alloys increased with decreasing lattice constants associated with increasing Cr content. The main phase of the as-cast Ti–xV–Cr (Cr/Ti=40/25) alloys containing more than 15%V was a b.c.c. phase. These b.c.c. alloys exhibited a 2.4 mass% protium capacity. Heat-treatment over 1673 K was effective on stabilizing the b.c.c. structure for the Ti–xV–Cr (Cr/Ti=2/3) alloys with low V content. The Ti–5V–57.5Cr alloy heat-treated at 1673 K for 1 h yields a high capacity of 2.8 mass% protium, which is the highest value at 313 K reported so far. The alloy is economically promising since it contains low amounts of expensive V metal.

New V-based alloys with high protium absorption and desorption capacity

Abstract The hydrogen absorption properties of V–Zr–Ti–M(M=Fe, Mn, Ni) alloys were examined in order to develop the alloys with high hydrogen capacity. It was found that the best composition among the studied alloys is the V–Zr–Ti–Ni system. A suitable amount of the vanadium in the V–Zr–Ti–Ni system was studied in correlation with its hydrogen absorption properties, and turned out to be around 75 at.%V. Substitution with Zr improved the hydrogen absorption properties in forming the grain boundary network phases of C14 Laves phase. Heat treating the alloys (homogenize and quench) drastically improves the plateau region of PCT curves. Zr addition also improves the properties for V–Zr–Ti–Cr alloys.

Protium absorption properties of Ti–V–Cr–Mn alloys with a b.c.c. structure

The protium absorption–desorption properties of Ti–V–Cr–Mn alloys were studied by varying the contents of Mn, Ti and V in the alloys. It was found that Mn addition of <10 at% was effective in improving the slope of the plateau region without any reduction of the protium absorption–desorption capacity. For Ti–V–Cr–Mn alloys, the α-Ti phase, which does not contribute to the protium storage capacity, appeared along grain boundaries during heat treatment. The formation of the α-Ti phase was found to be suppressed by reducing the Ti content and increasing the V content. By inhibiting the appearance of the α-Ti phase along the grain boundary, the protium absorption–desorption capacity of Ti–xV–Cr–Mn (x=45, 55; Ti/Cr/Mn=24:31:10) heat-treated alloys exhibited 2.7 mass% H, the highest value reported so far.

Role of intermetallics in hydrogen storage materials

Abstract Many intermetallics such as AB5, AB2, A2B show the excellent hydrogen absorbing properties. Specially, LaNi5 compound has been extensively studied for the negative electrode of Ni–MH batteries. In todays use for the negative electrode of Ni–MH batteries, alloys must be single phase. But it is well known that it is difficult to make non-stoichiometric LaNi5 alloys as single phase because of its limited solid-solution range. The present study describes the rapid quenching effects on the solid-solution range of La(Ni, M)x (x=3–5) alloys prepared by melt spinning in discussing their hydrogen absorption properties. It is found that the single phase with CaCu5 crystal structure extends to LaNi4.65 compositional alloys. For fuel cell automobile, the alloys with high content of hydrogen must be developed. Strong candidates for this will be Vanadium based BCC alloys, which would be hard to be activated. Since it is well known that the Laves phases undergo easy activation treatment, it may be possible that the BCC phases will be easily activated if the alloys contain such Laves phases. The present study also found that Zr addition is effective on forming Laves phase as grain boundary phases in BCC phases in V–Ti–Zr–Ni alloys, and on improving the hydrogen absorbing properties. The study will be also extended to future hydrogen absorbing alloys with high capacity.

Crystal structure and protium absorption properties of La-rich La(Ni, M)x (x=3–4.7) (M=Al, Co, Mn, Si) melt-spun ribbons

Abstract The present study describes the rapid quenching effects on the solid-solution range of La(Ni, M) x ( x =3–4.7) (M=Al, Co, Mn, Si) alloys prepared by melt-spinning and discusses their protium (hydrogen atom) absorption properties. It is found that the single phase with CaCu 5 crystal structure extends to LaNi 4.65 compositional alloys. When x in LaNi 5− x becomes smaller, the alloy acquires better protium absorption properties including easier activation, better flatness of plateau region and a good hydrogen storage capacity comparable to those of LaNi 5 homogenized sample. On adopting a melt-spinning technique it is easy to form single phase of CaCu 5 type-structure for La-rich non-stoichiometric La–Ni–M alloys such as La(Ni, M) x ( x =3–4.7) (M=Al, Mn, Si) alloys. The readiness of forming a single phase CaCu 5 type-structure in melt-spun La–Ni–M alloys is in order of Al≥Si>Mn>Co.The phases formed and protium absorption properties of La-rich LaNi 4.5 M 0.2 melt-spun alloys are studied. It was found that LaNi 4.5 M 0.2 alloys had better protium absorption properties such as easier activation than the LaNi 4.65 binary alloy and as good hydrogen storage capacity as that of homogenized LaNi 5 .

Microstructures and Electrical-Mechanical Properties of Cu–Cr–Zr and Cu–Cr–Zr–Ti Alloys Heat-treated in Hydrogen

The effect of hydrogen heat treatment on the electrical and mechanical properties of Cu-0.2Cr-xZr (x = 0, 0.15, 0.6, 1.2) and Cu-0.2Cr-0.6Zr-0.5Ti alloys was investigated. After hydrogen heat treatment, Cu5Zr precipitates decomposed with the formation of ZrH2 and TiH2. For the Cu-Cr-Zr ternary alloys, high conductivity (i.e., >90% IACS) was achieved in Cu-0.2Cr-0.15Zr alloy after hydrogen heat treatment at 450°C while maintaining high tensile strength, more than 450 MPa.