JP2022020892A - HYDROGEN STORAGE ALLOY POWDER OF LOW Co CONTENT - Google Patents

Abstract

To save a raw material cost by reducing a Co content or adding no Co, and to keep life properties of a negative pole by suppressing particle size reduction of an alloy caused by repeated storage and discharge of hydrogen in a CaCu5 type hydrogen storage alloy used for a negative pole of a nickel hydrogen battery.SOLUTION: A hydrogen storage alloy has a CaCu5 type crystal structure and a component composition represented by general formula MmNiaMnbAlcCod (where Mm is misch metal, 4.30≤a≤4.70, 0.25≤b≤0.45, 0.35≤c≤0.45, 0≤d≤0.14 (preferably, 0.02≤d≤0.14), 5.20≤a+b+c+d≤5.50). The hydrogen storage alloy has a Young's modulus of 120 GPa or more and 127 GPa or less, which is obtained by measuring a single crystal phase of a particle cross section with a nanoindenter.SELECTED DRAWING: Figure 2

Description

The present invention relates to a hydrogen storage alloy having a CaCu5 type crystal structure used as a negative electrode of a nickel hydrogen battery and having a low Co content. Further, the present invention relates to a battery using this hydrogen storage alloy as a negative electrode.

Nickel-metal hydride batteries using a hydrogen storage alloy for the negative electrode were commercialized in the early 1990s and have been widely used since then.

Nickel-metal hydride batteries were used as a power source for mobile phones and laptop computers when they were first commercialized, but after that, they were gradually replaced by smaller and lighter lithium-ion batteries, and now they are inexpensive and highly safe. And, because of its good balance with the energy density per volume, it is used in toys, digital cameras, electrically power assisted bicycles, power tools, and even hybrid vehicles.

The hydrogen storage alloy used in such a nickel-metal hydride battery is an alloy that reacts with hydrogen to form a metal hydride. This hydrogen storage alloy can reversibly store and release a large amount of hydrogen near room temperature.

As hydrogen storage alloys, AB5 type alloys typified by LaNi5, AB2 type alloys typified by ZrV _0.4 Ni _1.5 , and various types of alloys such as AB type, A2B type, and AB3 type are known. Has been done. These alloys have an affinity for hydrogen with element groups (rare earth elements, Ca, Mg, Ti, Zr, V, Nb, Pt, Pd, etc.) that have a high affinity for hydrogen and play a role in increasing hydrogen occlusion. Although it is relatively low and has a small occlusion, it is composed of a combination with an element group (Ni, Mn, Co, Al, etc.) that plays a role in promoting the hydrogenation reaction and lowering the reaction temperature.

Among these, an AB5 type hydrogen storage alloy having a CaCu5 type crystal structure, for example, mischmetal (hereinafter referred to as “Mm”), which is a rare earth-based mixture, is used for the A site, and Ni, Mn, Co, for the B site. An alloy using an element such as Al can form a negative electrode with a material that is relatively inexpensive as compared with alloys having other compositions.

In the AB5 type hydrogen storage alloy, the ratio of the B site atomic weight to the A site atomic weight (AB ratio) and the substitution amount of Co, Mn, Al, etc. for a part of Ni are adjusted to charge and discharge the negative electrode. Various characteristics such as capacity, input / output characteristics, and cycle life can be adjusted. The AB5 type hydrogen storage alloy having such characteristics makes it possible to manufacture nickel-metal hydride storage batteries according to various uses.

In order to popularize and expand hybrid vehicles, it is necessary to keep the manufacturing cost of nickel-metal hydride batteries low and further improve the life characteristics and input / output characteristics of the negative electrode. In order to achieve this purpose, research and development of AB5 type hydrogen storage alloy is being actively carried out. In particular, in the AB5 type hydrogen storage alloy in which the amount of Co used, which is an expensive rare metal, is reduced as much as possible, studies are being conducted focusing on the hardness of the alloy for the purpose of maintaining and improving the life characteristics.

For example, in Patent Document 1, as a hydrogen storage alloy for a secondary battery, (R _x My) · (Ni _a Co _b Mn _c Al _d ) _z ( _where R is a rare earth mixed metal containing 75% ≧ La, M. Is any one metal selected from Ti, Zr, and Hf, x + y = 1, 0.002 ≦ y <0.01, 3.5 <a <4.5, 0.6 ≦ b <1.0, It is a hydrogen storage alloy for a secondary battery represented by c ≦ 0.2, 0.2 ≦ d ≦ 0.4, 5.1 ≦ z <5.3), and the micro Vickers hardness of this hydrogen storage alloy is determined. It is proposed to be over 400

Further, in Patent Document 2, the general formulas are (R _x My) and (Ni _{a Cob} Mn _c Al _{d )} (where R is a rare earth mixed metal containing 75% La ≧, and M is selected from Ti, Zr, and Hf. Any one of the metals, x + y = 1, 0.002≤y <0.01, 3.5 <a <4.5, 0.6≤b <1.0, c≤0.2, 0. It is a hydrogen storage alloy for a secondary battery represented by 2 ≦ d ≦ 0.4 and 5.1 ≦ (a + b + c + d +), and it is proposed that the micro Vickers hardness of this hydrogen storage alloy is 400 or more.

Further, in Patent Document 3, A _x R _1-x (My Ni _{1-y ) n} (in the formula, A is Y, Gd, Tb, Dy or a mixture thereof, R is La, Ce, Pr, Nd or these. , M represents Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, W, B or a mixture of these elements; x, y and n are 0.01 ≦ x ≦ 0.1, respectively. 0.01 ≦ y ≦ 0.5, 4.9 ≦ n ≦ 5.4), and it is proposed that the Vickers hardness (Hv) of the alloy is 900 kg / mm ² or more. Has been done.

Further, Patent Document 4 proposes a method for producing a hydrogen storage alloy powder capable of removing impurities that cause a short circuit. In this production method, a raw material for a hydrogen storage alloy is melted to form a molten metal. The hydrogen storage alloy ingot obtained by cooling the hydrogen storage alloy ingot is crushed, and then magnetic separation is performed using a magnet to remove magnetic deposits. More specifically, for example, in the general formula _{MmNi a} Mn _b Al _c Cod (however, 4.0 ≦ a ≦ 4.7, 0.3 ≦ b ≦ 0.7, 0.20 ≦ c00.50). , 0 <d ≦ 0.80), the alloy raw material is melted to form a molten metal, and this molten metal is poured into, for example, a water-cooled mold at a casting temperature of, for example, 1350 to 1550 ° C. Cast and cool at a given cooling rate. Then, if necessary, heat treatment is performed in an inert gas atmosphere at, for example, 1000 to 1200 ° C. for 3 to 6 hours. Next, it has been proposed to coarsely crush the hydrogen storage alloy ingot and remove impurities causing a short circuit in the subsequent magnetic separation step (see paragraphs 0017, 0025 to 0030).

Japanese Unexamined Patent Publication No. 10-261413 Japanese Unexamined Patent Publication No. 2008-21809 Japanese Unexamined Patent Publication No. 2001-266864 Japanese Unexamined Patent Publication No. 2010-255104

As mentioned above, studies have been made so far, but in recent years the transaction price of Co, which is a rare metal, has soared, and in order to maintain or reduce the raw material cost of AB5 type hydrogen storage alloy containing Co, it is necessary to maintain or reduce it. It is necessary to reduce the Co content as much as possible. However, when the Co content of the AB5 type hydrogen storage alloy is reduced, the alloy is promoted to be micronized due to repeated storage and release of hydrogen, and the life characteristic of the negative electrode tends to be deteriorated. No effective solution to the problem has been found to achieve both the reduction of Co content and the life characteristics of the negative electrode.

In Patent Document 1, as a hydrogen storage alloy for a secondary battery, an amount of a rare earth mixed metal containing 75% or more of La at (R _x My) and (Ni _{a Cob Mn c Al d} ) _z and an element _M are added. Is 0.02 or more and less than 0.01, and the micro Vickers hardness of the alloy cannot be set to 400 or more unless it is within this range as the reason for the addition amount of the element M. Therefore, at the time of hydrogen storage / release, that is, charge / discharge. It is said that sometimes the alloy becomes fragile, leading to a decrease in cycle life. However, the amount of Co in this hydrogen storage alloy is set to 0.6 to 1.0, which is 0.6 to 0.9 in the examples, but there is a problem that the amount of Co is too large. Further, Patent Document 1 relates to the problem of suppressing pulverization of the hydrogen storage alloy and specific means for solving the problem from the viewpoint of improving the life characteristics when the hydrogen storage alloy is used as the negative electrode active material. There is no description.

In Patent Document 2, as a hydrogen storage alloy for a secondary battery, La (R _x My) and ( _{Nia Cob Mn c Al d ) in} rare earth metals is 75% or more, and the element M added to the La site. The addition amount of the alloy is 0.02 or more and less than 0.01, and the micro Vickers hardness of the alloy cannot be 400 or more unless it is in this range as the reason for the addition amount of the element M. Therefore, at the time of hydrogen storage / release, that is, It is necessary to prolong the cycle life because the alloy is easily cracked during charging and discharging. However, the amount of Co in this hydrogen storage alloy is set to 0.6 to 1.0, which is 0.6 to 0.9 in the examples, but there is a problem that the amount of Co is too large. Also in this patent document, from the viewpoint of improving the life characteristics when the hydrogen storage alloy is used as the negative electrode active material, the problem of suppressing the pulverization of the hydrogen storage alloy and the specific means for solving the problem are described. not.

In Patent Document 3, A _x R _1-x (My Ni _{1-y ) n} (in the formula, A is Y, Gd, Tb, Dy or a mixture thereof, R is La, Ce, Pr, Nd or these. In the mixture of, M represents Co, Al, Mn, Fe, Cu, Zr, Ti, Mo, W, B or a mixed element thereof. X is preferably 0.03 to 0.05, y is 0.01 to 0.01. It is said that a hydrogen storage alloy having an Hv of 900 kg / mm ² can be obtained when 0.5 and n are 4.9 or more and 5.4 or less. However, the Co amount of this hydrogen storage alloy is 0.50 to 0 according to the examples. It is .75 mol%, which also has a problem that the amount of Co is too large. Also, from the viewpoint of improving the life characteristics when the hydrogen storage alloy is used as the negative electrode active material, the fine powder of the hydrogen storage alloy is also present in this patent document. There is no description about the problem of suppression of conversion and specific means for solving it.

Patent Document 4 discloses that impurities (iron and Cr) that cause a short circuit can be effectively removed by providing a magnetic selection step in the hydrogen storage alloy manufacturing process to remove impurities. From the viewpoint of improving the life characteristics when the hydrogen storage alloy is used as the negative electrode active material, there is no description about the problem of suppressing the pulverization of the hydrogen storage alloy and the specific means for solving the problem.

The present invention has been made in view of the above problems, and for a Co-containing AB5 type hydrogen storage alloy, the raw material cost is suppressed by reducing the Co content, and then the fine powder of the alloy is repeatedly stored and released. The problem is to maintain the life characteristics when used as a negative electrode active material by suppressing the formation.

In order to solve the above problems, the present inventor has studied diligently, and in the AB5 type hydrogen storage alloy containing Co having a predetermined composition, the raw material cost is suppressed by reducing the Co content or adding no Co, and then pulverizing. As a result of investigating in detail the relationship between the mechanical properties of the single crystal phase of the alloy particle cross section obtained in 1 and the battery characteristics, the Young's ratio obtained when the single crystal phase of the particle cross section of the hydrogen storage alloy was measured with a nanoindenter. By controlling the young rate to 120 GPa or more and 127 GPa or less, it is possible to suppress the reduction of hydrogen storage capacity and to suppress the atomization of the alloy due to repeated storage and release of hydrogen, and the negative electrode activity. We have found that it is possible to obtain an AB5 type hydrogen storage alloy that maintains the life characteristics when used as a material, and completed the present invention.

The gist of the present invention is as follows.

(1) General formula MmNiaMnbAlcCod (In the formula, Mm is a misch metal, 4.30 ≦ a ≦ 4.70, 0.25 ≦ b ≦ 0.45, 0.35 ≦ c ≦ 0.45, 0 ≦ d ≤0.14, 5.20≤a + b + c + d≤5.50), which is a hydrogen storage alloy having a CaCu5 type crystal structure.
In Mm, the total of La and Ce is in the range of 90% by mass or more and 100% by mass or less with respect to the total mass of Mm, and the single crystal phase of the particle cross section of the hydrogen storage alloy after heat treatment is subjected to nanoindenter. A hydrogen storage alloy characterized in that the young ratio obtained when measured is 120 GPa or more and 127 GPa or less.
(2) The hydrogen storage alloy powder according to (1) above, wherein d is 0.02 ≦ d ≦ 0.14.
(3) When the hydrogen storage amount (H / M) is 0.85 or more and 1.00 or less, the difficulty of pulverization is 0.50 or more and 0.60 or less, and the equilibrium pressure is 0.040 MPa or more and 0.070 MPa or less. The hydrogen storage alloy powder according to (1) or (2) above, which is characterized by being present.
(4) A negative electrode characterized in that the hydrogen storage alloy according to any one of (1) to (3) above is used as a negative electrode active material.
(5) A battery using the negative electrode according to (4) above.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain an AB5 type hydrogen storage alloy in which the raw material cost is suppressed by reducing the Co content and the atomization of the alloy due to repeated storage and release of hydrogen is suppressed. It has a remarkable effect of being able to obtain an AB5 type hydrogen storage alloy suitable for a negative electrode for a nickel-metal hydride battery because of its excellent life characteristics.

The secondary electron image (a) and the orientation analysis image (b) by EBSD about the particle cross section obtained by embedding the crushed powder of a hydrogen storage alloy with a resin and then polishing are shown. In the orientation analysis image (b) by EBSD, the particles on the left side (single crystal phase 1), the particles on the upper center (single crystal phase 2) and the particles on the right side (single crystal phase 3) are obtained by orientation analysis by EBSD. Since the cross section of each particle has the same color, it indicates that each particle has one orientation, that is, it consists of a single crystal phase. Further, the Young's modulus of the single crystal phase is measured by setting the vicinity of the central portion of the particles as the measurement point. 3 is a graph showing the relationship between the degree of difficulty in micronization and the Young's modulus obtained from the nanoindenter in Examples 1 to 4 and Comparative Examples 1 to 3.

The present invention will be described in detail below.

The present inventor paid attention to the mechanical properties of the AB5 type hydrogen storage alloy having a low Co content, and the larger the young ratio obtained when the single crystal phase of the particle cross section of the hydrogen storage alloy was measured with a nanoindenter, the finer the powder. It was found to be excellent in the difficulty of conversion. The Young's modulus indicates the rigidity of the alloy crystal, and it can be interpreted that the rigidity of the alloy crystal makes it difficult for the alloy crystal to crack even if hydrogen is stored. Therefore, in order to improve the difficulty of micronization and obtain excellent life characteristics as a negative electrode for a nickel-metal hydride battery, it is necessary to use the alloy crystal having a large Young's modulus.
Specifically, when the Young's modulus obtained when the single crystal phase of the particle cross section of the hydrogen storage alloy is measured by a nanoindenter is 120 GPa or more, the alloy is excellent as a negative electrode for a nickel hydrogen battery. If it is less than 120 GPa, cracking of the alloy crystal tends to proceed when hydrogen storage and release are repeated, that is, the difficulty of micronization decreases and sufficient characteristics cannot be obtained as a negative electrode for a nickel hydrogen battery. If the Young's modulus becomes large, the difficulty of pulverization becomes better, but if the Young's modulus becomes too large, the processability of the alloy deteriorates and it takes too much time to pulverize the alloy that can be used as the negative electrode for nickel-metal hydride batteries. Therefore, the upper limit of the Young's modulus was 127 GPa.

The AB5 type hydrogen storage alloy in the present invention has a CaCu5 type crystal structure and has a general formula MmNiaMnbAlcCod (Mm is a misch metal, 4.30 ≦ a ≦ 4.70, 0.25 ≦ b ≦ 0.45, 0. It is represented by .35 ≦ c ≦ 0.45, 0 ≦ d ≦ 0.14, 5.20 ≦ a + b + c + d ≦ 5.50), and the molar ratio d of Co is preferably as small as possible in order to reduce the raw material cost. 0 ≦ d ≦ 0.14. However, if the Co content is less than 0.02, the difficulty of micronization may not be maintained even if the Young's modulus is increased, so 0.02 ≦ d ≦ 0.14 is preferable.

In the AB5 type hydrogen storage alloy, the metal constituting the A site will be described.
In the present invention, as the metal constituting the A site, mischmetal (Mm) in which La or a part or all of La is a rare earth metal mixture is used. In Mm, it is preferable that La and Ce occupy a ratio within the range of 90% by mass or more and 100% by mass or less with respect to the total mass of Mm, and more preferably, La is 70 to 96% by mass and Ce is 4. It is in the range of about 30% by mass, more preferably in the range of 74 to 94% by mass for La and 6 to 26% by mass for Ce.

Next, the metals constituting the B site will be described. In the present invention, Ni, Mn, Al, and Co are used as the metals constituting the B site. The molar ratio of these metals preferably satisfies the following conditions.
Ni molar ratio (a) 4.30 ≤ a ≤ 4.70
Mn molar ratio (b) 0.25 ≤ b ≤ 0.45
Al molar ratio (c) 0.35 ≤ c ≤ 0.45
Co molar ratio (d) 0 ≤ d ≤ 0.14
AB ratio 5.20 ≤ (a + b + c + d) ≤ 5.50
More preferable conditions are as follows.
Ni molar ratio (a) 4.40 ≤ a ≤ 4.70
Mn molar ratio (b) 0.25 ≤ b ≤ 0.41
Al molar ratio (c) 0.38 ≤ c ≤ 0.42
Co molar ratio (d) 0.02 ≤ d ≤ 0.11
AB ratio 5.25 ≤ (a + b + c + d) ≤ 5.46

The reason why the ratio of La and Ce of Mm and the molar ratio of Ni, Mn and Al are set as described above is that the hydrogen storage amount (H / M) of the AB5 type hydrogen storage alloy is 0.85 to 1.00. This is because consideration is given to ensuring the charge / discharge capacity, facilitating the initial activation by setting the equilibrium pressure to 0.040 to 0.070 MPa, and widening the plateau range in the PCT curve as much as possible.

In the hydrogen storage alloy, the Young's modulus obtained when the single crystal phase of the 100 μm alloy particle cross section obtained by pulverization is measured by a nanoindenter is preferably 120 GPa or more and 127 GPa or less.
Further, the hydrogen storage alloy undergoes crystal growth during casting, but the grain size of the alloy cross section observed by EBSD shows an average of 134 μm even on a cooling surface where the grain size is relatively small with respect to the free surface. Since the particles obtained by mechanically pulverizing the hydrogen storage alloy crack preferentially from the grain boundaries, the obtained particles having a size of about 100 μm are almost single crystals. If the particles are embedded in resin and polished to the center of the 100 μm particle cross section, the single crystal phase is selected. Therefore, the Young's modulus measurement by nanoindentation should be performed at this part. conduct.

In the hydrogen storage alloy, the hydrogen storage amount (H / M) is 0.85 to 1.00, the pulverization difficulty is 0.50 to 0.60, and the equilibrium pressure is 0.040 to 0.070 MPa. be.
Here, the difficulty of pulverization is defined as "the particle size (D ₅₀ ) of the hydrogen storage alloy powder after 10 hydrogen storage and release cycles in an environment of holding temperature 45 ° C. and hydrogen pressure adjustment 1.82 MPa" as "hydrogen storage alloy". It is a value divided by "initial particle size of powder (D ₅₀ )". The equilibrium pressure is the equilibrium hydrogen pressure (MPa) at a measurement temperature of 45 ° C. on the hydrogen release side at H / M = 0.5.

The reason why the degree of difficulty in micronization is set to 0.50 to 0.60 is that if it is too high, it is difficult to perform initial activation, and if it is too low, the life characteristics of the battery cannot be ensured. The difficulty of micronization tends to be in phase with the hydrogen storage amount (H / M), and the difficulty of micronization tends to be lower as the H / M is larger, and the difficulty of micronization tends to be higher as the H / M is smaller. The reason why the H / M is set to 0.85 to 1.00 is to secure the target charge / discharge capacity when the negative electrode for the nickel-metal hydride battery is manufactured.

The hydrogen-storing alloy powder of the present invention is a hydrogen-storing alloy having the above-mentioned CaCu5 type crystal structure, and the young ratio obtained when the single crystal phase of the alloy particle cross section is measured by a nanoindenter is 120 GPa or more and 127 GPa or less. Any manufacturing method can be used as long as it can be used, and for example, it can be manufactured by a melting method using a vacuum melting furnace or an atmospheric melting furnace, a gas phase synthesis method, an arc melting method, a plasma melting method, or the like. The produced alloy may be heat-treated or pulverized.
As an example of the manufacturing method, a case of manufacturing through a weighing step, a mixing step, a casting step, a heat treatment step, a cooling step, and a crushing step will be described. In the weighing step, each raw material of the hydrogen storage alloy is weighed so as to have a desired alloy composition. In the mixing step, a plurality of weighed raw materials are mixed. In the casting step, the mixed raw material is put into a high-frequency heating and melting furnace, the mixed raw material is melted to form a molten metal, and the molten metal is held for a predetermined time, and then the molten metal is poured into a mold for casting. The molten metal temperature at the time of casting depends on the alloy composition, but is, for example, a temperature in the range of 1150 ° C. to 1550 ° C. (casting temperature = molten metal temperature in the crucible at the start of casting). This step aims to completely mix a plurality of types of raw materials, but internal strain occurs due to volume shrinkage when the alloy solidifies. In order to reduce this internal strain as much as possible, the casting thickness of the alloy should be 35 mm or more, and the cooling time to 500 ° C. or less should be 10 minutes or more. By setting this casting thickness, the alloy is slowly solidified and internal strain can be reduced. At this time, the Young's modulus of the cast alloy is 50 GPa or more and 110 GPa or less.
The alloy after casting is heat-treated to remove the above-mentioned internal strain. If necessary, the heat treatment may be performed in a non-oxidizing atmosphere in the heat treatment step. The heat treatment temperature is determined according to the alloy composition for the purpose of completely removing strain, and may be, for example, 1060 ° C to 1150 ° C.
The heat treatment temperature is preferably slightly lower than the melting point, but if it is too low, the structure will not be homogenized during the heat treatment, and if the temperature is too high, a segregated phase will appear, resulting in local composition deviation. Homogenization of the crystal structure and local composition deviation affect the toughness of the alloy, which affects the decrease in Young's modulus. In order to prevent this, 1060 ° C to 1150 ° C is preferable in the current composition range.
The heat treatment time is, for example, 8 to 13 hours, although it depends on the size of the ingot (hydrogen storage alloy piece) after casting. Heat treatment is a process that affects the homogeneity of the structure, and if this time is short, sufficient homogeneity of the crystal structure cannot be obtained, and this local decrease in homogeneity affects the toughness of the alloy, and Young's modulus. Leads to a decline in.
In the cooling step, the heat-treated casting is cooled. The cooling method can be either free cooling or forced cooling.
The cooling rate should be appropriately set in order to obtain the required alloy properties according to the alloy composition.
By this step, Young's modulus becomes 120 GPa or more and 127 GPa or less.
In the crushing step, which is the next step, the cast alloy or the further heat-treated alloy ingot is coarsely pulverized, finely pulverized, or the like to obtain hydrogen storage alloy powder having a required particle size. For example, it is pulverized to a size that passes through a sieve of 500 μm to obtain a hydrogen storage alloy powder.
In the above-mentioned manufacturing method, the young ratio of the alloy can be appropriately adjusted such as the alloy composition, the molten metal holding temperature / time, the casting temperature, the heat treatment temperature / time / cooling rate of the cast alloy, and the like. For example, as an example in the alloy composition, Young's modulus can be increased by increasing the [Cu] / [Ca] ratio (x value of ABx) in the CaCu5 type crystal structure.

The hydrogen storage alloy powder thus obtained measures the hydrogen storage amount (H / M) and the equilibrium pressure by a PCT (hydrogen pressure-composition-isothermal diagram) characteristic evaluation device.

The Young's modulus measurement by the nano indenter in the present invention will be described.
The pulverized powder having a diameter of 40 to 200 μm of the hydrogen storage alloy is embedded in a resin and mirror-polished for nanoindenter observation, and adjusted so that the diameter is 31 to 32 mm and the height is 34 mm or less. As a method of resin embedding and polishing, for example, 80 mL of a resin (# 105) manufactured by Marumoto Strias Co., Ltd. and 1.2 mL of a curing agent (M agent) are used as the resin for embedding. As the container used for resin embedding, a SeriForm product having a diameter of 30 mm manufactured by Streeter is used. As an automatic polishing device, a device in which AMO-210 was incorporated in a refine polisher HV manufactured by Refine Tech Co., Ltd. was used. As a condition, the water resistant papers 600 to 800, 1000 and 2000 are polished at 200 N and 300 rpm for about 3 minutes according to the surface condition of the resin. After that, 1 μm of alumina and 0.05 μm of alumina are polished at 100 N and 200 rpm for about 2 minutes, respectively, and the finish is polished with only the buff for about 100 N, 200 rpm and about 30 seconds. After that, a method according to the international standard ISO14577, specifically, an InForce50 head is attached to an iMicroro type nanoindenter manufactured by Nanomechanics, and the continuous rigidity measurement method (CSM / CSR) is used to measure 5 points per particle. Young's modulus is measured at 5 points, a maximum pushing load of 50 mN, and a Young's modulus calculation depth of 30 to 40 nm. At this time, it is desirable to select and observe a flat surface without nests or missing parts.
The hydrogen storage alloy undergoes crystal growth during casting, but the grain size of the alloy cross section observed by EBSD shows an average of 134 μm even on a cooling surface where the grain size is relatively small with respect to the free surface. Since the particles obtained by mechanically pulverizing the hydrogen storage alloy crack preferentially from the grain boundaries, the obtained particles having a size of about 100 μm are almost single crystals. If the particles are embedded in resin and polished to the center of the 100 μm particle cross section, the single crystal phase is selected. Therefore, the Young's modulus measurement by nanoindentation should be performed at this part. (See FIG. 1 (b)).

The method for measuring the difficulty of micronization in the present invention will be described.
PCT (hydrogen pressure-composition-isothermal diagram) characteristics are sold by commercially available evaluation equipment based on JIS H7201 "Measurement method of pressure-isothermal wire (PCT line) of hydrogen storage alloy", for example, Suzuki Shokan Co., Ltd. Using the PCT characteristic measuring device and a sample holder made by SUS316 with an outer diameter of 12.7 mm and a length of 91 mm, it was obtained from the measurement of 5 g of the sample under the environment of "holding temperature 45 ° C. and hydrogen pressure adjustment 1.82 MPa". The value obtained by dividing "the particle size of the hydrogen storage alloy powder after 10 hydrogen storage and release cycles (D ₅₀ )" by "the initial particle size of the hydrogen storage alloy powder (D ₅₀ )" was indexed as the difficulty of pulverization.

Hereinafter, description will be given based on examples of the present invention. The present invention is not limited to the examples.

(Example 1-4)
La and Ce, Ni, Mn, Al, and Co metal raw materials as Mm were weighed so as to have the alloy composition of the final product shown in Table 1.
These raw materials were placed in an aluminal pot in a high-frequency melting furnace and evacuated to 27 Pa, and then argon gas was introduced to create an atmosphere of argon gas.
Then, it was heated and melted by a high-frequency heating device, and the molten metal was held in the range of 1550 to 1600 ° C. for 20 to 30 minutes, and then the molten metal in the temperature range of 1300 to 1400 ° C. was poured into a water-cooled iron mold for casting. The casting thickness at this time was 40 mm.
Next, the cast alloy was crushed into chunks, placed in a stainless steel container, and heat-treated at 1100 ° C. for 12.5 hours under an argon atmosphere.
Then, the temperature was lowered from around 900 ° C. to around 500 ° C. after the heat treatment at a cooling rate of 4.45 ° C./min, and then to 100 ° C. at a cooling rate of 1 ° C./min.
The obtained alloy was coarsely pulverized by a crusher under an argon gas atmosphere, subsequently pulverized using a cutting mill under an inert atmosphere, and subsequently made into a particle size (500 μm or less) passing through a sieve mesh of 500 μm.
As described above, the hydrogen storage alloy powders shown in Examples 1 to 4 in Table 1 were prepared.
The heat treatment conditions are also shown in Table 1.

(Measurement of PCT characteristics)
With respect to the obtained hydrogen storage alloy powder, the hydrogen storage amount (H / M) and the equilibrium pressure were measured by a PCT characterization device. The results are shown in Table 2.

(Measurement of difficulty in micronization)
The difficulty of micronization was measured using a PCT characterization device. The results are shown in Table 2.

(Measurement of Young's modulus by nanoindentation method)
The hydrogen storage alloy powder prepared as described above was embedded in a resin so as to have a diameter of 31 mm and a height of 30 mm, and a mirror-polished sample was used with a nanoindenter (using Nanomechanics iMicro type nanoindenter, InForce 50 head). The Young's modulus was measured by the continuous rigidity measurement method (CSM / CSR) at 5 points per particle, 5 points for the indentation test, a maximum indentation load of 50 mN, and a Young's modulus calculation depth of 30 to 40 nm. The results are shown in FIGS. 2 and 2.
The hydrogen storage alloy undergoes crystal growth during casting, but the grain size obtained by EBSD observation of the cross section of the alloy is about 100 to 200 μm even on a cooling surface where the grain size is relatively small with respect to the free surface. Since the particles obtained by mechanically pulverizing the hydrogen storage alloy crack preferentially from the grain boundaries, the obtained particles having a size of about 100 μm are almost single crystals. If the particles are embedded in resin and polished to the center of the 100 μm particle cross section, the single crystal phase is selected. Therefore, the Young's modulus measurement by nanoindentation should be performed at this part. (See FIG. 1 (b)).

(Comparative Example 1)
A hydrogen storage alloy was prepared by the same method as in Examples except that the composition was set to Comparative Example 1 in Table 1.
(Comparative Example 2)
A hydrogen storage alloy was prepared by the same method as in Examples except that the heat treatment time was set to 5 hours in the composition of Comparative Example 2 in Table 1.
(Comparative Example 3)
A hydrogen storage alloy was produced by the same method as in Examples except that the heat treatment temperature was set to 1050 ° C. in the composition of Comparative Example 3 in Table 1.
(Comparative Example 4)
A hydrogen storage alloy was produced by the same method as in Examples except that the cooling rate from 900 to 500 ° C. was lowered at 10 ° C./min in the composition of Comparative Example 4 in Table 1.
In Comparative Examples 1 to 4 above, the hydrogen storage amount (H / M), the equilibrium pressure, the difficulty of micronization, and the Young's modulus were measured by the same method as in the examples. The results are shown in Table 2.

表中の熱処理の冷却速度は900℃から500℃までの冷却速度を示す。

The cooling rate of the heat treatment in the table shows the cooling rate from 900 ° C to 500 ° C.

From Table 2 and FIG. 2, it can be seen that Young's modulus correlates with the degree of difficulty in micronization. The hydrogen storage alloy of the present invention has a Young's modulus of 120 GPa or more, which is higher than that of the hydrogen storage alloy of the comparative example (conventional), and thus shows a high degree of difficulty in micronization. Furthermore, it was confirmed that the H / M was relatively high at 0.92 or more and the equilibrium pressure was also relatively low.
From this, the hydrogen storage alloy of the present invention has a low Co content, a low raw material cost, excellent initial characteristics, excellent difficulty in pulverization, excellent life characteristics, and is an alloy suitable for a negative electrode for a nickel hydrogen battery. Was confirmed.

(1) General formula MmNiaMnbAlcCod (In the formula, Mm is a misch metal, 4.30 ≦ a ≦ 4.70, 0.25 ≦ b ≦ 0.45, 0.35 ≦ c ≦ 0.45, 0 ≦ d ≤0.14 (excluding the range of 0≤d≤0.06) 5.20≤a + b + c + d≤5.50), which is a hydrogen storage alloy powder having a CaCu5 type crystal structure.
In Mm, the total of La and Ce is in the range of 90% by mass or more and 100% by mass or less with respect to the total mass of Mm, and the single crystal phase of the particle cross section of the hydrogen storage alloy powder is measured by a nanoindenter. A hydrogen storage alloy powder having a young ratio of 120 GPa or more and 127 GPa or less.
( 2 ) When the hydrogen storage amount (H / M) is 0.85 or more and 1.00 or less, the difficulty of pulverization is 0.50 or more and 0.60 or less, and the equilibrium pressure is 0.040 MPa or more and 0.070 MPa or less. The hydrogen storage alloy powder according to (1) above, which is characterized by being present.
( 3 ) A negative electrode characterized in that the hydrogen storage alloy according to (1) or (2) above is used as a negative electrode active material.
( 4 ) A battery characterized by using the negative electrode according to ( 3 ) above.

The present inventor pays attention to the mechanical properties of the AB5 type hydrogen storage alloy having a low Co content, and the single crystal phase of the particle cross section of the hydrogen storage alloy powder (hereinafter, may be simply referred to as “hydrogen storage alloy”). It was found that the larger the young rate obtained when measuring with a nanoindenter, the more difficult it is to atomize. The Young's modulus indicates the rigidity of the alloy crystal, and it can be interpreted that the rigidity of the alloy crystal makes it difficult for the alloy crystal to crack even if hydrogen is stored. Therefore, in order to improve the difficulty of micronization and obtain excellent life characteristics as a negative electrode for a nickel-metal hydride battery, it is necessary to use the alloy crystal having a large Young's modulus.
Specifically, when the Young's modulus obtained when the single crystal phase of the particle cross section of the hydrogen storage alloy is measured by a nanoindenter is 120 GPa or more, the alloy is excellent as a negative electrode for a nickel hydrogen battery. If it is less than 120 GPa, cracking of the alloy crystal tends to proceed when hydrogen storage and release are repeated, that is, the difficulty of micronization decreases and sufficient characteristics cannot be obtained as a negative electrode for a nickel hydrogen battery. If the Young's modulus becomes large, the difficulty of pulverization becomes better, but if the Young's modulus becomes too large, the processability of the alloy deteriorates and it takes too much time to pulverize the alloy that can be used as the negative electrode for nickel-metal hydride batteries. Therefore, the upper limit of the Young's modulus was 127 GPa.

Claims (5)

General formula _{MmNi a} Mn _b Al _c Cod (In the formula, Mm is a misch metal, 4.30 ≦ a ≦ 4.70, 0.25 ≦ b ≦ 0.45, 0.35 ≦ c ≦ 0.45 , 0 ≦ d ≦ 0.14, 5.20 ≦ a + b + c + d ≦ 5.50), which is a hydrogen storage alloy having a CaCu5 type crystal structure.
In Mm, the total of La and Ce is in the range of 90% by mass or more and 100% by mass or less with respect to the total mass of Mm, and the single crystal phase of the particle cross section of the hydrogen storage alloy after heat treatment is subjected to nanoindenter. A hydrogen storage alloy characterized in that the young ratio obtained when measured is 120 GPa or more and 127 GPa or less. The hydrogen storage alloy powder according to claim 1, wherein d is 0.02 ≦ d ≦ 0.14. The hydrogen storage amount (H / M) is 0.85 or more and 1.00 or less, the pulverization difficulty is 0.50 or more and 0.60 or less, and the equilibrium pressure is 0.040 MPa or more and 0.070 MPa or less. The hydrogen storage alloy powder according to claim 1 or 2, wherein the hydrogen storage alloy powder is characterized. A negative electrode using the hydrogen storage alloy powder according to any one of claims 1 to 3 as a negative electrode active material. A battery according to claim 4, wherein the negative electrode is used.

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