U.S. patent application number 11/257315 was filed with the patent office on 2006-02-16 for hydrogen storage materials having excellent kinetics, capacity, and cycle stability.
Invention is credited to Michael A. Fetcenko, Jun Im, Feng Li, Taihei Ouchi, Stanford R. Ovshinsky, Melanie Reinhout, Kwo Young.
Application Number | 20060032561 11/257315 |
Document ID | / |
Family ID | 34678082 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060032561 |
Kind Code |
A1 |
Young; Kwo ; et al. |
February 16, 2006 |
Hydrogen storage materials having excellent kinetics, capacity, and
cycle stability
Abstract
A BCC phase hydrogen storage alloy capable of storing
approximately 4.0 wt. % hydrogen and delivering reversibly up to
3.0 wt. % hydrogen at temperatures up to 110.degree. C. The
hydrogen storage alloys also possess excellent kinetics whereby up
to 80% of the hydrogen storage capacity of the hydrogen storage
alloy may be reached in 30 seconds and 80% of the total hydrogen
storage capacity may be desorbed from the hydrogen storage alloy in
90 seconds. The hydrogen storage alloys also have excellent
stability which provides for long cycle life.
Inventors: |
Young; Kwo; (Troy, MI)
; Fetcenko; Michael A.; (Rochester, MI) ; Ouchi;
Taihei; (Rochester, MI) ; Im; Jun; (Troy,
MI) ; Ovshinsky; Stanford R.; (Bloomfield Hills,
MI) ; Li; Feng; (Troy, MI) ; Reinhout;
Melanie; (Shelby Twp, MI) |
Correspondence
Address: |
ENERGY CONVERSION DEVICES, INC.
2956 WATERVIEW DRIVE
ROCHESTER HILLS
MI
48309
US
|
Family ID: |
34678082 |
Appl. No.: |
11/257315 |
Filed: |
October 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10741222 |
Dec 19, 2003 |
|
|
|
11257315 |
Oct 24, 2005 |
|
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Current U.S.
Class: |
148/668 ;
420/900 |
Current CPC
Class: |
C22C 14/00 20130101;
C22C 30/00 20130101; C22C 27/025 20130101; Y10S 420/90 20130101;
C22C 27/06 20130101; Y02E 60/327 20130101; C01B 3/0031 20130101;
Y02E 60/32 20130101 |
Class at
Publication: |
148/668 ;
420/900 |
International
Class: |
C22F 1/18 20060101
C22F001/18 |
Claims
1. A process for producing a hydrogen storage alloy, said process
comprising: 1) forming a hydrogen storage alloy having two or more
elements; 2) annealing said hydrogen storage alloy to form a
substantially single phase BCC structure; 3) quenching said
annealed hydrogen storage alloy at a cooling rate in the range of
10.sup.2 to 10.sup.3.degree. C./second; and 4) inhibiting the
formation of said oxides on the surface of said hydrogen storage
alloy during quenching and/or removing said oxides from the surface
of said hydrogen storage alloy after quenching.
2. The process according to claim 1, wherein said hydrogen storage
alloy is formed via arc melting, cold wall induction melting, or
levitation melting techniques.
3. The process according to claim 1, wherein said hydrogen storage
alloy is annealed at a temperature in the range of 1350.degree. C.
to 1450.degree. C.
4. The process according to claim 1, wherein said hydrogen storage
alloy is quenched in liquid argon, liquid nitrogen, or water.
5. The process according to claim 1, wherein said oxides on the
surface of said hydrogen storage alloy are removed via etching or
mechanical grinding.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of and is entitled
to the benefit of the earlier filing date and priority of,
co-pending U.S. patent application Ser. No. 10/741,222, which is
assigned to the same assignee as the current application, entitled
"HYDROGEN STORAGE MATERIALS HAVING EXCELLENT KINETICS, CAPACITY,
AND CYCLE STABILITY," filed Dec. 19, 2003, the disclosure of which
is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to hydrogen storage alloys
utilized for the reversible storage of hydrogen. More particularly,
the present invention relates to hydrogen storage alloys having
excellent absorption and desorption kinetics.
BACKGROUND
[0003] Hydrogen storage is a technology critical to a wide variety
of applications, some of the most prevalent being fuel cells,
portable power generation, and hydrogen combustion engines. Such
applications would benefit substantially from hydrogen storage
alloys capable of absorbing and desorbing higher amounts of
hydrogen as compared to present day commercially available hydrogen
storage alloys. Hydrogen storage alloys having the hydrogen
absorption and desorption characteristics of the present invention
will benefit such applications by providing longer operating life
and/or range on a single charge for hydrogen power generators, fuel
cells, and hydrogen internal combustion engines.
[0004] In the past considerable attention has been given to the use
of hydrogen as a fuel or fuel supplement. While the world's oil
reserves are being rapidly depleted, the supply of hydrogen remains
virtually unlimited. Hydrogen can be produced from coal, natural
gas and other hydrocarbons, or formed by the electrolysis of water.
Moreover hydrogen can be produced without the use of fossil fuels,
such as by the electrolysis of water using nuclear or solar energy.
Furthermore, hydrogen, although presently more expensive than
petroleum, is a relatively low cost fuel. Hydrogen has the highest
density of energy per unit weight of any chemical fuel and is
essentially non-polluting since the main by-product of burning
hydrogen is water.
[0005] While hydrogen has wide potential application as a fuel, a
major drawback in its utilization, especially in mobile uses such
as the powering of vehicles, has been the lack of acceptable
lightweight hydrogen storage medium. Conventionally, hydrogen has
been stored in a pressure-resistant vessel under a high pressure or
stored as a cryogenic liquid, being cooled to an extremely low
temperature. Storage of hydrogen as a compressed gas involves the
use of large and heavy vessels. In a steel vessel or tank of common
design only about 1% of the total weight is comprised of hydrogen
gas when it is stored in the tank at a typical pressure of 136
atmospheres. In order to obtain equivalent amounts of energy, a
container of hydrogen gas weighs about thirty times the weight of a
container of gasoline. Additionally, transfer is very difficult,
since the hydrogen is stored in a large-sized vessel. Furthermore,
storage as a liquid presents a serious safety problem when used as
a fuel for motor vehicles since hydrogen is extremely flammable.
Liquid hydrogen also must be kept extremely cold, below
-253.degree. C., and is highly volatile if spilled. Moreover,
liquid hydrogen is expensive to produce and the energy necessary
for the liquefaction process is a major fraction of the energy that
can be generated by burning the hydrogen.
[0006] Alternatively, certain metals and alloys have been known to
permit reversible storage and release of hydrogen. In this regard,
they have been considered as a superior hydrogen-storage material,
due to their high hydrogen-storage efficiency. Storage of hydrogen
as a solid hydride can provide a greater volumetric storage
density. than storage as a compressed gas or a liquid in pressure
tanks. Also, hydrogen storage in a solid hydride presents fewer
safety problems than those caused by hydrogen stored in containers
as a gas or a liquid. Solid-phase metal or alloy system can store
large amounts of hydrogen by absorbing hydrogen with a high density
and by forming a metal hydride under a specific
temperature/pressure or electrochemical conditions, and hydrogen
can be released by changing these conditions. Metal hydride systems
have the advantage of high-density hydrogen-storage for long
periods of time, since they are formed by the insertion of hydrogen
atoms to the crystal lattice of a metal. A desirable hydrogen
storage material must have a high storage capacity relative to the
weight of the material, a suitable desorption temperature/pressure,
good kinetics, good reversibility, resistance to poisoning by
contaminants including those present in the hydrogen gas, and be of
a relatively low cost. If the material fails to possess any one of
these characteristics it will not be acceptable for wide scale
commercial utilization.
[0007] The hydrogen storage capacity per unit weight of material is
an important consideration in many applications, particularly where
the hydride does not remain stationary. A low hydrogen storage
capacity relative to the weight of the material reduces the mileage
and hence the range of a hydrogen fueled vehicle making the use of
such materials. A low desorption temperature is desirable to reduce
the amount of energy required to release the hydrogen. Furthermore,
a relatively low desorption temperature to release the stored
hydrogen is necessary for efficient utilization of the available
exhaust heat from vehicles, machinery, fuel cells, or other similar
equipment.
[0008] Good reversibility is needed to enable the hydrogen storage
material to be capable of repeated absorption-desorption cycles
without significant loss of its hydrogen storage capabilities. Good
kinetics are necessary to enable hydrogen to be absorbed or
desorbed in a relatively short period of time. Resistance to
contaminants to which the material may be subjected during
manufacturing and utilization is required to prevent a degradation
of acceptable performance.
[0009] The prior art hydrogen storage materials include a variety
of metallic materials for hydrogen-storage, e.g., Mg, Mg--Ni,
Mg--Cu, Ti--Fe, Ti--Mn, Ti--Ni, Mm--Ni and Mm--Co alloy systems
(wherein, Mm is Misch metal, which is a rare-earth metal or
combination/alloy of rare-earth metals). None of these prior art
materials, however, has had all of the properties required for a
storage medium with widespread commercial utilization.
[0010] Of these materials, the Mg alloy systems can store
relatively large amounts of hydrogen per unit weight of the storage
material. However, heat energy must be supplied to release the
hydrogen stored in the alloy, because of its low hydrogen
dissociation equilibrium pressure at room temperature. Moreover,
release of hydrogen can be made, only at a high temperature of over
250.degree. C. along with the consumption of large amounts of
energy.
[0011] The rare-earth (Misch metal) alloys have their own problems.
Although they typically can efficiently absorb and release hydrogen
at room temperature, based on the fact that it has a hydrogen
dissociation equilibrium pressure on the order of several
atmospheres at room temperature, their hydrogen-storage capacity
per unit weight is only about 1.2 weight percent.
[0012] The Ti--Fe alloy system which has been considered as a
typical and superior material of the titanium alloy systems, has
the advantages that it is relatively inexpensive and the hydrogen
dissociation equilibrium pressure of hydrogen is several
atmospheres at room temperature. However, since it requires a high
temperature of about 350.degree. C. and a high pressure of over 30
atmospheres for initial hydrogenation, the alloy system provides
relatively low hydrogen absorption/desorption rate. Also, it has a
hysteresis problem which hinders the complete release of hydrogen
stored therein.
[0013] Hydrogen storage alloys have various crystal structures
which play an important role in the alloys ability to absorb and
desorb hydrogen. Some of the crystal structures include body
centered cubic (BCC), face centered cubic (FCC), or C-14 Laves
phase. Hydrogen storage alloys may also change crystal structure
upon absorption/desorption of hydrogen. The crystal structure of
the BCC phase hydrogen storage alloys, upon absorption of hydrogen,
may change to an FCC crystal structure. When this change in crystal
structure occurs, excess energy (heat) may be needed to desorb the
hydrogen stored within the alloy. Reduced cycling may also be
realized due to degradation of the alloy resulting from changes in
the crystal structure. Another disadvantage of the change in
crystal structure is that the structure does not completely revert
back to a BCC crystal structure upon desorption of hydrogen. Upon
desorption of hydrogen, the alloy has a combination BCC/FCC crystal
structure. This adversely affects the hydrogen storage properties
of the alloy, because all the benefits of having a BCC alloy will
not be realized. Although the original BCC crystal structure may be
restored by heating the alloy, this is not practical for most
systems utilizing BCC alloys due to their low temperature
design.
[0014] BCC alloys are widely used for the storage of hydrogen and
have been the subject of multiple patents. Iba et al. (U.S. Pat.
No. 5,968,291) discloses Ti--V based BCC phase hydrogen storage
alloys comprising two solid solutions having a periodical structure
formed by spinodal decomposition. While the alloys disclosed in Iba
et al. are able to achieve hydrogen storage capacities of
approximately 3.5 weight percent hydrogen, they are only able to
achieve approximately 2.0 weight percent reversible hydrogen
storage, which makes them unsuitable for many applications. For
example, in vehicle applications, alloys having a low reversible
hydrogen storage capacity adversely affect the range of the vehicle
or require additional weight and space considerations for onboard
metal hydride storage to obtain minimum range requirements. Such is
the case with portable power applications as well.
[0015] Sapru et al. (U.S. Pat. No. 6,616,891) discloses BCC phase
hydrogen storage alloys capable of absorbing up to 4.0 weight
percent hydrogen while capable of desorbing up to 2.8 weight
percent hydrogen. However, Sapru et al. is only able to obtain
these hydrogen storage characteristics at temperatures of
150.degree. C. The alloys disclosed by Sapru et al. are Ti--V based
with the addition of various modifier elements which improve the
reversibility of the hydrogen storage alloys. While the alloys
disclosed in Sapru et al. have demonstrated excellent hydrogen
absorption/desorption properties at temperatures up to 150.degree.
C., there is still a need to provide such properties at lower
temperatures. The ability to operate at lower temperatures will
provide many additional opportunities for hydrogen to be the fuel
of choice for a wide variety of applications.
[0016] Another problem with prior art BCC alloys is that while they
may initially have a good hydrogen storage capacity, these alloys
have very poor stability. Upon increased cycling, the poor
stability of the BCC hydrogen storage alloys causes a significant
reduction in the hydrogen storage capacity of the alloys, which has
resulted in BCC alloys being overlooked for a wide variety of
hydrogen storage applications.
[0017] Under the circumstances, a variety of approaches have been
made to solve the problems of the prior art and to develop an
improved material which has a high hydrogen storage efficiency with
excellent reversibility, a proper hydrogen dissociation equilibrium
pressure, a high absorption/desorption rate, and excellent phase
stability resulting in increased cycle life. By making such
improvements in hydrogen storage alloys, hydrogen
SUMMARY OF THE INVENTION
[0018] The present invention discloses a hydrogen storage alloy
which absorbs at least 80% of its hydrogen storage capacity within
180 seconds, desorbs at least 80% of its total hydrogen storage
capacity within 180 seconds, and reversibly stores at least 2.2
weight percent hydrogen at temperatures up to 110.degree. C. The
hydrogen storage alloy may also absorb at least 80% of its hydrogen
storage capacity within 30 seconds and desorb at least 80% of its
total hydrogen storage capacity within 90 seconds at temperatures
up to 110.degree. C. At least 85% of the hydrogen storage alloy
reverts to a BCC or BCT crystal structure from a FCC crystal
structure upon desorption of hydrogen from the hydrogen storage
alloy.
[0019] The lattice constant of the hydrogen storage alloy is in the
range of 3.015 to 3.045 angstroms. For high pressure applications,
the lattice constant of the hydrogen storage alloy is preferably in
the range of 3.015 to 3.028 angstroms. For low pressure
applications, the lattice constant of said hydrogen storage alloy
is in the range of 3.028 to 3.045 angstroms. The surface of said
hydrogen storage alloy may be substantially free of any oxides. The
hydrogen storage alloy may have a cycle life greater than 700
cycles. The hydrogen storage alloy reversibly stores up to 2.83
weight percent hydrogen at 90.degree. C., and up to 3.01 weight
percent hydrogen at 110.degree. C.
[0020] The hydrogen storage alloy comprises 8.0 to 45 atomic
percent titanium, 5.0 to 75 atomic percent vanadium, and 10 to 65
atomic percent chromium. The hydrogen storage alloy may further
comprises one or more modifier elements selected from nickel,
manganese, molybdenum, aluminum, iron, silicon, magnesium,
ruthenium, or cobalt, wherein the modifier elements are present in
an amount greater than 0 up to 16 atomic percent. The hydrogen
storage alloy may have a single phase BCC structure, which may be
formed by cooling at a quench rate in the range of 10.sup.2 to
10.sup.3.degree. C./second.
[0021] The present invention also disclosed a process for producing
a hydrogen storage alloy, said process comprising 1) forming a
hydrogen storage alloy having two or more elements, 2) annealing
the hydrogen storage alloy to form a substantially single phase BCC
structure, 3) quenching the annealed hydrogen storage alloy at a
cooling rate in the range of 10.sup.2 to 10.sup.3.degree.
C./second, and 4) inhibiting the formation of the oxides on the
surface of the hydrogen storage alloy during quenching and/or
removing said oxides from the surface of the hydrogen storage alloy
after quenching.
[0022] The hydrogen storage alloy may be formed via arc melting,
cold wall induction melting, or levitation melting techniques. The
hydrogen storage alloy may be annealed at a temperature in the
range of 1350.degree. C. to 1450.degree. C. and quenched in liquid
argon, liquid nitrogen, or water. The oxides on the surface of the
hydrogen storage alloy may be removed via etching or mechanical
grinding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1, is a PCT plot of a medium vanadium content alloy
accordance with the present invention showing the change in crystal
structure as hydrogen is absorbed and desorbed from the alloy.
[0024] FIG. 2 shows the cycle stability for low vanadium content
alloy in accordance with the present invention.
[0025] FIG. 3, is a plot comparing the cycle stabilities for a
medium vanadium content alloy and a low vanadium content alloy in
accordance with the present invention.
[0026] FIG. 4, is a plot showing the relationship between the
reversible hydrogen storage capacities at 90.degree. C. and the
lattice constant for hydrogen storage alloys in accordance with the
present invention.
[0027] FIG. 5, is a plot showing the relationship between the
equilibrium pressure at 1.5% storage and the lattice constant for
hydrogen storage alloy in accordance with the present
invention.
[0028] FIG. 6, is a x-ray diffraction analysis of hydrogen storage
alloys in accordance with the present invention produced by
different melting methods.
[0029] FIG. 7, is a PCT plot of hydrogen storage alloys in
accordance with the present invention produced by different melting
methods.
[0030] FIG. 8, is a schematic of an apparatus for
annealing/quenching the alloys of the present invention.
[0031] FIG. 9, shows scanning electron micrographs of hydrogen
storage alloys in accordance with the present invention produced
with different annealing temperatures.
[0032] FIG. 10, is a x-ray diffraction analysis of hydrogen storage
alloys in accordance with the present invention produced with
different post annealing quench rates.
[0033] FIG. 11, is a PCT plot of hydrogen storage alloys in
accordance with the present invention produced with different post
annealing quench rates.
[0034] FIG. 12, shows the absorption/desorption rate for a low
vanadium alloy in accordance with the present invention.
[0035] FIG. 13, shows the absorption/desorption rate for a medium
vanadium alloy in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention discloses hydrogen storage alloys
generally having a single phase body centered cubic (BCC)
structure, although more than one BCC phase may be present. These
alloys are capable of storing approximately 4.0 wt. % hydrogen and
delivering reversibly up to 3.0 wt. % hydrogen at temperatures
ranging from 90.degree. C. to 110.degree. C. The hydrogen storage
alloys also possess excellent kinetics whereby up to 80% of the
hydrogen storage capacity of the hydrogen storage alloy may be
reached in 30 seconds and 80% of the total hydrogen storage
capacity may be desorbed from the hydrogen storage alloy in 90
seconds. The hydrogen storage alloys also have excellent stability
which provides for long cycle life.
[0037] The hydrogen storage alloys may be generally composed of
titanium, vanadium, and chromium. The alloys generally include 8.0
to 45 atomic percent titanium, 5.0 to 75 atomic percent vanadium,
and 10 to 65 atomic percent chromium. The hydrogen storage alloys
are classified as 1) high vanadium content, 2) low vanadium
content, or 2) medium vanadium content. The high vanadium content
alloys exhibit a BCC structure after melting and cooling. This
family of alloys, however, has the lowest reversible capacity as
compared to low vanadium content and medium vanadium content
alloys. The low vanadium content alloys normally have a stable
Laves phase when cooling to room temperature after melting without
the addition of modifier elements. The BCC crystal structure of
these alloys only exists at a narrow temperature window above
1370.degree. C. Therefore an annealing/quenching process may be
used to obtain the BCC form of this material. Although by adding a
proper amount of modifier elements with a controlled melting method
a clean BCC structure may be obtained directly from melting, a post
annealing/quench is still recommended. The low vanadium content
alloys have a much better cycle life as compared to the high
vanadium and medium vanadium content alloys. The medium vanadium
content alloys have a much better reversible storage capacity as
compared to the high vanadium content and low vanadium content
alloys. As with the low vanadium content alloys, the medium content
alloys are preferably annealed and quenched after melting to obtain
a BCC structure. However, such steps may be omitted by inclusion of
certain modifier elements with a controlled melting method.
[0038] The hydrogen storage alloys of the present invention may
include one or more modifier elements selected from nickel,
manganese, molybdenum, aluminum, iron, silicon, magnesium,
ruthenium, and cobalt. Such elements may be included in the
hydrogen storage alloy in the range of 0-16 atomic percent. Some of
the modifier elements may also be available as impurities in
vanadium. Vanadium containing such impurities is cheaper in cost
and can result in cost savings when producing such alloys.
Preferred alloys of the present invention are shown by atomic
percent in Table 1. TABLE-US-00001 TABLE 1 Alloy Sample V Ti Cr Ni
Mn Mo Al Fe Si Mg Ru Co 1 80.00 10.00 10.00 2 75.00 10.00 15.00 3
75.00 10.00 5.00 10.00 4 75.00 10.00 10.00 5.00 5 5.00 33.00 62.00
6 7.50 33.00 59.50 7 33.00 64.50 2.50 8 33.00 62.00 5.00 9 2.50
33.00 59.50 2.50 2.50 10 26.01 32.76 25.85 14.77 0.59 11 40.00
58.00 2.00 12 40.00 56.00 4.00 13 40.00 50.00 10.00 14 2.00 40.00
58.00 15 4.00 40.00 56.00 16 10.00 40.00 50.00 17 6.00 43.00 51.00
18 5.00 58.00 37.00 19 80.00 10.00 10.00 20 23.00 28.00 42.00 4.00
1.00 2.00 21 74.50 10.00 12.50 3.00 22 75.50 10.00 11.00 1.50 2.00
23 46.88 52.88 0.12 0.084 24 23.00 30.00 42.00 2.00 1.00 2.00 25
23.00 32.00 38.50 5.00 0.50 1.00 26 62.50 37.50 27 77.00 8.00 12.00
0.50 2.50 28 23.00 30.00 42.00 3.00 2.00 29 20.00 33.33 46.67 30
46.67 33.33 20.00 31 33.00 47.00 20.00 32 40.00 40.00 20.00 33
26.00 33.00 26.00 15.00 34 26.00 31.00 28.00 15.00 35 26.00 28.00
31.00 15.00 36 10.13 40.35 48.46 1.05 37 40.00 45.00 15.00 38 8.00
40.00 52.00 39 5.00 40.00 50.00 5.00 40 40.00 49.00 1.00 10.00 41
10.00 40.00 49.00 1.00 42 50.00 50.00 43 33.30 33.30 33.30 44 40.00
48.00 10.00 2.00 45 10.00 40.00 48.00 2.00 46 74.50 10.00 11.50
4.00 47 74.00 10.00 11.00 5.00 48 74.00 10.00 10.00 6.00 49 9.00
40.00 50.00 1.00 50 9.00 40.00 50.00 1.00 51 9.00 40.00 50.00 1.00
52 8.50 40.00 50.00 0.50 0.50 0.50 53 67.50 10.00 12.50 7.00 3.00
54 67.50 10.00 12.50 3.00 7.00 55 67.50 10.00 12.50 3.00 7.00 56
65.50 10.00 12.50 3.00 3.00 3.00 3.00 57 23.00 30.00 40.00 2.00
3.00 2.00 58 23.00 30.00 40.00 3.00 2.00 2.00 59 23.00 30.00 38.00
2.00 3.00 2.00 2.00 60 23.00 30.00 41.00 3.00 3.00 61 23.00 30.00
40.00 3.00 4.00 62 23 30 42 5 63 23 30 40 4 3 64 23 30 39 4 4 65 23
30 39 5 3 66 23 30 39 3 5 67 23 30 42 2 3 68 50 21 29 69 30 31 39
70 9 40 50 1 71 8 40 50 0.5 0.5 0.5 0.5 72 67.5 10 12.5 3 7 73 66.5
10 12.5 2 3 2 2 2 74 23 30 40 3 2 2 75 23 30 38 1.5 3 1.5 1.5
1.5
[0039] During absorption/desorption cycling of the alloys of the
present invention, the crystal structure of the alloys changes
between BCC phase and FCC phase. Shown in FIG. 1, is a PCT plot of
a medium vanadium content alloy 10
(V.sub.26Ti.sub.32.7Cr.sub.25.9Mn14.8 MO.sub.0.6) in accordance
with the present invention showing the change in crystal structure
as hydrogen is absorbed and desorbed from the alloy. In typical BCC
phase hydrogen storage alloys the BCC crystal structure is
transformed into a FCC crystal structure upon absorption of
hydrogen and the FCC crystal structure converts into a BCC plus FCC
crystal structure after desorption of the stored hydrogen. While
the FCC phase allows for high hydrogen storage, the stored hydrogen
is not able to be released at useful temperatures, therefore the
reversibility of the alloys is adversely affected resulting in a
decrease in cycling. It is possible, however, to convert the
crystal structure from BCC plus FCC back to BCC by heating the
alloy above 300.degree. C., but this is not practical for most low
temperature applications.
[0040] The alloys of the present invention are able to cycle back
and forth between the original body centered cubic (BCC) crystal
structure (sometimes combined with a body centered tetragonal (BCT)
crystal structure) and the face centered cubic (FCC) crystal
structure while leaving substantially no remnants of the FCC
structure when the stored hydrogen has been desorbed from the
alloy. Upon desorption of hydrogen, the hydrogen storage alloys of
the present invention are able to revert back to the BCC and/or BCT
phase while leaving less than 15% of the alloy in the FCC phase.
Preferably, the hydrogen storage alloys of the present invention
are able to revert back to the BCC and/or BCT phase while leaving
less than 10% of the alloy in the FCC phase. Most preferably, the
hydrogen storage alloys of the present invention are able to revert
back to the BCC and/or BCT phase while leaving less than 5% of the
alloy in the FCC phase.
[0041] The ability of the hydrogen storage alloys of the present
invention to cycle back and forth between the BCC/BCT and FCC
crystal structures allows the alloys of the present invention to
achieve increased cycle life. While not wishing to be bound by
theory, the present inventors believe that inclusion of the
modifier elements through the principles of atomic engineering have
resulted in increased cycle life of the alloys by disrupting the
transformation of FCC crystal structure to a FCC plus BCC/BCT
crystal structure upon desorption of hydrogen from the hydrogen
storage alloy. The modifier elements are able to stabilize the
BCC/BCT crystal structure by giving the crystal structure a lower
energy state. Normally the FCC crystal structure is
thermodynamically desirable as opposed to the BCC/BCT crystal
structure, but with the inclusion of the modifier elements lowering
the energy state of the BCC/BCT crystal structure, the BCC/BCT
crystal structure becomes more desirable resulting in the ability
for the FCC crystal structure to revert back to the original
BCC/BCT crystal structure upon desorption of the stored hydrogen.
The ability to revert back to a BCC/BCT phase from the FCC phase
allows the hydrogen storage alloys of the present invention to
retain their hydrogen storage capacities through extended cycling
resulting in excellent cycle life. The alloys of the present
invention are able to exhibit cycle stability for 700+ cycles. FIG.
2 shows the cycle stability for low vanadium content alloy 16
(V.sub.10Ti.sub.40Cr.sub.50) and FIG. 3 shows the cycle stability
for medium vanadium content alloy 10
(V.sub.26Ti.sub.32.7Cr.sub.25.9Mn.sub.14.8Mo.sub.0.6) as compared
to the low vanadium content alloy 16 (V.sub.10Ti.sub.40Cr.sub.50).
After the initial 10% drop in reversible capacity, both the total
and reversible capacity remain constant for low vanadium content
alloy 16 above 800 cycles. The medium vanadium content alloy 10,
however, showed a much degraded cycle performance. It is believed
that the degradation in cycle performance can be attributed to the
FCC-BCC phase transition becoming less reversible with cycling. The
medium vanadium content alloy family, although providing for a
higher storage capacity, has a lower cycle life than low vanadium
content alloys.
[0042] The lattice constant is another important consideration
which directly relates to hydrogen storage characteristics of the
hydrogen storage alloys of the present invention. The reversible
hydrogen storage capacities at 90.degree. C. for a hydrogen storage
alloys in accordance with the present invention are plotted against
their respective BCC lattice constants in FIG. 4. As the BCC
lattice becomes larger, the hydrogen occupied sites within the
alloy become more stable and therefore more reluctant to allow the
removal of hydrogen from the bulk of the alloy resulting in a lower
reversible capacity. Preferably, the hydrogen storage alloys of the
present invention have a lattice constant in the range of 3.015
angstroms to 3.045 angstroms. Hydrogen storage alloys having
lattice constants in this range allow for higher hydrogen storage
capacity and higher reversibility by providing the hydrogen with
greater access to and from bonding sites within the hydrogen
storage alloy. Hydrogen storage alloys having a lattice constant
outside of this range have an increase in equilibrium pressure.
With a smaller lattice constant, hydrogen may not be able to access
as many bonding sites within the hydrogen storage alloy resulting
in decreased hydrogen storage capacity and reversibility. With a
higher lattice constant the quantum tunneling between storage sites
becomes too easy and hydrogen is easily removed in the presence of
a concentration gradient resulting in decreased hydrogen storage
capacity. As shown in FIG. 5, a hydrogen storage alloy having a
lattice constant in the range of 3.015 angstroms to 3.028 angstroms
is preferred for high pressure applications and a hydrogen storage
alloy having a lattice constant in the range of 3.028 angstroms to
3.045 angstroms is preferred for low pressure applications.
[0043] The alloys of the present invention may be produced using
arc melting, levitation melting, cold wall induction melting, melt
spinning, or gas atomization techniques, all of which are well
known in the art. Preferably, the alloys of the present invention
are produced by arc melting, cold wall induction melting, or
levitation melting techniques. With regard to cold wall induction
melting and levitation induction melting, cold wall induction
melting is able to process more material with less power, while
levitation induction melting is able to produce materials with
fewer contaminants, such as oxides. Other methods may be used
provided they the quench rate required to form the micro-structural
or micro-chemical variation within the hydrogen storage alloy
giving rise to high hydrogen storage capacity and
reversibility.
[0044] After the alloys are produced via various melting
techniques, the alloys are annealed to increase the packing density
of the alloy and remove voids within the alloy structure. By
annealing the alloy, the hydrogen storage capacity and
reversibility of the hydrogen storage alloys are increased. The
hydrogen storage alloys may be annealed for at least 5 minutes at a
temperature in the range of 1300.degree. C. to 1500.degree. C.,
preferably in the range of 1350.degree. C. to 1450.degree. C.
[0045] After annealing, to obtain the hydrogen storage
characteristics and the fast kinetics earlier described, the
hydrogen storage alloys of the present invention are quenched at a
rate of 10.sup.2 to 10.sup.3.degree. C./second to freeze in the
desired microstructure. Preferably the alloys are cooled using a
low oxygen, quick quench. When quenching the alloys of the present
invention, alloys formed with a faster quench rate have been found
to exhibit improved hydrogen storage characteristics as compared to
alloy formed using a slower quench rate. When utilizing a fast
quench rate, the hydrogen storage alloys obtain a substantially
uniform single phase BCC crystal structure.
[0046] During the melting or quenching of the hydrogen storage
alloy an oxide coating may form on the exterior of the alloy
particles. While not wishing to be bound by theory, the present
inventors believe that the formation of the oxide coating adversely
affects the total hydrogen storage capacity of the hydrogen storage
alloy while having little or no effect on the reversibility of the
alloy. To prevent reduction in the hydrogen storage capacity of the
alloy, the oxide coating may be removed from the alloy particles or
may be inhibited from forming altogether. To inhibit the oxide
coating from forming during melting, the alloy may be melted in a
copper crucible instead of various other crucibles, such as
aluminum oxide crucibles, which allow oxygen to enter the alloy
from the crucible material at high temperatures. Crucibles composed
of materials other than copper may also be used provided they do
not allow oxygen contained in the crucible material to react with
the molten alloy. To inhibit formation of the oxide coating during
quenching, the alloy may be quenched in a low oxygen environment.
Instead of quenching the alloy in water, the alloy may be quenched
in liquid nitrogen, liquid argon, oil, or other media having a low
oxygen content. While using these low oxygen content media may
prevent or hinder oxide formation on the surface of the alloy
particles, the quench rate will be affected due to the differences
in heat capacity between the various media, which may be
detrimental to the hydrogen storage characteristics of the alloy.
When using quenching media allowing the formation of an oxide
coating on the surface of the alloy particles, the oxide coating
may be removed from the particles via etching or mechanical
grinding. These methods may be preferred when a certain quench rate
not obtainable with low oxygen content quenching media is
desired.
EXAMPLE 1
[0047] To determine the effect of melting techniques on the alloys
of the present invention, several 5 g samples of alloy 16
(V.sub.10Ti.sub.40Cr.sub.50) in accordance with the present
invention were prepared with different melt techniques and
subsequently tested for hydrogen storage characteristics. The
samples were prepared using arc melting with water cooled copper
basin (a), induction furnace with MgO crucible (b) , and
melt-spinner with boron nitride crucible (c). The samples prepared
by arc melting and induction melting were annealed in argon gas for
5 minutes at 1400.degree. C. and quick quenched in water. The melt
spin sample was not annealed. For purposes of comparing PCT data,
an additional sample (b1) was produced by induction melting in a
MgO crucible and annealed for 20 minutes at 1400.degree. C.
followed by a quick quench in water. The samples were then etched
in 2% HF+10% HCl (50%) solution for 10 minutes in an ultrasonic
bath to substantially remove any oxide formed on the surface of the
ingots. An X-ray diffraction analysis of the samples is shown in
FIG. 6 and a PCT plot is shown in FIG. 7 for the samples. The
sample produced via arc melting shows the purest BCC structure
while the other samples show secondary phases such and Laves and
titanium phases along with the BCC phase. The sample produced via
arc melting exhibited the higher total and reversible hydrogen
storage capacities. The induction melting sample showed higher
plateau pressure and inferior hydrogen storage capacity. The melt
spinning sample showed the worst storage capacity among all four
samples.
[0048] The arc melting sample was prepared using a Discovery 201T
arc melter. This system is composed of a water-cooled tungsten
anode, a water-cooled copper mold as a cathode, and a vacuum
chamber with a mechanical pump. All elements used in the alloy
formulation were pure and free of surface contamination. The
pre-weighed elements were loaded on top of the water-cooled copper
mold in the vacuum chamber of the arc-melter and the arc-melter was
evacuated to 20 micron and flushed with argon gas three times to
obtain an oxygen free environment. Then for further purification of
the arc-melter chamber, a 15 g piece of titanium was melted and
cooled three times as an oxygen getter.
[0049] The melting temperature controls for the alloying process
were based on the element with the highest melting point. The
alloying process for the sample was composed of five consecutive
twenty second melting and turning over sequences to obtain a
homogeneous sample. The alloy sample was cooled in the water cooled
copper mold during and after the melting process. After the alloy
was prepared, the alloy samples were annealed and quenched.
[0050] The apparatus for the annealing/quenching of the alloy is
shown in FIG. 8. The apparatus utilizes a type 59300 high
temperature tube furnace as the heat zone 1. One horizontal arm of
cross quartz tube was inserted through the tube furnace with argon
gas 2 continuously flowing therethrough. A magnetically coupled rod
3 was used to move the alloy ingots 4 into and out of the heat zone
1. The alloy ingots 4 were first loaded through the top end 5 of
the apparatus through a passage exposed by removing the removable
cap 6. The bottom end of the apparatus was immersed into a
quenching zone 7 filled with water, liquid argon, liquid nitrogen,
or another quenching agent. The alloy ingot 4 was heat-treated at
1673.degree. C. for 5-20 minutes in an argon atmosphere and then
quickly removed from the heat zone 1. The boat was then immediately
turned over dropping the alloy ingot into the quench zone 7.
EXAMPLE 2
[0051] To determine the effect of the annealing temperature on the
alloys of the present invention, several samples of alloy 28
(V.sub.23Ti.sub.30Cr.sub.42Mn.sub.3Fe.sub.2) were prepared via arc
melting (as earlier described). Shown in FIG. 9, are scanning
electron micrographs of alloy 28 samples annealed at 1200.degree.
C. (a) , 1300.degree. C. (b), 1400.degree. C. (c) and 1450.degree.
C. (d) . Annealing at 1400.degree. C. provides the alloy with a
packed microstructure substantially free from voids. Samples
annealed at 1200.degree. C. and 1300.degree. C. underwent phase
segregation and the sample annealed at 1450.degree. C. showed the
formation of secondary phases. The absorption and desorption
characteristics of these alloys are summarized below in Table 2.
TABLE-US-00002 TABLE 2 Annealing Absorption Annealing Temp. Time at
10.degree. C. Desorption at 90.degree. C. 1200.degree. C. 5 minutes
3.06% 2.32% 1300.degree. C. 5 minutes 3.36% 2.56% 1400.degree. C. 5
minutes 3.57% 2.82% 1450.degree. C. 5 minutes 3.41% 2.71%
EXAMPLE 3
[0052] To determine the effect of the annealing duration on the
alloys of the present invention, several samples of alloy 28
(V.sub.23Ti.sub.30Cr.sub.42Mn.sub.3Fe.sub.2) were prepared via arc
melting (as earlier described). Annealing times of 5 minutes, 10
minutes, and 20 minutes were performed on alloy samples at
1400.degree. C. Upon testing, the length of annealing was found to
not have as dramatic effect on the hydrogen storage capacity of the
hydrogen storage alloy as the annealing temperature. The absorption
and desorption characteristics of these alloys are summarized below
in Table 3. TABLE-US-00003 TABLE 3 Annealing Absorption Temp
Annealing Time at 10.degree. C. Desorption at 90.degree. C.
1400.degree. C. 5 minutes 3.57% 2.82% 1400.degree. C. 10 minutes
3.50% 2.78% 1400.degree. C. 20 minutes 3.53% 2.83%
EXAMPLE 4
[0053] To determine the effect of the quenching media on the alloys
of the present invention, several samples of alloy 16
(V.sub.10Ti.sub.40Cr.sub.50) were prepared via arc melting (as
earlier described) and quenched using different quenching media.
The alloys were quenched in water, liquid nitrogen, liquid argon,
and oil. After quenching, each of the samples were cleaned using a
HF/HCl solution. The hydrogen storage measurement results showed no
significant difference in the hydrogen absorption and desorption
characteristics based on the difference in quenching media, except
the oil quenched sample suffered from carbon pick-up as seen from
the Auger-Electron spectroscopy depth profile. The absorption and
desorption characteristics of these alloys are summarized below in
Table 4. TABLE-US-00004 TABLE 4 Annealing Absorption Desorption
Quenching Media Condition at 10.degree. C. at 90.degree. C. Water
1400.degree. C. for 5 min 3.67% 2.61% Liquid Nitrogen 1400.degree.
C. for 5 min 3.66% 2.63% Liquid Argon 1400.degree. C. for 5 min
3.67% 2.62% Oil 1400.degree. C. for 5 min 3.59% 2.55%
EXAMPLE 5
[0054] To determine the effect of the quenching speed on the alloys
of the present invention, three samples of alloy 16
(V.sub.10Ti.sub.40Cr.sub.50) were prepared via arc melting (as
earlier described), annealed at 1400.degree. C. for 5 minutes, and
cooled at different rates. The three samples included a control
(a), a slow cooled sample (b), and a quick quench sample (c). The
control sample was a 10 g ingot, which was annealed at 1400.degree.
C. for 5 minutes and quenched in water. The quick quenched sample
was a 10 g ingot which was ground into several pieces smaller than
the control sample to allow faster quenching in water as compared
to the control sample. The slow cooled sample was a 10 g ingot that
was allowed to cool at room temperature after annealing. The XRD
patterns for the three samples are plotted in FIG. 10. Both the
control (a) and the quick quench samples (c) showed a pure BCC
phase while the slow cool sample exhibited a typical Laves phase
structure (b). The quick quenched sample has an identical Lattice
constant as the control (3.051 .ANG.), but a larger crystallite
size (196 .ANG. vs. 169 .ANG.). PCT isotherms measured for all
three samples (a, b, c) are shown in FIG. 11. The quick quench
sample exhibited the best hydrogen storage capacity and
reversibility of the three samples while the slow cooled sample
having a Laves phase exhibited the worst hydrogen storage capacity
and reversibility of the three samples.
EXAMPLE 6
[0055] To determine the effect of etching the alloys of the present
invention, four samples of alloy 16 (V.sub.10Ti.sub.40Cr.sub.50)
were prepared via arc melting (as earlier described). Sample 1 is
an as cast ingot without any post-treatment (annealing or
quenching). Sample 2 was annealed at 1400.degree. C. and quenched
without any surface cleaning. Sample 3 was annealed at 1400.degree.
C. for 5 minutes and water quenched with subsequent mechanical
filing to remove the surface oxide from the ingot. Sample 4 was
annealed at 1400.degree. C. for 5 minutes, water quenched, with
subsequent etch in HF+HCl, which was able to remove more of the
surface oxide than the mechanical filing. After preparation, the
samples were tested for hydrogen absorption and desorption
characteristics. Removal of the surface oxide of the ingots made no
significant difference in the reversible storage capacities of the
alloys, however, the total hydrogen storage capacity of the alloys
improved with more of the surface oxide being removed from the
surface of the ingot. The hydrogen storage measurements for the
samples are summarized below in Table 5. TABLE-US-00005 TABLE 5
Process Absorption at 30.degree. C. Desorption at 90.degree. C. As
cast 3.36% 2.49% Annealed w/water quench 3.49% 2.63% (WQ) Annealed
w/WQ and 3.63% 2.62% mechanically filed Annealed w/WQ and acid
3.80% 2.66% etch
EXAMPLE 7
[0056] To compare the hydrogen storage capacities between the
alloys of the present invention based on vanadium content, one
alloy from each family (low vanadium, medium vanadium, high
vanadium) was selected and tested for hydrogen storage capacity.
The samples selected were V.sub.10Ti.sub.40Cr.sub.50 (low vanadium
content), V.sub.80Ti.sub.10Cr.sub.10 (high vanadium content), and
V.sub.23Ti.sub.30Cr.sub.41Mn.sub.3Fe.sub.3 (medium vanadium
content). The samples were prepared via arc melting (as earlier
described), annealed at 1400.degree. C. for 5 minutes, water
quenched, and acid etched. The samples were first activated in 3MPa
hydrogen with cooling from 300.degree. C. to 30.degree. C., the
hydrogen pressure was increased from 3MPa to 10MPa, and then cooled
down to 10.degree. C. to measure the total hydrogen storage
capacity. To desorb the hydrogen, the samples were heated to
90.degree. C. and PCT measurements were performed followed by a
second measurement at 110.degree. C. The hydrogen storage
capacities for the alloys are shown below in Table 6.
TABLE-US-00006 TABLE 6 Absorption Absorption Desorption Desorption
Alloy sample at 30.degree. C. at 10.degree. C. at 90.degree. C. at
110.degree. C. low vanadium 3.66% 3.69% 2.63% 2.71% high vanadium
3.65% 3.68% 2.45% medium 3.49% 3.57% 2.84% 3.01% vanadium
EXAMPLE 8
[0057] To compare the absorption/desorption rates of the alloys of
the present invention, a sample of a low vanadium alloy
(V.sub.10Ti.sub.40Cr.sub.50) and a sample of a medium vanadium
alloy (V.sub.23Ti.sub.30Cr.sub.41Mn.sub.3Fe.sub.3) were prepared
via arc melting (as described earlier), annealed at 1400.degree. C.
for 5 minutes, and water quenched. The absorption/desorption rate
for the low vanadium alloy is shown in FIG. 12, and the
absorption/desorption rate for the medium vanadium alloy is shown
in FIG. 13. The absorption/desorption rate for the low vanadium
alloy were better than the absorption/desorption rate for the
medium vanadium content alloy, however, in either case 80%
absorption and desorption for either alloy can be obtained within 3
minutes.
[0058] While there have been described what are believed to be the
preferred embodiments of the present invention, those skilled in
the art will recognize that other and further changes and
modifications may be made thereto without departing from the spirit
of the invention, and it is intended to claim all such changes and
modifications as fall within the true scope of the invention.
* * * * *