JP7518045B2 - Hydrogen storage alloy - Google Patents

Description

The present invention relates to a hydrogen storage alloy having a _CaCu5 type crystal structure for use as the negative electrode of a nickel-metal hydride battery. The present invention further relates to a negative electrode using this hydrogen storage alloy and a battery using this negative electrode.

Nickel-metal hydride batteries, which use a hydrogen storage alloy in the negative electrode, were commercialized in the early 1990s and have since become widely used.

When nickel-metal hydride batteries were first commercialized, they were used as power sources for mobile phones and laptops, but they have since been gradually replaced by smaller, lighter lithium-ion batteries. Today, due to their low cost, high safety, and good balance of energy density per volume, nickel-metal hydride batteries are used in toys, small devices, and even hybrid cars.

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

Known hydrogen storage alloys include _AB5 type alloys such as _LaNi5 , _AB2 type alloys such as _{ZrV0.4Ni1.5 ,} as well as AB type, _A2B type, _AB3 type, etc. These alloys are composed of a combination of an element group (rare earth elements, Ca, Mg, Ti, Zr, V, Nb, Pt, Pd, etc.) that has a high affinity with hydrogen and plays a role in increasing the amount of hydrogen storage, and an element group (Ni, Mn, Co, Al, etc.) that has a relatively low affinity with hydrogen and a small amount of storage, but promotes the hydrogenation reaction and plays a role in lowering the reaction temperature.

Among these, _AB5 type hydrogen storage alloys having a _CaCu5 type crystal structure, for example, alloys using misch metal (hereinafter referred to as "Mm"), which is a rare earth mixture, in the A site and elements such as Ni, Mn, Co, and Al in the B site, can form a negative electrode using relatively inexpensive materials compared to alloys of other compositions.

In the _AB5 type hydrogen storage alloy, various characteristics of the negative electrode using it, such as the charge/discharge capacity, input/output characteristics, and cycle life, can be adjusted by adjusting the ratio of the B site atomic weight to the A site atomic weight (AB ratio) and the amount of part of Ni substituted with Co, Mn, Al, etc. The _AB5 type hydrogen storage alloy with such characteristics makes it possible to manufacture nickel-metal hydride storage batteries for various applications.

In order to popularize hybrid vehicles, it is necessary to reduce the manufacturing cost of nickel-metal hydride batteries and further improve the life characteristics and input/output characteristics of the negative electrode. To achieve this goal, research and development of _AB5 type hydrogen storage alloys is being actively conducted. In particular, research is being conducted on the "columnar crystals" in the alloy with the aim of maintaining and improving the life characteristics of _AB5 type hydrogen storage alloys.

For example, Patent Document 1 proposes producing a hydrogen storage alloy by melting misch metal Mm, Ni, Co, and Al metals and cooling them to form columnar crystals, with the average crystal grain size being 10 microns or less.

Patent Document 2 also proposes a hydrogen storage alloy electrode made of hydrogen storage alloy powder, 90% or more of which is columnar, using a roll quenching device that ejects molten hydrogen storage alloy onto a roll whose surface is made of a material that promotes epitaxial crystal growth.

Furthermore, Patent Document 3 proposes a hydrogen storage alloy consisting of an alloy having a composition represented by the general formula A Nia Mnb Alc Cod Me (wherein A is at least one element selected from rare earth elements including Y (yttrium), M is at least one element selected from W, Ta, Mo, Nb, In, Ga, Sn, Zn, Cr, V, Ti, Zr and Hf, 3.5≦a≦5, 0.1≦b≦1, 0≦c≦1, 0.1≦d≦1, 0<e≦0.6, 4.5≦a+b+c+d+e≦6), at least a part of which has a columnar crystal structure, the average minor axis of the columnar crystals being 5 to 30 μm, and the proportion of the columnar crystals being 70% or more by area ratio.

Japanese Patent Application Publication No. 3-188236 Japanese Patent Application Publication No. 7-245102 Japanese Patent Application Publication No. 7-268519

As mentioned above, studies have been conducted so far, but in recent years, the trading price of Co, which is a rare metal, has been rising, and in order to maintain or reduce the raw material cost of _AB5 type hydrogen storage alloys containing Co, 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 pulverized due to repeated absorption and release of hydrogen, and the life characteristics of the negative electrode tend to decrease. In order to achieve both a reduction in the Co content and the life characteristics of the negative electrode, studies have been conducted focusing on columnar crystals, but an effective solution to the problem has not been found.

Patent Document 1 proposes producing a hydrogen storage alloy by melting misch metal Mm, Ni, Co, and Al metals and cooling them to form columnar crystals with an average crystal grain size of 10 microns or less. However, there is an issue that the Co content of the hydrogen storage alloy is 0.4 to 1.4, which is still too high.

Patent Document 2 proposes a hydrogen storage alloy electrode made of hydrogen storage alloy powder in which 90% or more of the crystals are columnar, using a roll quenching device in which molten hydrogen storage alloy is sprayed onto a roll whose surface is made of a material that promotes epitaxial crystal growth. However, the document only describes that the productivity of the roll quenching method is low, and that the columnar crystal ratio is examined under a microscope, and does not disclose or suggest the specific type of crystal grains.

Patent Document 3 describes an alloy having a composition represented by the general formula A Nia Mnb Alc Cod Me (wherein A is at least one element selected from rare earth elements including Y (yttrium), M is at least one element selected from W, Ta, Mo, Nb, In, Ga, Sn, Zn, Cr, V, Ti, Zr and Hf, 3.5≦a≦5, 0.1≦b≦1, 0≦c≦1, 0.1≦d≦1, 0<e≦0.6, 4.5≦a+b+c+d+e≦6), and this alloy is at least partially composed of a column. Although a hydrogen storage alloy has been proposed that has a columnar crystal structure, with the average short diameter of the columnar crystals being 5 to 30 μm and the proportion of columnar crystals being 70% or more by area ratio, the manufacturing method is a roll quenching method, which poses productivity issues. Furthermore, the observation of the columnar crystal structure was performed by assembling a nickel-hydrogen battery, charging and discharging it for 10 cycles, disassembling the battery, removing the negative electrode, and observing it with a scanning electron microscope; it was not an observation of the structure of the hydrogen storage alloy after it was manufactured.

The present invention has been made in consideration of the above problems, and aims to suppress raw material costs by reducing the Co content in a Co-containing _AB5 type hydrogen storage alloy, and to maintain the life characteristics when used as a negative electrode active material by suppressing pulverization of the alloy due to repeated absorption and release of hydrogen.

The present inventors have conducted intensive research in order to solve the above problems, and have found that in a Co-containing _AB5 type hydrogen storage alloy, the Co content can be reduced to reduce raw material costs, while pulverization of the alloy due to repeated absorption and release of hydrogen can be suppressed, and the life characteristics can be maintained when used as a negative electrode active material by preparing a negative electrode active material from an alloy with larger crystal grains than ever before, thereby completing the present invention.

The gist of the present invention is as follows:

(1) A compound represented by the general formula MmNiaMnbAlcCod (wherein Mm is a misch metal, 4.30≦a≦4.70, 0.25≦b≦0.45, 0.25≦c≦0.45, 0≦d≦0.14, 5.20≦a+b+c+d≦5.55), having a _CaCu5 type crystal structure, in which the sum of the proportions of La and Ce in Mm is in the range of 90% by mass or more and 100% by mass or less with respect to the total Mm,
A hydrogen storage alloy characterized in that the average crystal grain size in the ingot state, as measured by electron backscatter diffraction, is 180 μm or more in terms of circle equivalent diameter (however, this does not include the cases where, when the hydrogen storage alloy powder is made into a particle size that passes through a 20 μm mesh screen, the pulverization difficulty is 0.58 and the hydrogen storage capacity is 0.92, the pulverization difficulty is 0.55 and the hydrogen storage capacity is 0.90, and the pulverization difficulty is 0.52 and the hydrogen storage capacity is 0.89).
(2) A hydrogen storage alloy as described in (1), characterized in that the ratio of the major axis to the minor axis of the crystal grains measured by electron backscatter diffraction on a plane of the alloy parallel to the crystal growth direction is 1.2 or more.
(3) A hydrogen storage alloy according to (1) or (2), characterized in that the ratio of the major axis to the minor axis of the crystal grains measured by electron backscatter diffraction on a plane perpendicular to the crystal growth direction is 2.0 or less.
(4) The hydrogen storage alloy according to any one of (1) to (3), characterized in that a segregation phase not containing La is present, the average size of the segregation phase is 0.2 times or less the average crystal grain size in terms of circle equivalent diameter, and the segregation phase is scattered at grain boundaries of the crystal grains.
(5) A hydrogen storage alloy according to any one of (1) to (4), characterized in that the pulverization difficulty index when pulverized to a size passing through a 500 μm mesh is 0.46 to 0.60 .
(6) A negative electrode, characterized in that a hydrogen storage alloy powder obtained by pulverizing the hydrogen storage alloy according to any one of (1) to (5) above is used as a negative electrode active material.
(7) A battery using the negative electrode according to (6).

According to the present invention, it is possible to obtain an AB5 type hydrogen storage alloy in which the raw material cost is reduced by reducing the Co content, and the alloy has larger crystal grains than ever before, thereby suppressing the pulverization of _the alloy due to repeated absorption and release of hydrogen.

FIG. 2 is a plot of the relationship between the average crystal grain size measured by EBSD method and the difficulty of pulverization for the hydrogen storage alloys obtained in the examples. FIG. 2 is a plot of the relationship between the ratio of the average size of segregation phases/average crystal grain size and the difficulty of pulverization for the hydrogen storage alloys obtained in the examples.

The present invention is described in detail below.

The present inventors have intensively studied a method for maintaining the pulverization resistance without decreasing the hydrogen storage capacity in an _AB5 type hydrogen storage alloy having a low Co content. As a result, they have found that by making the crystal grains larger than ever before, the pulverization resistance does not decrease even with a low Co content, and life characteristics are maintained when used in a negative electrode. Specifically, the above effect can be obtained when the average crystal grain size measured by electron backscatter diffraction (EBSD) is 180 μm or more in equivalent circle diameter.

The reason for this is merely speculation, but we believe it is as follows.
(1) By increasing the crystal grain size, the uniformity of the alloy structure and composition of the _AB5 type hydrogen storage alloy, which has a hexagonal _CaCu5 type crystal structure, is improved.
(2) Although the Co content, which is effective in suppressing pulverization, was reduced, the difficulty of pulverization was maintained. This is because the improved uniformity of the composition allowed the expansion and contraction of the crystal lattice accompanying the absorption and release of hydrogen to proceed more uniformly, thereby mitigating the stress generated in each crystal.
(3) It is considered that the growth of large crystal grains led to a decrease and uneven distribution of the segregated phase, which is difficult to absorb and release hydrogen.

The _AB5 type hydrogen storage alloy of the present invention has a parent phase of a _CaCu5 type crystal structure. The parent phase of the _CaCu5 type crystal structure is represented by the general formula MmNiaMnbAlcCod (Mm is a misch metal, 4.30≦a≦4.70, 0.25≦b≦0.45, 0.25≦c≦0.45, 0≦d≦0.14, 5.20≦a+b+c+d≦5.55).

The metal constituting the A site in the _AB5 type hydrogen storage alloy will now be described. In the present invention, La or a misch metal (Mm) in which part or all of La is a rare earth metal mixture is used as the metal constituting the A site. In Mm, La and Ce preferably occupy a ratio within a range of 90 mass% to 100 mass% relative to the total mass of Mm, more preferably 70 to 96 mass% of La and 4 to 30 mass% of Ce, and even more preferably 74 to 94 mass% of La and 6 to 26 mass% of 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. It is preferable that the molar ratio of these metals 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) 0.25≦c≦0.45
Co molar ratio (d) 0≦d≦0.14
AB ratio 5.20≦(a+b+c+d)≦5.55

In the hydrogen storage alloy of the present invention, the average crystal grain size measured by grain analysis using electron backscatter diffraction (EBSD) is 180 μm or more in equivalent circle diameter. More preferably, the average crystal grain size is 250 μm or more in equivalent circle diameter. Here, the average crystal grain size may be any size as long as it has grown to a certain size or more, and the shape of the crystal grains is not important, but the upper limit is when only one crystal grain is observed in the maximum field of view observable with an electron microscope. For example, when observed at a low magnification of 100 times (see the examples described later), the maximum diameter of the crystal grain size is 1000 μm.

In addition, in the present invention, it is more preferable that the ratio of the long diameter/short diameter of the crystal grains measured by electron backscattering diffraction on the alloy in the plane parallel to the crystal growth direction is 1.2 or more. In an alloy formed of a long diameter/short diameter = 1, that is, a circular (spherical) crystal, the stress generated by the mutual crystal growth tends to cause crystal distortion. As a result, as described above, when the crystal grains become larger, the uniformity of the composition improves and the crystals tend to be easily cracked due to the expansion and contraction of the crystal lattice accompanying the hydrogen absorption and release. However, in an alloy formed of crystals with a long diameter/short diameter > 1, the crystal distortion caused by the stress generated by the mutual crystal growth is smaller than the above. As a result, the crystals are less likely to crack even if the expansion and contraction of the crystal lattice accompanying the hydrogen absorption and release occurs. This effect is most noticeable when the long diameter/short diameter ratio is 1.2 or more. More preferably, it is 1.4 or more.

Furthermore, it is more preferable that the ratio of the major axis/minor axis of the crystal grains measured by electron backscattering diffraction on the plane perpendicular to the crystal growth direction is 2.0 or less. Large crystal grains and a small ratio of the major axis/minor axis means that the nuclei are generated and grown uniformly (equally spaced and without bias) on the mold surface, and the stress between the crystal grains caused by the crystal growth is minimized. As a result, the crystal distortion caused by the stress caused by the crystal growth is smaller. This makes it difficult for the crystal to crack even if the crystal lattice expands and contracts due to the absorption and release of hydrogen. Therefore, it is more preferable that the ratio of the major axis/minor axis is 1.5 or less, and ideally, the ratio of the major axis/minor axis is 1.

It is also preferable that a segregation phase that does not contain lanthanum (La) is present, the average size of the segregation phase is 1/5 or less of the average crystal grain size of the hydrogen storage alloy in terms of circle equivalent diameter, and the segregation phase is scattered at the grain boundaries of the crystal grains. Although it is preferable that the amount of segregation phase is small, it is practically impossible to reduce the amount of segregation phase to zero. However, by optimizing the state in which the segregation phase exists, it is possible to suppress the grain size reduction. It is preferable that the average distance between the scattered intervals of the segregation phase is 1/3 or more and 1/1 or less of the average crystal grain size of the hydrogen storage alloy.

The hydrogen storage alloy of the present invention preferably has a hydrogen storage capacity (H/M) of 0.85 to 1.00, a pulverization difficulty of 0.46 to 0.60, and a plateau pressure of 0.04 to 0.07 MPa. Here, the plateau pressure refers to the equilibrium hydrogen pressure (MPa) at H/M = 0.5 on the hydrogen release side at a measurement temperature of 45°C. By setting the plateau pressure within this range, when the hydrogen storage alloy powder is used as a negative electrode, the negative electrode has a large charge/discharge capacity, a long life characteristic, and is easy to initially activate. In addition, the effect of the hydrogen storage alloy of the present invention is to suppress pulverization of the alloy due to repeated absorption and release of hydrogen, which is determined by the pulverization difficulty. The pulverization difficulty is a value obtained by dividing the particle size of the hydrogen storage alloy powder after 10 hydrogen absorption and desorption cycles under an environment of a holding temperature of 45° C. and a hydrogen pressure adjustment of 1.82 MPa by the initial particle size of the hydrogen storage alloy powder. The reason for setting the pulverization difficulty within the above range is that if it is too high, initial activation is difficult and the input/output characteristics of the battery are reduced, and conversely, if it is too low, the life characteristics of the battery are not ensured. Furthermore, the pulverization difficulty is correlated with the hydrogen storage amount (H/M), and the higher the H/M, the lower the pulverization difficulty tends to be, and the lower the H/M, the higher the pulverization difficulty tends to be, so it is necessary to consider it in combination with H/M. The reason for setting H/M to 0.85 to 1.00 is to ensure the target charge/discharge capacity when a negative electrode for a nickel-metal hydride battery is manufactured. As described below, the hydrogen storage capacity (H/M) and plateau pressure can be measured using a PCT (hydrogen pressure-composition-isotherm) characteristic evaluation device.

The hydrogen storage alloy may be produced by any method as long as it satisfies the crystal size and other requirements of the present invention. For example, it may be produced by melting in 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, and the resultant product may be heat-treated again. The resulting hydrogen storage alloy may be crushed to a specified particle size and used as hydrogen storage alloy powder for use in the negative electrode of a nickel-metal hydride battery, or the like.

An example of the manufacturing method is shown below. After weighing out a predetermined amount, each metal raw material is mixed and melted by high-frequency heating, poured into a mold, cooled and solidified to obtain an ingot. The ingot may be heat-treated as is or after being appropriately divided. In order to obtain the large crystal size of the present invention, depending on the composition, it is preferable to use conditions that allow crystal growth (grain growth) as much as possible when pouring into a mold to form crystals. For example, it is preferable to solidify from the mold side to grow crystals (unidirectional solidification). To achieve this, it is necessary to efficiently remove heat from the mold side. For example, unidirectional solidification can be achieved by increasing the heat removal from the opposite side of the mold to delay solidification, water-cooling the mold, increasing the heat capacity of the mold, increasing the thermal conductivity of the mold, or by appropriately combining these to increase the efficiency of heat removal from the side of the mold.

Of these, regarding the thermal conductivity of the mold, it is preferable to use a mold with a thermal conductivity of 43 W/(m·°C) or more at 20°C. More preferably, a mold with a thermal conductivity of 52 W/(m·°C) or more at 20°C is used. Furthermore, by subjecting the solidified material to heat treatment, the crystals can be grown to a sufficient size even if the crystal size after solidification is insufficient. The crystal structure after solidification is more regularly arranged, so that the crystals can be grown by the subsequent heat treatment more easily. Furthermore, since the ease of crystal growth differs depending on the alloy composition, the above-mentioned means are appropriately used to produce the desired alloy in order to obtain large crystals.
Therefore, the hydrogen storage alloy of the present invention can form the desired crystal grains, crystal form, and segregation phases by appropriately selecting and combining the composition, casting conditions such as the molten metal temperature and solidification method, and heat treatment conditions.

In obtaining the hydrogen storage alloy of the present invention, the alloy is produced through a weighing process, a mixing process, a casting process, a heat treatment process, a cooling process, and a crushing process in the same manner as in the general method, except for the above. In the weighing process, each raw material of the hydrogen storage alloy is weighed so as to obtain a predetermined alloy composition. In the mixing process, the weighed raw materials are mixed. In the casting process, the mixed raw materials are charged into a high-frequency heating melting furnace, and the mixed raw materials are melted to form a molten metal. This molten metal is poured into a mold, for example, and cast at a temperature in the range of 1150°C to 1550°C (casting temperature = molten metal temperature in the crucible at the start of casting). Here, the casting temperature is preferably in the range of 1200°C to 1450°C, more preferably 1300°C to 1400°C, and even more preferably in the range of 1340°C to 1360°C. Here, in cooling by the mold, it is desirable to set the conditions such that the molten metal generates heterogeneous nuclei on the mold surface side and rapidly solidifies, and crystals easily grow from the nuclei. For this purpose, it is preferable to use a mold with a thermal conductivity of 43 W/(m·°C) or more at 20°C. It is even more preferable to use a mold with a thermal conductivity of 52 W/(m·°C) or more at 20°C.

In the heat treatment process, the alloy after casting is heat-treated at a temperature of 950°C to 1200°C in a non-oxidizing atmosphere. In the hydrogen storage alloy according to this embodiment, the heat treatment temperature is preferably 1000°C to 1150°C. The heat treatment time depends on the size of the ingot (hydrogen storage alloy piece) after casting, but is generally several hours to several dozen hours, and the time can be set so that the specified temperature is reached up to the center of the ingot. In the cooling process, the heat-treated casting is cooled. The cooling method may be natural cooling or air cooling. There is no particular restriction on the cooling rate. In the crushing process, the ingot obtained in this way is coarsely crushed and finely crushed to produce hydrogen storage alloy powder of the required particle size. For example, the ingot can be crushed to a size that passes through a 500 μm sieve to produce hydrogen storage alloy powder.

The _AB5 type hydrogen storage alloy in the present invention has a _CaCu5 type crystal structure and is represented by the general formula MmNiaMnbAlcCod (Mm is Misch metal, 4.30≦a≦4.70, 0.25≦b≦0.45, 0.25≦c≦0.45, 0.0≦d≦0.14, 5.20≦a+b+c+d≦5.55), and the molar ratio d of Co is preferably as small as possible in order to reduce raw material costs, and is set to 0≦d≦0.14. Generally, when the Co content is 0, sufficient difficulty in pulverization is not obtained, but sufficient difficulty in pulverization is obtained if the requirements of the present invention are satisfied. On the other hand, when the Co content exceeds 0.14, sufficient difficulty in pulverization is obtained, but this does not lead to reduction in raw material costs. In the present invention, Co may be contained, and it is preferable that 0.01≦d≦0.10, and more preferably 0.01≦d≦0.05.

The reason why the ratio of La, Ce and the molar ratio of Ni, Mn and Al in Mm are set as described above is that the hydrogen storage capacity (H/M) of the _AB5 type hydrogen storage alloy is set to 0.85 to 1.00 to ensure the charge/discharge capacity, the plateau pressure is set to 0.04 to 0.07 MPa to facilitate initial activation, and the plateau region in the PCT curve is set as wide as possible. In Mm, the total of La and Ce is set to a range of 90 mass% to 100 mass% with respect to the total mass of Mm, so that the hydrogen storage capacity (H/M) can be set to 0.85 to 1.00 to ensure the charge/discharge capacity. As described above, the ratio (a) of Ni is in the range of 4.30 to 4.70, but when a negative electrode is produced using the hydrogen storage alloy powder as an active material, the output characteristics are easily maintained and the life characteristics are not particularly deteriorated. As described above, the Mn ratio (b) is in the range of 0.25 to 0.45, and within this range, the difficulty of pulverizing the hydrogen storage alloy powder can be easily maintained. As described above, the Al ratio (c) is in the range of 0.25 to 0.45, and within this range, the hysteresis in the PCT characteristics is small, and the deterioration of the charge/discharge efficiency of the hydrogen storage alloy powder can be suppressed, and the decrease in the hydrogen storage capacity of the hydrogen storage alloy powder can be suppressed.

The method of measuring the crystal grain size by electron backscatter diffraction (EBSD) in the present invention will be described. In an ingot of a hydrogen storage alloy, a piece of about several mm square is sampled from the vicinity of the cooling surface, embedded in a resin with a diameter of 1 inch for observation with a scanning electron microscope (SEM), mirror-polished, and polished using colloidal silica with a particle size of 60 nm as a final finish. A secondary electron image is observed with an SEM at a magnification of about 100 to 300 times. At this time, it is preferable to select and observe a flat surface without voids or missing parts. In an arbitrary surface, in a field where an electron beam diffraction pattern called a Kikuchi pattern can be clearly observed, LaNi ₅ is selected as the crystal type, EBSD map analysis is performed, and the crystal phase is displayed in a color-coded manner. At this time, the angle for distinguishing the crystal grain boundaries is 5°. Then, an analysis of the crystal grains is performed, and the crystal grain size can be measured in terms of the circle equivalent diameter from the area of each detected crystal grain. The average crystal grain size of the present invention is the average value obtained by observing 10 fields of view and measuring each of them.
The long and short diameters of each crystal cut parallel to the crystal growth direction and observed in the same manner as above are measured, the long and short diameter ratios are calculated, and the average value is the long and short diameter ratio of the crystal grains parallel to the crystal growth direction of the present invention. Also, the long and short diameters of each crystal cut perpendicular (vertical) to the crystal growth direction and observed in the same manner as above are measured, the long and short diameter ratios are calculated, and the average value is the long and short diameter ratio of the crystal grains perpendicular to the crystal growth direction of the present invention.

The segregation phase does not contain lanthanum La, that is, it is not a _LaNi5 crystal phase, and can be distinguished from the main phase _LaNi5 phase as a portion that is not a LaNi5 crystal in EBSD analysis or a portion that does not contain La in EDS analysis. The size of the segregation phase can be determined by area analysis using energy dispersive X-ray spectroscopy (EDS) to recognize the distribution (state of existence) and individual morphology of the segregation phase, and the size of the segregation phase is calculated as a circle equivalent diameter from the morphology (area).

The method for measuring the difficulty of pulverization in the present invention will be described (this method was followed in the following examples). Using a PCT (hydrogen pressure-composition-isotherm) characteristic evaluation device, the value obtained by dividing "the particle size of the hydrogen storage alloy powder after 10 hydrogen absorption/desorption cycles under an environment of a holding temperature of 45°C and a hydrogen pressure adjustment of 1.82 MPa" by "the initial particle size of the hydrogen storage alloy powder" was used as an index of the difficulty of pulverization. (That is, the closer the difficulty of pulverization is to 1, the more difficult it is for the hydrogen storage alloy powder to be pulverized, and the closer it is to 0, the more easily the hydrogen storage alloy powder is pulverized.) In determining the difficulty of pulverization, the "initial particle size of the hydrogen storage alloy powder" refers to the average particle size _D50 measured using a particle size distribution analyzer 7997SRA manufactured by Leeds & Northrup. "The particle size of the hydrogen storage alloy powder after 10 hydrogen absorption/desorption cycles under an environment of a holding temperature of 45°C and a hydrogen pressure adjustment of 1.82 MPa" refers to the average particle size D50 measured with a particle size distribution analyzer 7997SRA manufactured by Leeds & Northrup Co. after 10 hydrogen absorption/desorption cycles under an environment of a holding temperature of 45°C and a hydrogen pressure adjustment of 1.82 MPa using a fully automatic PCT measurement device (1/2 inch straight tube sample cell, sample amount _{3 g} ) manufactured by Suzuki Shokan Co., Ltd. The activation treatment of the hydrogen storage alloy powder in the fully automatic PCT measurement device was performed under an environment of an activation temperature of 80°C and a hydrogen pressure of 1.82 MPa, and the hydrogen absorption/desorption cycle of the hydrogen storage alloy powder in the same device was performed under an environment of a holding temperature of 45°C, a hydrogen absorption pressure of 1.82 MPa, and a hydrogen desorption pressure of 0 MPa.

The present invention will be described below based on examples. Note that the present invention is not limited to these examples.

(Examples 1 to 10, Comparative Examples 1 to 3)
The metal raw materials of Ni, Mn, Al, Co, and La and Ce as Mm were weighed so as to obtain the alloy composition shown in Table 1. The raw materials were placed in a crucible in a melting furnace, which was evacuated to a vacuum, and then placed in an argon gas atmosphere. The raw materials were then heated and melted using a high-frequency heating device, and the molten metal at the temperature in Table 1 was poured into a mold with the thermal conductivity in Table 1 to perform casting, and heat treatment was performed at the temperature and time in Table 1 in an inert atmosphere to produce a hydrogen storage alloy. In casting, crystal growth was promoted when the heat capacity of the mold was excessive for the solidification of the molten metal, and heat removal by the mold was effectively performed. On the other hand, for example, in casting, if the heat capacity of the mold for the solidification of the molten metal was reduced and the thermal conductivity was also reduced, nucleation occurred from everywhere in the molten metal, and the molten metal was solidified without large crystal growth. Of course, crystal growth occurs by the subsequent heat treatment, but the degree of crystal growth varies depending on the temperature and time.

Specifically, in Examples 1 to 4, the composition contains cobalt Co, so there is a tendency for grain growth to occur relatively easily, but by casting at a high molten metal temperature and increasing the thermal conductivity of the mold, heat is removed from the mold better and non-uniform nucleation occurs more easily. Because the molten metal temperature is high, solidification does not occur immediately on the side opposite the mold (the free surface side), and solidification proceeds from the mold surface side where the nuclei are generated, resulting in grain growth. In other words, large crystal grains are already formed during casting. Furthermore, the crystal grains can be made larger by using high temperature and long time heat treatment conditions.
In addition, in Examples 5 to 10, although the composition does not contain cobalt (Co), the AB ratio is increased to make it easier for grains to grow. Compared to Examples 5 to 8, in which the thermal conductivity of the mold is low, Examples 9 to 10, in which the thermal conductivity of the mold is high and the melting temperature is high, are able to form large crystal grains for the reasons mentioned above. In Examples 5 to 8, grain growth is also possible under the heat treatment conditions of high temperature and long time.
On the other hand, in Comparative Examples 1 and 2, the composition is relatively easy for grain growth to occur, but because the mold has low thermal conductivity and the molten metal temperature is low, uniform nucleation occurs during solidification, making grain growth difficult. The subsequent heat treatment is also performed at a relatively low temperature for a short time, so grain growth does not occur. In Comparative Example 3, the composition does not contain cobalt Co and has a small AB ratio, so grain growth is difficult, and large crystal grains could not be obtained even when a mold with high thermal conductivity was used. The subsequent heat treatment is also performed at a low temperature for a short time, so grain growth does not occur.
In this manner, hydrogen storage alloys (ingots) according to Examples 1 to 10 and Comparative Examples 1 to 3 were obtained.

The obtained alloy ingots were coarsely crushed in an inert atmosphere using a crusher, then crushed in an inert atmosphere using a cutting mill, and the hydrogen storage alloy powders were measured for their PCT properties and difficulty in pulverization to obtain a particle size that passes through a 500 μm sieve (500 μm or less). The alloy compositions obtained by analyzing each of the obtained hydrogen alloy powders were consistent with those in Table 1.

(Measurement of PCT characteristics)
The hydrogen storage capacity (H/M) and plateau pressure of the obtained hydrogen storage alloy powder were measured using a PCT characteristic evaluation device.

(Measurement of Difficulty of Micronization)
The degree of difficulty in pulverization was measured using a particle size distribution analyzer and a PCT characteristic evaluation device. The details are as described above, and the results are summarized in Table 2.

(Measurement of average grain size, etc.)
A piece of about several mm square was sampled from the cooling surface side of the hydrogen storage alloy (ingot), and after embedding in resin, mirror polishing, and polishing with colloidal silica so that the parallel and perpendicular planes to the crystal growth direction could be observed, the average crystal grain size of the hydrogen storage alloy was measured using an electron backscatter diffraction analyzer (EBSD) manufactured by Oxford Instruments Ltd. attached to a scanning electron microscope (manufactured by JEOL Ltd., JSM-7900F). The observation magnification of the secondary electron image at this time was 100 times. The EBSD analysis software used was AZtec manufactured by Oxford Instruments Ltd. Table 2 shows the average crystal grain size by grain analysis of each hydrogen storage alloy obtained in Examples 1 to 10 and Comparative Examples 1 to 3, and the ratio of the major axis/minor axis of the crystal grain in the case of the parallel plane and the perpendicular plane to the crystal growth direction.

(Measurement of size of segregation phase, etc.)
The segregation phase was confirmed as described above, and the morphology of the segregation phase was observed and the size thereof was measured using an energy dispersive X-ray spectroscopy (EDS) manufactured by Oxford Instruments Ltd. attached to a scanning electron microscope (manufactured by JEOL Ltd., JSM-7900F), and the ratio of the average size of the segregation phase not containing La in terms of the circle equivalent diameter to the average crystal grain size of each hydrogen storage alloy was calculated. The results are shown in Table 2. It was confirmed that the segregation phase not containing La was scattered at the grain boundaries of the crystal grains in each hydrogen storage alloy in each of the hydrogen storage alloys according to Examples 1 to 10 and Comparative Examples 1 to 3.

As shown in Table 2, the average crystal grain size is larger in the Examples than in the Comparative Examples, and the difficulty of pulverization is higher. In detail, as shown in FIG. 1, the difficulty of pulverization tends to increase as the average crystal grain size increases, and the Comparative Examples could not obtain a difficulty of pulverization comparable to that of the Examples of the present invention. In addition, the difficulty of pulverization tends to increase as the ratio of the long axis/short axis of the crystal in a plane parallel to the crystal growth direction increases. The difficulty of pulverization tends to increase as the ratio of the long axis/short axis of the crystal in a plane perpendicular to the crystal growth direction decreases. Furthermore, with regard to the segregation phase, as shown in FIG. 2, the difficulty of pulverization tends to increase as the ratio of the segregation phase size/main crystal (mother crystal) size decreases.

As described above, according to the present invention, by increasing the size of the crystal grains and forming a hydrogen storage alloy with a specified shape, it is possible to obtain an AB5 type hydrogen storage alloy that suppresses pulverization of the alloy due to repeated absorption and release of hydrogen while suppressing raw material costs by reducing the Co content. Therefore, when used in the negative electrode of a nickel-metal hydride battery, it is possible to improve the life characteristics and input/output characteristics.

The hydrogen storage alloy of the present invention has a low Co content, low raw material costs, and excellent initial characteristics and life characteristics, making it an ideal alloy for the negative electrode of a nickel-metal hydride battery.

Claims (7)

The formula is MmNiaMnbAlcCod (wherein Mm is a misch metal, 4.30≦a≦4.70, 0.25≦b≦0.45, 0.25≦c≦0.45, 0≦d≦0.14, 5.20≦a+b+c+d≦5.55), and has a _CaCu5 type crystal structure, in which the sum of the proportions of La and Ce in Mm is in the range of 90% by mass or more and 100% by mass or less with respect to the total Mm,
A hydrogen storage alloy characterized in that the average crystal grain size in the ingot state, as measured by electron backscatter diffraction, is 180 μm or more in terms of circle equivalent diameter (however, this does not include the cases where, when the hydrogen storage alloy powder is made into a particle size that passes through a 20 μm mesh screen, the pulverization difficulty is 0.58 and the hydrogen storage capacity is 0.92, the pulverization difficulty is 0.55 and the hydrogen storage capacity is 0.90, and the pulverization difficulty is 0.52 and the hydrogen storage capacity is 0.89). The hydrogen storage alloy according to claim 1, characterized in that the ratio of the major axis/minor axis of the crystal grains measured by electron backscatter diffraction on the alloy in a plane parallel to the crystal growth direction is 1.2 or more. The hydrogen storage alloy according to claim 1 or 2, characterized in that the ratio of the major axis/minor axis of the crystal grains measured by electron backscatter diffraction on the plane perpendicular to the crystal growth direction is 2.0 or less. The hydrogen storage alloy according to any one of claims 1 to 3, characterized in that a segregation phase not containing La is present, the average size of the segregation phase is 0.2 times or less the average crystal grain size in terms of circle equivalent diameter, and the segregation phase is scattered at the grain boundaries of the crystal grains. 5. The hydrogen storage alloy according to claim 1, wherein the difficulty of pulverization when pulverized to a size passing through a sieve with an opening of 500 μm is 0.46 to 0.60 . A negative electrode characterized in that the hydrogen storage alloy powder obtained by pulverizing the hydrogen storage alloy according to any one of claims 1 to 5 is used as a negative electrode active material. A battery using the negative electrode described in claim 6.

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