Alloys or metallic compositions – Rare earth metal base singly or in combination
Reexamination Certificate
1999-09-28
2001-03-27
King, Roy V. (Department: 1742)
Alloys or metallic compositions
Rare earth metal base singly or in combination
C420S900000
Reexamination Certificate
active
06207104
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a novel hydrogen storage alloy for use in apparatus for storing and transporting hydrogen and secondary batteries of hydrides, and also to a process for the production thereof.
Hydrogen storage alloys are materials for absorbing and desorbing a great amount of hydrogen reversibly, and binary intermetallic compounds and V-series solid solution alloys have been known, which include AB
5
-type such as LaNi
5
, AB
2
-type such as ZrMn
2
, AB-type such as TiFe and A
2
B-type such as Ti
2
Ni. Since hydrogen intrudes in the form of atom (H) into the interstices in crystal lattice of such hydrogen storage alloys to form metal hydride in an unstable bonding state, the alloys can absorb and desorb hydrogen repeatedly under a relatively mild condition to do with the temperature and the hydrogen pressure.
Further, the hydrogen absorbing characteristics of the alloys can be changed by substituting at least a portion of elements A and/or B with other elements. For example, in LaNi
5
series alloys put to practical use such as hydrogen storage media or nickel-metal hydride batteries, metallographic structures and hydrogen storage characteristics are controlled so as to satisfy particular requirements in particular applications by substituting La with a misch metal (Mm) which is a mixture of rare-earth elements in the A-sites and a portion of Ni with Co, Al, Mn, etc. in the B-sites, thereby making it multiple components.
However, in the practical hydrogen storage alloys, the rechargeable hydrogen storage capacity is as small as about H/M=1 in atomic ratio between hydrogen (H) and metal (M), that is, 1 to 2% in weight ratio. Such a small rechargeable hydrogen storage capacity per unit weight gives a significant drawback as hydrogen storage media.
Mg
2
Ni has been known as a lightweight hydrogen storage alloy. The alloy is superior to the hydrogen storage alloy above-mentioned in that the hydrogen storage capacity is as large as 3.6mass %. However, a high temperature is necessary to make the Mg
2
Ni alloys desorb hydrogen since its hydrogen dissociation pressure is 1 atm at about 250° C. This is because strongly basic Mg tends to easily donate electrons to become anionic (H
−
) and form a hydride in a strong bonding state. Therefore, the hydrogen dissociation pressure changes scarcely even when Mg and/or Ni are partially substituted with other elements, which is different from the LaNi
5
series alloys in which hydrogen is absorbed atomically as H. Then, it has been said impossible to greatly lower the hydrogen desorbing temperature of the Mg
2
Ni series alloys.
In addition to Mg
2
Ni, there are several binary alloys consisting of lightweight Mg or Ca as a main ingredient and forming hydrides. However, any of the alloys above-mentioned changes into an amorphous state or decomposes disproportionately into a stable hydride such as MgH
2
or CaH
2
. For example, as reported in the treatise (Journal of Alloys and Compounds, vol 253-254 (1997), p. 313), LaMg
2
absorbs hydrogen to form LaMg
2
H
7
, which shows a very large amount of hydrogen storage capacity in the Laves phase alloys, of H/M≧2 in atomic ratio, that is, about 3.5% in weight ratio. However, it decomposes into La and MgH
2
during the hydrogen desorption process, that is, hydrogen absorption and desorption do not progress reversibly.
Alloy hydrides are generally in a metastable state. Accordingly, the intermetallics' hydrides change into an amorphous state to be more stable thermodynamically or decompose disproportionately into hydrides of elemental metals while hydrogen absorption and desorption progress at a high temperature. LaNi
5
is peculiar because it absorbs and desorbs hydrogen reversibly near room temperature, and Mg
2
Ni is the only alloy that stably absorbs and desorbs hydrogen at a high temperature of 250° C. Accordingly, two characteristics are required for new hydrogen storage alloys to be developed in the future: 1) No denaturation or decomposition occurs in absorbing and desorbing hydrogen and 2) hydrogen desorption is possible at room temperature, in other words, 1) high stability to hydrogenation of alloy and 2) instability of formed hydrides.
In alloys showing a high desorbing temperature, namely, low hydrogen dissociation pressure, hydrogen atoms at special interstices in crystal lattices make strongly bonds with metal atoms to form a stable hydride. Accordingly, the hydrogen desorbing temperature does not lower so much through “partial substitution method” of substituting a portion of constituent atoms with other atoms, namely, through forming a pseudo-binary alloy by merely making it having multiple components. Further, although a method of disturbing the metallographic structure by a mechanical treatment has often been also attempted in recent years, this is not effective essentially, for example, because it decreases the rechargeable hydrogen storage capacity. Accordingly, in order to enhance the stability to hydrogenation and instability of hydrides, it is necessary to strengthen bonds between metal atoms and greatly lower the chemical binding strength between metal and hydrogen atoms through drastical change of the composition and the crystal structure.
SUMMARY OF THE INVENTION
In view of the present state of the art and requirements to be demanded in the future as above-mentioned, it is a primary object of the present invention to provide a novel hydrogen storage alloy having a large amount of rechargeable hydrogen storage capacity per unit weight and being excellent in the reversibility of hydrogen absorption and desorption, through enhancement of the stability of an alloy hydride comprising a light metal element as main ingredient and, at the same time, increase of the hydrogen dissociation pressure of the hydride.
In view of the present state of the art and the requirements to be demanded in the future as above-described, the present inventors have made earnest studies on the metallographic and crystallographic structure of hydrogen storage alloys, and investigated on reactivity thereof with hydrogen. As a result thereof, they found that a novel ternary alloy having a specific phase provides an excellent performance as a hydrogen storage material, to accomplish the present invention.
The foregoing object can be attained in accord with the invention by an AB
2
C
2
-type ternary hydrogen storage alloy comprising as main ingredient an AB
x
C
y
phase in which A comprises at least one of a rare-earth element and Ca, B mainly comprises Mg, and C comprises at least one of transition metal elements of Cu, Ni, Co, Fe, Cr, Mn, Ti, V and Zn, x and y representing values within the ranges of 1.5≦x≦2.5 and 1.5≦y≦3.5, respectively.
In the AB
2
C
2
ternary hydrogen storage alloys, each of x and y for AB
x
C
y
in the crystal structure can be 2 in which A, B and C are as above-described.
As stated above, the ternary hydrogen storage alloy of the invention has a structure comprising three kinds of elements and includes a pseudoternary crystal system wherein the elements are partially or entirely substituted in accordance with the definition described above.
In the AB
2
C
2
ternary hydrogen storage alloy, the element constituting each of the phases or a portion thereof can be substituted with each of elements defined below. Namely, a preferred embodiment of the invention may include the AB
2
C
2
phase with a hexagonal crystal structure in which the A-site is occupied with at least one of the rare-earth element and Ca, the B-site is mainly occupied with Mg and the C-site is occupied with at least one of transition metal elements of Cu, Ni, Co, Fe, Cr, Mn, Ti, V and Zn.
A preferred embodiment of the invention may include the AB
2
C
2
phase with a tetragonal ThCr
2
Si
2
-type crystal structure in which the Th-site is occupied with at least one of rare-earth element and Ca, the Cr-site is mainly occupied with Mg and the Si-site is occupied with at least one of transition metal elements of Cu, Ni, Co, Fe, Cr, Mn, Ti, V and Zn.
In
Kadir Abdulkarim
Sakai Tetsuo
Tanaka Hideaki
Uehara Itsuki
Agency of Industrial Science & Technology
Arent Fox Kintner & Plotkin & Kahn, PLLC
King Roy V.
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