Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Reexamination Certificate
1999-03-10
2001-09-04
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
C420S900000
Reexamination Certificate
active
06284066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing hydrogen absorbing alloy powder suitable for a negative electrode of an alkaline storage battery, as well as a hydrogen absorbing alloy electrode formed of the hydrogen absorbing alloy powder produced thereby. More specifically, it relates to a process for producing a hydrogen absorbing alloy suitable to the application of a nickel-hydrogen storage battery which excels in properties such as a cycle life and a high rate discharge property.
2. Description of the Related Art
Since a hydrogen absorbing alloy which can charge and discharge hydrogen was discovered, the application thereof has been progressively developed. An alkaline storage battery containing a negative electrode formed of the hydrogen absorbing alloy has been made practicable, and the hydrogen absorbing alloy used for it has been improved one after another.
Although the LaNi
5
alloy investigated initially (Japanese Provisional Patent Publication No. 51-13934/1976) has an advantage that the absorbed amount of hydrogen is large, it also has disadvantages such as expensive La metal and a tendency to become impalpable powder in progress of repetition of charge and discharge of hydrogen. Further, it has such a disadvantage that it is easily corroded by an alkaline solution or an acid solution. Accordingly, when said hydrogen absorbing alloy is applied in an alkaline storage battery, despite an high initial capacity, the capacity becomes no more than a half of the initial capacity after the 50 or more times of charge and discharge cycles. Consequently, the hydrogen absorbing alloy has an disadvantage of being incapable of withstanding a longer period of use.
The disadvantages have been improved by substituting a part of La of the LaNi
5
alloy by Ce, Pr, Nd or other rare earth elements, and/or by substituting a part of Ni of the LaNi
5
alloy by metal such as Co, Al, Mn or the like (Japanese Provisional Patent Publication Nos. 53-48918/1978, 54-64014/1979, 60-250558/1985, 61-91862/1986, and 61-233969/1986).
SUMMARY OF THE INVENTION
The hydrogen absorbing alloy having Mm has an advantage that Mm is inexpensive. As the applications for the hydrogen absorbing alloy have been expanded widely recently, however, the hydrogen absorbing alloy having Mm which also owns a good high rate discharge property has been sought.
In order to improve the high rate discharge property, a surface treatment or alloy (an alkaline treatment or an acid treatment), a plating treatment, or an addition of B (boron), Mo or others have been carried out conventionally. However, it is difficult to maintain the surface activity of the alloy in use of the surface treatment such as the alkaline or the acid treatment, or in the addition of the other element.
Hence, it is the first object of the present invention to provide an inexpensive process for producing hydrogen absorbing alloy powder suitable for a nickel-metal hydride storage battery having a excellent high rate discharge property.
Moreover, it is the second object of the present invention to provide an inexpensive process for producing hydrogen absorbing alloy powder suitable for a nickel-metal hydride storage battery having a higher capacity and a longer cycle life for repetition of charge and discharge, as well as an excellent high rate discharge property.
In above view, the present inventors have studied the problems and found that the cycle life and the high rate discharge property can be improved by adding a rare earth metal oxide and/or hydroxide to a hydrogen absorbing alloy powder, then wet- or dry-mixing them, and subsequently treating thermally in an inert atmosphere or in a vacuum. Thus, the present invention has been completed.
According to the present invention, an ingot of hydrogen absorbing alloy is thermally treated at the temperature of 800 to 1100° C. in an inert atmosphere or in a vacuum, then cooled, and reduced to yield a hydrogen absorbing alloy powder. A rare earth metal oxide or hydroxide is added to the obtained hydrogen absorbing alloy powder, and a wet- or dry-mixed. Then, they are further thermally treated at 100 to 800° C. in an inert atmosphere or in a vacuum. As a result, a storage battery having an excellent high rate discharge property is obtained, since the surface activity of the obtained hydrogen absorbing alloy powder is not damaged by an alkaline electrolyte.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates a process for producing hydrogen absorbing alloy powder, comprising steps of adding a rare earth metal oxide and/or hydroxide to a hydrogen absorbing alloy powder, preferably to the hydrogen absorbing alloy powder obtained by a thermal treatment of an ingot of hydrogen absorbing alloy at the temperature of 800 to 1100° C. in an inert atmosphere or in a vacuum, then cooling, and pulverization; mixing the resultant in a wet or dry manner; and then further thermally treating the mixture in an inert atmosphere or in a vacuum.
Compositions or preparation methods for hydrogen absorbing alloy which can be used in the present invention are not particularly limited. It is preferably the intermetallic compound represented stoichiometrically by LaNi
5
, wherein a part of La is substituted by Ce, Pr, Nd or the other rare earth element and a part of Ni is substituted by Co, Mn, Al or the other element such as Fe, Cu, Si, Ti, Mo or Zr. In view of improving the cycle life, it is more preferable to substitute at least a part of Ni by Mn, further more preferable to substitute at least a part of Ni by Mn and Co.
According to the present invention, the hydrogen absorbing alloy having the said compositions can be used. And MmNi
5
-type hydrogen absorbing alloy having the following composition is preferably used. It is represented specifically by the general formula, (R)
n
(Ni)
5−x−y−z
(Mn)
x
(Al)
y
(Co)
z
, wherein R is La or a mixture of La and at least one rare earth element (selected from Ce, Pr, Nd or the like); n, x, y and z are each a positive number which represents an atomic ratio; and n is between 0.93 and 1.06, and x, y and z each satisfies 0<x≦0.6, 0<y≦0.6 and 0<z≦1.0.
Furthermore, other than the above composition, the compositions having a part of Ni substituted by Fe, Si, Cu, Mo, Ti, Zr or the like may be used. Among them, the compositions having 20% by weight or more of La in R are more preferable.
Although the preparation method for the ingot of hydrogen absorbing alloy is not limited, the casting of the liquid having each of metal components melted is preferable because of lower cost. Even when other methods including a roll rapid quenching method and an atomization method are used, the hydrogen absorbing alloys obtained may bring the same effects as or better effects than those obtained from the casting.
According to the present invention, the ingot of hydrogen absorbing alloy may be thermally treated. Although any one of the known methods of thermal treatments may be selectively used, it is preferable to use the treatment at 800 to 1100° C. for 5 to 20 hours in resistance-type furnace in a vacuum or in an inert atmosphere such as argon, helium or the like. The thermal treatment effects the removal of segregation and strain of metals in the alloy. If the temperature is less than 800° C., the removal of segregation of metals in the alloy may not be enough. If the temperature is more than 1100° C., the discrepancy in the composition may take place since metals having higher vapor pressures such as Mn come out. Accordingly, the temperature range of 800 to 1100° may be used.
The atmosphere of an inert gas such as argon or helium is used for eliminating the contamination by impurities such as oxygen. Accordingly, as long as the contamination is eliminated, any other methods may be used. That is, it is not limited to the inert atmosphere. A vacuum may be used in the same reason and not limited to the vacuum in the strict sense in the same reason. The “in a vacuum” is generally under the pressu
Ishii Masatoshi
Shinya Naofumi
Sugahara Hiroto
Coleman Henry D.
Sapone William J.
Shin-Etsu Chemical Co. , Ltd.
Sudol R. Neil
Wyszomierski George
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