U.S. patent application number 10/093021 was filed with the patent office on 2002-08-22 for palladium-boron alloys and methods for making and using such alloys.
Invention is credited to Imam, M. Ashraf, Miles, Melvin H..
Application Number | 20020114725 10/093021 |
Document ID | / |
Family ID | 26900250 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020114725 |
Kind Code |
A1 |
Miles, Melvin H. ; et
al. |
August 22, 2002 |
Palladium-boron alloys and methods for making and using such
alloys
Abstract
A palladium-boron composition and methods of making and using
same are provided. In one aspect, the invention comprises an alloy
comprising palladium and boron, the boron being in solid solution
in the palladium and the alloy having a two-phase structure,
wherein each phase of the two-phase structure has the same crystal
structure as the other phase and has a different set of lattice
parameters from the other phase such that the palladium is greatly
hardened by the presence of the smaller phase crystals within the
spaces between the larger phase crystals. The composition is
carefully prepared by a process wherein palladium and an amount of
boron sufficient to place the boron in solid solution, but
insufficient to combine with the palladium, are placed together and
repeatedly arc melted, cooled and turned over until sufficiently
mixed. The hardened composition can be used to create thinner
membranes for hydrogen purification and improved electrodes for
generation of heat energy, and other electrochemical processes.
Inventors: |
Miles, Melvin H.;
(Ridgecrest, CA) ; Imam, M. Ashraf; (Great Falls,
VA) |
Correspondence
Address: |
NAVAL RESEARCH LABORATORY
ASSOCIATE COUNSEL (PATENTS)
CODE 1008.2
4555 OVERLOOK AVENUE, S.W.
WASHINGTON
DC
20375-5320
US
|
Family ID: |
26900250 |
Appl. No.: |
10/093021 |
Filed: |
March 8, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10093021 |
Mar 8, 2002 |
|
|
|
09651270 |
Aug 30, 2000 |
|
|
|
60205255 |
May 19, 2000 |
|
|
|
Current U.S.
Class: |
420/463 ;
75/10.49 |
Current CPC
Class: |
C22C 1/02 20130101; B01D
71/022 20130101; B01D 2323/08 20130101; C22C 5/04 20130101; B01D
2323/12 20130101; C01B 3/505 20130101 |
Class at
Publication: |
420/463 ;
75/10.49 |
International
Class: |
C22C 005/04 |
Claims
What is claimed is:
1. An alloy comprising palladium and boron, wherein said boron is
in solid solution in said palladium, and wherein the alloy has a
two-phase structure, comprising a first phase and a second phase,
wherein each phase of the two-phase structure has the same crystal
structure as the other phase and has either smaller or larger
lattice parameters from the other phase.
2. An alloy according to claim 1, wherein the boron comprises about
0.1 to about 0.8 percent by weight of the alloy and wherein the
palladium comprises about 99.2 to about 99.9 percent by weight of
the alloy.
3. An alloy according to claim 1, wherein the amount of alloy in
the phase having smaller lattice parameters than the other phase
increases as the amount of boron in the alloy is increased.
4. An alloy according to claim 1, wherein the diameter of the
crystallites in the first phase is in the range of about 10 to
about 100 Angstroms.
5. An alloy consisting essentially of palladium and boron, wherein
said boron is in solid solution in said palladium, and wherein the
alloy has a two-phase structure, comprising a first phase and a
second phase, wherein each phase of the two-phase structure has the
same crystal structure as the other phase and has either smaller or
larger lattice parameters from the other phase.
6. A method of preparing a two-phase palladium-boron composition,
said method comprising the steps of: (a) placing boron in a
compartment evacuated of air and filled with another gas; (b)
placing palladium in a cavity overlying the boron in said
compartment, wherein the amount of boron is insufficient to form a
compound of boron in the palladium and is sufficient to react with
oxygen in the palladium while said boron is in solid solution with
said palladium; (c) melting the boron and palladium together to
form a mixture; (d) cooling the mixture for producing a solidified
mixture; (e) turning the mixture over for reversing the vertical
locations of the top and bottom portions of the mixture; and (f)
repeating steps (c) through (e) until a mixture at the desired
homogeneity is attained.
7. A method according to claim 6 wherein the palladium is
introduced in the form of palladium sponge and the boron is
introduced in the form of a powder.
8. A method according to claim 6 wherein the boron and palladium
are melted using electric arc means to create an electric arc for
melting the boron and palladium.
9. A method according to claim 6, wherein the method further
comprises swaging the mixed alloy for reducing the diameter of the
alloy.
10. A method according to claim 9, wherein the method further
comprises the steps of annealing the alloy for reducing residual
stress, and cooling the alloy.
11. A method according to claim 6, wherein steps a)-f) occur in the
order specified.
12. A method according to claim 8, wherein the boron and palladium
are placed on a copper hearth in a chamber, and wherein said copper
hearth is part of the arc melting means.
13. A method according to claim 6, wherein the melting of the boron
and palladium is carried out at a temperature between about
2079.degree. C. and about 2200.degree. C. for a time period between
about 4 and about 10 minutes.
14. A method according to claim 6, wherein steps c) to e) are
repeated about 3-10 times.
15. A method of preparing a palladium-boron composition according
to claim 10, wherein the annealing is performed between about
650.degree. C. and about 700.degree. C. for less than about three
hours.
16. An alloy according to claim 1 wherein said alloy is in the form
of a membrane.
17. A method of using the membrane of claim 16, wherein the
membrane is used for the purification of hydrogen.
18. An alloy according to claim 1 wherein said alloy is in the form
of an electrode.
19. A method of using the electrode of claim 18, wherein the
electrode is loaded with deuterium and wherein the electrode
generates energy in the form of heat.
Description
[0001] This application is a non-provisional application and claims
priority under a provisional application Serial No. 60/205,255,
filed on May 19, 2000.
FIELD OF THE INVENTION
[0002] The present invention generally relates to processes for the
production of a high-strength alloy that may be used as a gas
purification membrane, as an electrode for numerous applications
including the generation of heat energy or other electrochemical
processes, and more particularly to the preparation and use of
two-phase palladium-boron alloys which have greater strength and
hardness than other palladium metals or alloys and which thus can
be advantageously utilized in a variety of applications including
hydrogen purification membranes or electrodes.
BACKGROUND OF THE INVENTION
[0003] With increased use of electrical processes and hardware,
processes utilizing the excellent reliability and conductivity of
palladium, an extremely valuable but expensive metal, have become
of increasing importance, particularly when used as an electrode.
However, it has long been known that the hardness of palladium is
often less than optimal for many of these processes. Accordingly,
there has been a distinct need in this field to develop palladium
alloy electrodes that are harder and more resilient than pure
palladium while still offering the superior electrical
characteristics of pure palladium.
[0004] In addition, interest has increased in the quick and
efficient production of hydrogen, which has, because of its many
industrial and scientific applications, assumed greater importance.
Hydrogen is typically purified from surrounding gas by using a
membrane permeable to hydrogen, but not to the other gases. In this
process, the hydrogen passes through the membrane and is collected
on the other side. With respect to hydrogen production, there is
much interest in methods of increasing the hardness and durability
of these membranes which are again typically composed of palladium.
One proposed solution to overcoming the hardness problem would be
to harden the palladium metal without affecting its hydrogen
purification characteristics, which would allow for thinner
membranes than those of pure palladium. This would allow either the
same amount of hydrogen to be purified at a great cost savings, or
a larger amount of hydrogen could be purified for the same cost.
However, suitable methods for developing palladium or palladium
alloys with sufficient hardness have not yet been achieved.
[0005] Further, the demand for energy increases each year while the
world's natural energy sources such as fossil fuels are finite and
are being used up. Accordingly, the development of alternative
energy sources is very important and a number of potential new
energy sources are under study. Although there have been many
attempts to develop a palladium compound which can be utilized in
processes to generate heat, such as through the introduction of
aqueous deuterium, none of these attempts have been successful or
repeatable, and there is thus a distinct need to develop palladium
alloys which can be utilized for the generation of heat as a
potential energy source.
[0006] Previously, it has been known to prepare single-phase alloys
made of palladium and other minor elements. For example, the prior
art includes various palladium alloys which include boron, such as
Weber et al. U.S. Pat. No. 5,518,556 (a boron-containing surface
layer), Hough et al. U.S. Pat. No. 4,341,846 (an electroless
boron/palladium plating material), Smith Jr. et al., U.S. Pat. No.
4,396,577 (a brazing alloy containing boron, palladium and other
metals) and Prosen U.S. Pat. No. 4,046,561 (an alloy for porcelain
applications containing boron, palladium and other metals).
[0007] However, what is lacking in the prior art is a pure
boron/palladium composition of sufficient strength to be used as a
reactive structure rather than a coating material, and which may be
used in thin hydrogen purification membranes or as an electrode in
a heat-generating process. There thus remains a distinct need to
develop palladium alloys which can be utilized advantageously in a
variety of applications where pure palladium is unsuitable either
because of the expense or insufficient hardness.
SUMMARY OF THE INVENTION
[0008] In accordance with the invention, there is provided a
two-phase alloy comprised of palladium and boron wherein the boron
is in solid solution in the palladium and wherein each phase of the
two-phase structure has the same crystal structure as the other
phase but has a different set of lattice parameters from the other
phase. In addition, a method of preparing the two-phase alloy of
the invention is also provided wherein the boron in powder form is
preferably placed in an airless compartment, palladium in sponge
form is placed in the compartment overlying the boron, the boron
and palladium are melted together to form a mixture via a heating
apparatus such as an electric arc, the mixture is cooled to
solidification, turned over for complete mixing, and the melting,
cooling and turning process is preferably repeated until a mixture
with the desired homogeneity is attained. In the preferred process,
the amount of boron is such that it is insufficient to form a
compound of boron in the palladium, but sufficient to remain in
solid solution with the palladium.
[0009] In the particularly preferred embodiment, the composition of
the present invention comprises 0.1 to 0.8 by weight percent boron,
and 99.2 to 99.9 percent by weight percent palladium, and the
palladium and boron comprise at least 99.9% of the composition. It
is also preferred that the second phase forms crystallites which
are on average at least twice as large as the crystallites of the
first phase, and that the diameter of the crystallites in the first
phase is in the range of 10 to 100 Angstroms.
[0010] In a particularly preferred method or preparation in
accordance with the invention, the palladium and boron are placed
on a copper hearth in a mixing chamber which is part of an arc
melting means. The arc melting is then performed between about
2079.degree. C. and 2200.degree. C., for a period of between about
4 and 10 minutes. The melting, cooling and turning steps are
preferably repeated roughly 3-10 times. After a complete mixture
results from melting, turning, and cooling, the composition may
also be swaged to reduce the diameter of the alloy. The alloy is
annealed at elevated temperature to reduce the residual stress, and
then undergoes a final cooling to room temperature. The annealing
is performed between about 650 and 700.degree. C., and for less
than about three hours.
[0011] Preferably the alloy composition of the present invention
can be formed into a membrane for use in the purification of
hydrogen, or can be made into an electrode useful for numerous
purposes, including the loading of the electrode with deuterium for
the generation of heat energy, or other standard electrochemical
purposes.
[0012] Further features and advantages of the present invention
will be set forth in, or apparent from, the detailed description of
preferred embodiments which follows below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view of the process of preparing a
palladium-boron alloy in accordance with the present invention.
[0014] FIG. 2 is a transmission electron micrograph of a
palladium/0.62% boron alloy composition in accordance with the
present invention which shows the two phases.
[0015] FIG. 3a is a graphic representation of an X-ray diffraction
pattern of palladium/0.18% boron.
[0016] FIG. 3b is a graphic representation of an X-ray diffraction
pattern of palladium/0.38% boron.
[0017] FIG. 3c is a graphic representation of an X-ray diffraction
pattern of palladium/0.62% boron.
[0018] FIG. 3d is a graphic representation of an X-ray diffraction
pattern of pure palladium.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] In accordance with the present invention, there is provided
a composition comprising a two-phase alloy of palladium and boron
which has superior tensile strength and hardness when compared to
pure palladium, and which can thus be used in a variety of
applications, including use as a hydrogen purification membrane or
as an electrode in numerous electrochemical processes.
[0020] In the preferred process of producing the palladium/boron
alloy of the present invention, as shown in the flowchart of FIG.
1, there is a series of preferred steps for making the
boron-palladium composition with a two-phase crystalline structure.
In this preferred process, boron in powder form is first placed
within a structure capable of producing heat sufficient to melt
metal alloys, such as on a copper plate, or hearth, within a
compartment filled with argon, after evacuation of the compartment,
suitable for arc melting and cooling. Copper is preferably used as
the material for the hearth because of copper's excellent
conductivity for the arc melting phase. The copper hearth is cooled
with water from below to keep it from melting into the alloy as it
has a lower melting point than either boron or palladium. The
hearth, and the mixture resting upon it, are cooled by the transfer
of heat from the composition through the hearth to the coolant
water. The system is not reliant on air cooling because of its
water cooling means. The entire process preferably occurs in the
chamber which is filled with argon gas after evacuating the
chamber. Exposure to air cannot be allowed because oxygen or air
would oxidize the palladium and the boron, ruining the process. The
process is usually performed in a container with a noble gas,
typically argon. In this process, palladium is placed in the
compartment 12 so that it is overlying the boron powder in the
cavity. The palladium is preferably introduced in a pure palladium
sponge form.
[0021] The palladium-boron alloy produced in accordance with the
present invention preferably comprises about 0.05 to 2.0 percent by
weight boron, most preferably 0.1 to 0.8 weight percent, and from
about 98.0 to 99.95 percent by weight percent palladium, most
preferably from about 99.2 to 99.9 weight percent. The amount of
boron present in the cavity is fixed so that it is small enough not
to form a compound of boron in the palladium but is sufficient to
react with oxygen in the palladium while the boron is in solid
solution in the palladium. Very small amounts of the composite are
used herein. Typically, there will be 100 g of palladium and an
amount of boron in accordance with the ranges as set forth
above.
[0022] In the preferred process, after the palladium and boron are
placed within the cavity, a heating source such as an electric arc
is used to melt the boron and palladium together to form a mixture
14. Although there are several other well-known forms of heating
that would be suitable for the present invention, an electric arc
is preferably used because it offers rapid control which prevents
the Boron from vaporizing prematurely. The palladium is preferably
placed on top of the boron because of the differences in density.
The denser palladium prevents splatter or flying of the boron.
Additionally, the lower melting point of palladium allows it to
form a protective shell over the boron before the boron begins
melting. A melting arc is generated between a tungsten tip just
above the mixture and the conductive copper hearth upon which it
rests.
[0023] The melting point of boron, the higher of the pair, is
2079.degree. C. so the melting must be done at least at this
temperature. However, the temperature preferably should not exceed
about 2200.degree. C. or the boron will begin to vaporize. The
melting time should be about 5 minutes. A melting time greater than
10 minutes at the aforementioned temperature would result in
vaporization of a portion of the boron.
[0024] After the palladium and boron are melted, the mixture is
cooled The mixture solidifies upon cooling 16. The mixture is
preferably turned over after cooling and solidification 18 to
provide a more homogeneous mixture by preventing settling of the
boron and palladium into layers. The mixture is turned over with
the tungsten tip of the arc mechanism.
[0025] After the initial melting and cooling to bind the loose
boron powder to the palladium, the melting step 14, cooling step
16, and turning over step 18 (as shown in FIG. 1) are repeated as
often as necessary to eliminate any boron or palladium pockets, and
for thoroughly mixing the mixture into a solution of a desired
homogeneity to be attained suitable for commercial use. The steps
are preferably repeated anywhere from about 3-10 times, with about
4-5 times particularly preferred, and the solution is then mixed by
the use of a combination of gravity and random movement, followed
by repeated melting, cooling and turning cooled solid solution
over.
[0026] After a homogeneous mixture is prepared, the material
preferably undergoes an additional step of swaging 20, if so
desired, for reducing the alloy to a fixed diameter. A die is
preferably used and the metal is forced into a bar shape of the
appropriate diameter. Swaging is particularly useful when producing
an electrode from the raw alloy material.
[0027] After the material undergoes swaging 20, the rods are cut
into appropriate lengths and machined into usable electrode form.
The alloy is annealed 22 to reduce the residual stress. The alloy
begins at room temperature and is heated to approximately
650.degree. C. for approximately 2 hours. The time and temperature
in this step are important because too high a time or temperature
would result in a larger grain size of the composition which would
detract from hardness and render the composition ineffective. For
example, temperatures at 700.degree. C. and above would begin to
produce undesirable results in the final alloy. Grain size would
also become dangerously large after approximately 3 hours. After
annealing, the solution is cooled to room temperature in a final
cooling step 24.
[0028] The composition resulting from the above process is a boron
in solid solution in the palladium, with the alloy having a
two-phase structure. As shown in FIG. 2, the first and second
phase, 26 and 28, respectively, of the two-phase structure have the
same crystal structure but different sets of lattice parameters so
that the crystals of the second phase are larger than the crystals
of the first phase. The diameter of the crystallites in the first
phase 26 is in the range of 10 to 100 Angstroms, whereas the
diameter of the crystallites in the second phase 28 is much
larger.
[0029] The differing sizes of the crystals of the phases creates a
"miscibility gap" meaning that the miscibility of the two phases
with each other is high because the crystals of the smaller first
phase can easily rest in gaps between the larger crystals of the
second phase. This "filling" of the gaps of the larger crystals
binds the crystals of both phases together and results in a
hardened composition.
[0030] The amount of boron in the mixture appears to be critical.
It has been found that the amount of boron must be maintained below
2 weight percent of the mixture for solution. Anything more and it
will begin to bond with the palladium, preventing formation of the
two-phases.
[0031] This composition can show the same or better strength than
pure palladium with much less thickness. This is advantageous in
the creation of palladium hydrogen purification membranes because
less palladium would be needed to create a membrane and achieve the
same results using it. This is because sturdy membranes of much
less thickness are enabled by the present invention than would be
possible using palladium alone. The advantage herein is that
because of the additional hardness of the palladium-boron alloy of
the invention, a much smaller amount of expensive palladium may be
used to provide a membrane of the same capacity compared to
palladium alone. Palladium is one of the precious metals and is,
therefore, very costly. This would allow much greater membrane
capacity through reduced material costs. How much the thickness of
the membrane would be able to be decreased with the present
composition would depends upon such factors as design, gases to be
purified, and the extent of purification desired.
[0032] Preferably, the hardened palladium-boron alloy can be made
into a membrane to purify hydrogen. The openings in the palladium
are sufficient to allow hydrogen to pass but not other gases.
Therefore, palladium is commonly used to purify hydrogen using the
membrane, and the gas to be purified is usually placed on one side
and a vacuum is on the other. This pressure differential forces the
hydrogen through the membrane to towards the vacuum. The larger
gases cannot fit through the membrane and are left behind. Further,
the material would be advantageous for purifying hydrogen because
its increased strength offers increased overall membrane
reliability.
[0033] The hardened electrode would also be advantageous for use in
etching, polishing, electrochemical machining, semiconductor wafer
manufacture and other electrochemical processes in which use of a
hardened palladium cathode retaining superior palladium electrical
characteristics is advantageous.
[0034] An additional application of the alloy, which has been borne
out by experimental data is as an electrode in the generation of
energy in the form of heat. In a preferred process using the alloy
of the present invention in the form of an electrode, the electrode
in connected to a platinum cathode and immersed in water containing
deuterium. The immersed electrode is loaded with deuterium from the
surrounding electrolyte. As a current is applied, excess energy
from the loaded electrode in the form of heat is generated. Using
the palladium-boron electrode manufactured in accordance with the
present invention, excess enthalpy has been achieved, and this
result has been far more reproducible than in past experiments of
this type, which may result in a new energy source at low cost.
[0035] Although the invention has been described above in relation
to preferred embodiments thereof, it will be understood by those
skilled in the art that variations and modifications can be
effected without departing from the scope and spirit of the
invention.
EXAMPLES
[0036] Palladium-boron composition samples containing 0.18% boron,
0.38% boron and 0.62% boron were prepared in accordance with the
invention. Platinum sponge was used as a platinum source for its
high purity. The boron source was a powder commonly referred to as
five-nine boron (99.999% pure boron).
[0037] In accordance with the present method, approximately 100 g
of palladium and a corresponding amount of boron giving the
concentrations set forth above were measured. The boron powder and
palladium were placed on a copper hearth within a compartment. The
compartment is airless, with an argon atmosphere. The samples were
melted. A typical arc melting apparatus having 12 v and 300 amps
was used for the melting. The two elements used for the arc melting
are a tungsten tip immediately above the sample and the copper
hearth. The melting was performed at about 2100 c and the melting
time was about 5 minutes. The mixture was then cooled for
approximately 30 minutes and the mixture turned over. The melting,
cooling and turning steps were repeated 4-5 times.
[0038] To make electrodes from the composition, the alloy was
swaged to 0.4 cm diameter. The swaged rods were cut into 3.5 cm
lengths and machined into usable electrode form. The samples were
annealed at approximately 650 c for approximately 2 hours. The
samples were cooled for two hours until they returned to room
temperature.
[0039] Nine samples in electrode form were tested. This testing
centered around the generation of heat with the electrode. Each
palladium-boron electrode was connected to a platinum anode, and
the palladium-boron cathode was then immersed in water containing
deuterium. After immersion, the electrodes were then
electrochemically "loaded" with hydrogen. It is believed extra
loading was possible due to the two-phase structure brought about
by the solution of boron within the palladium. Of nine samples
tested, eight yielded positive results of heat. The results of
these experiments are more repeatable than any experiment of this
type completed thus far. Not surprisingly, amount of heat varied
with, and had a positive relationship to, boron content.
[0040] X-ray diffraction studies were carried out to characterize
the three compositions of the two-phase palladium/boron alloy in
accordance with the present invention. The diffractions were
obtained in a Phillips diffractometer with generator settings of 50
kV, 30 mA and a copper target. Two distinct phases of the same
cubic structure were found in all three compositions of the alloy.
Lattice parameters for the samples were measured. As can be seen in
FIG. 3, the two distinct phases have the same crystal structure but
different lattice parameters. The lattice parameter in a first
phase remains constant with changes in the boron content of the
alloy whereas the lattice parameter of a second phase increases
with an increase in the boron content. As the boron increased, the
amount of crystals in the second phase increases at the expense of
the first phase, as expected.
[0041] The 0.62% boron sample was studied with a transmission
electron microscope. FIG. 2 shows the Selected Area Diffraction
(SAD) pattern. Lattice parameters of the two phases 26, 28 measured
from x-ray diffraction and SAD are consistent, and confirmed the
production of the palladium/boron alloys in accordance with the
present invention.
* * * * *