Hydrogen-occluding alloy and method for production thereof

Abstract

A hydrogen storage alloy and its production method are disclosed, which has an extremely high effective hydrogen storage capacity in the pressure range from 0.001 to 10 MPa, and a variety of use. The alloy is principally of a body-centered cubic crystal structure, and represented by the compositional formula CraTibVcFedMeXf (M: Al etc.; X: La etc.; 30≦a≦70, 20≦b≦50, 5≦c≦20, 0≦d≦10, 0≦e≦10, and 0≦f≦10, a+b+c+d+e+f=100). The alloy contains 0.005 to 0.150 wt % of O2, and has hydrogen absorption-desorption capability of not less than 2.2% of its weight from 0 to 100° C. and from 0.001 to 10 MPa. The method includes step (a) of melting starting materials for the alloy, deoxidizing step (b) such as step (b1) of blowing Ar into the alloy melt, and casting step (c).

Description

FIELD OF ART

[0001] The present invention relates to hydrogen storage alloys that are capable of absorbing and desorbing hydrogen in a temperature range from room temperature to 100° C., and methods for producing the same. In particular, the present invention relates to hydrogen storage alloys useful for vehicle on-board or stationary hydrogen storage, and methods for producing the same.

BACKGROUND ART

[0002] Hydrogen, which reacts with oxygen to generate water without generating toxic substances, has been attracting attention as clean energy. Handling of hydrogen, however, is delicate due to its explosive reactivity with oxygen at a certain ratio. Storage alloys that absorb and store hydrogen in metals, have been noticed as being capable of safely storing more hydrogen than hydrogen cylinders.

[0003] Hydrogen storage alloys have recently been in use for production of anodes of rechargeable batteries, and their production has been sharply increasing. For keeping up with tightening of the vehicle emissions limit from the year 2004, leading vehicle manufacturers have been developing electric vehicles equipped with rechargeable batteries or proton-exchange membrane fuel cells, in which electricity is generated by methanol reforming to generate hydrogen followed by reaction with atmospheric oxygen. On such electric vehicles, hydrogen cylinders or hydrogen storage alloys are installed for supplying hydrogen in initial start-up and for compensation for load fluctuation.

[0004] Hybrid cars, which are equipped with a gasoline engine and a motor, are now on the market. Such hybrid cars employ AB5-type hydrogen storage alloys. For extending the travel distance per charge and lightening the vehicle body weight, improvement and development of alloys having a higher hydrogen storage capacity are strongly demanded.

[0005] The AB5-type hydrogen storage alloys now in general use have hydrogen storage capacities of about 1.4% of the alloy weight. As hydrogen storage alloys having higher hydrogen storage capacities than those of the AB5-type hydrogen storage alloys, Fe—Ti based alloys are conventionally known. The Fe—Ti based alloys are advantageous in relatively low price and the plateau pressure of 0.4 to 0.6 MPa at room temperature, but disadvantageous in hardness of activation. However, these alloys have hydrogen storage capacities of as high as 1.7% of the alloy weight, which is quite promising.

[0006] MgNi2 alloys are known to have a high hydrogen storage capacity. However, the operational temperature of these alloys is as high as 300° C., which is too high for general household use or home electric appliances.

[0007] As hydrogen storage alloys usable in the temperature range from room temperature to 100° C., those having a body-centered cubic structure (referred to as BCC hereinbelow) have been receiving attention. In the BCC structure, there is a space in the center of a tetrahedral or octahedral structure, in which space hydrogen is stored. The BCC alloys have been reported to have a theoretical hydrogen storage capacity of 4.0% of the alloy weight.

[0008] As BCC hydrogen storage alloys, JP-10-110225-A discloses a hydrogen absorbing alloy having a composition expressed by the general formula TixCryVz (x+y+z=100) wherein a body-centered cubic structure appears and a spinodal decomposition occurs with the exception of a Laves phase, and the structure has a regular periodical structure formed by the spinodal decomposition, and its apparent lattice constant is at least 0.2950 nm but is not greater than 0.3060 nm. JP-10-310833-A discloses a Ti—V—Cr based hydrogen storage alloy. JP-10-121180-A discloses an alloy of the formula Ti(100-a-b)—Cra—Xb (40<a<70, 0<b<20) having the BCC structure and containing Mo or W. JP-11-106859-A discloses a Ti—V—Cr based alloy to which one or more quaternary elements selected from the group consisting of Mn, Co, Ni, Zr, Nb, Hf, Ta, and Al are added, wherein the ratio of the components in atomic % is 14<Ti<60, 14<Cr<60, 9<V<60, 0<quaternary element <8 in total of 100%, and the metal structure is the BCC structure to improve the flatness of the plateau. The above-mentioned alloys have the BCC structure, but have the hydrogen storage capacities of only less than 2.5%.

[0009] Regarding hydrogen storage alloys having the BCC structure and containing Fe, JP-9-49034-A discloses a method for producing, from a Fe—V alloy as a starting material, a hydrogen storage alloy having the BCC structure composed of not less than three elements including at least V and Fe. The hydrogen storage capacity of the alloy obtained by this method, however, is below 2.5%. Japanese Patent No. 2743123 discloses a Ti—Cr—V—Fe hydrogen storage alloy, but this alloy also has the hydrogen storage capacity of not higher than 2.5%.

[0010] It has been reported that the storage capacity of hydrogen storage alloys is influenced by the oxygen content in the alloys (J. Alloys Comp. 265 (1998), p257-263). In the text of Special Public Symposium '99 (Dec. 17, 1999) of MH Riyou Kaihatsu Kenkyu Kai, it is reported that the oxygen concentration in the alloy was decreased from 1% to 0.06% by alloying a crude material produced by thermit process as a basic material containing 14 atom % of V, 1 atom % of Ni, and Nb, and other component elements and 5 atom % of misch metal (abbreviated as Mm hereinbelow) by arc melting under a reduced pressure argon atmosphere, to thereby succeed in remarkable improvement in the hydrogen storage capacity. However, the hydrogen storage capacity of the alloy system is still less than 2.0%.

[0011] Conventionally, performance of hydrogen storage alloys has been evaluated based on the maximum hydrogen storage capacity in repeated hydrogen absorption and desorption at a certain temperature, or on the hydrogen storage capacity measured from the origin taken under vacuum. However, in practical use of hydrogen storage alloys in fuel cells, what is important is not the maximum hydrogen storage capacity, but the amount of hydrogen that is involved in absorption and desorption in the pressure range from 0.001 to 10 MPa, that is, the available hydrogen (referred to as effective hydrogen storage capacity hereinbelow).

[0012] BCC alloys typically have two plateaus, and the maximum hydrogen storage capacity or the storage capacity in the first cycle of the BCC alloys containing V conventionally includes the hydrogen content in the first plateau appearing at lower pressure, which is actually not available for use. Thus the measured amount is far from the effective hydrogen storage capacity. Also in the conventional measurement from the origin taken under vacuum, the hydrogen content in an impractically lower pressure range is included in the measured amount, so that the amount thus measured is larger than the effective hydrogen storage capacity.

[0013] In sum, the hydrogen storage capacities of the BCC hydrogen storage alloys developed to date have been reported to exceed 2.5%, but these are evaluated in terms of the maximum hydrogen storage capacity, not in terms of the effective hydrogen storage capacity. Thus there is conventionally known no alloy containing not more than 20 atom % of V that has a hydrogen storage capacity exceeding 2.2% in terms of the effective hydrogen storage capacity in the pressure range from 0.001 to 10 MPa and the temperature range from room temperature to 100° C.

[0014] The BCC hydrogen storage alloys are produced by rapid quenching from the high temperature BCC region, in order to give the BCC structure to the alloy in the service temperature range. Thus alloys having a wide BCC region in the higher temperature range in the phase diagram are advantageous for production of hydrogen storage alloys. For extending the BCC region in the higher temperature range, V is added to the alloy composition, as typically in Ti—Cr—V based alloys, in which the BCC region is extended in proportion to the V content. However, V has two disadvantages in use as a principal component. One is the high cost of V metal. The higher the V content, the more expensive the resulting hydrogen storage alloy will be, and the use of the alloy may be limited. The other is the melting point of V, which is as high as 1910° C. For melting V metal in the Ti—Cr—V based alloy, the furnace temperature is raised, which causes reduction of the furnace refractory materials by a principal element Ti of the alloy. This shortens the life of the refractory materials of the melting furnace, and increases the oxygen content of the resulting alloy. Thus in the production of Ti—Cr—V based alloys, reduction in the amount of expensive V and lowering of the melting temperature are important factors.

[0015] Instead of V metal, inexpensive ferrovanadium (Fe—V) may be taken into consideration as a material of hydrogen storage alloys. However, Fe—V has a very high oxygen content of 0.5 to 1.5%, which leads to increase in the oxygen content and lowering of the hydrogen storage property of the resulting hydrogen storage alloy.

DISCLOSURE OF THE INVENTION

[0016] It is an object of the present invention to provide a hydrogen storage alloy having an extremely high effective hydrogen storage capacity in the pressure range from 0.001 to 10 MPa, and useful in a variety of use, and a method for producing such an alloy.

[0017] It is another object of the present invention to provide a method for producing a hydrogen storage alloy that enables, at a temperature lower than the melting temperature of V, ready production of a hydrogen storage alloy having an extremely high effective hydrogen storage capacity in the pressure range from 0.001 to 10 MPa and useful in a variety of use.

[0018] According to the present invention, there is provided a hydrogen storage alloy principally of a BCC crystal structure, and represented by the compositional formula CraTibVcFedMeXf, wherein M stands for one or more elements selected from the group consisting of Al, Mo, and W; X stands for one or more elements selected from the group consisting of La, Mm, Ca, and Mg; a, b, c, d, e, and f each denotes a value in atomic percent, and 30≦a≦70, 20≦b≦50, 5≦c≦20, 0≦d≦10, 0≦e≦10, and 0≦f≦10, provided that a+b+c+d+e+f=100,

[0019] wherein said alloy comprises 0.005 to 0.150 wt % of O2, and has hydrogen absorbing-desorbing capability of not less than 2.2% of its weight in a temperature range from 0 to 100° C. and in a pressure range from 0.001 to 10 MPa.

[0020] According to the present invention, there is also provided a method for producing the hydrogen storage alloy mentioned above, comprising:

[0021] melting step (a) of melting starting materials for the hydrogen storage alloy to prepare an alloy melt;

[0022] at least one deoxidizing step (b) selected from the group consisting of deoxidizing step (b1) of blowing argon gas into the alloy melt, deoxidizing step (b2) of retaining the alloy melt in the vacuum of not higher than 0.1 Pa, and deoxidizing step (b3) of adding and retaining in the alloy melt one or more elements selected from the group consisting of La, Mm, Ca, and Mg;

[0023] casting step (c) of solidifying the alloy melt; and optionally,

[0024] step (d) of retaining a solidified alloy in a temperature range from 1150 to 1450° C. for 1 to 180 minutes, followed by cooling to 400° C. or lower at a cooling rate of not lower than 100° C./sec.

PREFERRED EMBODIMENTS OF THE INVENTION

[0025] The present invention will now be discussed in detail.

[0026] The hydrogen storage alloy according to the present invention is principally of a body-centered cubic structure. “Principally” as used herein means that the alloy is of a body-centered cubic structure to the extent that no secondary phase different from BCC is clearly observed under an X-ray diffractometer.

[0027] The hydrogen storage alloy of the present invention is represented by the compositional formula CraTibVcFedMeXf, and contains a specific amount of O2. In the formula, M stands for one or more elements selected from the group consisting of Al, Mo, and W, and X stands for one or more elements selected from the group consisting of La, Mm, Ca, and Mg. a, b, c, d, e, and f each denotes a value in atomic percent, and 30≦a≦70, 20≦b≦50, 5≦c≦20, 0≦d≦10, 0≦e≦10, and 0≦f≦10, provided that a+b+c+d+e+f=100.

[0028] In the above formula, Ti, Cr, and Fe are indispensable elements for rendering the crystal structure of the alloy BCC, and must be contained at the particular ratio mentioned above.

[0029] In the formula, V is an expensive material, and if the V content exceeds 20 atom %, the price of the resulting hydrogen storage alloy will be too expensive to be marketed, whereas with the V content of less than 5 atom %, the BCC structure is hard to be obtained. If the Fe content exceeds 10 atom %, the hydrogen storage capacity of the alloy sharply drops. Thus, d representing the Fe content is preferably 1≦d≦10.

[0030] Among the elements of M in the formula, if the content of Al exceeds 10 atom %, the hydrogen storage capacity is adversely affected. Addition of up to 20 atom % of Mo or W to a Ti—Cr alloy renders the alloy BCC. However, for the Cr—Ti—V—Fe alloy of the present invention, in which a small amount of V and Fe are contained, if the content of Mo and/or W exceeds 10 atom %, the alloy is not rendered BCC, and the hydrogen storage capacity is lowered.

[0031] The component X in the formula, which is one or more elements selected from the group consisting of La, Mm, Ca, and Mg, is contained in the hydrogen storage alloy of the present invention when added as a deoxidizer during production of the alloy. X is usually added in an amount 1.5 times the amount of oxygen in the starting materials for the alloy, but if the resulting hydrogen storage alloy contains more than 10 atom % of X, the effective hydrogen storage capacity is less than 2.2%.

[0032] With the hydrogen storage alloy of the present invention, even if the ratio of M and/or X in the formula is 0, the desired effective hydrogen storage capacity may be achieved. When the present hydrogen storage alloy contains M and/or X, in other words, when 0<e≦10 and/or 0<f≦10 independently, e and f are preferably 1≦e≦10 and 1≦f≦10, independently. In other words, the hydrogen storage alloy of the present invention has three types, namely, those containing neither M or X, those containing either M or X, and those containing both M and X.

[0033] The present hydrogen storage alloy is represented by the above compositional formula, and contains O2 in an amount of not less than 0.005 wt % and not more than 0.150 wt %, preferably not less than 0.04 wt % and not more than 0.100 wt %. With the O2 content exceeding 0.150 wt %, the desired effective hydrogen storage capacity is hard to be obtained. On the other hand, the alloy containing less than 0.005 wt % of O2 is hard to be produced.

[0034] The hydrogen storage alloy of the present invention may contain inevitable components in addition to the above components, as long as the desired objects of the present invention are not impaired.

[0035] The present hydrogen storage alloy has the hydrogen absorption-desorption capability of not less than 2.2%, preferably not less than 2.4% of its weight in the temperature range from 0 to 100° C. and in the pressure range from 0.001 to 10 MPa. The upper limit of the hydrogen absorption-desorption capability is not particularly imposed, but is about 3.0%.

[0036] The present hydrogen storage alloy may preferably be produced by a method according to the present invention essentially including steps (a) to (c), and optionally step (d).

[0037] That is, the method of the present invention includes the melting step (a) of melting the starting materials for the present hydrogen storage alloy to prepare an alloy melt, at least one deoxidizing step (b) selected from the group consisting of deoxidizing step (b1) of blowing argon gas into the alloy melt, deoxidizing step (b2) of retaining the alloy melt in the vacuum of not higher than 0.1 Pa, and deoxidizing step (b3) of adding and retaining in the alloy melt one or more elements selected from the group consisting of La, Mm, Ca, and Mg, and casting step (c) of solidifying the alloy melt, and optionally step (d) of retaining the solidified alloy in a temperature range from 1150 to 1450° C. for 1 to 180 minutes, followed by cooling to 400° C. or lower at a cooling rate of not lower than 100° C./sec.

[0038] The starting materials for the hydrogen storage alloy in step (a) contain Cr, Ti, V, and Fe, and optionally component M that is one or more elements selected from the group consisting of Al, Mo, and W, and/or component X that is one or more elements selected from the group consisting of La, Mm, Ca, and Mg. The mixing ratio of the respective components may suitably be selected so that the resulting alloy has the desired composition as mentioned above.

[0039] Each of the starting materials may either be an unalloyed metal or an alloy. As an alloy, for example, a Fe—V, Cr—Ti, or Cr—V alloy having a lower melting point than that of V metal, may be used. V prepared by the thermit process for reducing the oxygen content in V metal may be used. In this case, since V prepared by this process usually contains Al, the amount of this residual Al must be included in the ratio defined in the desired composition mentioned above. Each of the starting materials may be melted in any order, either simultaneously or in several separate batches. Some of the starting materials may even be melted during the deoxidizing step (b) to be discussed later.

[0040] The starting materials for the alloy may be melted, for example, by arc melting or in a high frequency furnace. The melting is preferably performed in an argon atmosphere. The temperature for melting is not lower than the melting temperature of the starting materials, and the upper limit may preferably be 1700° C. For lowering the melting temperature, a Fe—V alloy, which has a lower melting point than that of V metal, may preferably be used. This Fe—V alloy has a high oxygen content, which causes lower hydrogen absorption-desorption capability, and is thus not suitable for producing alloys having high hydrogen absorption-desorption capability. According to the present invention, however, such an alloy may still be utilized as a starting material, since the present method includes the step for reducing the oxygen content of the objective alloy.

[0041] Step (b) includes at least one deoxidizing step selected from the group consisting of steps (b1), (b2), and (b3), and two or more of these steps may be performed.

[0042] In the deoxidizing step (b1), argon gas is blown into the alloy melt prepared instep (a) for deoxidization. For effective deoxidization, the argon gas is blown into the alloy melt preferably for longer than 10 seconds and shorter than 5 minutes. The amount of the argon gas to be blown may suitably be decided depending on the volume and amount of the alloy melt.

[0043] In the deoxidizing step (b2), the alloy melt prepared in step (a) is retained in the vacuum of not higher than 0.1 Pa for deoxidization. In the vacuum of higher than 0.1 Pa, efficient deoxidization may not be performed. The duration of the deoxidization may preferably be 1 to 5 minutes, but in view of the reactivity between the alloy melt and the crucible, the duration is preferably kept minimum.

[0044] In the deoxidizing step (b3), one or more elements selected from the group consisting of La, Mm, Ca, and Mg are added and retained in the alloy melt. When the starting materials for the alloy in step (a) contain one or more elements selected from the group consisting of La, Mm, Ca, and Mg, step (b3) may be performed simply by retaining the molten starting materials for a time period for allowing deoxidization, preferably for 1 to 5 minutes. Step (b3) may alternatively be performed, after the alloy melt is prepared, by adding to the alloy melt the desired amount of one or more elements selected from the group consisting of La, Mm, Ca, and Mg as a deoxidizer, melting, and retaining the resulting alloy melt for the desired period of time mentioned above. Here, the La, Mm, Ca, Mg, or mixtures thereof added as a deoxidizer may be or may not be included in the composition of the resulting alloy. When the resulting alloy does not contain these components, the alloy has a composition of the formula wherein the ratio of X is 0. When the resulting alloy contains these components, their amounts have to be adjusted to be in the range for X.

[0045] When step (b3) wherein the deoxidizer is added afterwards and melted is employed, it is preferred to perform this step after step (b1) and/or (b2) for effective reaction of the deoxidizer.

[0046] In the casting step (c), the alloy melt is solidified, which may be performed by a conventional casting method such as metal mold casting or strip casting. The cooling conditions may suitably be selected, but for their ready control, the strip casting may be preferred, which is also favorable to production of readily pulverizable alloy strips of 2 mm thick or less. The cooling conditions may be set, for example, to generate the BCC structure in the higher temperature range by controlling the cooling rate. However, such conditions are not necessary when step (d) to be discussed later is employed, and the cooling rate may be set at a lower rate.

[0047] When the optional step (d) is performed after the casting step (c), the alloy obtained in step (c) may be subjected to step (d) either in the as-cast form, or after suitable pulverization, homogenizing heat treatment, or aging heat treatment. When step (c) is followed by step (d), the cast alloy obtained in step (c) does not necessarily have the BCC structure, which may be generated in the following step (d).

[0048] In step (d), the alloy prepared in step (c) in the as-cast form or optionally after pulverization or various heat treatments, is retained in a temperature range from 1150 to 1450° C. for 1 to 180 minutes, preferably in a temperature range from 1200 to 1400° C. for 5 to 20 minutes, and then cooled to 400° C. or lower, preferably to around room temperature, at a cooling rate of not lower than 100° C./sec, preferably 500 to 1000° C./sec. Step (d) is particularly needed for generating the BCC structure desired of the present hydrogen storage alloy when the BCC structure is not obtained under the solidifying conditions in step (c).

[0049] The method of the present invention may optionally include other steps in addition to the above, as long as the objects of the present invention are not impaired.

[0050] The hydrogen storage alloy of the present invention is of the particular composition, principally of the BCC structure, and has the particular O2 content, which results in the high effective hydrogen storage capacity that has never been achieved by conventional alloys. Thus, the present alloy is extremely useful for the vehicle on-board purpose such as for electric vehicles and hybrid cars, as well as for the stationary hydrogen storage purpose. The method of the present invention uses the particular starting materials for the alloy and includes the deoxidizing step (b) and optionally step (d) of particular heat treatment and cooling. Thus the hydrogen storage alloy of the present invention that is useful in various purposes, may easily be produced at a temperature lower than the melting point of V.

EXAMPLES

[0051] The present invention will now be explained with reference to Examples and Comparative Examples, which do not intend to limit the present invention.

Examples 1-8 and Comparative Examples 1-2

[0052] Using V with 0.55 wt % oxygen content produced by the thermit process, a Cr—Ti—V—Fe or Cr—Ti—V—Fe—Al alloy was prepared. The obtained alloy was used as a basic component, and La, Mm, Ca, or Mg was measured out for preparing the objective composition shown in Table 1. The alloy and the measured metal in total of 20 g were each placed in a water-cooled copper mold, arc-melted in an argon atmosphere, turned up side down in the mold, and further melted. This operation was repeated three times, and La, Mm, Ca, or Mg was added to the alloy melt and retained, which is step (b3), to obtain a cast alloy.

[0053] 3 g of the cast alloy was measured out and placed in a PCT measuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.). Hydrogen absorption and desorption were repeatedly performed at 40° C. under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogen storage capacity was determined from the obtained PCT curve in the third cycle. The results are shown in Table 1.

[0054] Next, the obtained alloy was retained at 1400° C. for 10 minutes, cooled down to 300° C. at a cooling rate of 550 to 1000° C./sec, and further allowed to cool to room temperature. The composition of the resulting alloy was measured by infrared absorption with respect to the oxygen content, and by ICP atomic emission spectrochemical analysis with respect to the other elements. Further, 3 g of the obtained alloy was measured out and placed in a PCT measuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.). Hydrogen absorption and desorption were repeatedly performed at 40° C. under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogen storage capacity was determined from the obtained PCT curve in the third cycle. The ratio of the BCC phase in the alloy was measured by X-ray diffraction. The results are shown in Table 1.

[0055] It is seen from Table 1 that the alloys of the present invention exhibited the effective hydrogen storage capacities of not less than 2.2% even when the as-cast alloy had a lower capacity. On the other hand, all of the alloys of the Comparative Examples having conventional compositions exhibited the effective hydrogen storage capacities of less than 2.2%. 1 TABLE 1 Alloy Composition (atomic %) As-cast Alloy Heat-treated Alloy Basic Effective Effective Composition of Ratio of Hydrogen Ratio of Hydrogen Alloy Starting O2 Content BOC Phase Storage BOC Phase Storage Material La Mm Ca Mg (wt %) (%) Capacity (%) (%) Capacity (%) Example 1 Cr46Ti29V13Fe4A13  5 — — — 0.041 100 2.42 100 2.70 Example 2 — 5 — — 0.043 100 2.35 100 2.65 Example 3 — — 5 — 0.055 100 2.32 100 2.55 Example 4 — — — 5 0.052 100 2.12 100 2.53 Example 5 Cr48Ti32V13Fe  6 — — — 0.042 100 2.43 100 2.70 Example 6 Cr50Ti33V15Fe  1 — — — 0.042 100 2.43 100 2.70 Example 7 Cr55Ti34V6FeMo2  2 — — — 0.078 10 1.35 100 2.70 Example 8 Cr54Ti34V6Fe2Al  3 — — — 0.065 10 1.25 100 2.68 Comparative Cr53Ti27V13Fe4Al3 — — — — 0.160 100 1.70 100 2.10 Example 1 Comparative Cr43Ti28V13Fe2Al2 12 — — — 0.044 100 1.80 100 2.15 Example 2

Examples 9-15 and Comparative Examples 3-4

[0056] Fe—V with 0.55 wt % oxygen content produced by the thermit process and a Cr—Ti alloy were initially placed in a MgO crucible, melted at 1650° C., and retained under the vacuum of 0.08 MPa for 3 minutes. Then the atmosphere was replaced with argon, and pure argon was blown into the alloy melt by means of a lance. The alloy melt was retained under the vacuum of 0.08 MPa for 3 minutes again, the alloy composition was precisely adjusted, and La, Mm, Ca, or Mg was added. When reached 1680° C., the alloy melt was poured onto a copper roll rotating at 1 m/sec or 15 m/sec to produce alloy strips by strip casting. 3 g of the cast alloy was measured out and placed in a PCT measuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.). Hydrogen absorption and desorption were repeatedly performed at 40° C. under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogen storage capacity was determined from the obtained PCT curve in the third cycle. The results are shown in Table 2.

[0057] Next, the obtained alloy strips were retained at 1400° C. for 10 minutes, and water-cooled to room temperature at a cooling rate of 1000° C./sec. The composition of the basic alloy, amount of La, Mm, Ca, or Mg, and O2 content of each alloy were measured in the same way as in Examples 1 to 8. Further, 3 g of the obtained alloy was measured out and placed in a PCT measuring system (PCT-4SWIN manufactured by SUZUKI SHOKAN CO., LTD.). Hydrogen absorption and desorption were repeatedly performed at 40° C. under the hydrogen pressure of 0.01 to 5 MPa, and the effective hydrogen storage capacity was determined from the obtained PCT curve in the third cycle. The results are shown in Table 2.

[0058] It is seen from Table 2 that all of the alloys produced in Examples had the oxygen content of less than 0.1 wt %. Even though the effective hydrogen storage capacities of some of the as-cast alloys of the present invention determined from the PCT curve were less than 2.2%, the subsequent heat treatment improved the effective hydrogen storage capacities to over 2.2% in all the alloys of the present invention. 2 TABLE 2 Alloy Composition (atomic %) As-cast Alloy Heat-treated Alloy Basic Effective Effective Composition of Ratio of Hydrogen Ratio of Hydrogen Alloy Starting O2 Content BOC Phase Storage BOC Phase Storage Material La Mm Ca Mg (wt %) (%) Capacity (%) (%) Capacity (%) Example 9 Cr46Ti29V14Fe3Al3  5 — — — 0.053 100 2.40 100 2.60 Example 10 — 5 — — 0.061 100 2.20 100 2.43 Example 11 — — 5 — 0.077 100 2.20 100 2.45 Example 12 — — — 5 0.062 100 2.10 100 2.41 Example 13 Cr48Ti32V15Fe  4 — — — 0.052 100 2.40 100 2.62 Example 14 Cr55Ti34V7FeMo  2 — — — 0.075 20 1.40 100 2.60 Example 15 Cr54Ti34V7Fe2  3 — — — 0.065 20 1.25 100 2.58 Comparative Cr53Ti27V14Fe3Al3 — — — — 0.180 100 1.80 100 2.15 Example 3 Comparative Cr43Ti28V13Fe2Al2 12 — — — 0.054 100 1.70 100 2.10 Example 4

Comparative Example 5

[0059] A Cr49Ti31V15FeLa4 alloy was prepared by melting in a high frequency furnace a Fe—V alloy with 0.65 wt % oxygen content prepared by the thermit process, a Cr—V alloy, and metal Ti as main staring materials. Specifically, these starting materials were initially placed in a MgO crucible, melted at 1650° C., and poured onto a copper roll rotating at im/sec to produce alloy strips. Then the obtained alloy strips were retained at 1300° C. for 10 minutes, and rapidly quenched to room temperature, to thereby obtain an objective alloy. The obtained alloy was subjected to the measurement of the oxygen content and the determination of the PCT curve in the same way as in Examples 1-8. The results are shown in Table 3.

Example 16

[0060] The same starting materials as in Comparative Example 5 were melted at 1650° C., and retained under the vacuum of 0.06 MPa for 5 minutes. Then the atmosphere was replaced with argon, and pure argon was blown into the alloy melt by means of a lance. The alloy melt was retained under the vacuum of 0.06 MPa for 3 minutes again, and poured onto a copper roll rotating at 1 m/sec to produce alloy strips. The obtained alloy strips were retained at 1300° C. for 10 minutes, and rapidly quenched to room temperature, to thereby obtain an objective alloy. The obtained alloy was subjected to the measurement of the oxygen content and the determination of the PCT curve in the same way as in Examples 1-8. The results are shown in Table 3.

[0061] It is seen from Table 3 that even with the same alloy composition, the alloy of Comparative Example 5 produced by a conventional method exhibited a higher oxygen content and a lower effective hydrogen storage capacity, compared to those of the alloy of the present invention. 3 TABLE 3 As-cast Alloy Heat-treated Alloy Composition of Effective Effective Alloy Starting Ratio of Hydrogen Ratio of Hydrogen Material O2 Content BOC Phase Storage BOC Phase Storage (atomic %) (wt %) (%) Capacity (%) (%) Capacity (%) Example 16 Cr49Ti31V15FeLa4 0.068 100 2.12 100 2.42 Comparative Cr49Ti31V15FeLa4 0.180 100 1.85 100 2.15 Example 5

Claims

1. A hydrogen storage alloy principally of a body-centered cubic crystal structure, and represented by the compositional formula CraTibVcFedMeXf, wherein M stands for one or more elements selected from the group consisting of Al, Mo, and W; X stands for one or more elements selected from the group consisting of La, Mm (misch metal), Ca, and Mg; a, b, c, d, e, and f each denotes a value in atomic percent, and 30≦a≦70, 20≦b≦50, 5≦c≦20, 0<d≦10, 0≦e≦10, and 0≦f≦10, provided that a+b+c+d+e+f=100,

wherein said alloy comprises 0.005 to 0.150 wt % of O2, and has hydrogen absorption-desorption capability of not less than 2.2% of its weight in a temperature range from 0 to 100° C. and in a pressure range from 0.001 to 10 MPa.

2. The hydrogen storage alloy of claim 1, wherein e in the compositional formula satisfies 0<e≦10.

3. The hydrogen storage alloy of claim 1, wherein f in the compositional formula satisfies 0<f≦10.

4. The hydrogen storage alloy of claim 1, wherein e in the compositional formula satisfies 0<e≦10, and f satisfies 0<f≦10.

5. A method for producing a hydrogen storage alloy of claim 1, comprising:

melting step (a) of melting starting materials for a hydrogen storage alloy of claim 1 to prepare an alloy melt;

at least one deoxidizing step (b) selected from the group consisting of deoxidizing step (b1) of blowing argon gas into the alloy melt, deoxidizing step (b2) of retaining the alloy melt in vacuum of not higher than 0.1 Pa, and deoxidizing step (b3) of adding and retaining in the alloy melt one or more elements selected from the group consisting of La, Mm, Ca, and Mg; and

casting step (c) of solidifying the alloy melt.

6. The method of claim 5 further comprising, after step (c), step (d) of retaining a solidified alloy in a temperature range from 1150 to 1450° C. for 1 to 180 minutes, followed by cooling to 400° C. or lower at a cooling rate of not lower than 100° C./sec.

7. The method of claim 5, wherein a melting temperature in said melting step (a) is not higher than 1700° C.

8. The method of claim 5, wherein said starting materials in said melting step (a) comprise at least one of a Fe—V alloy, a Cr—Ti alloy, a Cr—V alloy, or V metal containing Al prepared by the thermit process.