U.S. patent application number 11/633466 was filed with the patent office on 2007-06-07 for hydrogen storage alloy and producing method thereof.
This patent application is currently assigned to THE JAPAN STEEL WORKS, LTD.. Invention is credited to Hideaki Ito, Toshiki Kabutomori, Toshio Takahashi, Hitohisa Yamada.
Application Number | 20070125457 11/633466 |
Document ID | / |
Family ID | 37889592 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070125457 |
Kind Code |
A1 |
Ito; Hideaki ; et
al. |
June 7, 2007 |
Hydrogen storage alloy and producing method thereof
Abstract
To produce a hydrogen storage alloy by melting a hydrogen
storage alloy having a body-centered cubic crystal structure
without using a refractory crucible and solidifying a molten alloy
by a unidirectional solidification process. The unidirectional
solidification is carried out by a cold crucible induction melting
method at a moving speed of a solid-liquid interface in the range
of 10 to 200 mm/hr by using a water-cooled metal crucible in a
vacuum or an inert gas atmosphere.
Inventors: |
Ito; Hideaki; (Hokkaido,
JP) ; Takahashi; Toshio; (Hokkaido, JP) ;
Yamada; Hitohisa; (Hokkaido, JP) ; Kabutomori;
Toshiki; (Hokkaido, JP) |
Correspondence
Address: |
SUGHRUE-265550
2100 PENNSYLVANIA AVE. NW
WASHINGTON
DC
20037-3213
US
|
Assignee: |
THE JAPAN STEEL WORKS, LTD.
Chiyoda-ku
JP
|
Family ID: |
37889592 |
Appl. No.: |
11/633466 |
Filed: |
December 5, 2006 |
Current U.S.
Class: |
148/404 ;
420/900; 75/10.11 |
Current CPC
Class: |
C01P 2002/76 20130101;
C22C 1/00 20130101; B22D 27/045 20130101; Y02E 60/10 20130101; C30B
21/02 20130101; C30B 29/52 20130101; C22C 27/025 20130101; H01M
8/04208 20130101; Y02E 60/50 20130101; H01M 4/383 20130101; F17C
11/005 20130101; C01B 3/0031 20130101; Y02E 60/32 20130101; H01M
8/065 20130101 |
Class at
Publication: |
148/404 ;
420/900; 075/010.11 |
International
Class: |
C22C 27/02 20060101
C22C027/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 5, 2005 |
JP |
2005-350250 |
Claims
1. A method of producing a hydrogen storage alloy, comprising:
melting a hydrogen storage alloy having a body-centered cubic
crystal structure without using a refractory crucible; and
solidifying a molten alloy by a unidirectional solidification
process.
2. The method of producing a hydrogen storage alloy according to
claim 1, wherein the hydrogen storage alloy is melted in a
water-cooled metal crucible.
3. The method of producing a hydrogen storage alloy according to
claim 1, wherein the metal crucible comprises copper.
4. The method of producing a hydrogen storage alloy according to
claim 1, wherein a speed of the unidirectional solidification
process is set at a moving speed of a solid-liquid interface in a
range of 10 to 200 mm/hr.
5. The method of producing a hydrogen storage alloy according to
claim 4, wherein the speed of the unidirectional solidification
process is set at the moving speed of a solid-liquid interface in a
range of 12 to 60 mm/hr.
6. The method of producing a hydrogen storage alloy according to
claim 1, wherein the hydrogen storage alloy is melted and
unidirectionally solidified in a vacuum or an inert gas
atmosphere.
7. The method of producing a hydrogen storage alloy according to
claim 1, wherein the unidirectional solidification is carried out
by a cold crucible induction melting method.
8. The method of producing a hydrogen storage alloy according to
claim 7, wherein the unidirectional solidification is carried out
by disposing a water-cooled metallic crucible along an axial
direction inside of an induction heating coil, and moving the
metallic crucible relatively to the induction heating coil in a
direction opposite to a solidification direction.
9. A hydrogen storage alloy having a body-centered cubic structure
that is produced by the method according to claim 1.
Description
[0001] This application claims priority from Japanese Patent
Application No. 2005-350250, filed on Dec. 5, 2005, the entire
subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a hydrogen storage alloy which can
reversibly absorb and release hydrogen and is used in a hydrogen
storage tank, a hydrogen feed system for fuel cell, a hydrogen
refining and recovering system, a hydrogen absorption material for
hydrogen gas actuator and an electrode material for nickel-hydrogen
battery. This invention can also be applied to a hydrogen storage
tank that stores and feeds hydrogen in a temperature range from
room temperature to 100.degree. C.
[0004] 2. Description of the Related Art
[0005] Hitherto, when hydrogen is stored and transported, a
hydrogen cylinder in which hydrogen gas is compressed and filled or
liquid hydrogen has been used. Recently, a technology that uses a
hydrogen storage alloy is gathering attention. A hydrogen storage
alloy can store hydrogen of a volume corresponding to 1000 times a
volume thereof and can reversibly release. Accordingly, a volume
space necessary for storing a definite amount of hydrogen is
smaller in comparison with the case where a high-pressure cylinder
or liquid hydrogen is used. Accordingly, hydrogen can be compactly
stored. Furthermore, the hydrogen storage alloy can reversibly
absorb and release hydrogen at ambient temperature under pressure
of 10 atm or less. Accordingly, it can be said a hydrogen storage
method that neither necessitates cooling to a very low temperature
like liquid hydrogen nor pressurizing by use of a compressor like a
high-pressure cylinder and a material that can realize safe and
convenient storage of hydrogen. Furthermore, application systems
such as a chemical heat pump and a heat transportation system that
make use of a reaction heat at the time of absorption and release
of hydrogen and an actuator that makes use of pressure difference
at the time of absorption and release of hydrogen are considered
and some of these are put into practical use.
[0006] When the hydrogen storage alloy is applied to various
application fields, how much hydrogen can be reversibly absorbed
and released is considered most important. As the hydrogen storage
alloys that have been put into practical use, an AB.sub.5 type
alloy typical in LaNi.sub.5 and an AB.sub.2 type alloy typical in
TiMn.sub.1.5 and TiCr.sub.2 are known. However, since a reversible
absorption/release amount thereof is 2% by weight or less, a
material larger in the storage amount of hydrogen is demanded from
a system user side. On the other hand, vanadium metal, as a simple
material, can reversibly absorb and release hydrogen. In the case
of simple vanadium metal, it is necessary to vacuum at a high
temperature of 400.degree. C. or more to let absorb hydrogen at
first (activate) and furthermore to apply hydrogen pressure of
several tens atm. Accordingly, it is difficult to put into
practical use for a system that is used in room temperature and
under low pressure. In order to improve the problem, Ti--Cr--Mn
system alloys and Ti--Cr--V system alloys that have a BCC structure
that is same in the crystal structure with that of vanadium metal
have been developed. In the alloys, the maximum hydrogen storage
amount is close to 4% by weight similarly to the vanadium metal.
However, a hydrogen amount that can be reversibly absorbed and
released at ambient temperature is only 2.5% by weight. In order to
improve this, various researches and developments have been
forwarded.
[0007] The hydrogen storage alloys having the BCC structure, unlike
intermetallic compounds that have a fixed value of metal atom ratio
like existing alloys typical in LaNi.sub.5, are materials in which
constituent elements are in solid solution at an arbitrary metal
atom ratio. Usually, the hydrogen storage alloy having the BCC
structure is produced in such a manner that raw material metals are
blended and melted at a predetermined ratio, followed by casting in
a mold, further followed by applying heat treatment to an ingot to
homogenize a crystal structure and components.
[0008] Hitherto, as a method of slowly lowering a temperature from
a molten state during production of a hydrogen storage alloy to
solidify, a method such as disclosed in JP-A-60-135538, in which a
La-Ni system alloy is grown into an aligned structure by a
directional solidification process or a method such as proposed in
JP-A-07-157833, in which a Ti--Cr system alloy, while allowing
growing unidirectionally, is solidified to carry a hydrogen-storing
composition with a non-hydrogen-storing composition is known.
However, in the methods, in order to inhibit the hydrogen storage
alloy from pulverization, two different kinds of phases of a
hydrogen-storing composition and a non-hydrogen-storing composition
are tried to obtain as an aligned structure. However, there is a
problem in that when the non-hydrogen-storing composition is
precipitated, the material as a whole is deteriorated in a hydrogen
storage amount. On the other hand, according to JP-A-06-212311, a
molten metal of a hydrogen storage alloy having a Laves phase
structure of AB.sub.2 type is unidirectionally solidified to form
an electrode material of single phase, and thereby a phase other
than an effective alloy phase is removed. Furthermore, in
JP-A-2003-277847, a method of solidifying a hydrogen storage alloy
having a body-centered cubic structure (BCC structure) while
gradually cooling at a cooling speed of 5.degree. C./min or less or
a method of accelerating homogenization of an alloy by a zone
melting process is proposed. However, the method intends as well to
reduce solidification/segregation and inclusions.
[0009] However, it is considered that, since the heat treatments in
existing producing methods cannot achieve sufficient
homogenization, the reversible hydrogen absorption and release
characteristics that the alloy latently has cannot be exerted. In
order to make the homogenization more complete, JP-A-2003-277847
proposes gradually cooling a molten metal at a cooling speed of
5.degree. C./min or less. However, in the hydrogen storage alloys
having the BCC structure, inclusion of impurity elements
deteriorates the hydrogen storage characteristics. Accordingly,
gaseous impurities such as oxygen, nitrogen and carbon are
particularly demanded to reduce as low as possible. Normally, when
a hydrogen storage alloy is melted, an oxide refractory crucible is
used. Accordingly, when a molten alloy is gradually cooled to
solidify, since a time during which the molten metal in a liquid
state is in contact with the crucible becomes longer, oxygen from
the refractory crucible comes in the molten alloy. Thus, an
improvement effect is not sufficiently obtained. On the other hand,
JP-A-2003-277847 discloses as well a method due to a floating zone
melting process, in which a refractory crucible is not used. The
floating zone melting process is a process where, owing to the
surface tension of a liquid in a molten region, solid portions up
and down are joined. When a sample size is made larger, since the
surface tension of the liquid cannot withstand a self-weight, the
continuity of the sample is discontinued. Accordingly, the method
is a method that cannot be expanded in scale to apply
industrially.
SUMMARY OF THE INVENTION
[0010] The invention relates to a producing method of a hydrogen
storage alloy having a BCC structure and a hydrogen storage alloy
that is produced according to the producing method, and intends to
grow, without including impurities from a refractory material, a
large unidirectionally grown grains to produce in an industrial
scale a BCC alloy that is excellent in the hydrogen storage
characteristics such as an increase in a reversible absorption and
release amount of hydrogen, the flattening of a plateau and an
improvement in the durability.
[0011] That is, among present inventions, according to a first
aspect of the invention, there is provided a method of producing a
hydrogen storage alloy, comprising: melting a hydrogen storage
alloy having a body-centered cubic crystal structure without using
a refractory crucible; and solidifying a molten alloy by a
unidirectional solidification process.
[0012] According to the first aspect of the invention, a high
melting point hydrogen storage alloy having a BCC structure can be
melted without coming into contact with a refractory and thereby
impurity elements such as Ca, Mg, Al and Si derived from the
refractory can be inhibited from contamination. In particular,
oxygen that deteriorates the hydrogen storage characteristics of
the alloy having a BCC structure can be inhibited from
contaminating from the refractory to increase an oxygen content in
the alloy. The smaller the oxygen content in the hydrogen storage
alloy is, the more desirable, for instance, 500 ppm or less being
preferable.
[0013] Furthermore, when the molten alloy is cooled from one side
to slowly unidirectionally solidify, homogeneous crystal grains
close to single crystal can be obtained and an aggregate of large
grains having directionality in the grain growth owing to the
unidirectional solidification can be obtained.
[0014] According to a second aspect of the invention, the hydrogen
storage alloy is melted in a water-cooled metal crucible. According
to a third aspect of the invention, the metal crucible comprises
copper.
[0015] In the second and third aspects of the invention, when a
refractory crucible is avoided to use and a crucible made of metal
such as copper is used while cooling with water to melt an alloy,
the impurities, in particular, oxygen can be assuredly inhibited
from contaminating from the refractory crucible into the hydrogen
storage alloy.
[0016] According to a fourth aspect of the invention, a speed of
the unidirectional solidification is set at a moving speed of a
solid-liquid interface in a range of 10 to 200 mm/hr.
[0017] In the fourth aspect of the invention, when the
unidirectional solidification speed is set at an appropriate moving
speed of solid-liquid interface, the crystallinity can be improved
and the impurities can be drastically reduced, and thereby the
hydrogen storage characteristics can be largely improved.
[0018] According to a fifth aspect of the invention, the speed of
the unidirectional solidification process is set at the moving
speed of a solid-liquid interface in a range of 12 to 60 mm/hr.
[0019] According to a sixth aspect of the invention, the hydrogen
storage alloy is melted and unidirectionally solidified in a vacuum
or an inert gas atmosphere.
[0020] In the invention, since the refractory crucible is not used,
oxygen is inhibited from contaminating from the crucible into the
alloy. On the other hand, as a source of oxygen in the hydrogen
storage alloy as an impurity, oxygen present in an atmosphere of a
melting furnace can be considered as well. In the sixth aspect of
the invention, a process from melting to solidification is carried
out in a vacuum or inert gas atmosphere, and, thereby, oxygen is as
far as possible inhibited from contaminating from the atmosphere
into the hydrogen storage alloy. Thereby, oxygen contained in the
produced hydrogen storage alloy can be suppressed to a sum total of
oxygen amounts contained in melt raw materials of the respective
alloy components. Accordingly, the characteristics deterioration
can be suppressed to the minimum level.
[0021] According to a seventh aspect of the invention, the
unidirectional solidification process is carried out by a cold
crucible induction melting process.
[0022] In the seventh aspect of the invention, in a state where a
contact with a water-cooled crucible is suppressed minimum by use
of an electromagnetic force, a hydrogen storage alloy can be melted
and furthermore a melting raw material can be continuously supplied
and a solidified alloy can be readily drawn.
[0023] According to an eighth aspect of the invention, the
unidirectional solidification is carried out by disposing a
water-cooled metallic crucible along an axial direction inside of
an induction heating coil, and moving the metallic crucible
relatively to the induction heating coil in a direction opposite to
a solidification direction.
[0024] In the eighth aspect of the invention, when the water-cooled
metal crucible is moved relatively to the induction heating coil in
an axial direction, while controlling the moving speed of the
solid-liquid interface, the unidirectional solidification can be
applied.
[0025] According to a ninth aspect of the invention, a hydrogen
storage alloy having a body-centered cubic structure that is
produced by the method according to the first aspect of the
invention.
[0026] According to the ninth aspect of the invention, an aggregate
structure of large grains that less contain the impurities such as
oxygen that deteriorates the hydrogen storage characteristics, are
homogeneous and have the directionality in the grain growth can be
obtained. Thereby, excellent hydrogen storage characteristics can
be exerted.
[0027] As mentioned above, according to the producing method of a
hydrogen storage alloy of the invention, since a hydrogen storage
alloy having a body-centered cubic structure as a crystal structure
is melted without using a refractory crucible and a molten alloy
thereof is solidified by the unidirectional solidification process.
Therefore, impurities can be suppressed as low as possible and
large grains having the directionality can be obtained.
Accordingly, it is possible to industrially produce a BCC
structured hydrogen storage alloy that is larger in the hydrogen
storage amount than that of an alloy produced according to an
existing method and smaller in the plateau gradient and the
hysteresis and excellent in the durability.
[0028] Furthermore, a hydrogen storage alloy of the invention,
since the hydrogen storage alloy is produced according to a
producing method of the invention and has a body-centered cubic
structure as the crystal structure, has a large hydrogen storage
amount and excellent plateau characteristics and the
durability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a cross sectional diagram of a cold crucible
furnace used in a producing method of one embodiment of the
invention;
[0030] FIG. 2 is a substitution photograph for a drawing showing a
structure of a hydrogen storage alloy obtained according to example
1 of the invention;
[0031] FIG. 3 is a substitution photograph for a drawing showing a
structure of a hydrogen storage alloy obtained according to example
4 of the invention;
[0032] FIG. 4 is a diagram showing hydrogen equilibrium
pressure-composition-isotherms of hydrogen storage alloys in
examples and comparative examples of the invention;
[0033] FIG. 5 is a diagram showing maximum hydrogen storage amounts
of hydrogen storage alloys in examples and comparative examples of
the invention; and
[0034] FIG. 6 is a diagram showing remaining ratio of hydrogen
transfer amounts due to repetition of hydrogen absorption and
release of hydrogen storage alloys in examples and comparative
examples of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Embodiments of the invention will be described below.
[0036] FIG. 1 shows a cold crucible furnace that is used in a
producing method according to the invention of a hydrogen storage
alloy. In the melting furnace, a water-cooled crucible 2 made of
copper is disposed on an elevatable crucible table 3. In an
external periphery of the water-cooled crucible 2, induction
heating coil 4 is disposed and a metal material accommodated in the
water-cooled crucible 2 can be melted by induction heating.
[0037] One embodiment of a producing method of the invention of a
hydrogen storage alloy, which uses the cold crucible furnace, will
be described below.
[0038] A hydrogen storage alloy raw material 1 such as Ti--Cr--Mn
system alloy or Ti--Cr--V system alloy having a BCC structure is
prepared at a desired component ratio, accommodated in the
water-cooled crucible 2, followed by oscillating the induction
heating coil 4 to melt the hydrogen storage alloy raw material 1 in
the water-cooled crucible. At that time, the hydrogen storage alloy
raw material 1 is disposed under a vacuum atmosphere or under an
inert gas atmosphere. By the melting, a molten metal 1a of the
hydrogen storage alloy is obtained. In the next place, a bottom
portion of the water-cooled crucible 2 is water-cooled to
unidirectionally solidify the hydrogen storage alloy melt 1a down
up from the bottom side to gradually obtain a unidirectionally
solidified alloy 1b. At this time, the crucible table 3 is lowered
to move the water-cooled crucible 2 in a direction opposite to a
solidification direction relative to the induction heating coil 4
to control the solidification speed at a moving speed of the
solid-liquid interface from 10 to 200 mm/hr. When the moving speed
of a solidification interface exceeds 200 mm/hr, in an obtained
alloy, the homogeneity cannot be improved, grain growth and
reduction of the impurities cannot be sufficiently achieved.
Accordingly, an advantage of the characteristics improvement cannot
be obtained. Furthermore, when the moving speed of the
solidification interface is less than 10 mm/hr, owing to the
equilibrium partitioning between a solid phase and a liquid phase,
an alloy composition tends to vary and it takes a long time to
produce an alloy to remarkably lower the production efficiency.
Accordingly, the moving speed of solid-liquid interface is shown as
a desirable range. In view of a balance between the efficiency when
the alloy is produced and the hydrogen storage characteristics of
the obtained alloy the moving speed of the solid-liquid interface
is preferably set at 12 mm/hr at the minimum and 60 mm/hr at the
maximum. According to the above process, the hydrogen storage alloy
1b can be efficiently produced. Furthermore, when grains are grown
in one direction like this, impurity elements in the melt, which
remains in a solid phase during the rapid solidification, can be
exhausted in a liquid phase and thereby an amount of impurities in
the produced alloy can be reduced. In the hydrogen storage alloy
having a BCC structure, the number of hydrogen occupation sites
where hydrogen can be stable in the alloy is larger than other
alloys. Accordingly, a large hydrogen storage amount is exhibited.
However, owing to the deterioration of the crystallinity and
existence of impurities, energy levels of the hydrogen occupation
sites are broadened, resulting in a decease in the hydrogen
absorption amount, an increase in the plateau gradient and an
increase in the hysteresis. The invention can improve the hydrogen
storage characteristics of the hydrogen storage alloy having a BCC
structure and has an effect as well on the alloy deterioration
observed as a decrease in the hydrogen storage amount when an
absorption/release of hydrogen is repeated.
[0039] The hydrogen storage alloy obtained according to the above
process can be obtained as an aggregate of large grains that less
contain the impurities, in particular, oxygen, are homogeneous in
the structure and have the directionality in the grain growth can
be obtained. The alloy is large in the reversible absorption and
release amount of hydrogen, flat in the plateau and excellent in
the durability, as well.
EXAMPLE 1
[0040] In the beginning, 3 kg of raw material metals is blended so
that a composition may be Ti: Cr: V=7.5: 13.5: 79 by atomic ratio
and the blend is primarily melted with an intention of alloying of
raw materials with a water-cooled crucible having a diameter of 100
mm under an Ar gas atmosphere in a cold crucible furnace shown in
FIG. 1, In order to promote the homogenization of alloy components,
a primarily molten ingot is cut into substantially 20 mm cubes and
the cubes are melted again with a water-cooled crucible having a
diameter of 80 mm under an Ar gas atmosphere in the cold crucible
furnace. When all material in the water-cooled crucible is melted,
the water-cooled crucible is lowered in an arrow mark direction
shown in FIG. 1 at a speed of 12 mm/hr to allow unidirectionally
solidifying at the moving speed of solid-liquid interface of 12
mm/hr. After cooling to room temperature, an ingot is taken out. A
vertical cross sectional structure of the ingot is shown in FIG. 2.
In an upward direction from a bottom portion of the crucible, large
grains having a width of 3 to 5 mm are grown. That is, a
unidirectionally grown structure can be obtained.
EXAMPLE 2
[0041] Similarly to example 1, a primary melting and a re-melting
are carried out and thereafter a water-cooled crucible is lowered
at a speed of 30 mm/hr to unidirectionally solidify at a moving
speed of the solid-liquid interface of 30 mm/hr. After cooling to
room temperature, an ingot is taken out. When observing a vertical
cross sectional structure of the ingot, large grains having a width
of 3 to 4 mm are grown in an upward direction from a bottom portion
of the crucible. That is, a unidirectionally grown structure can be
obtained.
EXAMPLE 3
[0042] Similarly to example 1, a primary melting and a re-melting
are carried out and thereafter a water-cooled crucible is lowered
at a speed of 60 mm/hr to unidirectionally solidify at a moving
speed of the solid-liquid interface of 60 mm/hr. After cooling to
room temperature, an ingot is taken out. When observing a vertical
cross sectional structure of the ingot, grains having a width of
substantially 2 mm are grown in an upward direction from a bottom
portion of the crucible. That is, a unidirectionally grown
structure can be obtained.
EXAMPLE 4
[0043] Similarly to example 1, a primary melting and a re-melting
are carried out and thereafter a water-cooled crucible is lowered
at a speed of 120 mm/hr to unidirectionally solidify at a moving
speed of the solid-liquid interface of 120 mm/hr. After cooling to
room temperature, an ingot is taken out. A vertical cross sectional
structure of the ingot is shown in FIG. 4. When observing a
vertical cross sectional structure of the ingot, grains are grown
in an upward direction from a bottom portion of the crucible in
some parts. However, magnitude of grains is substantially 1 to 2
mm. That is, large grain growth is not achieved.
COMPARATIVE EXAMPLE 1
[0044] Raw material metals of 50 g is blended so that a composition
ratio of Ti: Cr: V may be 8: 13: 79 by atomic ratio, followed by
melting in an arc melting furnace with a water-cooled hearth made
of copper in an Ar gas atmosphere. After the melting, a molten
metal is cooled as it is to room temperature in the water-cooled
hearth.
COMPARATIVE EXAMPLE 2
[0045] Raw material metals of 10 kg are blended so that a
composition ratio of Ti: Cr: V may be 7.5: 13.5: 79 by atomic
ratio, followed by melting by the high frequency induction heating
with a calcia crucible in an Ar gas atmosphere. A molten metal is
poured into a 17 mm thick steel mold in an Ar gas atmosphere and
cooled to room temperature, followed by taking out of the melting
furnace.
[0046] Alloy components of the respective alloys obtained in the
examples 1 through 4 and comparative examples 1 and 2, Ti, Cr and V
and impurities of O, N and C are analyzed. Results thereof are
shown in Table 1. In the melting process where the refractory
crucible is not used like in examples 1 through 4 and comparative
example 1, oxygen is less contained, and an amount of impurities in
the alloy can be suppressed low. In particular, in examples 1
through 3, since contents of O and C are less than other alloys, it
is shown that, by conducting the unidirectional solidification, the
impurities in the alloys can be reduced.
[0047] Further, in examples 1 through 3 and comparative example 1,
alloys are melted and produced so as to obtain the same
composition. Here, in examples 1 through 3, Ti is less contained
than other alloys. Accordingly, it is found that when the
solidification speed is slowed the alloy composition varies.
[0048] Then, the hydrogen storage characteristics of the alloys are
measured and results are shown in FIG. 4. From each of the alloy
ingots of examples 1 through 4 and comparative examples 1 and 2, a
sample of substantially 5 g is cut out. The sample is introduced
into a hydrogenation characteristics measurement equipment and a
sample vessel is vacuumed with a rotary vacuum pump, followed by
cooling the sample vessel to 5.degree. C. in a water bath, further
followed by adding hydrogen gas at 4.5 MPa to activate. After
sufficient absorption of hydrogen by the activation, followed by
vacuuming at 100.degree. C. for 1 hour, and with this state as an
original point, a hydrogen equilibrium
pressure-composition-isotherm is measured. In FIG. 4, PCI curves at
20.degree. C. are shown of the respective samples. An arc-melted
sample according to comparative example 1, in spite of a low
content of oxygen that is the impurity, shows a large plateau
gradient and hysteresis. Comparative example 2 shows large plateau
gradient and hysteresis similarly to comparative example 1.
Furthermore, because of the use of the refractory crucible, oxygen
is contained much, resulting in lowering a hydrogen storage
amount.
[0049] Example 2 is most abundant in the hydrogen storage amount,
slight in the plateau gradient and very small in the hysteresis,
that is, very excellent in the characteristics. Example 4 is,
though slightly larger than example 1 in the hysteresis, excellent
in the flatness of the plateau and has a larger hydrogen storage
amount than comparative example 1.
[0050] In the next place, the maximum hydrogen storage amounts when
hydrogen is absorbed with the vacuuming at 100.degree. C. for 1
hour as an original point are shown in block in FIG. 5. It is found
that, in comparison with comparative examples 1 and 2, examples 1
through 4 where grains are grown according to the unidirectional
solidification show larger hydrogen storage amount.
[0051] FIG. 6 shows remaining ratio of hydrogen transfer amount
when absorption and release of hydrogen is repeated of the alloy
samples. A vertical axis shows C.sub.200/C.sub.1 that is a value
obtained by dividing a hydrogen transfer amount after 200 cycles by
a hydrogen transfer amount at an initial time and shows a ratio at
which the hydrogen storage amount of the initial alloy is
maintained after 200 cycles. In comparison with comparative
examples 1 and 2, in examples 1 through 4 where grains are grown by
conducting the unidirectional solidification, all samples are small
in the degradation of the hydrogen storage amount. In particular,
in examples 1 through 3 where coarse grains are observed grown,
excellent repetition durability is shown.
[0052] In the above, the invention has been described with
reference to the embodiments and examples. However, the invention
is not restricted to the content of the description and can be
appropriately modified within a range that does not deviate from
the scope of the invention.
[0053] For example, in the embodiments, the hydrogen storage alloy
is melted by use of the electromagnetic force. However, as a method
that, without using a refractory, industrially melts a metal, an
arc melting process that uses a water-cooling hearth and an
electron beam melting process can be cited. The melting processes
can be applied as well when the unidirectional solidification is
appropriately carried out thereafter. That is, as far as the
refractory crucible is not used, the melting process is not
particularly restricted.
[0054] Further, in the embodiments, the unidirectional
solidification is applied when the water-cooled metal crucible is
moved relatively to the induction heating coil in the axial
direction. However, as far as the crucible and the induction
heating coil are movable relatively, either one of these or both
thereof may be moved to change relative position. TABLE-US-00001
TABLE 1 Metal Atom Ratio (at %) Impurity Element (ppm) Sample Ti Cr
V O N C Example 1 5.88 13.44 80.68 253 123 40 Example 2 5.19 12.96
81.85 294 132 30 Example 3 5.30 12.60 82.10 281 125 60 Example 4
7.22 13.56 79.22 338 133 190 Comparative 8.11 12.25 79.46 633 250
120 Example 1 Comparative 7.32 13.56 79.11 3,340 181 160 Example
2
* * * * *