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  Hydrogen-absorbing alloy for alkaline storage battery, and alkaline storage batteryHydrogen-absorbing alloy for alkaline storage battery, and alkaline storage battery description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070184346, Hydrogen-absorbing alloy for alkaline storage battery, and alkaline storage battery. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001]The priority applications Numbers Japanese Patent Applications 2006-32036, 2006-237931 and 2007-6537 upon which this patent application are hereby incorporated by reference. BACKGROUND OF THE INVENTION [0002]1. Field of the Invention [0003]This invention relates to a hydrogen-absorbing alloy for alkaline storage battery, and to an alkaline storage battery employing a positive electrode, a negative electrodes using the hydrogen-absorbing alloy for alkaline storage battery, and an alkaline electrolyte. More particularly, a feature of the invention is an improvement in a hydrogen-absorbing alloy for alkaline storage battery used for a negative electrode in an alkaline storage battery, which utilizes, for the purpose of enhancing hydrogen-absorbing capability of the hydrogen-absorbing alloy and improving a cycle life of the alkaline storage battery. [0004]2. Description of Related Art [0005]Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel metal hydride batteries using a hydrogen-absorbing alloy as a material for their negative electrodes have drawn considerable attention from the viewpoints that they have higher capacity than nickel-cadmium storage batteries and, being free of cadmium, they are more environmentally safe. [0006]As the nickel metal hydride batteries have been used in various portable devices, demands for further higher performance in the nickel metal hydride batteries have been increasing. [0007]In the nickel metal hydride batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu.sub.5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni have been generally used for their negative electrodes. [0008]However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to increase the capacity of the alkaline storage batteries further. [0009]In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a hydrogen-absorbing alloy having a crystal structure other than CaCu.sub.5 type, by adding magnesium to the rare earth-nickel hydrogen-absorbing alloy (see Japanese Published Unexamined Patent Application No. 2002-69554). [0010]Nevertheless, problems in producing the above-described hydrogen-absorbing alloy containing rare earth elements, nickel and magnesium by a metal mold casting process which is a conventional commonly used method, have been that it is difficult to obtain a hydrogen-absorbing alloy having an uniform structure, due to differences in melting point and specific gravity between magnesium and the other elements, and characteristics of the hydrogen-absorbing alloy such as corrosion resistance and a cycle life of the alkaline storage battery are degraded. [0011]In this connection, there has been proposed a rapid quenching method to produce a hydrogen-absorbing alloy containing rare earth elements, nickel and magnesium with an uniform structure (see Japanese Published Unexamined Patent Application No. 2004-115870). [0012]However, in producing the hydrogen-absorbing alloy containing the rare earth elements, nickel and magnesium by the rapid quenching method, there have still been the following problems. If a composition of the hydrogen-absorbing alloy is not suitable, the cycle life of the alkaline storage battery is degraded and hydrogen-absorbing capability of the hydrogen-absorbing alloy is lowered. SUMMARY OF THE INVENTION [0013]The present invention is directed to a solution to the above problems associated with an alkaline storage battery using a hydrogen-absorbing alloy for alkaline storage battery containing rare earth elements, nickel and magnesium, as its negative electrode. [0014]A first object of the invention is to obtain a hydrogen-absorbing alloy for alkaline storage battery containing rare earth elements, nickel and magnesium with a uniform structure and a suitable composition, and to improve hydrogen-absorbing capability thereof. [0015]An another object of the invention is to improve a cycle life of the alkaline storage battery using the hydrogen-absorbing alloy for alkaline storage battery as its negative electrode, and to obtain an alkaline storage battery with a high capacity. [0016]The hydrogen-absorbing alloy for alkaline storage battery according to the present invention is produced by a rapid cool using a rapid quenching method and has a composition represented by a general formula Ln.sub.1-xMg.sub.xNi.sub.a-b-cAl.sub.bZ.sub.c, where Ln is at least an element selected from rare earth elements including Y, Zr, and Z is at least one element selected from the group consisting of V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Ga, Zn, Sn, In, Cu, Si, P, and B, 0.05.ltoreq.x<0.25, 2.8.ltoreq.a.ltoreq.3.9, 0<b.ltoreq.0.30, and 0.ltoreq.c.ltoreq.0.25. [0017]Herein, in order to produce the hydrogen-absorbing alloy for alkaline storage battery by the rapid cool, a publicly known rapid quenching method may be used. A rapid quenching method is a method for forming an alloy ingot by superrapidly cooling an alloy melt. One of examples is a roll method, such as, a single roll method and a twin roll method, which is the method to eject the alloy melt on a roll rotating fast. While a cooling rate is about 10 K/s in a conventional metal mold casting process, the cooling rate is not less than 10.sup.2 K/s to 10.sup.3 K/s, generally, about 10.sup.4 K/s to 10.sup.6 K/s, in a rapid quenching method. [0018]The hydrogen-absorbing alloy for alkaline storage battery produced by the rapid cool using the rapid quenching method whose composition is represented by the above general formula has an uniform structure of Ce.sub.2Ni.sub.7 crystal structure or a similar structure with a high crystallinity. [0019]Even in the hydrogen-absorbing alloy for alkaline storage battery produced as above, if the ratio of Mg is too large, an uniform alloy structure is not attained and corrosion resistance is deteriorated. On the other hand, if the ratio of Mg is too small, maintenance of the Ce.sub.2Ni.sub.7 crystal structure or the similar structure becomes difficult because of drastic change of its crystal structure and hydrogen-absorbing capability is drastically lowered. For these reasons, it is necessary that x indicating the ratio of Mg in the general formula be 0.05.ltoreq.x<0.25, and preferably be 0.05.ltoreq.x.ltoreq.0.20. [0020]The hydrogen-absorbing alloy for alkaline storage battery has the following tendency that if the ratio of Mg is large, the capacity is increased and if the ratio of Al is large, the corrosion resistance is improved. As a result of observation of the alloy structure by EPMA, Al concentration is low in parts where Mg concentration is high, and Mg concentration is low in parts where Al concentration is high, which means a concentration dispersion of Mg is the opposite of that of Al. It shows that Mg and Al are not easily mixed each other due to their dispositions. Therefore, if a total amount of Mg and Al in the alloy structure is large, segregation occurs easily. For these reasons, the use of rapid quenching method having a superior effect for uniform of alloy structure has a greater effect in the hydrogen-absorbing alloy for alkaline storage battery wherein the total amount of Mg and Al in the alloy structure is large. Particularly, the effect is greater in the hydrogen-absorbing alloy for alkaline storage battery wherein the total amount of Mg and Al, (x+b) in the above general formula is not less than 0.30, making it possible to attain a hydrogen-absorbing alloy for alkaline storage battery having a high capacity and a superior corrosion resistance. [0021]In the hydrogen-absorbing alloy for alkaline storage battery, a, b and c satisfy 2.8.ltoreq.a.ltoreq.3.9, 0<b.ltoreq.0.30, and 0.ltoreq.c.ltoreq.0.25 in the above general formula. It is because if a, b and c are out of the above ranges, the crystal structure of the hydrogen-absorbing alloy for alkaline storage battery is drastically changed and the hydrogen-absorbing capability thereof is lowered as described above. Continue reading about Hydrogen-absorbing alloy for alkaline storage battery, and alkaline storage battery... Full patent description for Hydrogen-absorbing alloy for alkaline storage battery, and alkaline storage battery Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Hydrogen-absorbing alloy for alkaline storage battery, and alkaline storage battery patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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