JPH10310833A - Hydrogen storage alloy excellent in durability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy, suitable for hydrogen storage and transport, heat transport and cooling system, gaseous hydrogen purification, etc., having high hydrogen occluding capacity and long-term repeated hydrogen absorbing and releasing life (hardly causing pulverization), usable at temps. in the vicinity of room temp., excellent in oxidation resistance, and capable of handling with ease in the air, and its production. SOLUTION: The hydrogen storage alloy, having a chemical composition represented by formula Tia V1-a-b Crb where the values of (a) and (b) are 0.3-0.5 and 0.1-0.45, respectively, and also having a structure in which the average crystalline grain size of the main phase is regulated to $40 μm, is produced by means of rapid solidification. The oxidation resistance of this hydrogen storage alloy can be remarkably improved by applying Ni coating to the surface of the alloy and then applying heat treatment at 400-750 deg.C or by applying Ni coating to the surface of the alloy by means of mechanical alloying to form an Ni-addition layer composed essentially of Ti-Ni compound on the surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to hydrogen having the characteristics of high hydrogen absorption (hydrogen storage capacity), low degradation of characteristics due to repeated hydrogen absorption and release, use at temperatures near room temperature, and relatively low cost. The present invention relates to an occlusion alloy and a method for producing the same. The hydrogen storage alloy of the present invention having the above characteristics is most suitable for hydrogen gas storage and hydrogen gas purification.

[0002]

2. Description of the Related Art Hydrogen gas is a cleaner energy source than conventional fossil fuels because it does not form carbon dioxide gas or nitrogen oxides during combustion. A high-pressure hydrogen gas container is often used for storage and transportation, but a large high-pressure gas container must be used to store a large amount of hydrogen gas because hydrogen gas has a low liquefaction temperature. And the disadvantage of increased weight.

Since a hydrogen storage alloy can reversibly absorb and release hydrogen gas, if a hydrogen storage alloy capable of storing a large amount of hydrogen gas is used for storing and transporting hydrogen, a large amount of hydrogen can be stored at a higher density than a high-pressure gas container. It has the advantage that it can be stored, is lighter than a high-pressure gas container that stores the same volume of hydrogen, is easy to transport, and is resistant to mechanical shock. For this reason, research and development of hydrogen storage alloys for the purpose of storing and transporting hydrogen have been conventionally performed.

Since the absorption of hydrogen by the hydrogen storage alloy is a hydrogenation reaction of the alloy and the release of hydrogen is a decomposition reaction of the hydride, heat is generated and heat is absorbed when hydrogen gas is absorbed and released. Utilizing this property, hydrogen storage alloys can also be used in heat transport systems or heating or cooling systems (eg, heat pumps).

[0005] The hydrogen absorption and desorption rate of the hydrogen storage alloy is higher than that of other gas components, and there is a difference between hydrogen and its isotope. A hydrogen storage alloy can also be used for purifying hydrogen gas from a gas containing a gas component.

In any of the above-mentioned applications, the hydrogen storage alloy is repeatedly subjected to absorption and release of hydrogen gas during use. In order to obtain a practical reaction rate in absorbing and releasing hydrogen gas, it is necessary to increase the surface area by powdering the hydrogen storage alloy. The powdered hydrogen storage alloy is filled in a predetermined container and used.

When the hydrogen storage alloy absorbs and releases hydrogen gas, in addition to the above-mentioned heat generation and heat absorption, expansion and contraction of about 10 to 20% of the volume also occurs. Therefore, if hydrogen gas is repeatedly absorbed and released during use, cracks are formed in the powder of the hydrogen storage alloy, and the powder particles are broken, resulting in pulverization into fine powder. As the pulverization progresses, the fine powder becomes clogged in the container, causing a problem that the hydrogen gas does not easily flow or the fine powder moves into the gas pipe due to the flow of the hydrogen gas. Under such circumstances, suppression of the pulverization of the alloy powder in repeated use has become a very serious problem in practical use of a hydrogen storage alloy.

[0008] At the time of manufacturing a hydrogen storage device using a hydrogen storage alloy, it is impossible to prevent the alloy from coming into contact with the atmosphere. When the hydrogen storage alloy is exposed to the atmosphere and oxidized, an oxide film is formed on the surface of the alloy, and the oxide film hinders hydrogen absorption, so that the hydrogen storage amount decreases. In particular, in V alloys, hydrogen absorption is easily hindered by oxidation. In order to be able to use the once oxidized V alloy, the hydrogen storage alloy must be heated to a high temperature in high-pressure hydrogen, and a hydrogen cylinder that can withstand such a treatment is not only expensive but also has a small size. And an increase in weight is inevitable. Therefore, improvement of the oxidation resistance of the hydrogen storage alloy is also a practically important problem.

As described above, the hydrogen storage alloy has a wide range of applications, but the hydrogen absorption amount is the most important characteristic in any application. Also, depending on the application, absorption of hydrogen at a relatively low temperature near room temperature (eg, 150 ° C or less)
It is also important that the release occurs. Furthermore, the price of the alloy is also very important in practical use in applications where a large amount of hydrogen storage alloy is used.

[0010] For example, AB ₅ type hydrogen storage alloys commercialized represented by LaNi ₅ or MmNi ₅ was most prior it is expensive, is for small secondary batteries utilize less of the hydrogen storage alloy can be used However, it is difficult to use it for applications requiring a large amount of hydrogen storage alloy, such as for hydrogen storage.

Japanese Patent Publication No. 59-38293 describes a Ti-Cr-V hydrogen storage alloy as a relatively inexpensive and high capacity hydrogen storage alloy. As an alloy production method, only the arc method is specifically described. JP-A-7-252560 also describes a hydrogen storage alloy composed of similar components. JP-A-7-268513 and JP-A-7-268514 describe similar hydrogen-absorbing alloys based on Ti-V-Ni.

Japanese Patent Application Laid-Open No. 60-190570 discloses that the influence of contamination by an impurity gas in the atmosphere can be reduced by coating copper and / or nickel metal on a hydrogen storage alloy powder by wet electroless plating. Is explained.

[0013]

The above-mentioned Ti-Cr-V-based hydrogen storage alloy and Ti-V-Ni-based alloy should theoretically have a large amount of hydrogen absorption and have excellent hydrogen storage capacity. ,
It has been experienced that it is not. this is,
It is considered that such an alloy system generates a second phase such as TiCr ₂ having a small amount of hydrogen absorption, which causes a decrease in hydrogen storage capacity.

For example, a method described in JP-B-59-38293 is disclosed.
When the Ti-Cr-V-based hydrogen storage alloy is manufactured by the arc method as described in this publication, the solidification rate is low,
As a second phase precipitate, a considerable amount of TiCr ₂ intermetallic compound having a low hydrogen storage amount is generated, and the hydrogen storage capacity is reduced.
Further, there is a problem that the alloy powder is cracked from the second phase as a starting point during repetition of hydrogen absorption / desorption, which promotes pulverization.

In the embodiment of Japanese Patent Application Laid-Open No. 7-252560, in order to reduce the above-mentioned second phase, a method of holding at a high temperature of 1200 to 1400 ° C. to form a cubic single-phase structure, and then immediately quenching by water cooling is used. Has been taken. However, in such a method, the crystal grains are coarsened during high-temperature heating and holding,
Even if the amount of the precipitates of the second phase is reduced, the strength of the material itself is weakened due to the coarsening, and it is rather easy to pulverize. In addition, since a large ingot is used for mass production on an industrial scale, a sufficient cooling rate cannot be obtained even with water cooling after heating, and coarse precipitates are formed and the hydrogen storage capacity is reduced.

The Ti-V-Ni-based hydrogen storage alloys described in JP-A-7-268513 and JP-A-268514 have a structure in which a Ti-Ni intermetallic compound is contained in a matrix composed of a Ti-V-based solid solution alloy. A three-dimensional network skeleton is formed in the field. Since such a grain boundary phase enhances the reactivity of the alloy with hydrogen, even if there is some oxidation in the base layer, hydrogen gas can be absorbed and released through the grain boundary phase. However, since a large number of second phases having a three-dimensional network structure are formed, the formation of the second phase having a low hydrogen storage capacity reduces the hydrogen storage capacity of the entire alloy, and the second phase is used as a starting point. There is a problem that pulverization occurs.

Further, when the surface of the hydrogen storage alloy is coated with Cu or Ni metal as described in JP-A-60-190570,
Since the coating itself has no hydrogen storage capacity, the amount of hydrogen absorbed is reduced by the amount of the coating.

The present invention has a high hydrogen storage capacity and a long-term repetitive hydrogen absorption / desorption life which can be applied to hydrogen storage / transport, heat transport / cooling systems, hydrogen gas purification, etc., and can be used at a temperature near room temperature. It is another object of the present invention to provide a relatively inexpensive hydrogen storage alloy which does not deteriorate its hydrogen storage characteristics even when stored in the atmosphere.

[0019]

Means for Solving the Problems The present inventors have a high hydrogen storage capacity and an excellent repetitive hydrogen absorption / release life,
As a result of conducting research on hydrogen storage alloys that can be used at relatively low temperatures near room temperature (below 150 ° C), when the Ti-Cr-V-based hydrogen storage alloy is manufactured by the rapid solidification method to reduce the crystal grain size, We found that we could solve the problem,
The present invention has been reached.

Here, the present invention is represented by the formula of Ti _a V _1-ab Cr _b , wherein the value of a is 0.3 to 0.5 and the value of b is 0.1 to 0.45
Wherein the average crystal grain size of the main phase is 40 μm or less.

The hydrogen storage alloy is represented by the following formula: Ti _a V _1-ab Cr _b , where a is 0.3 to 0.5 and b is 0.1 to 0.4.
It can be produced by melting an alloy having a chemical composition of 5 by a rapid solidification method.

In a preferred embodiment of the present invention, the hydrogen storage alloy has a Ni-added layer mainly composed of a Ti-Ni compound on the surface. This Ni-added layer is coated with Ni on the surface of the hydrogen storage alloy produced by the rapid solidification method and then heat-treated at a temperature of 400 to 750 ° C, or the surface of the hydrogen storage alloy is coated with Ni by mechanical alloying. Can be formed.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The feature of the hydrogen storage alloy of the present invention is a chemical composition represented by Ti _a V _1-ab Cr _b (where a: 0.3 to 0.5, b: 0.1 to 0.45)
And, the average crystal grain size of the main phase is as fine as 40 μm or less,
It is two points.

The main phases of this hydrogen storage alloy are Ti, V, Cr
Is a solid solution layer composed of the three elements, and the crystal form is body-centered cubic. The average crystal grain size of the fine main phase can be achieved by manufacturing the hydrogen storage alloy by a rapid solidification method such as a roll quenching method or a gas atomizing method.
For example, when the cooling rate at the time of solidification is slow as in the arc melting method, the crystal grains grow during the solidification and become coarse, so that the average crystal grain size of the main phase exceeds 40 μm.

The Ti— of the present invention that satisfies the above conditions
V-Cr-based hydrogen storage alloys absorb a very large amount of 0.1 MPa hydrogen gas near the atmospheric pressure even at a temperature range of 150 ° C or less, because the amount of the second phase such as TiCr _{2 having} a low hydrogen storage capacity is small. The amount of absorbed hydrogen is calculated as the H / M ratio (the ratio of the number of absorbed hydrogen atoms to the number of absorbed metal atoms in the alloy).
1.5 or more. In addition, since the temperature at which the equilibrium hydrogen pressure of 0.1 MPa is as low as 150 ° C or lower, it can be used near room temperature of 150 ° C or lower, and is resistant to pulverization and durable.
It becomes a hydrogen storage alloy with extremely excellent (lifetime). Further, since it does not contain a rare earth metal, the raw material cost of the alloy is not so high.

The crystal form of the precipitate of the second phase is hexagonal or cubic, which is present at the grain boundary or inside of the body-centered cubic which is the main phase of the hydrogen storage alloy of the present invention. .
First, the reason why the chemical composition of the hydrogen storage alloy of the present invention is limited as described above will be described. As can be seen from the above chemical composition formula, the amounts of the respective elements are all atomic ratios, and the total is 1.

Titanium (Ti) : As the amount of Ti increases, the lattice size of the main phase of the body-centered cubic alloy increases, and the amount of hydrogen absorbed increases.
In order to obtain a high hydrogen absorption of H / M of 1.2 or more, Ti of 0.3 or more is required, and if the amount of Ti is less than this, the hydrogen absorption becomes low. If too much titanium is added, the amount of absorbed hydrogen further increases, but the hydrogen equilibrium pressure becomes too low, making it unusable near room temperature and atmospheric pressure.

For this reason, Cr is added as an element for increasing the hydrogen equilibrium pressure. However, if the Ti content exceeds 0.5, the hydrogen equilibrium pressure cannot be increased to near atmospheric pressure even if Cr is added, and the hydrogen absorption is not increased.・ Durability (alloy life) against pulverization due to repeated release also deteriorates. From the viewpoint of the balance between the amount of hydrogen absorbed and the life of the alloy, the amount of Ti added is 0.3 to 0.
It is preferably in the range of 45, more preferably in the range of 0.3 to 0.4.

Chromium (Cr) : As the amount of added Cr increases, the amount of absorbed hydrogen increases, but the extent is not as large as that of titanium. Cr
The main purpose of the addition is to control the hydrogen equilibrium pressure. Therefore, the addition amount varies depending on the Ti addition amount, the intended use temperature, and the hydrogen equilibrium pressure. If Cr is less than 0.1, the hydrogen equilibrium pressure at room temperature becomes lower than atmospheric pressure in the case of Ti0.3, and hydrogen cannot be reversibly absorbed and released at room temperature. On the other hand,
If it exceeds 0.45, the amount of the TiCr ₂ phase precipitated as the second phase increases, and not only the hydrogen absorption amount decreases, but also the life for repeated hydrogen absorption / release decreases. The amount of Cr added should be 0.2-0.45 from the viewpoint of the balance between hydrogen absorption and alloy life.
Is more preferably in the range of 0.3 to 0.4.

Vanadium (V): In a binary alloy of Ti—Cr,
V is added because a large amount of TiCr ₂ is formed as the second phase, the hydrogen absorption amount and the alloy life are reduced, and the hydrogen equilibrium pressure is too low to use at room temperature. By adding V, a large body-centered cubic phase is obtained, and the amount of hydrogen absorption increases. The added amount of V is automatically determined by the added amounts of Ti and Cr.

Even with the Ti-Cr-V ternary chemical composition described above, the amount of hydrogen absorbed by this alloy varies depending on the manufacturing method and the average crystal grain size of the main phase, and the cooling during solidification When the speed becomes slow and the average crystal grain size of the main phase exceeds 40 μm,
Even with the same chemical composition, the amount of hydrogen absorption decreases. This is because, as the solidification rate decreases, the rate at which precipitates such as TiCr ₂ are formed as the second phase increases, and since the precipitates themselves have a small amount of hydrogen absorption, the amount of the second phase formed increases. This is because the hydrogen absorption of the alloy as a whole decreases.

When the rate at which this precipitate is formed as the second phase increases, the proportion of the main phase of the alloy phase of the body-centered cubic crystal increases.
Since the amounts of Ti and Cr decrease, not only the hydrogen absorption of the main phase decreases, but also the hydrogen absorption and
The hydrogen equilibrium pressure, which is the equilibrium gas pressure for the release reaction, decreases, and it becomes impossible to release reversibly absorbed hydrogen.

In the hydrogen storage alloy of the present invention that satisfies both of the above conditions, precipitates such as TiCr ₂ do not
Although it is inevitable to precipitate as a phase, these problems are substantially eliminated because the amount thereof is small.

Further, the hydrogen storage alloy of the present invention is less susceptible to pulverization (which can be determined by a reduction in the average powder particle diameter) caused by expansion and contraction of the alloy volume due to repeated absorption and release of hydrogen, and has a high durability. Very good at. This excellent durability is based on Mm, which is known as a rare earth hydrogen storage alloy.
Significantly better than Ni ₅ intermetallics.

However, the durability against pulverization also varies depending on the production method and the average crystal grain size of the main phase, as in the case of the amount of hydrogen absorbed. If it exceeds 40 μm, pulverization is likely to occur even with the same chemical component. As described above, when the cooling rate is reduced in this manner, the amount of generation of the second phase, TiCr ₂ , increases. Therefore, it is estimated that the pulverization is mainly caused by grain boundary fracture starting from the second phase. You.

From the above findings, in the hydrogen storage alloy of the present invention, the average crystal grain size of the main phase is limited to 40 μm or less. To further improve the resistance to pulverization (alloy life)
The average crystal grain size of the main phase is preferably 20 μm. In addition, the precipitate formed as the second phase has an average crystal grain size of 5
If it is less than μm, pulverization hardly occurs, and if it is less than 2 μm, it is hardly pulverized.

The hydrogen storage alloy of the present invention in which the average crystal grain size of the main phase is 40 μm or less can be produced by melting a molten alloy having a predetermined composition by a rapid solidification method. The rapid solidification method is not particularly limited as long as the hydrogen storage alloy of the present invention having the above average crystal grain size is obtained. In general, any solidification method that provides a cooling rate of 10 ³ ° C / sec or more may be used.

As the rapid solidification method that can be employed in the present invention,
A rotating electrode method, a method of pouring molten alloy on a rotating drum or a roll (eg, a single roll or twin roll quenching method),
A method of thin casting on a water-cooled copper plate, a gas atomizing method, and the like can be given. Of these, the rotating electrode method and the atomizing method can produce spherical powder of a hydrogen storage alloy, which eliminates the need for a pulverizing step for pulverization, and has a substantially spherical powder shape and a high packing density. This is advantageous in that: In the case of another method, the obtained hydrogen storage alloy is pulverized into powder as required. Hydrogen crushing,
Any of mechanical pulverization can be adopted, and both may be used in combination.

The hydrogen storage alloy of the present invention has an average particle size of 10 to
It is appropriate to use a powder form of about 50 μm. As a result, the surface area increases, and the hydrogen absorption / desorption reaction is promoted. If necessary, adjust the average particle size by classification.

The hydrogen storage alloy of the present invention produced by the rapid solidification method has a minute rapid cooling strain. Although this quenching strain does not cause a particularly significant adverse effect on the durability (pulverization) of the hydrogen storage alloy of the present invention, the quenching strain may be removed by heat-treating the hydrogen storage alloy if desired. This heat treatment is preferably performed in a vacuum or in an inert gas to prevent oxidation of the alloy.

The heat treatment conditions need to be set so that the average crystal grain size of the alloy main phase does not become larger than 40 μm during the heat treatment. This condition will vary depending on the average crystal grain size of the main phase of the hydrogen storage alloy produced by the rapid solidification method, but will usually be in the range of 400 to 750 ° C for 2 to 20 hours.

However, as will be described later, in order to improve the oxidation resistance of the hydrogen storage alloy of the present invention, when a Ni-added layer mainly composed of a Ti—Ni compound is formed on the alloy surface, this layer is formed. A heat treatment may be performed in the process, and the quenching strain is also removed during this heat treatment. In this case, the heat treatment only for the purpose of removing the quenching strain is not required. It is not desirable to perform the heat treatment twice on the hydrogen storage alloy of the present invention from the viewpoint of increasing the average crystal grain size.

When the hydrogen storage alloy of the present invention is left in the atmosphere, the hydrogen absorption measured at a low temperature near room temperature (eg, 70 ° C.) decreases. Even when the alloy is left in the atmosphere, it is heated to 500 ° C. in high-pressure hydrogen gas (20 atm) and reactivated, the hydrogen absorption increases, and the absorption before recovery is restored.
That is, it is considered that the surface of the alloy is oxidized when the alloy is left in the air, and the oxide film acts as an obstacle to reduce the amount of hydrogen absorbed at low temperatures.

In applications such as storage or purification of hydrogen gas, it is inevitable that the hydrogen storage alloy is exposed to the atmosphere, and the operating temperature is relatively low at 150 ° C. or less. It is desirable to improve the oxidation resistance of the hydrogen storage alloy of the present invention in order to avoid a decrease in absorption.

As a result of examining this point, Japanese Unexamined Patent Publication No.
As described in 190570, it has been found that coating the surface of the hydrogen storage alloy of the present invention with Ni improves the oxidation resistance of the alloy. However, although this technique is effective in improving oxidation resistance, Ni itself coated on the alloy surface has almost no hydrogen storage capacity, so that the amount of hydrogen absorbed per unit weight of the alloy decreases. Therefore, as a result of further study, the Ni coating layer on the alloy surface was reacted with the Ti-V-Cr alloy serving as the base material.
By changing to a Ni-added layer mainly composed of a Ti-Ni compound, this Ni-added layer has a greater hydrogen storage capacity than pure Ni, so that the hydrogen storage alloy has an oxidation resistance with almost no reduction in hydrogen absorption. It has been found that it can be applied.
Therefore, in a preferred embodiment, the hydrogen storage alloy of the present invention
The alloy surface has an additional Ni layer mainly composed of a Ti-Ni compound.

The method of coating Ni on the alloy surface may be a physical method (including a method of mixing Ni fine powder and alloy powder, a method corresponding to mechanical alloying by mixing with a ball mill or the like), and a chemical method. (E.g., electrolytic Ni plating, electroless
Ni plating), and there is no particular limitation. The coating amount of Ni varies depending on the average particle size of the powder of the hydrogen storage alloy, but is usually 1 to 20% by weight, preferably 5 to 10% by weight based on the hydrogen storage alloy. Before this Ni coating,
If necessary, the hydrogen storage alloy may be pickled with a non-oxidizing acid such as hydrofluoric acid or hydrochloric acid to remove an oxide layer on the surface of the alloy.

After the surface of the hydrogen storage alloy is coated with Ni, heat treatment is performed to cause the Ni in the surface coating to react with the Ti component in the base alloy, thereby converting the Ni layer into a Ti—Ni compound having a high hydrogen storage capacity. By changing the surface, Ni mainly composed of Ti-Ni compound
An additional layer is formed. Since the Ni-added layer takes in Cr from the base material, it has better oxidation resistance than the binary intermetallic compound of Ti-Ni.

This heat treatment is preferably performed in a vacuum or in an inert gas to prevent oxidation of the alloy.
The heat treatment conditions are set so that during the heat treatment, the main phase of the base alloy does not become coarse until the average crystal grain size of the main phase exceeds 40 μm. From this viewpoint, the heat treatment temperature is in the range of 400 to 750 ° C. If the heat treatment temperature exceeds 750 ° C., the precipitates of the second phase having an average crystal grain size are coarsened, and the amount of hydrogen absorbed is reduced, and the powder is liable to be finely divided by repeating the absorption and release of hydrogen. On the other hand, when the temperature is lower than 400 ° C., the formation reaction of the Ti—Ni compound hardly proceeds. The preferred heat treatment temperature is 450-600 ° C.

However, when Ni coating is performed by a method corresponding to mechanical alloying, for example, in a ball mill for a long time (eg, 100 to 1000 hours), the Ni
Since the coating has already reacted with Ti in the base metal alloy to form a Ni-added layer mainly composed of a Ti-Ni compound, it is not necessary to perform heat treatment for the reaction.

[0050]

[Example] To produce a test alloy, high-frequency melting (5 kg / c
h), button arc melting (button size: 250 g / ch and 50 g
/ ch), single-roll quenching using a copper roll (20 g / ch), Ar gas atomization (10 g / ch), and a rotating electrode method (500 g / ch). The raw materials used for preparing the alloy melt were sponge titanium having a purity of 99% by weight, vanadium having a purity of 98% by weight, and chromium having a purity of 99% by weight.

In a method other than gas atomization and a rotating electrode, in which powder is directly obtained, the obtained alloy is heated at 300 ° C. and 2.5 MPa.
After hydrogenation for 5 hours in hydrogen gas, mechanical pulverization,
Powdered. Each alloy powder was used by selecting a powder having a size of 100 μm or less through a sieve. A part of the gas atomized material was heat-treated in an argon atmosphere in order to increase the average crystal grain size. The method of evaluating the properties of the test alloy will be described below.

Hydrogen Gas Absorption / Desorption Characteristics The hydrogen gas absorption / desorption characteristics were measured using a Siebert's type apparatus. The test was performed by first testing the test alloy at 200 ° C and 2.5 MPa
After activating it by leaving it in hydrogen gas for 12 hours, 300
The dehydrogenation treatment was performed at a temperature of 80 ° C., and hydrogen was absorbed at a temperature of 80 ° C. Before the activation, the test alloy was pickled with a 5 vol% hydrofluoric acid aqueous solution in order to eliminate the effect of oxidation of the alloy powder surface during mechanical pulverization. The amount of hydrogen absorbed was measured by measuring the amount of hydrogen absorbed at 0.5 MPa in the release curve of the first cycle, and evaluated by the H / M value that was the ratio of the number of metal atoms constituting the alloy to the number of absorbed hydrogen atoms. / M was 1.5 or more.

The effect of pulverization due to repeated hydrogen absorption and release was determined by measuring the increase in powder having a particle size of 20 μm or less after 300 cycles of the above-described hydrogen absorption and release characteristics measurement test. evaluated. For particle size measurement, a laser diffraction type particle size distribution measuring device was used. Since there was a difference in the particle size distribution of the powder depending on the production method, the evaluation was based on the amount of particles having a particle size of 20 μm or less before the test, and the increase rate of the fine powder compared to the amount was calculated by the following formula. If the rate of increase in fine powder is 15% or less, the test passes.

[0054]

(Equation 1)

Crystal grain size test The grain size of the main phase of the alloy was measured by embedding the alloy before pulverization in an epoxy resin, polishing and then etching with a mixed acid of 0.4 vol% hydrofluoric acid and 1 vol% nitric acid. Observation was performed with a microscope, and the average value of the measurement results of 20 randomly selected crystal grains was defined as the average crystal grain size. Since the particle size of the precipitate of the second phase was fine, it was measured using an SEM (secondary electron microscope), and the average value was obtained in the same manner as above.

Oxidation resistance The oxidation resistance of the hydrogen-absorbing alloy having a Ni-added layer formed by coating the surface with Ni was evaluated by standing for one week in a constant temperature and constant humidity air atmosphere at a temperature of 25 ° C. and a humidity of 65%. Sibeltz-type hydrogen absorption
2.5 MP at 70 ° C without activation using release test equipment
The hydrogen gas absorption test of a was performed, and the reduction rate of the hydrogen absorption amount compared with the hydrogen adsorption amount of the alloy before forming the Ni additional layer was calculated by the following equation. If the rate of decrease in the amount of absorbed hydrogen is 10% or less, the test passes.

[0057]

(Equation 2)

(Example 1) This example is an example in which the performance of a hydrogen storage alloy was examined by changing the alloy composition. As a method for producing the hydrogen storage alloy, only the rapid solidification method was used. Table 1 shows the measurement results of the average crystal grain size of the main phase, the amount of absorbed hydrogen, and the rate of increase in fine powder.

[0059]

[Table 1]

As can be seen from Table 1, all of the hydrogen storage alloys of the present invention having an alloy composition within the range of the present invention have H / M
Has a high hydrogen absorption of 1.5 or more and a low pulverization rate of 15% or less, indicating that the hydrogen absorption is large and the deterioration due to repeated hydrogen absorption and release is small.

On the other hand, the alloy of Comparative Example No. 9, which contains a large amount of Ti and Cr and does not contain V, has a large amount of hydrogen absorption, but is very easily pulverized. N whose Ti and Cr contents are out of the range of the present invention
In the alloys of Comparative Examples o to 8 to 11, the pulverization is moderate but the hydrogen absorption is small.

Example 2 This example is an example in which the influence of the average crystal grain size of the main phase of the hydrogen storage alloy produced by various manufacturing methods on the performance of the hydrogen storage alloy was examined. The alloy had the same chemical composition as Ti = 0.40, V = 0.30, and Cr = 0.30. In order to examine the influence of the crystal grain size, a test alloy heat-treated after gas atomization was also prepared. Table 2 shows the test results.

[0063]

[Table 2]

As can be seen from Table 2, when a hydrogen storage alloy is produced by the rapid solidification method, an alloy having a fine structure in which the average crystal grain size of the main phase is about 20 μm or less can be obtained. When the hydrogen storage alloy having this fine structure was heat-treated, the crystal grain size became coarse. However, if the average crystal grain size of the main phase was 40 μm or less, both the hydrogen absorption and the fine powder increase rate were acceptable. However, a test alloy having an average crystal grain size of not more than 20 μm and not subjected to heat treatment after rapid solidification showed excellent characteristics with a fine powder increase rate of not more than 10%.

If the average crystal grain size of the main phase is 40 μm or less, the average crystal grain size of the second phase precipitate is also 5 μm or less.
It can be seen from Table 2 that the range of the suppression of the pulverization is preferably 2 μm or less.

On the other hand, although the button arc melting material had a relatively large hydrogen absorption amount, the fine powder increase rate was large (Nos. 5 and 6). In the case of high frequency melting material, both hydrogen absorption and fine powder increase rate were rejected (No.7). When heat treatment was performed on the gas atomized material, if the heat treatment conditions were set so that the average crystal grain size exceeded 40 μm, the rate of increase in fine powder was rejected, and when the crystal grain size was excessively large, the hydrogen absorption amount was significantly reduced.

(Embodiment 3) In this embodiment, Ti-Ni
The improvement of the oxidation resistance of the hydrogen storage alloy when a Ni-added layer mainly composed of a compound is formed will be exemplified. All of the tested hydrogen storage alloys were powders produced by the Ar gas atomizing method. The chemical composition of the alloy is Ti = 0.40, V =
The same composition of 0.30 and Cr = 0.30 was used (this is alloy A). The other example (alloy B) has Ti = 0.30, V = 0.60,
Cr = 0.10. In order to examine the influence of the crystal grain size, a test alloy heat-treated after gas atomization was also prepared.

For the Ni coating of the hydrogen storage alloy powder for forming the Ni additional layer, both a physical method and a chemical method were employed. In the physical method, Ni fine powder having a particle size of about 1 μm is used, and after blending 10% by weight with respect to the alloy powder,
Mix evenly in a mortar or mix for a long time in a ball mill. The chemical method used a commercially available electroless Ni plating solution to form an approximately 10% by weight Ni plating layer on the surface of the alloy powder. Of course, the same Ni
A plating layer can be formed.

After applying Ni coating by these methods, heat treatment is performed in an argon atmosphere, and the Ni coating layer is reacted with the alloy powder to form an alloy, whereby the alloy surface is mainly composed of a Ti-Ni compound. A Ni additional layer was formed. However, in the method of mechanically coating Ni powder with a ball mill, heat treatment was not performed because alloying of the Ni coating occurred by mechanical alloying by performing the ball mill mixing for as long as 100 hours. Further, as a comparative example, a test material simply coated with Ni without performing this heat treatment was also manufactured.

The oxidation resistance of the hydrogen-absorbing alloy powder produced by the gas atomization method, on which the Ni-containing layer was formed on the surface, was examined after standing for one week in the air under the predetermined conditions as described above. The test results were compared with the method for forming the Ni-added layer.
Coating method, lower stage is heat treatment condition), average crystal grain size of main phase, T
Presence or absence of formation of i-Ni compound phase (confirmed by X-ray diffractometer)
Are shown in Table 3.

[0071]

[Table 3]

As can be seen from Table 3, according to the present invention, Ni
Applying a coating, and forming a Ni-added layer mainly composed of a Ti-Ni compound on the alloy surface by reacting the Ni coating with the alloy component, oxidation of the hydrogen storage alloy of the present invention in the atmosphere is suppressed, Even after one week of storage, the decrease in hydrogen absorption was suppressed to 10% or less. As a result, the handling of the alloy powder in the atmosphere is simplified, and the reactivation treatment when the surface is oxidized becomes unnecessary.

On the other hand, in the comparative example, when no Ni coating was applied, the hydrogen absorption of the alloy powder after standing for one week was reduced by 30% (Test No. 5). However, even if the Ni coating is applied, if the Ni coating is not reacted with the alloy components by heat treatment or mechanical alloying, the hydrogen absorption of the alloy powder after standing for one week is reduced by 23 to 26% (Test No. 6, 7). In other words, it can be seen that the oxidation resistance is hardly improved with the Ni coating alone as compared with the case without the Ni coating. Also, if the heat treatment temperature after Ni coating is too high and the average crystal grain size of the main phase exceeds 40 μm,
Due to the effect of coarsening, the amount of hydrogen absorbed decreased significantly.

[0074]

The hydrogen storage alloy of the present invention has a large amount of hydrogen absorption and absorbs and releases hydrogen at a relatively low temperature near room temperature (eg, 150 ° C. or lower), so that it is easy to use for various applications.
Even if hydrogen absorption / release is repeated for a long period of time, it is difficult to pulverize, so that a high amount of hydrogen absorption is maintained for a long period of time (that is, it has a long life) and is relatively inexpensive.

Further, when a Ni-added layer mainly composed of a Ti-Ni compound is formed on the surface of the alloy, the oxidation resistance of the alloy is remarkably improved, and the decrease in the elemental absorption when used in the atmosphere is extremely small. Therefore, it can be easily handled in the atmosphere, and the reactivation process is not required. The hydrogen storage alloy of the present invention is most suitable for hydrogen storage and transportation, heat transportation and cooling systems, and hydrogen gas purification applications.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 27/02 101 C22C 27/02 101Z (72) Inventor Naoshi Maeda 4-33 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries Inside the corporation

Claims (5)

[Claims] 1. A chemical composition represented by the formula Ti _a V _1-ab Cr _b , wherein a has a value of 0.3 to 0.5 and b has a value of 0.1 to 0.45, and has an average crystal grain size of the main phase. Is a hydrogen storage alloy having a particle size of 40 μm or less. 2. The hydrogen storage alloy according to claim 1, having a Ni-added layer mainly composed of a Ti—Ni compound on the surface. 3. An alloy having a chemical composition represented by the formula Ti _a V _1-ab Cr _b , wherein the value of a is 0.3 to 0.5 and the value of b is 0.1 to 0.45, is produced by a rapid solidification method. Consisting of
A method for producing a hydrogen storage alloy according to claim 1. 4. An alloy having a chemical composition represented by the formula Ti _a V _1-ab Cr _{b wherein} the value of a is 0.3 to 0.5 and the value of b is 0.1 to 0.45 is produced by a rapid solidification method. 3. The method for producing a hydrogen storage alloy according to claim 2, wherein the surface of the hydrogen storage alloy is coated with Ni and then heat-treated at a temperature of 400 to 750C. 5. An alloy having a chemical composition represented by the formula Ti _a V _1-ab Cr _b , wherein the value of a is 0.3 to 0.5 and the value of b is 0.1 to 0.45, is produced by a rapid solidification method. 3. The method for producing a hydrogen storage alloy according to claim 2, wherein the surface of the hydrogen storage alloy is coated with Ni by a mechanical alloying method.