Alloys or metallic compositions – Containing over 50 per cent metal but no base metal
Reexamination Certificate
2001-12-27
2004-08-10
Jenkins, Daniel (Department: 1742)
Alloys or metallic compositions
Containing over 50 per cent metal but no base metal
C148S198000, C148S198000, C148S559000, C148S562000
Reexamination Certificate
active
06773667
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydrogen storage material and a process of producing the same. More particularly, it relates to a hydrogen storage material which is, while with a minimized cobalt content, excellent in insusceptibility to grain size reduction and hydrogen storage characteristics (PCT characteristics) and exhibits not only excellent initial activity that is an important characteristic for use in a battery but high output characteristics for use in electric tools, etc. or low-temperature characteristics for use in hybrid electric vehicles, and a process for producing the same.
BACKGROUND ART
Nickel-metal hydride storage batteries (secondary batteries) having a hydrogen storage material in the anode have recently been attracting attention as high capacity alkali storage batteries supplanting nickel-cadmium storage batteries. The hydrogen storage materials that are currently used widely are hydrogen storage alloys composed of five elements, i.e., Mm (misch metal, a mixture of rare earth elements), Ni, Al, Mn, and Co.
Compared with La-based alloys, the Mm—Ni—Mn—Al—Co alloys enable constructing an anode out of relatively cheap materials and provide closed nickel-metal hydride storage batteries having a long cycle life and a controlled inner pressure rise which is caused by gas generated in case of an overcharge and have therefore been used widely as an electrode material.
The Mm—Ni—Mn—Al—Co alloys in current use are designed to have a prolonged cycle life by preventing the alloys from reducing their grain size. It is generally known that about 10% by weight of Co (0.6 to 1.0 in an atomic ratio) is required to prevent the grain size reduction. It is also accepted that a given amount of Co is necessary for securing excellent hydrogen storage characteristics and anticorrosion.
However, the material cost increases with the Co content, which is problematical from the aspect of material cost. Taking into consideration application of the hydrogen storage material to large batteries, such as the power source of electric vehicles, and the ever expanding market of nickel-metal hydride storage batteries, in particular, the material cost is weighty in choosing anode materials and has been a matter of concern.
To settle the above problem, JP-A-9-213319 proposes altering the composition of the Mm—Ni—Mn—Al—Co alloy and adding thereto a small amount of an additional element. Use of the hydrogen storage material powder disclosed therein as an anode makes it feasible to reduce the Co content and yet to suppress deterioration of the anode caused by the alloy's reduction in grain size below a certain level and thereby to extend the cycle life of the battery.
Because the alloy of the composition disclosed in JP-A-9-323319 does not always secure stability in its characteristics, the present inventors have proposed in JP-A-11-152533 a composition and a production process for obtaining satisfactory initial activity, whereby a low-Co alloy has now come to be used in special applications.
However, where the hydrogen storage materials disclosed in the above publications (JP-A-9-213319 and JP-A-11-15253) are used, output characteristics, especially output in low temperature, are insufficient for electric tools needing high output characteristics or for hybrid electric vehicles.
Accordingly, an object of the present invention is to provide a hydrogen storage material of which the production cost is reduced by extremely decreasing its cobalt content and which exhibits excellent insusceptibility to grain size reduction, excellent hydrogen storage characteristics, satisfactory output characteristics, and satisfactory storage characteristics and a process for producing the same.
DISCLOSURE OF THE INVENTION
As a result of extensive studies, the present inventors have found that the above object is accomplished by a hydrogen storage material of AB structure having a specific nonstoichiometric composition (B site rich), particularly a composition having 4.1<Ni£4.3 and 0.4<Mn£0.6, and the c-axis of which is in a given range. They have also found that such a hydrogen storage material is obtainable with the above-described specific composition when a casting temperature and heat treating conditions satisfy a given relationship.
The present invention has been reached based on the above findings and provides a hydrogen storage material which is an AB
5
type hydrogen storage alloy having a CaCu
5
type crystal structure represented by general formula:
MmNi
a
Mn
b
Al
c
Co
d
wherein Mm is a misch metal, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, and 5.2≦a+b+c+d≦5.45,
or general formula:
MmNi
a
Mn
b
Al
c
Co
d
X
e
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, 0<e≦0.1, and 5.2≦a+b+c+d+e≦5.45,
characterized in that the lattice length on the c-axis is 406.2 pm or more.
The present invention also provides a preferred process for producing the hydrogen storage material of the present invention which comprises heat-melting hydrogen storage alloy raw materials, casting the melt, and heat treating the resulting alloy in an inert gas atmosphere to produce an AB
5
type hydrogen storage material having a CaCu
5
type crystal structure represented by the following general formulae, characterized in that the casting temperature is 1350 to 1550° C., the pouring temperature is 1200 to 1450° C., and conditions of said heat treating are 1040 to 1080° C. and 1 to 6 hours.
General formula:
MmNi
a
Mn
b
Al
c
Co
d
wherein Mm is a misch metal, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, and 5.2≦a+b+c+d≦5.45,
or general formula:
MmNi
a
Mn
b
Al
c
Co
d
X
e
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, 0<e≦0.1, and 5.2≦a+b+c+d+e≦5.45.
The Best Mode For Carrying Out The Invention:
The hydrogen storage material according to the present invention is an AB
5
type hydrogen storage alloy having a CaCu
5
type crystal structure represented by general formula:
MmNi
a
Mn
b
Al
c
Co
d
wherein Mm is a misch metal, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, and 5.2≦a+b+c+d≦5.45,
or general formula:
MmNi
a
Mn
b
Al
c
Co
d
X
e
wherein Mm is a misch metal, X is Cu and/or Fe, 4.1<a≦4.3, 0.4<b≦0.6, 0.2≦c≦0.4, 0.1≦d≦0.4, 0<e≦0.1, and 5.2≦a+b+c+d+e≦5.45.
In the above formulae, Mm is a misch metal, a mixture of rare earth elements such as La, Ce, Pr, Nd, and Sm. The hydrogen storage material is an AB
5
type hydrogen storage alloy having a CaCu
5
type crystal structure having a B site-rich nonstoichiometric composition ranging from AB
5.2
to AB
5.45
.
In this hydrogen storage material, the compositional ratio (atomic ratio) of Ni
a
Mn
b
Al
c
Co
d
fulfills the following relationships. The ratio of Ni: 4.1<a≦4.3. The ratio of Mn: 0.4<b≦0.6. The ratio of Al: 0.2≦c≦0.4. The ratio of Co: 0.1≦d≦0.4. (a+b+c+d) is in a range of from 5.2 to 5.45.
The compositional ratio (atomic ratio) of Ni
a
Mn
b
Al
c
Co
d
X, (wherein X is Cu and/or Fe) satisfies the following relationships. The ratio of Ni: 4.1<a≦4.3. The ratio of Mn: 0.4<b≦0.6. The ratio of Al: 0.2≦c≦0.4.The ratio of Co: 0.1≦d≦0.4. The ratio of X: 0<e≦0.1. (a+b+c+d+e) is in a range of from 5.2 to 5.45.
As described above, the ratio of Ni, a, is more than 4.1 and up to 4.3, desirably from 4.15 to 4.25. If a is 4.1 or less, the output characteristics are not satisfactory. If it exceeds 4.3, deterioration in insusceptibility to grain size reduction or life characteristics is observed.
The ratio of Mn, b, is more than 0.4 and up to 0.6. If b is 0.4 or less, the plateau pressure increases, and the hydrogen storage capacity is reduced. If it exceeds 0.6, the alloy undergoes considerable corrosion
Kikugawa Shingo
Sakaguchi Yoshiki
Yasuda Kiyotaka
Jenkins Daniel
Mitsui Mining & Smelting Company Ltd.
Young & Thompson
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