Metal treatment – Stock – Nickel base
Reexamination Certificate
2000-07-06
2002-04-16
King, Roy (Department: 1742)
Metal treatment
Stock
Nickel base
C148S555000, C148S675000, C420S455000, C420S900000
Reexamination Certificate
active
06372059
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydrogen storage material and a process for 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 discharge characteristics for use in electric tools or low-temperature characteristics for use in hybrid electric vehicles, and a process for producing the same.
BACKGROUND ART
Nickel-hydrogen storage batteries (secondary batteries) having a hydrogen storage material in the anode have recently been attracting attention as high capacity alkaline storage batteries taking the place of nickel-cadmium storage batteries. The hydrogen storage materials that are currently used widely are composed of five elements, i.e., Mm (misch metal), 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-hydrogen storage batteries having a long cycle life and a suppressed 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 currently used 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 of the alloy. 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 application of the hydrogen storage material to large batteries into consideration, such as the power source of electric vehicles, and the ever expanding market of nickel-hydrogen 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, Japanese Patent Application Laid-Open No. 213319/97 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 extent the cycle life of the battery.
Because the alloy of the composition disclosed in the 213319/97 does not always secure stability in its characteristics, the present inventors have proposed in Japanese Patent Application Lain-Open No. 152533/99 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 (Laid-Open No. 213319/97 and Laid-Open No. 152533/99 are used, the batteries have insufficient discharge characteristics particularly in low temperature and cannot be used for electric tools needing high discharge characteristics or for hybrid electric vehicles.
DISCLOSURE OF THE INVENTION
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 discharge characteristics, and satisfactory initial activation and a process for producing the same.
As a result of extensive studies, the present inventors have found that the above object is accomplished by a hydrogen storage material of AB
5
structure having a specific stoichiometric composition (B site rich), particularly a composition of 4.0<Ni≦4.3 and 0.25≦Mn≦0.4, 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 denotes a misch metal, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, and 5.05≦a+b+c+d≦5.25, or general formula:
MmNi
a
Mn
b
Al
c
Co
d
X
e
wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and 5.05≦a+b+c+d+e≦5.25,
characterized in that the lattice length on the c-axis is 404.9 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 raw materials of a hydrogen storage material, 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 1230 to 1430° C., and conditions of said heat treating are 1070 to 1100° C. and 1 to 6 hours. General formula:
MmNi
a
Mn
b
Al
c
Co
d
wherein Mm denotes a misch metal, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, and 5.05≦a+b+c+d≦5.25, or general formula:
MmNi
a
Mn
b
Al
c
Co
d
X
e
wherein Mm denotes a misch metal, X is Cu and/or Fe, 4.0<a≦4.3, 0.25≦b≦0.4, 0.25≦c≦0.4, 0.3≦d≦0.5, 0<e≦0.1, and 5.05≦a+b+c+d+e≦5.25.
The Best Mode for Carrying out the Invention:
In the above formulae, Mm donates 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 of AB
5.05
to AB
5.25
.
In this hydrogen storage material, the compositional ratio (atomic ratio) of Ni
a
Mn
b
M
c
Co
d
fulfills the following relationships. The ratio of Ni: 4.0<a≦4.3. The ratio of Mn: 0.25≦b≦0.4. The ratio of Al: 0.25≦c≦0.4. The ratio of Co: 0.3≦d≦0.5. (a+b+c+d) is in a range of from 5.05 to 5.25.
The compositional ratio (atomic ratio) of Ni
a
Mn
b
Al
c
Co
d
X
c
(wherein X is Cu and/or Fe) satisfies the following relationships. The ratio of Ni: 4.0<a≦4.3. The ratio of Mn: 0.25≦b≦0.4. The ratio of Al: 0.25≦c≦0.4. The ratio of Co: 0.3≦d≦0.5. The ratio of X: 0<e≦0.1. (a+b+c+d+e) is in a range of from 5.05 to 5.25.
As described above, the ratio of Ni, a, is from 4.0 to 4.3, desirably from 4.1 to 4.2. If a is less than 4.0, the discharge 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 from 0.25 to 0.4. If b is less than 0.25. the plateau pressure increases, and the hydrogen storage capacity is reduced. If it exceeds 0.4, the alloy undergoes considerable corrosion so that the battery voltage greatly decreases during storage.
The ratio of Al, c, is from 0.25 to 0.4. If c is smaller than 0.25, the plateau pressure, which is the hydrogen release pressure of a hydrogen storage material, increases to deteriorate energy efficiency in charges and discharges. If it exceeds 0.4, the hydrogen storage capacity is reduced.
The ratio of Co, d, is 0.3 to 0.5. If d is less than 0.3, the hydrogen storage characteristics or the resistance to grai
Kikugawa Shingo
Mukai Daisuke
Sakaguchi Yoshiki
Uchiyama Akira
Yasuda Kiyotaka
King Roy
Mitsui Mining & Smelting Co. Ltd.
Wilkins, III Harry D.
Young & Thompson
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