Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Reexamination Certificate
2001-03-19
2002-11-19
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
C420S900000
Reexamination Certificate
active
06482277
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode capable of reversibly carrying out the electrochemical charge/discharge of a hydrogen gas which is used as a negative electrode of an nickel-metal hydride battery, and more particularly to surface-treatment of the hydrogen-absorbing alloy particle containing a rare-earth element and nickel.
2. Description of the Related Art
In recent years, in order to realize the alkaline storage battery with high energy density, the nickel-metal hydride battery equipped with a hydrogen-absorbing alloy electrode has been put into practice. Previously known hydrogen-absorbing alloys which can be used for the nickel-metal hydride battery are Ti—Ni alloy, La (or Mm)—Ni alloy, etc.
The hydrogen-absorbing alloy used in the hydrogen-absorbing alloy electrode is ground alloy which is obtained by grinding an alloy ingot, flake or spherical particle mechanically and electrochemically, or spherical or similar shape (elliptical) particle which is made by an atomization, a rotary disk method, a rotary nozzle method, etc. Meanwhile, the surface of the hydrogen-absorbing alloy particle is very active. Therefore, when it is exposed to air atmosphere just a little, it immediately reacts with oxygen in the air. As a result, the alloy surface is oxidized to form an oxide layer. The oxide layer attenuates the surface activity of the alloy, and particularly gives rise to reduction of the initial discharging capacity of the battery. Therefore, after the battery has been assembled, it must be subjected to charging/discharging several cycles to several tens cycles to remove the oxide layer and activate the particle surface, thereby realizing the desired discharging capacity. This required very great labor and time.
In order to obviate such inconvenience, a technique for removing an oxide layer on the surface of the hydrogen-absorbing alloy particle has been proposed in JP-A-5-225975 which discloses an acidic treating method of immersing the hydrogen-absorbing alloy particle in hydrochloric acid (HCl).
The acidic treating method proposed by JP-A-5-225975 was efficient to remove the layer of the rare-earth metal oxide of the oxide layer on the surface of the hydrogen-absorbing alloy particle, but not so efficient to remove nickel oxide or nickel hydroxide, thus presenting a problem that the nickel hydroxide is newly created.
Further, by the acidic treatment using hydrochloric acid, chloride ions were left on the surface of the hydrogen-absorbing alloy particle so that rinsing treatment must be made to remove the chloride ions after the acidic treatment. During the rinsing treatment, an inconvenience that the activated surface of the hydrogen-absorbing alloy is oxidized again by oxygen in water was presented.
In order to obviate such inconvenience, a surface treatment of a hydrogen absorbing alloy particle in an atmosphere of a hydrogen gas maintained at high temperature was proposed by JP-A-9-237628 in which before preparing a hydrogen-absorbing alloy electrode, the hydrogen-absorbing alloy particle containing nickel with a nickel oxide or nickel hydroxide is held for a suitable time in an atmosphere of a hydrogen gas (also referred to as hydrogen gas atmosphere) maintained at a temperature where absorbing of the hydrogen gas does not substantially occur so that the layer on the hydrogen-absorbing alloy particle is reduced by the hydrogen gas to expose the nickel activated surface onto the surface of the hydrogen-absorbing alloy particle.
In the hydrogen gas reduction treatment technique for the nickel-containing hydrogen-absorbing alloy proposed by JP-A-9-237628, before the hydrogen gas reduction treatment, the alloy is previously subjected to alkaline treatment to remove the oxide on the surface of the hydrogen-absorbing alloy particle, or previously subjected to acidic treatment to generate a layer of nickel oxide or nickel hydroxide so that the quantity of nickel to be reduced by the hydrogen gas reduction treatment is increased.
However, since the acidic treatment or alkaline treatment which is carried out before the hydrogen reduction treatment belong to a wet type, the nickel-containing hydrogen-absorbing alloy particle must be dried before it is subjected to the hydrogen gas reduction treatment. This presented a problem of making the manufacturing process complicate and increasing the treatment time.
When the nickel-containing hydrogen-absorbing alloy particle is dried before it is subjected to the hydrogen gas reduction treatment, a problem occurred that a part of the surface of the hydrogen-absorbing alloy particle is oxidized during drying so that rare-earth oxide or nickel oxide is formed. In this case, another problem occurred that the nickel oxide of the oxides thus formed can be reduced by the hydrogen gas reduction treatment, but the rare-earth oxide cannot be reduced so that the porosity of the surface of the hydrogen-absorbing alloy particle is decreased, thus attenuating the battery capacity.
Still another problem occurred that where the activated surface of nickel is exposed to the particle surface of the hydrogen-absorbing alloy particle, if the activated surface is exposed to air again, it is oxidized again so that it is difficult to maintain the activity.
A further problem occurred that the since the surface of the hydrogen-absorbing alloy particle is activated greatly by the hydrogen gas reduction treatment, the hydrogen-absorbing alloy particle thus treated must be held in water so that the active surface is lost.
SUMMARY OF THE INVENTION
This invention has been accomplished to solve the problems described above, and aims at providing a treatment method which can maintain the activity of the surface of a hydrogen-absorbing alloy particle is activated by hydrogen gas reduction treatment, thereby obtaining a hydrogen-absorbing alloy electrode with an excellent discharging capacity.
In order to attain the above object, the method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises:
a step of holding a hydrogen-absorbing alloy particle in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur;
a step of naturally cooling the atmosphere from the temperature where absorbing of the hydrogen gas does not substantially occur to the temperature where the equilibrium hydrogen pressure of the hydrogen-absorbing alloy is equal to the hydrogen pressure in the atmosphere of the hydrogen gas and thereafter vacuum-evacuating the atmosphere of the hydrogen gas so that the hydrogen-absorbing alloy particle is cooled to room temperature while the hydrogen gas is exhausted;
a step of exhausting the hydrogen gas and cooling the atmosphere to room temperature and thereafter introducing at least one kind of gas selected from the group consisting of argon, nitrogen and carbon dioxide into the atmosphere, thereby returning the atmosphere to normal atmospheric pressure; and
a step of immersing the hydrogen-absorbing alloy particle thus acquired in solution containing an oxidation inhibiting agent.
Preferably, said step of holding comprises a step of reducing an oxide or hydroxide residing on the surface of a hydrogen-absorbing alloy particle while the alloy particle is held in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur.
In the case thwhereat the hydrogen-absorbing alloy is pressurized by hydrogen at a prescribed temperature, a metallic phase with the hydrogen solid-solved (&agr; phase) is first created. When the pressure is further raised, the hydrogen generates a hydroxide (&bgr; phase) where the hydrogen occupies the site of a metallic lattice. At this time, there is a region where the a phase and the &bgr; phase exist commonly, which is referred to as a plateau region. At a high temperature, the pressure of generating the plateau region (plateau pressure) is raised so that the hydroxide cannot be generated. In t
Hirota Yohei
Imoto Teruhiko
Ise Tadashi
Murakami Takayuki
King Roy
McGuthry-Banks Tima
Nixon & Vanderhye P.C.
Sanyo Electric Co,. Ltd.
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