Hydrogen storage alloy, preparation process thereof, and hydrogen storage device

Abstract

The hydrogen storage alloy has, as a main phase thereof, a bcc structure phase having a composition represented by Ti.sub.xCr.sub.yV.sub.zX.sub.w wherein 3/2.ltoreq.y/x.ltoreq.3/1, 50.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, and x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe. The hydrogen storage device is a device using the alloy. The preparation process of a hydrogen storage alloy includes the steps of: melting/casting raw materials mixed to give the composition represented by Ti.sub.xCr.sub.yV.sub.zX.sub.w; heat-treating an ingot obtained in the melting/casting step; and subjecting the heat-treated ingot to a hydrogen storing/releasing treatment at least once to activate the ingot.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a hydrogen storage alloy capable of storing and releasing hydrogen in a reversible manner, a preparation process of the hydrogen storage alloy, and a hydrogen storage device using the hydrogen storage alloy.

[0002] Hydrogen energy has recently been drawing attention as a clean alternative energy in view of environmental problems such as global warming due to emission of a carbon dioxide gas or energy problems such as depletion of petroleum resources. For industrialization of the hydrogen energy, it is important to develop technologies for storing and transporting hydrogen with safety. There are some candidates for the storage method of hydrogen. Among them, a method of using a hydrogen storage material capable of storing and releasing hydrogen in a reversible manner are considered as the safest means for storing/transporting hydrogen. It is expected as a hydrogen storage medium to be installed on fuel cell cars.

[0003] As the hydrogen storage material, carbon materials such as activated carbon, fullerene, and nanotube, and hydrogen storage alloys such as LaNi.sub.5 and TiFe are known. Of these, hydrogen storage alloys are promising as hydrogen storage materials for storing/transporting hydrogen because of a high hydrogen density per unit volume compared with carbon materials.

[0004] As hydrogen storage alloys that can store/release hydrogen at about a room temperature and therefore permit easy control, and are suited for practical use, LaNi.sub.5, TiFe alloy, V--Ti--Cr alloy, and the like are known. Of these, LaNi.sub.5 and TiFe alloy have a problem that a hydrogen storage amount per weight is small. On the other hand, the V--Ti--Cr alloy is characterized in that it can store a larger amount of hydrogen per weight than the above two.

[0005] There have been various proposals on the V--Ti--Cr alloy.

[0006] For example, Patent Document 1 discloses a Ti.sub.xCr.sub.yV.sub.z alloy (x=from 5 to 70, y=from 20 to 70, z=from 10 to 30) and a Ti.sub.25Cr.sub.35V.sub.40 alloy having a regular periodic structure formed by spinodal decomposition.

[0007] According to the document,

[0008] (1) the Ti.sub.xCr.sub.yV.sub.z alloy has a maximum hydrogen absorption/release amount (1.4H/M) when the apparent lattice constant of two phases formed by spinodal decomposition is around 0.3040 nm; and

[0009] (2) the Ti.sub.25Cr.sub.35V.sub.40 alloy after heat-treatment has a hydrogen storage amount (maximum hydrogen storage amount) of about 3.7 wt %.

[0010] Patent Document 2 discloses a Ti.sub.28.3Cr.sub.50.3V.sub.19.2Fe.sub.1.7Al.sub.0.5 alloy, a Ti.sub.30Cr.sub.50V.sub.20 alloy, a Ti.sub.30Cr.sub.50V.sub.19Cu.sub.1 alloy, a Ti.sub.25Cr.sub.50V.sub.20Fe.sub.4Ni.sub.1 alloy, and a Ti.sub.25Cr.sub.55V.sub.5Mo.sub.10Fe.sub.5 alloy.

[0011] According to the document,

[0012] (1) when the alloy after homogenization heat-treatment is cooled at a cooling rate of from 10 to 200.degree. C./hour to a mixed phase region of a BCC structure phase and a C15 Laves phase, the precipitation of needle-like .alpha.-Ti in crystal grain is suppressed, crystal completeness of the BCC structure is improved, and the effective hydrogen transfer amount increases, and

[0013] (2) when the alloy after homogenization heat-treatment is subjected to water quenching treatment, the effective hydrogen amount is in the range from 235 to 262 cc/g, while when the alloy after homogenization heat-treatment is slowly cooled at a predetermined cooling rate, the effective hydrogen amount is in the range from 261 to 281 cc/g.

[0014] Patent Document 3 discloses a V.sub.70Ti.sub.12Cr.sub.18 alloy, a V.sub.40Ti.sub.24Cr.sub.36 alloy, and a V.sub.60Ti.sub.16Cr.sub.24 alloy.

[0015] According to the document, when the alloy composition is optimized, hydrogen absorbed in the alloy in a low-pressure plateau region or a lower plateau region of a sloping plateau becomes unstable, making it possible to release hydrogen from these regions.

[0016] Patent Document 4 discloses a Ti.sub.15.0Cr.sub.34.7V.sub.49.8Al.sub.0.5 alloy.

[0017] The document describes that addition of Al to a Ti--Cr--V alloy improves plateau flatness.

[0018] Patent Document 5 discloses a hydrogen storage alloy available by mechanical milling of a V-10% Ti-20% Cr alloy in a hydrogen atmosphere.

[0019] According to the document, the milling treatment simultaneously achieves reduction in the particle size and homogenization of the composition. This results in a reduction in the hysteresis.

[0020] Patent Document 6 discloses a Ti.sub.20Cr.sub.45V.sub.30Mo.sub.5 alloy, a Ti.sub.25Cr.sub.50V.sub.20Mo.sub.5 alloy, a Ti.sub.25Cr.sub.40V.sub.25Mo.sub.10 alloy, and a Ti.sub.25Cr.sub.40V.sub.20Mo.sub.15 alloy.

[0021] According to the document, hydrogen release characteristics in a low-temperature region can be improved by optimizing the chemical composition and the crystal structure and lattice constant of the main phase.

[0022] In order to put a hydrogen storage alloy into practical use, the initial value of a hydrogen amount (effective hydrogen amount) that can be stored and released in a reversible manner should be high and time-dependent deterioration in the effective hydrogen amount should be less (in other words, cycle durability is excellent). Conventional alloys, however, are excellent in only one of the initial effective hydrogen amount and the cycle durability. An alloy excellent in both has not been developed yet.

[Patent Document 1] Japanese Patent Application Laid-Open No. H10-110225

[Patent Document 2] Japanese Patent Application Laid-Open No. 2004-169102

[Patent Document 3] Japanese Patent Application Laid-Open No. 2000-345273

[0023] [Patent Document 4] Japanese Patent Application Laid-Open No. H11-106859

[Patent Document 5] Japanese Patent Application Laid-Open No. 2001-11560

[Patent Document 6] Japanese Patent Application Laid-Open No. 2006-188737

SUMMARY OF THE INVENTION

[0024] An object of the invention is to provide a hydrogen storage alloy having a high initial effective hydrogen amount and excellent cycle durability; and a preparation process of the hydrogen storage alloy.

[0025] Another object of the invention is to provide a hydrogen storage device using a hydrogen storage alloy having a high initial effective hydrogen amount and excellent cycle durability.

[0026] In order to overcome the above-described problems, a hydrogen storage alloy according to the invention has, as a main phase thereof, a bcc structure phase having a composition represented by the following formula (1):

Ti.sub.xCr.sub.yV.sub.zX.sub.w (1)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe.

[0027] A preparation process of a hydrogen storage alloy according to the invention includes: a melting/casting step of melting/casting raw materials mixed to give a composition represented by the following formula (1):

Ti.sub.xCr.sub.yV.sub.zX.sub.w (1)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe; a heat-treating step of heat-treating an ingot obtained in the melting/casting step; and an activation step of activating the heat-treated ingot by subjecting the ingot to hydrogen storing/releasing treatment at least once.

[0028] A hydrogen storage device according to the invention has the hydrogen storage alloy according to the invention, a container for placing the hydrogen storage alloy therein, and a heat exchanger for controlling the temperature of the hydrogen storage alloy placed in the container.

[0029] When the Cr/Ti ratio (y/x) of a Ti--Cr--V alloy is optimized, an initial effective hydrogen amount can be increased while keeping a plateau pressure within a practically adequate range. In addition, when the amount of V is optimized, cycle durability can be improved while maintaining the initial effective hydrogen amount at a high level. Further, when the alloy contains predetermined amounts of Al, Si, and/or Fe, the plateau pressure can be raised and at the same time, cycle durability can be improved without causing a drastic decrease in the maximum hydrogen amount.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 includes X-ray diffraction patterns of alloys after heat-treatment obtained in Examples 1 to 4;

[0031] FIG. 2 is a pressure-composition isotherm of the alloy obtained in Example 1 during an initial hydrogen desorption process at a room temperature;

[0032] FIG. 3 illustrates cycle durability of each of alloys obtained in Examples 1 and 4, and Comparative Examples 1 and 3 at a room temperature (Example 1 and Comparative Example 3) and 0.degree. C. (Example 4 and Comparative Example 1 (.largecircle., .diamond-solid., .DELTA., and .box-solid. are measured values, each curve shows an approximate curve of the measured value);

[0033] FIG. 4 illustrates the relationship between a y/x ratio of a Ti.sub.xCr.sub.yV.sub.z alloy (z=75) and an initial effective hydrogen amount or cycle durability;

[0034] FIG. 5 illustrates the relationship between a y/x ratio of a Ti.sub.xCr.sub.yV.sub.z alloy (z=65) and an initial effective hydrogen amount or cycle durability;

[0035] FIG. 6 illustrates the relationship between a y/x ratio of a Ti.sub.xCr.sub.yV.sub.z alloy (z=50) and an initial effective hydrogen amount or cycle durability;

[0036] FIG. 7 illustrates the relationship between a y/x ratio of a Ti.sub.xCr.sub.yV.sub.z alloy (z=40) and an initial effective hydrogen amount or cycle durability;

[0037] FIG. 8 illustrates the relationship between z of a Ti.sub.xCr.sub.yV.sub.z alloy (y/x=1.5) and an initial effective hydrogen amount or cycle durability;

[0038] FIG. 9 illustrates the relationship between z of a Ti.sub.xCr.sub.yV.sub.z alloy (y/x=2.0) and an initial effective hydrogen amount or cycle durability; and

[0039] FIG. 10 illustrates the relationship between z of a Ti.sub.xCr.sub.yV.sub.z alloy (y/x=2.5) and an initial effective hydrogen amount or cycle durability.

DETAILED DESCRIPTION OF THE INVENTION

[0040] One embodiment of the present invention will hereinafter be described specifically.

1. Hydrogen Storage Alloy

[0041] The hydrogen storage alloy according to the invention has, as a main phase thereof; a bcc structure phase having a composition represented by the following formula (1))

Ti.sub.xCr.sub.yV.sub.zX.sub.w (1)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe;

1.1. Composition of Alloy

1.1.1. y/x

[0042] The term "maximum hydrogen amount" as used herein means the maximum hydrogen amount that can be taken out from the alloy theoretically. In addition, the term "effective hydrogen amount" as used herein means a hydrogen amount that can be stored and released in a reversible manner within a range of from 0.01 to 10 MPa.

[0043] The symbol x represents an amount (mol %) of Ti contained in the alloy. The symbol y represents an amount (mol %) of Cr contained in the alloy. The y/x means a molar ratio of the amount of Cr to the amount of Ti (Cr/Ti) in the alloy.

[0044] With a decrease in the y/x ratio (in other words, with an increase in the Ti amount), the maximum hydrogen amount increases. When the y/x ratio becomes too small, however, a plateau pressure decreases. An excessive reduction in the plateau pressure leads to a reduction in the effective hydrogen amount because pressure reduction is necessary in order to release hydrogen from the alloy. Accordingly, the y/x ratio should be 3/2 or greater. The y/x ratio is more preferably 3/1.65 or greater.

[0045] With an increase in the y/x ratio (in other words, with an increase in the Cr amount), a plateau pressure increases and release of hydrogen is facilitated. When the plateau pressure becomes too large, however, a high pressure should be applied to store hydrogen. In addition, when the y/x ratio becomes too large, the maximum hydrogen amount decreases. Accordingly, the y/x ratio should be 3/1 or less. The y/x ratio is more preferably 3/1.2 or less, more preferably 3/1.35 or less.

1.1.2. z

[0046] The symbol z represents an amount (mol %) of V contained in the alloy. With a decrease in z (in other words, with a decrease in the amount of V), the cycle durability deteriorates. In order to achieve higher cycle durability, z should be 40 mol % or greater. The z is more preferably 50 mol % or greater, still more preferably 55 mol % or greater, still more preferably 57.5 mol % or greater, still more preferably 60 mol % or greater.

[0047] On the other hand, an excessive increase in the amount of V causes a decrease in the initial amount of the effective hydrogen amount (initial effective hydrogen amount). Accordingly, z should be 75 mol % or less.

[0048] In order to obtain a hydrogen storage alloy especially excellent in the initial effective hydrogen amount, z is preferably less than 70 mol %. The z is more preferably 68 mol % or less.

[0049] On the other hand, in order to obtain a hydrogen storage alloy especially excellent in cycle durability, z preferably exceeds 70 mol % but not greater than 75 mol %. The z is more preferably 71 mol % or greater, and still more preferably 72 mol % or greater.

1.1.3. w

[0050] The symbol w represents an amount (mol %) of an element X contained in the alloy. The "X" represents any one or more elements selected from Al, Si, and Fe.

[0051] The element X is not an essential element, but addition of it can raise the plateau pressure and at the same time, improve the cycle durability without causing a drastic reduction in the maximum hydrogen amount.

[0052] When w becomes too large, on the other hand, the effective hydrogen amount decreases drastically. Accordingly, w should be 5 mol % or less. The w is more preferably 3 mol % or less, still more preferably 2 mol % or less.

1.2. Specific Example of Alloy Composition

[0053] The hydrogen storage alloy represented by the formula (1) has a relatively high initial effective hydrogen amount and relatively high cycle durability. Further optimization of the components enables to obtain a hydrogen storage alloy having improved initial effective hydrogen amount and/or cycle durability. The following are specific examples of such an improved alloy.

1.2.1. First Specific Example

[0054] A first specific example of a hydrogen storage alloy has, as a main phase thereof, a bcc structure phase having a composition represented by the following formula (2):

Ti.sub.xCr.sub.yV.sub.zX.sub.w (2)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 50.ltoreq.z.ltoreq.70 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents at least one element selected from Al, Si, and Fe.

[0055] In the formula (2), an increase in z causes an increase in the initial effective hydrogen amount. When z becomes too large, however, the initial effective hydrogen amount sometimes decreases. In the formula (2), an increase in z leads to improvement in the cycle durability.

[0056] In order to achieve both a high initial effective hydrogen amount and high cycle durability, z is preferably 55 mol % or greater. The z is more preferably 57.5 mol % or greater, and still more preferably 60 mol % or greater.

[0057] Similarly, in order to achieve both a high initial effective hydrogen amount and high cycle durability, z is preferably 68 mol % or less.

[0058] In the formula (2), with an increase in the y/x ratio, the initial effective hydrogen amount increases. When the y/x ratio becomes too large, however, the initial effective hydrogen amount decreases. Similarly in the formula (2), an increase in the y/x ratio leads to improvement in the cycle durability. However, an excessive increase in the y/x ratio sometimes causes deterioration in the cycle durability.

[0059] In order to achieve both a high initial effective hydrogen amount and high cycle durability, the y/x ratio is preferably 3/1.65 or greater.

[0060] Similarly, in order to achieve both a high initial effective hydrogen amount and high cycle durability, the y/x ratio is preferably 3/1.2 or less, and more preferably 3/1.35 or less.

[0061] Further, in the formula (2), an initial effective hydrogen amount and cycle durability can be satisfied at a high level by carrying out optimization of the y/x ratio and z in the formula (2) simultaneously. The following are preferable ranges of the y/x ratio and z.

(a) 3/2<y/x.ltoreq.3/1.2, 55.ltoreq.z<70 mol % (b) 3/2<y/x.ltoreq.3/1.2, 57.5.ltoreq.z<70 mol % (c) 3/2<y/x.ltoreq.3/1.35, 57.5.ltoreq.z<70 mol % (d) 3/1.65.ltoreq.y/x.ltoreq.3/1.35, 57.5.ltoreq.z<70 mol % (e) 3/2<y/x.ltoreq.3/1.2, 60.ltoreq.z.ltoreq.68 mol % (f) 3/2<y/x.ltoreq.3/1.35, 60.ltoreq.z.ltoreq.68 mol % (g) 3/1.65.ltoreq.y/x.ltoreq.3/1.35, 60.ltoreq.z.ltoreq.68 mol %

1.2.2. Second Specific Example

[0062] The second specific example of a hydrogen storage alloy has, as a main phase thereof, a bcc structure phase having a composition represented by the following formula (3). The hydrogen storage alloy represented by the formula (3) has an adequate initial effective hydrogen amount and high cycle durability.

Ti.sub.xCr.sub.yV.sub.zX.sub.w (3)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 70.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents at least one element selected from Al, Si, and Fe.

[0063] In the formula (3), an increase in z leads to improvement in cycle durability. An excessive increase in z however deteriorates the initial effective hydrogen amount.

[0064] In order to achieve both a high initial effective hydrogen amount and high cycle durability, z is preferably 71 mol % or greater. The z is more preferably 72 mol % or greater.

[0065] Similarly, in order to achieve both a high initial effective hydrogen amount and high cycle durability, z is preferably 74 mol % or less.

[0066] In the formula (3), the greater the y/x ratio, the greater the initial effective hydrogen amount. An excessive increase in the y/x ratio, however, deteriorates the initial effective hydrogen amount. In the formula (3), an increase in the y/x ratio leads to improvement in the cycle durability. However, an excessive increase in the y/x ratio may rather cause deterioration in the cycle durability.

[0067] In order to achieve both a high initial effective hydrogen amount and high cycle durability, the y/x ratio is preferably 3/1.65 or greater.

[0068] Similarly, in order to achieve both a high initial effective hydrogen amount and high cycle durability, the y/x ratio is preferably 3/1.2 or less, more preferably 3/1.35 or less.

[0069] Further, an initial effective hydrogen amount and cycle durability can be satisfied at a high level by simultaneously optimizing the y/x ratio and z in the formula (3).

[0070] When priority is given to the initial effective hydrogen amount, the following are preferable ranges of the y/x ratio and z.

(a) 3/2.ltoreq.y/x.ltoreq.3/1.2, 70<z.ltoreq.75 mol % (b) 3/1.65.ltoreq.y/x.ltoreq.3/1.35, 71.ltoreq.z.ltoreq.75 mol %

[0071] When priority is given to the cycle durability, on the other hand, the following are preferable ranges of the z and y/x ratio.

(a) 3/1.35.ltoreq.y/x.ltoreq.3/1, 70.ltoreq.z.ltoreq.75 mol % (b) 3/1.2.ltoreq.y/x.ltoreq.3/1, 71.ltoreq.z.ltoreq.75 mol %

1.3. bcc Structure Phase

[0072] The hydrogen storage alloy having, as a main phase thereof, a bcc structure phase represented by the formula (1) can be obtained by mixing raw materials so as to give the above-described composition and melting and casting the resulting mixture. The hydrogen storage alloy is preferably composed only of a bcc structure phase, but may contain an inevitable impurity. Examples of the inevitable impurity include pure Ti and TiCr.sub.2 (Laves phase). An inevitable impurity adversely affecting the hydrogen storage/release characteristics is preferably as small as possible.

[0073] In the invention, the term "has, as a main phase thereof, a bcc structure phase" means that the hydrogen storage alloy contains the bcc structure phase in an amount of 80 vol. % or greater. The amount of the bcc structure phase is more preferably 90 vol. % or greater.

1.4 Particle Size

[0074] The particle size of the hydrogen storage alloy has an influence on the storage/release characteristics of hydrogen. In general, an excessive decrease in the particle size of the alloy leads to an increase in the surface area thereof, which results in an increase in the surface oxidized layer and a decrease in the hydrogen storage amount. By pulverizing treatment for many hours with a view to decreasing the particle size, a strain is introduced into the hydrogen storage alloy. This results in a decrease in the hydrogen storage amount or deterioration in the plateau flatness. Accordingly, the particle size of the hydrogen storage alloy is preferably 0.1 mm or greater prior to the activation treatment.

[0075] An excessive increase in the particle size of the hydrogen storage alloy, on the other hand, decreases the surface area. This requires the activation treatment for many hours, at a high temperature, and/or at a high pressure. It also increases the frequency of the activation treatment. Accordingly, the particle size of the hydrogen storage alloy is preferably 10 mm or less prior to the activation treatment.

[0076] The term "particle size of the hydrogen storage alloy" means the size of a sieve opening to be used in a classification test with a sieve (mesh).

2. Preparation Process of Hydrogen Storage Alloy

[0077] The preparation process of a hydrogen storage alloy according to the invention has a melting/casting step, a heat-treatment step, and an activation step.

2.1. Melting/Casting Step

[0078] The melting/casting step is a step of melting/casting raw materials which have been mixed to give a composition represented by the formula (1). The details of the formula (1) have already been described above so that description on them is omitted.

Ti.sub.xCr.sub.yV.sub.zX.sub.w (1)

wherein 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe.

[0079] No particular limitation is imposed on the melting/casting method of the raw materials and various methods such as arc melting and high-frequency induction melting can be employed.

[0080] In order to prevent deterioration of alloy properties due to incorporation of a large amount of oxygen, the melting/casting of the raw materials is performed preferably in a non-oxidizing atmosphere such as inert gas atmosphere, reducing gas atmosphere, or vacuum (1.times.10.sup.-1 to 1.times.10.sup.-6 Torr (13.3 to 1.33.times.10.sup.-4 Pa)). Although no particular limitation is imposed on the melting temperature and melting time, those permitting to obtain a uniform melt are preferred.

2.2 Heat-Treatment Step

[0081] The heat-treatment step is a step of heat-treating the ingot obtained in the melting/casting step.

[0082] In general, the bcc structure phase of a TiCrV alloy is a high-temperature equilibrium phase. Heat-treatment (homogenization heat-treatment) in a high temperature region where the bcc structure phase is stable becomes necessary in order to reduce solidification segregation of each component (particularly, dendrite-like solidification segregation of Ti and V components) formed during melting/casting. The homogenization heat-treatment can increase the plateau flatness and thereby improving the storage/release characteristics of hydrogen.

[0083] Heat-treatment is performed at a temperature of preferably 1200.degree. C. or greater in order to diffuse the structural components for a short period of time and homogenizing the components.

[0084] The heat-treatment temperature is preferably not greater than the melting point of the alloy in order to suppress partial melting of the alloy. The heat-treatment temperature is more preferably a temperature lower by 20 to 100.degree. C. than the melting point of the alloy.

[0085] The longer heat-treatment time is generally preferred to achieve a sufficient homogenizing effect. Specifically, it is preferably 1 hour or greater.

[0086] Heat-treatment for too many hours, on the other hand, does not bring about an effect corresponding to an increase in the heat-treatment time and is therefore of no practical use so that heat-treatment time is preferably 24 hours or less.

[0087] The heat-treatment is conducted preferably in a non-oxidizing atmosphere such as inert gas atmosphere, reducing gas atmosphere, or vacuum (1.times.10.sup.-1 to 1.times.10.sup.-6 Torr (13.3 to 1.33.times.10.sup.-4 Pa)).

2.3 Activation Step

[0088] The activation step is a step of activating the heat-treated ingot by subjecting the ingot to hydrogen storing/releasing treatment at least once.

[0089] The activation treatment is performed by reducing the pressure while heating the ingot to a predetermined temperature and then bringing the ingot into contact with pressurized hydrogen.

[0090] An excessively low activation treatment temperature makes it difficult to store hydrogen in the ingot. The activation temperature is therefore preferably 300.degree. C. or greater.

[0091] When the activation treatment temperature becomes excessively high, there is a possibility of the structure, which has been homogenized, becoming inhomogeneous. Accordingly, the activation treatment temperature is preferably 450.degree. C. or less.

[0092] Although no particular limitation is imposed on the pressure upon pressure reduction and the hydrogen pressure upon hydrogen storage, they may be pressures at which full activation can be carried out. The pressure upon pressure reduction is usually about 1.times.10.sup.-4 Torr (about 1.33.times.10.sup.-2 Pa). The hydrogen pressure upon hydrogen storage is usually about 50 atom (5.07 MPa).

[0093] In the hydrogen storage alloy according to the invention, the activation treatment is required to be performed at least once. It is generally necessary to repeat, several times, the activation treatment for obtaining a hydrogen storage alloy, but in the hydrogen storage alloy according to the invention, it is possible to achieve sufficient activation by the activation treatment only once.

3. Hydrogen Storage Device

[0094] The hydrogen storage device according to the invention is equipped with the hydrogen storage alloy of the invention, a container, and a heat exchanger.

[0095] Details of the hydrogen storage alloy according to the invention have already been described so that the description on it is omitted here.

[0096] The container is for storing the hydrogen storage alloy therein. It is sufficient that the container can maintain the inner part thereof at a predetermined temperature and a predetermined pressure at storage/release of hydrogen.

[0097] The heat exchanger is for controlling the temperature of the hydrogen storage alloy placed in the container. In general, heat absorption or heat generation occurs upon storage/release of hydrogen. Accordingly, it is necessary to maintain the temperature of the hydrogen storage alloy within a predetermined range in order to stably store/release hydrogen. No particular limitation is imposed on the structure of the heat exchanger and heat exchangers having various structures can be used.

4. Effects of Hydrogen Storage Alloy and Preparation Process Thereof, and Hydrogen Storage Device

[0098] Optimization of the Cr/Ti ratio (y/x) in the Ti--Cr--V alloy enables to increase the initial effective hydrogen amount while maintaining the plateau pressure within a practically adequate range.

[0099] Optimization of the amount of V, on the other hand, enables to improve the cycle durability while maintaining the initial effective hydrogen amount at a high level. In addition, a relatively large amount of hydrogen can be stored/released even by the single activation treatment.

[0100] Further, addition of one or more elements selected from Al, Si, and Fe to the Ti--Cr--V alloy enables to increase the plateau pressure and improve the cycle durability without drastically reducing the maximum hydrogen amount. As a result, the effective hydrogen amount can be increased.

EXAMPLES

Examples 1 to 6, Comparative Examples 1 to 3

1. Preparation of Sample

[0101] Raw materials mixed at a predetermined ratio were arc-melted to obtain an ingot. The ingot thus obtained was heat-treated at 1300 to 1350.degree. C. in an Ar atmosphere. The alloy thus heat-treated is then subjected to activation treatment once at 300 to 450.degree. C.

2. Test Method

2.1. X-Ray Diffraction

[0102] The X-ray diffraction measurement of the alloy subjected to the heat-treatment was performed. From the resulting X-ray diffraction pattern, a lattice constant was determined.

2.2. Hydrogen Storage/Release Characteristics

[0103] The pressure-composition isotherm measurement of the alloy subjected to the activation treatment was performed for 10 cycles at a temperature from -20.degree. C. to a room temperature. Based on the initial effective hydrogen amount and the effective hydrogen amount at 10th cycle, a maintenance ratio (=(effective hydrogen amount at 10th cycle).times.100/initial effective hydrogen amount (%)) was determined.

[0104] Further, the pressure-composition isotherm measurement was performed for 50 to 100 cycles at a room temperature or 0.degree. C. to investigate a change in the effective hydrogen amount.

3. Results

[0105] Evaluation results are shown collectively in Table 1. The composition and heat-treatment conditions of each sample are also shown in Table 1.

[0106] It is apparent from Table 1 that:

[0107] (1) each of the alloys obtained in Comparative Examples 1 and 2 has a relatively large initial effective hydrogen amount but has a low maintenance ratio (cycle durability) of the effective hydrogen amount at 10th cycle;

[0108] (2) the alloy obtained in Comparative Example 3 has high cycle durability but has a small initial effective hydrogen amount; and

[0109] (3) each of the alloys obtained in Examples 1 to 6 has a large initial effective hydrogen amount and high cycle durability.

TABLE-US-00001 Heat- Effective hydrogen amount treatment Lattice Initial Amount at maintenance temp. Activation Crystal Constant Measured amount 10th cycle ratio No. Composition (.degree. C.) treatment Structure (nm) at (mass %) (mass %) (%) Example 1 V.sub.65.0Ti.sub.11.7Cr.sub.23.3 1350 400.degree. C. .times. bcc single 0.3026 r.t. 2.51 2.38 95 once phase Example 2 V.sub.64.0Ti.sub.11.7Cr.sub.23.3Si.sub.1.0 1350 400.degree. C. .times. bcc single 0.3020 0.degree. C. 2.32 2.26 97 once phase Example 3 V.sub.63.5Ti.sub.11.6Cr.sub.23.1Al.sub.1.8 1350 400.degree. C. .times. bcc single 0.3029 0.degree. C. 2.36 2.25 95 once phase Example 4 V.sub.63.9Ti.sub.11.7Cr.sub.23.4Fe.sub.1.0 1350 450.degree. C. .times. bcc single 0.3023 0.degree. C. 2.42 2.37 98 once phase Example 5 V.sub.75.0Ti.sub.8.3 Cr.sub.16.7 1300 400.degree. C. .times. bcc single -- r.t. 2.37 2.31 97 once phase Example 6 V.sub.40.0Ti.sub.20.0Cr.sub.40.0 1300 400.degree. C. .times. bcc main -- 10.degree. C. 2.54 2.29 90 once phase Comp. Ex. 1 V.sub.20.0Ti.sub.25.0Cr.sub.50.0Mo.sub.5.0 1300 300.degree. C. .times. bcc main 0.3019 0.degree. C. 2.20 1.81 82 once phase Comp. Ex. 2 V.sub.25.0Ti.sub.25.0Cr.sub.50.0 1300 300.degree. C. .times. bcc main -- -20.degree. C. 2.50 2.10 84 once phase Comp. Ex. 3 V.sub.75.0Ti.sub.10.0Cr.sub.10.0Mo.sub.5.0 1300 300.degree. C. .times. bcc main -- r.t. 1.99 1.89 95 once phase

[0110] FIG. 1 includes X-ray diffraction patterns of the alloys obtained in Examples 1 to 4. It has been confirmed from FIG. 1 that each of the alloys obtained in Examples 1 to 4 had a bcc single phase.

[0111] FIG. 2 shows a pressure-composition isotherm of the alloy obtained in Example 1 during the initial hydrogen desorption process at a room temperature. The initial effective hydrogen amount of the alloy obtained in Example 1 is 2.51 mass %.

[0112] FIG. 3 illustrates cycle durability of each of alloys obtained in Examples 1 and 4, and Comparative Examples 1 and 3 at a room temperature (Example 1 and Comparative Example 3) and 0.degree. C. (Example 4 and Comparative Example 1).

[0113] It is understood from FIG. 3 that in the alloy obtained in Comparative Example 1, a reduction amount of the effective hydrogen amount due to an increase in the number of cycles is relatively large, while in the alloys obtained in Examples 1 and 4, a reduction amount of the effective hydrogen amount due to an increase in the number of cycles is relatively small.

[0114] On the other hand, it has been found that in the alloy obtained in Comparative Example 3, a reduction amount of the effective hydrogen amount with an increase in the number of cycles is relatively small but the initial effective hydrogen amount is smaller than those of the other examples.

Example 7

1. Preparation of Sample

[0115] In a similar manner to Example 1, various Ti.sub.xCr.sub.yV.sub.z alloys different in y/x ratio and z were prepared.

2. Test Method

[0116] In a similar manner to Example 1, an initial effective hydrogen amount and an effective hydrogen amount at 10th cycle were measured at from 0 to 50.degree. C. and a maintenance ratio (cycle durability) was determined based on them.

3. Results

[0117] FIGS. 4 to 10 show initial effective hydrogen amounts and cycle durability of various Ti.sub.xCr.sub.yV.sub.z alloys. FIGS. 4 to 10 reveal the following findings:

[0118] (1) Within a range of 3/1.65.ltoreq.y/x.ltoreq.3/1, the initial effective hydrogen amount reaches the maximum value and at the same time, an increase in the y/x ratio leads to improvement in cycle durability.

[0119] (2) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1 and 65.ltoreq.z.ltoreq.75, the initial effective hydrogen amount becomes 2.2 mass % or greater and the cycle durability becomes 94% or greater.

[0120] (3) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1.2 and 50.ltoreq.z<65, the initial effective hydrogen amount becomes 2.2 mass % or greater and the cycle durability becomes 91% or greater.

[0121] (4) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1 and 50.ltoreq.z.ltoreq.75, the initial effective hydrogen amount becomes 2.2 mass % or greater and the cycle durability becomes 91% or greater.

[0122] (5) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1.2 and 40.ltoreq.z<50, the initial effective hydrogen amount becomes 1.8 mass % or greater and the cycle durability becomes 90% or greater.

[0123] (6) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1.5 and 40.ltoreq.z<50, the initial effective hydrogen amount becomes 2.3 mass % or greater and the cycle durability becomes 90% or greater.

[0124] (7) Within a range of 3/1.5.ltoreq.y/x.ltoreq.3/1.2 and 65.ltoreq.z.ltoreq.75, the initial effective hydrogen amount becomes 2.3 mass % or greater and the cycle durability becomes 95% or greater.

[0125] (8) Within a range of 3/1.5.ltoreq.y/x.ltoreq.3/1.2 and 50.ltoreq.z.ltoreq.65, the initial effective hydrogen amount becomes 2.2 mass % or greater and the cycle durability becomes 92% or greater.

[0126] (9) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1.2 and 65.ltoreq.z.ltoreq.75, the initial effective hydrogen amount becomes 2.3 mass % or greater and the cycle durability becomes 94% or greater.

[0127] (10) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1.5 and 50.ltoreq.z.ltoreq.65, the initial effective hydrogen amount becomes 2.3 mass % or greater and the cycle durability becomes 91% or greater.

[0128] (11) Within a range of 3/2<y/x.ltoreq.3/1 and 50.ltoreq.z<70 mol %, the hydrogen storage alloy has adequate cycle durability and a high initial effective hydrogen amount.

[0129] Particularly, within a range of 3/1.65.ltoreq.y/x.ltoreq.3/1.35 and 60.0.ltoreq.z.ltoreq.68 mol %, the resulting hydrogen storage alloy can achieve both cycle durability and an initial effective hydrogen amount at a high level.

[0130] (12) Within a range of 3/2.ltoreq.y/x.ltoreq.3/1 and 70<z.ltoreq.75 mol %, the hydrogen storage alloy has an adequate initial effective hydrogen amount and high cycle durability.

[0131] Particularly, within a range of 3/1.65.ltoreq.y/x.ltoreq.3/1.35 and 71.ltoreq.z.ltoreq.75 mol %, the resulting hydrogen storage alloy has a high initial effective hydrogen amount.

[0132] Within a range of 3/1.2.ltoreq.y/x.ltoreq.3/1 and 71.ltoreq.z.ltoreq.75 mol %, the resulting hydrogen storage alloy has considerably high cycle durability.

Example 8

[0133] A V.sub.40Ti.sub.18.4Cr.sub.41.6 alloy was prepared by arc melting V, Ti, and Cr and then heat-treating the resulting ingot at 1300.degree. C. in an Ar atmosphere. The X-ray diffraction analysis of the resulting alloy was performed. It revealed that the alloy had, as a main phase thereof, a BCC phase. Pressure-composition isotherm measurement of the heat-treated alloy at 0.degree. C. was performed 10 cycles. The initial effective hydrogen amount was 2.29 mass %. On the other hand, the effective hydrogen amount at the 10th cycle was 2.07 mass % (90% of the initial amount).

Comparative Example 4

[0134] A V.sub.40Ti.sub.25Cr.sub.35 alloy was prepared by arc melting V, Ti, and Cr and then heat-treating the resulting ingot at 1260.degree. C. in an Ar atmosphere. The X-ray diffraction analysis of the resulting alloy was performed. It revealed that the alloy had a BCC single phase. Pressure-composition isotherm measurement of the heat-treated alloy at 50.degree. C. was performed 10 cycles. The initial effective hydrogen amount was 2.26 mass %. On the other hand, the effective hydrogen amount at the 10th cycle was 2.00 mass % (88% of the initial amount).

Comparative Example 5

[0135] A V.sub.20Ti.sub.35Cr.sub.45 alloy was prepared by arc melting V, Ti, and Cr and then heat-treating the resulting ingot at 1350.degree. C. in an Ar atmosphere. The X-ray diffraction analysis of the resulting alloy was performed. It revealed that the alloy had, as a main phase thereof, a BCC phase. Pressure-composition isotherm measurement of the heat-treated alloy at 50.degree. C. was performed 10 cycles. The initial effective hydrogen amount was 2.17 mass %. On the other hand, the effective hydrogen amount at the 10th cycle was 1.81 mass % (84% of the initial amount).

Comparative Example 6

[0136] A V.sub.25Ti.sub.42Cr.sub.33 alloy was prepared by arc melting V, Ti, and Cr and then heat-treating the resulting ingot at 1260.degree. C. in an Ar atmosphere. The X-ray diffraction analysis of the resulting alloy was performed. It revealed that it had, as a main phase thereof, a BCC phase. Pressure-composition isotherm measurement of the heat-treated alloy at 50.degree. C. was performed 10 cycles. The initial effective hydrogen amount was 0.34 mass %. On the other hand, the effective hydrogen amount at the 10th cycle was 0.28 mass % (83% of the initial amount).

Comparative Example 7

[0137] A V.sub.21Ti.sub.50Cr.sub.29 alloy was prepared by arc melting V, Ti, and Cr and then heat-treating the resulting ingot at 1260.degree. C. in an Ar atmosphere. The X-ray diffraction analysis of the resulting alloy was performed. It revealed that the alloy had, as a main phase thereof, a BCC phase. Pressure-composition isotherm measurement of the heat-treated alloy at 50.degree. C. was performed 10 cycles. The initial effective hydrogen amount was 0.26 mass %. On the other hand, the effective hydrogen amount at the 10th cycle was 0.23 mass % (88% of the initial amount).

[0138] Although some embodiments of the invention have been described herein specifically, it is to be understood that the invention is not limited to or by these embodiments and that various changes and modifications may be effected therein without departing from the scope of the invention.

[0139] The hydrogen storage alloy and preparation process thereof according to the invention can be used, respectively, as a hydrogen storage medium to be used as a hydrogen storage unit for fuel cell system or a hydrogen storage body for chemical heat pump, actuator, or metal-hydrogen storage battery and preparation process of the hydrogen storage alloy.

Claims

1. A hydrogen storage alloy comprising, as a main phase thereof, a bcc structure phase having a composition represented by the following formula (1): Ti.sub.xCr.sub.yV.sub.zX.sub.w (1) wherein, 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, and x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe.

2. The hydrogen storage alloy according to claim 1, wherein 3/2<y/x.ltoreq.3/1 and 50.ltoreq.z<70 mol %.

3. The hydrogen storage alloy according to claim 2, wherein 55.ltoreq.z<70 mol %.

4. The hydrogen storage alloy according to claim 2, wherein 3/2<y/x.ltoreq.3/1.2.

5. The hydrogen storage alloy according to claim 2, wherein 3/2<y/x.ltoreq.3/1.2 and 55.ltoreq.z<70 mol %.

6. The hydrogen storage alloy according to claim 2, wherein 3/1.65<y/x.ltoreq.3/1.35 and 60.ltoreq.z.ltoreq.68 mol %.

7. The hydrogen storage alloy according to claim 1, wherein 3/2.ltoreq.y/x.ltoreq.3/1 and 70<z.ltoreq.75 mol %.

8. A preparation process of a hydrogen storage alloy comprising: A melting/casting step of melting/casting raw materials mixed to give a composition represented by the following formula (1): Ti.sub.xCr.sub.yV.sub.zX.sub.w (1) wherein 3/2.ltoreq.y/x.ltoreq.3/1, 40.ltoreq.z.ltoreq.75 mol %, 0.ltoreq.w.ltoreq.5 mol %, x+y+z+w=100 mol %, and X represents any one or more selected from Al, Si, and Fe; a heat-treating step of heat-treating an ingot obtained in the melting/casting step; and an activation step of activating the heat-treated ingot by subjecting the ingot to a hydrogen storing/releasing treatment at least once.

9. A hydrogen storage device, comprising: the hydrogen storage alloy according to claim 1, a container for placing the hydrogen storage alloy therein, and a heat exchanger for controlling the temperature of the hydrogen storage alloy placed in the container.