Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making porous product
Reexamination Certificate
2000-10-10
2001-10-23
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making porous product
C419S008000, C419S010000, C419S035000, C419S036000, C419S037000, C419S048000, C427S216000, C427S217000, C427S220000
Reexamination Certificate
active
06306339
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a hydrogen storage material comprising particles of a hydrogen storage alloy and a method for manufacturing the same. The present invention is also directed to an electrode of a hydrogen storage alloy for use in negative electrodes of nickel-metal hydride storage batteries as a specific application of the hydrogen storage material and a method for manufacturing the electrode.
Recently, there is a wide application of hydrogen storage alloys which can absorb therein and desorb therefrom hydrogen in a reversible manner. For example, it becomes possible to reserve or convey hydrogen safely in an ordinary vessel if only a hydrogen storage alloy material is included in the vessel. Since hydrogen storage alloys allow selective absorption and desorption of hydrogen, they can also be used for refining hydrogen. Their other application includes a converter for a variety of energy by utilizing exothermic and endothermic reactions of the hydrogen storage alloys during their hydrogen absorption and desorption. They can also be applied as electrode materials for the clean nickel-metal hydride storage batteries affording a high energy density which should be replaced with conventional nickel-cadmium storage batteries. Those nickel metal-hydride storage batteries have been utilized as the power sources for a variety of portable electronic equipment, electric vehicles, etc.
The hydrogen storage alloy is in nature collapsed and pulverized into fine particles when it is forced to absorb therein and desorb therefrom hydrogen repeatedly. The hydrogen storage alloy, therefore, has a drawback that when it is used for storing, conveying or refining hydrogen, those pulverized particles become fugacious out of the alloy together with hydrogen gas, reducing the amount of reserved hydrogen or clogging a filter included in a refining device. The use of this hydrogen storage alloy for electrode has a drawback that pulverization of the alloy into fine particles will take place if charge and discharge operations are repeated for an electrode including such alloy, which in turn causes the pulverized particles to fall off from the surface of an electrode substrate, thereby decreasing the discharge capacity of a battery using the electrode, resulting in impaired life of the battery. The hydrogen storage alloy is disadvantageously poor in thermal conductivity, which restricts its use as an energy converter.
Proposed methods for solving these problems include: 1) to add a resin binder to hydrogen storage alloy particles and pressure-mold the resultant mixture to a hydrogen storage alloy material or 2) to plate particles of a hydrogen storage alloy with a metal film and subsequently pressure-mold the plated alloy particles into a hydrogen storage alloy material. In the former method, the binding force between the particles can be enhanced by increasing the amount of a binder, but this disadvantageously decreases the amount of hydrogen storage per unit weight of the alloy. In the latter method, although it depends on the kind of metal used for plating, the resultant alloy material is insufficient in strength despite strong to a certain degree.
SUMMARY OF THE INVENTION
In view of the above-mentioned problems, the object of the present invention is to provide a hydrogen storage material which does not develop any collapse due to pulverization of particles of an alloy used for the material caused by repeated absorption and desorption of hydrogen in and out of the alloy, thereby permitting its repeated use.
Another object of the present invention is to provide a hydrogen storage material having a large hydrogen occluding capacity and exceptional electric and thermal conductivities.
Still another object of the present invention is to provide a long cycle life hydrogen storage alloy electrode developing no collapse due to pulverization of the alloy particles even after repeated charge and discharge cycles, by establishing mutual contact of the alloy particles to substantially eliminate resistance inside the electrode, thereby preventing voltage drops due to electrode resistance.
The present invention provides a method for manufacturing a hydrogen storage alloy material comprising a step of pressure-molding particles of a hydrogen storage alloy, each surface of the particles being covered with a plated metal film having microgranules of a thermoplastic resin, at a temperature which is higher than a glass transition temperature or a melting point of and below a thermal decomposition temperature of the thermoplastic resin, thereby manufacturing a porous hydrogen storage material.
The hydrogen storage material in accordance with the present invention is a porous body having a three-dimensionally communicating space facing most of the hydrogen storage alloy particles directly or via plated metal films, the space being occupied in part by the thermoplastic resin so as to effect firm binding between the hydrogen storage alloy particles.
Because of this structure, the hydrogen storage material in accordance with the present invention can have a large hydrogen storage capacity per unit weight of the material and does not collapse even when it is forced to absorb therein and desorb therefrom hydrogen repeatedly, thus permitting recurrent use. The metal films plated on the hydrogen storage alloy particles impart electric and thermal conductivities to the material.
The hydrogen storage alloy electrode in accordance with the present invention comprising such hydrogen storage alloy material is thus large in electric capacity per unit weight of the electrode and endures repeated charge and discharge operations with no development of a collapse, thereby allowing repeated use. In addition, because of the electric conductivity imparted by the metal films plated on the alloy particles, resistance in the electrode can substantially be prevented and voltage drops due to electrode resistance can also be eliminated, which enables high-rate charge and discharge operations for a battery including the electrode.
The estimated mechanism that the hydrogen storage material in accordance with the present invention does not collapse even after it repeatedly absorbs therein and desorbs therefrom hydrogen is that the thermoplastic resin incorporated in the metal films plated on the hydrogen storage alloy particles enters a space between those alloy particles and effectively functions as a binder when it is pressure-molded together with the alloy particles at a temperature higher than a glass transition metal or a melting point of the resin, and that the metal films plated on the hydrogen storage alloy particles are clasped with each other complexly so as to be firmly bonded to each other mechanically when they are molded together with the particles at a high pressure.
As mentioned previously, the method for manufacturing a hydrogen storage alloy material in accordance with the present invention comprises the steps of preparing hydrogen storage alloy particles covered with a plated metal film having microgranules of a thermoplastic resin, and pressure-molding the hydrogen storage alloy particles at a temperature which is higher than a glass transition temperature or a melting point of and below a thermal decomposition temperature of the thermoplastic resin, thereby obtaining a porous body of the hydrogen storage alloy particles being bonded with each other firmly via the thermoplastic resin.
Another method for manufacturing a hydrogen storage material in accordance with the present invention comprises the steps of preparing hydrogen storage alloy particles covered with a plated metal film having microgranules of a thermoplastic resin and a porous metal substrate, positioning the hydrogen storage alloy particles on one or both surfaces of the porous metal substrate, and pressure-molding the hydrogen storage alloy particles and the porous metal substrate at a temperature which is higher than a glass transition temperature or a melting point of and below a thermal decomposition temperature of the
Kiyokawa Hajime
Kiyokawa Tadashi
Takashima Masayuki
Akin Gump Strauss Hauer & Feld L.L.P.
Kiyokawa Plating Industries Co., Ltd.
Mai Ngoclan
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