U.S. patent application number 13/267064 was filed with the patent office on 2012-02-02 for amorphous alloy.
This patent application is currently assigned to Korea Institute of Science and Technology. Invention is credited to Eric FLEURY, Jayaraj Jayamani, Do-hyang Kim, Ki-bae Kim, Kwang-youn Kim, Yu-chan Kim, Mee-soon Lee, Hyun-kwang Seok.
Application Number | 20120024431 13/267064 |
Document ID | / |
Family ID | 37836003 |
Filed Date | 2012-02-02 |
United States Patent
Application |
20120024431 |
Kind Code |
A1 |
FLEURY; Eric ; et
al. |
February 2, 2012 |
AMORPHOUS ALLOY
Abstract
The present invention relates to an amorphous alloy and a method
for manufacturing thereof. The amorphous alloy according to the
present invention includes has a chemical formula of
Ni.sub.100-a-b-c-d-e-fNb.sub.aZr.sub.bTi.sub.cTa.sub.dM.sub.eI.sub.f,
wherein the M is at least one selected from a group of Sn and Si,
wherein the I is at least one selected from a group of C and O, and
wherein the a, b, c, d, e, and f are satisfied with the
compositions of 10.0 wt %.ltoreq.a.ltoreq.25.0 wt %, 5.0 wt
%.ltoreq.b.ltoreq.25.0 wt %, 5.0 wt %.ltoreq.c.ltoreq.10.0 wt %,
0.0 wt %.ltoreq.d.ltoreq.25.0 wt %, 0.0 wt %.ltoreq.e.ltoreq.6.5 wt
%, 0.0 wt %.ltoreq.f.ltoreq.0.5 wt %, respectively.
Inventors: |
FLEURY; Eric; (Seoul,
KR) ; Jayamani; Jayaraj; (Seoul, KR) ; Kim;
Ki-bae; (Seoul, KR) ; Lee; Mee-soon; (Seoul,
KR) ; Seok; Hyun-kwang; (Seoul, KR) ; Kim;
Yu-chan; (Seoul, KR) ; Kim; Kwang-youn;
(Seoul, KR) ; Kim; Do-hyang; (Seoul, KR) |
Assignee: |
Korea Institute of Science and
Technology
Seoul
KR
|
Family ID: |
37836003 |
Appl. No.: |
13/267064 |
Filed: |
October 6, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12066124 |
Aug 12, 2008 |
8070891 |
|
|
PCT/KR0505/004678 |
Dec 30, 2005 |
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13267064 |
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Current U.S.
Class: |
148/403 |
Current CPC
Class: |
C22C 45/02 20130101;
C22C 45/04 20130101; C22C 1/002 20130101 |
Class at
Publication: |
148/403 |
International
Class: |
C22C 45/04 20060101
C22C045/04; C22C 45/00 20060101 C22C045/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2005 |
KR |
10-2005-0084067 |
Claims
1. The amorphous alloy having a chemical formula of
Ni.sub.100-a-b-c-d-e-fNb.sub.aZr.sub.bTi.sub.cTa.sub.dM.sub.eI.sub.f,
wherein the M is at least one selected from a group of Sn and Si,
wherein the I is at least one selected from a group of C and O, and
wherein the a, b, c, d, e, and f are satisfied with the
compositions of 10.0 wt %.ltoreq.a.ltoreq.25.0 wt %, 5.0 wt
%.ltoreq.b.ltoreq.25.0 wt %, 5.0 wt %.ltoreq.c.ltoreq.10.0 wt %,
0.0 wt %.ltoreq.d.ltoreq.25.0 wt %, 0.0 wt %.ltoreq.e.ltoreq.6.5 wt
%, 0.0 wt %.ltoreq.f.ltoreq.0.5 wt %, respectively.
2. The amorphous alloy of claim 1, wherein Vickers microhardness of
the amorphous alloy is 1000 kgf/mm.sup.2 or more, and strength of
the amorphous alloy is in a range from 2500 MPa to 4000 MPa.
3. The amorphous alloy of claim 1, wherein the amorphous alloy is
used as a material for a bipolar plate of a fuel cell.
Description
CROSS-REFERENCES TO RELATED APPLICATION
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 12/066,124, which was filed on Mar. 7, 2008,
which is a National Stage application of PCT/KR2005/004678 filed on
Dec. 30, 2005, which claims priority to Korean Patent Application
No. 10-2005-0084067 filed on Sep. 9, 2005, the contents of which
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] The present invention relates to an amorphous alloy, and
more specifically to an amorphous alloy with good strength and high
corrosion resistance.
[0004] (b) Description of the Related Art
[0005] A fuel cell has been spotlighted as an alternative energy
source due to global warming, pollution, and depletion of oil
resources. The fuel cell is an electric generator. In the fuel
cell, reactants continuously flow into the system while products
are continuously discharged from the system. At the same time,
electric energy is generated. That is, oxygen and hydrogen are
continuously supplied to the fuel cell and a chemical reaction
occurs, thereby generating electrical energy.
[0006] Various types of fuel cells have been developed. The fuel
cells can be classified as high temperature fuel cells and low
temperature fuel cells depending on electrolytes in a unit cell and
operational temperature. The high temperature fuel cells include
molten carbonate fuel cells, solid oxide fuel cells, and so on. The
low temperature fuel cells include phosphoric acid fuel cells,
polymer electrolyte fuel cells, alkaline fuel cells, and so on.
[0007] In particular, a polymer electrolyte membrane fuel cell
(hereinafter referred to as a "PEMFC"), one of the solid polymer
electrolyte fuel cells (hereinafter referred to as a "SPEFC") that
is a low temperature fuel cell, is compact and light-weight, and
has an advantage to be capable of operating at a low temperature.
The PEMFC generates electric power from hydrogen and oxygen through
a polymer membrane consisting of a membrane electrode assembly
(hereinafter referred to as an "MEA"). The MEA is made by combining
the polymer membrane with electrodes located on each side
thereof.
[0008] Each electrode support is made of carbon cloth supporting
electrode materials such as carbon black including platinum
catalysts. A plurality of the MEAs with a thickness of several tens
to several hundreds of micrometers are filled between
multi-functional bipolar plates in order to obtain sufficient
electric power. Therefore, several tens to several hundreds of unit
cells are serially connected to each other and form a fuel cell
stack.
[0009] The bipolar plates of the PEMFC are exposed to severe
operating conditions in which current and stress are applied and
corrosion occurs. Therefore, materials for the bipolar plates are
required to have properties of gas impermeability, high strength,
corrosion resistance, and good electrical conductivity. Currently
used bipolar plates mainly made of a carbon do not have such
properties. In particular, thin metallic bipolar plates with high
strength and low cost are required since bulk graphite bipolar
plates are weak.
[0010] Therefore, research relating to bipolar plates made of
graphite composites and stainless steels as a replacing material of
the bipolar plates has been undertaken. In addition, as a method of
manufacturing bipolar plates, research relating to a thermal
nitriding surface treatment, TiN coating on stainless steel, thin
PVD (physical vapor deposition) coatings, etc., has been
pursued.
[0011] However, there is a problem in that metallic ions are
dissolved and the MEA is poisoned if the electrons are lost in the
bipolar plates. Furthermore, metallic oxides grow in a cathode
since the metallic bipolar plates obtain electrons and then a
recovery reaction occurs. Therefore, there is a problem in that
electrical resistance of the surface increases while performance of
the fuel cell is deteriorated. Materials with high strength and
corrosion resistance have been seriously required in other
circumstances in addition to materials for the fuel cells.
SUMMARY OF THE INVENTION
[0012] This present invention is contrived to solve the
aforementioned problems, and provides an amorphous alloy with high
strength and good corrosion resistance.
[0013] In addition, the present invention provides a method for
manufacturing the aforementioned amorphous alloy.
[0014] An amorphous alloy according to the present invention has a
chemical formula of
Fe.sub.100-a-b-c-d-e-f-gCr.sub.aMo.sub.bC.sub.cB.sub.dY.sub.eM.sub.fI.sub-
.g. The M is at least one selected from a group consisting of Al,
Co, N, and Ni, the I is at least one selected from a group
consisting of Mn, P, S, and O as impurities, and the a, b, c, d, e,
f, and g are satisfied with the compositions of 16.0 wt
%.ltoreq.a.ltoreq.22.0 wt %, 15.0 wt %.ltoreq.b.ltoreq.27.0 wt %,
2.0 wt %.ltoreq.c.ltoreq.3.5 wt %, 1.0 wt %.ltoreq.d.ltoreq.1.5 wt
%, 1.0 wt %.ltoreq.e.ltoreq.3.5 wt %, 0.25 wt %.ltoreq.f.ltoreq.3.0
wt %, and 0.0 wt %.ltoreq.g.ltoreq.0.5 wt %, respectively.
[0015] It is preferable that the above amorphous alloy according to
the present invention includes N in a range from 0.4 wt % to 1.0 wt
%.
[0016] The N may be substantially 0.8 wt %.
[0017] An oxide film may be formed on a surface of the amorphous
alloy, and the oxide film may include the N.
[0018] It is preferable that Vickers microhardness of the amorphous
alloy is 1000 kgf/mm.sup.2 or more, and that strength of the
amorphous alloy is in a range from 2500 MPa to 4000 MPa.
[0019] It is preferable that a wetting angle of the amorphous alloy
is in a range from 80.degree. to 100.degree..
[0020] It is preferable that the amorphous alloy is used as a
material for a bipolar plate of a fuel cell.
[0021] Another amorphous alloy according to the present invention
has a chemical formula of
Ni.sub.100-a-b-c-d-e-fNb.sub.aZr.sub.bTi.sub.cTa.sub.dM.sub.eI.sub.f.
Here, the M is at least one selected from a group of Sn and Si, the
I is at least one selected from a group of C and O, and the a, b,
c, d, e, and f are satisfied with the compositions of 10.0 wt
%.ltoreq.a.ltoreq.25.0 wt %, 5.0 wt %.ltoreq.b.ltoreq.25.0 wt %,
5.0 wt %.ltoreq.c.ltoreq.10.0 wt %, 0.0 wt %.ltoreq.d.ltoreq.25.0
wt %, 0.0 wt %.ltoreq.e.ltoreq.6.5 wt %, 0.0 wt
%.ltoreq.f.ltoreq.0.5 wt %, respectively.
[0022] A method for manufacturing the amorphous alloy according to
the present invention is a method of manufacturing the amorphous
alloy having the above composition. The method for manufacturing
the amorphous alloy according to the present invention includes
steps of manufacturing a mixture in which each of elements are
mixed together to have a composition having a chemical formula of
Fe.sub.100-a-b-c-d-e-f-gCr.sub.aMo.sub.bC.sub.cB.sub.dY.sub.eM.sub.fI.sub-
.g, arc melting the mixture at a temperature of 3000.degree. C. or
more, manufacturing the amorphous alloy by suction casting the
melted mixture, processing a luster finish on a surface of the
amorphous alloy, and annealing the amorphous alloy for 5 to 15
minutes.
[0023] It is preferable that the glass transition temperature
(T.sub.g) of the amorphous alloy is in the range from 550.degree.
C. to 610.degree. C. in the manufacturing of the amorphous alloy by
suction casting the melted mixture.
[0024] The amorphous alloy may be annealed at a temperature of
0.6T.sub.g to 0.8T.sub.g in the annealing of the amorphous alloy
for 5 to 15 minutes.
[0025] Another method for manufacturing the amorphous alloy
according to the present invention is a method for manufacturing
the amorphous alloy having the above composition. The method for
manufacturing the amorphous alloy according to the present
invention includes steps of manufacturing a mixture in which each
of elements are mixed together to have a composition having a
chemical formula of
Ni.sub.100-a-b-c-d-e-fNb.sub.aZr.sub.bTi.sub.cTa.sub.dM.sub.eI.sub.f,
arc melting the mixture at a temperature of 3000.degree. C. or
more, manufacturing the amorphous alloy by suction casting the
melted mixture, processing a luster finish on a surface of
amorphous alloy, and annealing the amorphous alloy for 5 to 15
minutes.
Advantageous Effects
[0026] Since the Fe-based and Ni-based amorphous alloys according
to the present invention have high strength and good corrosion
resistance, they are suitable for being used as a bipolar
plate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is an XRD graph of the Fe-based and Ni-based
amorphous alloys.
[0028] FIG. 2 shows DSC (differential scanning calorimetry) traces
of the Fe-based and Ni-based amorphous alloys.
[0029] FIG. 3 is a potentio-dynamic graph measuring an
Fe.sub.50Cr.sub.18Mo.sub.8C.sub.14B.sub.6Y.sub.2Al.sub.2 alloy, a
Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2 alloy, and
stainless steel under 1M H.sub.2SO.sub.4+2 ppm F.sup.- at
75.degree. C. with hydrogen bubbling.
[0030] FIG. 4 is a potentio-dynamic graph measuring stainless
steel, a Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2
alloy, and an Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2
alloy under 1M H.sub.2SO.sub.4+2 ppm F.sup.- at 75.degree. C. with
air bubbling.
[0031] FIG. 5 is a potentio-dynamic graph measuring an
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Ni.sub.2 alloy and
a Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy under 1M
H.sub.2SO.sub.4+2 ppm F.sup.-, at 80.degree. C. with hydrogen
bubbling.
[0032] FIG. 6 is a potentio-dynamic graph measuring an
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2CO.sub.2 alloy and
a Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy under 1M
H.sub.2SO.sub.4+2 ppm F.sup.-, at 80.degree. C. with air
bubbling.
[0033] FIG. 7 is a graph illustrating a variation of chemical
compositions at a surface of an
Fe.sub.50Cr.sub.18Mo.sub.8C.sub.14B.sub.6Y.sub.2Al.sub.2 alloy.
[0034] FIG. 8 is a graph illustrating a variation of chemical
compositions at a surface of an
Fe.sub.50Cr.sub.18Mo.sub.8C.sub.14B.sub.6Y.sub.2Al.sub.2 alloy
after it is etched under 1M H.sub.2SO.sub.4+2 ppm F.sup.-,
80.degree. C. with air bubbling.
[0035] FIG. 9 is a graph illustrating a variation of chemical
compositions at a surface of an
Fe.sub.50Cr.sub.18Mo.sub.8C.sub.14B.sub.6Y.sub.2Al.sub.2 alloy
after it is etched under 1M H.sub.2SO.sub.4+2 ppm F.sup.-,
80.degree. C. with air bubbling.
[0036] FIGS. 10(A) and 10(B) are graphs illustrating a variation of
contact resistance of stainless steel, graphite, and Fe-based and
Ni-based amorphous alloys caused by a compaction force.
[0037] FIG. 11 is a graph illustrating a variation of a viscosity
of an Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2
alloy and a Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2
alloy depending on their temperature.
[0038] FIG. 12 is a schematic perspective view of a stack of a fuel
cell provided with a bipolar plate made of amorphous alloys
according to the present invention.
[0039] FIG. 13 is a graph illustrating a current density variation
of Fe.sub.45-xCr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2M.sub.x
alloys under 1M H.sub.2SO.sub.4+2 ppm F.sup.-, 80.degree. C. with
hydrogen bubbling at a cathode.
[0040] FIG. 14 is a graph illustrating a current density variation
of Fe.sub.45-xCr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2M.sub.x
alloys under 1M H.sub.2SO.sub.4+2 ppm F.sup.-, 80.degree. C. with
air bubbling at an anode.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Hereinafter, embodiments of the present invention will be
described with reference to the attached drawings. The embodiments
are merely to illustrate the present invention and the present
invention is not limited thereto.
[0042] An amorphous alloy according to the present invention can be
applied to a bipolar plate of fuel cells and so on. Hereinafter,
although the amorphous alloy is mainly explained by using it as a
material for the bipolar plate, this is merely to explain a use of
the amorphous alloy, and the present invention is not limited
thereto. Therefore, the amorphous alloy according to the present
invention can be used in an environment which requires high
strength and good corrosion resistance.
[0043] Since the amorphous alloy has very slow crystal nucleation
rate and growth rate, it can be supercooled at a temperature that
is much lower than its melting point. Since a supercooled liquid
has a high viscosity, the atoms are not rearranged to form a
crystal but maintain irregular arrangement to form an amorphous
phase. Therefore, the amorphous alloy has a high forming ability at
a supercooled liquid region. In addition, since the amorphous alloy
has properties of high strength, high hardness, soft magnetism,
good wear resistance, good corrosion resistance and so on, it is
suitable to be applied to the bipolar plate, etc.
[0044] In contrast with the amorphous alloy, a general alloy is
quickly crystallized at a temperature that is directly below
melting point when it is cooled in a liquid phase. In this case,
most of the alloys do not form single crystals, but form a
polycrystals that have various shapes and sizes.
[0045] In addition, the amorphous alloy has a high elastic
deforming ability. A stress and strain rate of the amorphous alloy
has a linear relationship with a slope of a range from 0.9 to 1.0.
The slope is much greater than that of a general alloy, which is
0.3. This means that the amorphous alloy has ideal forming ability
even at an under-cooled liquid phase where a deformation rate is
high. The amorphous alloy is easily deformed at a relatively low
temperature even by a low stress in comparison to a general
alloy.
[0046] In particular, when the amorphous alloy is used in the
bipolar plate, it is easy to form gas channels for circulating
hydrogen, oxygen, and water on the amorphous alloys at a
supercooled liquid phase. The gas channels can be formed by a
micropore forming method at a temperature between a glass
transition temperature (T.sub.g) and a crystallization temperature
(T.sub.x).
[0047] In a PEMFC, water is generated by a mixing reaction between
hydrogen and oxygen. Hydrogen cations are generated at an anode
while oxygen electrons are generated at a cathode. Electrons exit
from hydrogen atoms generate electric power, while the hydrogen
cations pass through a polymer electrolyte membrane and are
combined with oxygen electrons to generate water molecules.
[0048] The polymer electrolyte membrane is usually made of
perfluorinated sulfonic acid containing sulfur and fluorine. In
addition, S and F ions are removed from an electrode, forming a
solution similar to that of H.sub.2SO.sub.4+2 ppm F.sup.-. Reaction
temperature is limited to about 80.degree. C. in the PEMFC in order
to prevent vapor from being generated. In the bipolar plate, one
side thereof is exposed to hydrogen gas with a potential of -0.1V
while the other side thereof is exposed to oxygen gas with a
potential of 0.6V. Therefore, in spite of some variables, polymer
electrolyte membrane fuel cells operate under a condition in which
a relatively constant potential is maintained with a value of about
-0.1V and 0.6V at the anode and cathode, respectively.
[0049] A method for manufacturing the amorphous alloy will be
explained below.
[0050] First, elements that have desired compositions are mixed
together. For example, in a case of an Fe-based amorphous alloy,
each element is mixed to make a chemical composition of the
Fe-based amorphous alloy to be
Fe.sub.100-a-b-c-d-e-f-gCr.sub.aMo.sub.bC.sub.cB.sub.aY.sub.eM.sub.fI.-
sub.g. Here, the M is at least one selected from a group consisting
of Al, Co, N, and Ni, and the I is at least one selected from a
group consisting of Mn, P, S, and O as impurities. The a, b, c, d,
e, f, and g are satisfied with the compositions of 16.0 wt
%.ltoreq.a.ltoreq.22.0 wt %, 15.0 wt %.ltoreq.b.ltoreq.27.0 wt %,
2.0 wt %.ltoreq.c.ltoreq.3.5 wt %, 1.0 wt %.ltoreq.d.ltoreq.1.5 wt
%, 1.0 wt %.ltoreq.e.ltoreq.3.5 wt %, 0.25 wt %.ltoreq.f.ltoreq.3.0
wt %, and 0.0 wt %.ltoreq.g.ltoreq.0.5 wt %, respectively. In
addition, in a case of the Ni-based amorphous alloy, each element
is mixed to make a chemical composition of the Ni-based amorphous
alloy to be
Ni.sub.100-a-b-c-d-e-fNb.sub.aZr.sub.bTi.sub.cTa.sub.dM.sub.eI.sub.f.
Here, the M is at least one selected from a group consisting of Sn
and Si, and the I is at least one selected from a group consisting
of C and O. The a, b, c, d, e, and f are satisfied with the
compositions of 10.0 wt %.ltoreq.a.ltoreq.25.0 wt %, 5.0 wt
%.ltoreq.b.ltoreq.25.0 wt %, 5.0 wt %.ltoreq.c.ltoreq.10.0 wt %,
0.0 wt %.ltoreq.d.ltoreq.25.0 wt %, 0.0 wt %.ltoreq.e.ltoreq.6.5 wt
%, 0.0 wt %.ltoreq.f.ltoreq.0.5 wt %.
[0051] Next, a plate-shaped Fe-based bulk amorphous alloy or a
Ni-based bulk amorphous alloy is manufactured by suction casting
the above mixtures using a suction casting method. More
specifically, after the mixtures are arc melted at 3000.degree. C.
or more, they are suctioned by using a vacuum pump while being kept
in an arc melting mold by surface tension. Then, a plate-shaped
sample with a width of 5-8 mm and a length of 5-10 mm is
manufactured by filling the mixture in a copper mold. The suction
molding method is used in order to obtain an amorphous alloy with a
desired shape.
[0052] The amorphous alloy according to the present invention may
be manufactured by using a rapid solidification method in addition
to the aforementioned suction molding. It is possible to use plane
casting or die casting methods considering quality and shape of the
manufactured products.
[0053] Luster finishing is applied to a surface of the amorphous
alloy manufactured by the aforementioned method so that surface
roughness is lowered and microporosity is not generated. Next, the
bipolar plate is annealed at a temperature of about 0.6T.sub.g to
0.8T.sub.g for about 5 to 15 minutes in order to release residual
stresses.
[0054] Among the amorphous alloys, Fe-based and Ni-based amorphous
alloys are the same as stainless steel in terms of electrical
resistance. However, strength and corrosion resistance of the
amorphous alloys are three times or more those of the stainless
steel. Therefore, if the bipolar plate is made of the amorphous
alloys, electric power generation efficiency is not reduced even if
the fuel cell is used for a long time. In particular, since the
Fe-based amorphous alloys are not only inexpensive but also have
low contact resistance and high corrosion resistance, it is
suitable for the bipolar plate. On the other hand, since the
Fe-based amorphous alloy is a soft magnetic material, the Fe-based
amorphous alloy may be used for a transformer, a magnetic head,
etc.
[0055] Considering corrosion characteristics of a metallic alloy,
an oxide film, which is referred to as a passive film, is formed on
a surface of the alloy. The oxide film increases contact resistance
that is required in the bipolar plate. Therefore, it is necessary
to not increase contact resistance by forming the oxide film as
thin as possible. The oxide film formed on the bipolar plate is
thinner than that formed on a bipolar plate made of a stainless
steel. Therefore, the oxide film formed on the bipolar plate has
high corrosion resistance and low contact resistance.
[0056] Since the amorphous alloy according to the present invention
includes a predetermined amount of N, corrosion resistance thereof
is greatly improved. Therefore, it is better to use the amorphous
alloy as a material of the bipolar plate. The N significantly
influences the corrosion resistance under an HCl solution and an
H.sub.2SO.sub.4 solution which is similar to the environment of the
fuel cell.
[0057] In particular, the N improves the corrosion resistance of
the amorphous alloy in a solution including Cl. The N significantly
influences the corrosion resistance under an HCl solution and an
H.sub.2SO.sub.4 solution. If an amount of N is 0.1 wt % or more,
the corrosion resistance of an alloy increases. If an amount of N
is 0.2 wt % or less, although the alloy has an amorphous structure,
it is difficult to manufacture ingots since the N is easily
vaporized. Therefore, it is preferable that the content of the N is
controlled in a range from 0.4 wt % to 1.0 wt %, and it is more
preferable that the content of N is maintained at 0.8 wt %.
[0058] As described above, a bipolar plate, which is suitable for a
peripheral environment of the fuel cell, can be manufactured by
adding a suitable amount of N. When the bipolar plate is
manufactured by using the amorphous alloy including the
aforementioned amount of N, performance of the fuel cell is good
since the bipolar plate has good corrosion resistance.
[0059] Especially, if both Mo and N are added to the Fe-based
amorphous alloy according to the present invention, synergetic
effects in the corrosion resistance and glass forming ability
occur. Therefore, it is preferable that both Mo and N are
added.
[0060] Since the Fe-based amorphous alloy according to the present
invention includes both Cr and Mo, it has good corrosion
resistance. Considering a formation of the amorphous phase and
corrosion resistance, it is preferable that the Fe-based amorphous
alloy includes Cr at 16.0 wt % to 22.0 wt % and Mo at 15.0 wt % to
27.0 wt %. Cr contributes to improvement of the corrosion
resistance and Mo contributes to improvement of formation of an
amorphous phase. Composition ranges of Cr and Mo depend on a
process reaction. The Fe-based amorphous alloy has a corrosion
resistance that is better than that of the stainless steel in an
HCl solution.
[0061] If the content of Cr included in the Fe-based amorphous
alloys is less than 16.0 wt %, it is difficult to use it as the
bipolar plate since its corrosion resistance is low. In addition,
if the content of Cr is more than 22.0 wt %, it is difficult to
form an amorphous phase.
[0062] If the content of Mo included in the Fe-based base amorphous
alloy is less than 15.0 wt %, it is difficult to form an amorphous
phase. Further, if the content of Mo is more than 27.0 wt %,
corrosion resistance of the Fe-based amorphous alloy is reduced. In
particular, considering both amorphous phase forming ability and
corrosion resistance, it is most preferable that about 15 wt % of
the Mo is added.
[0063] Under an environment of the fuel cell including 1M
H.sub.2SO.sub.4+2 ppm F.sup.- at 80.degree. C. with hydrogen
bubbling and air bubbling, the Fe-based amorphous alloy including
Al and N has good corrosion resistance. If the Fe-based amorphous
alloy includes N at 2.0 wt % or less, its corrosion resistance is
improved.
[0064] It is preferable that the Fe-based amorphous alloys include
carbon at 2.0 wt % to 3.5 wt %. If the content of carbon is less
than 2.0 wt % or greater than 3.5 wt %, it is difficult to form an
amorphous phase.
[0065] It is preferable that the Fe-based amorphous alloy includes
boron at 1.0 wt % to 1.5 wt %. If the content of boron is less than
1.0 wt % or greater than 1.5 wt %, it is difficult to form an
amorphous phase since a supercooling region is narrowed.
[0066] It is preferable that the Fe-based amorphous alloy includes
yttrium at 1.0 wt % to 3.5 wt %. If the content of yttrium is less
than 1.0 wt % or greater than 3.5 wt %, the amorphous forming
ability is reduced. Particularly, it is preferable that the
Fe-based amorphous alloy includes rare earth metals such as yttrium
(Y), gadolinium (Gd), dysprosium (Dy), etc., at an amount of 2 wt %
to 3 wt % in order to improve amorphous forming ability of the
bipolar plate by removing an oxygen effect. As a result, an
amorphous phase has a plate shape with a thickness of 5 mm to 10 mm
depending on the alloy composition. The bipolar plate may be easily
manufactured by using the plate with the aforementioned
thickness.
[0067] It is preferable that the Fe-based amorphous alloy includes
at least one element selected from a group consisting of Al, Co, N,
and Ni at 0.25 wt % to 3.0 wt %. If the at least one element
selected from the group consisting of Al, Co, N, and Ni is present
at less than 0.25 wt %, corrosion resistance is reduced. If it is
greater than 3.0 wt %, it is difficult to form an amorphous
phase.
[0068] In addition, it is preferable that the Fe-based amorphous
alloy includes at least one element selected from a group
consisting of Mn, P, S, and O at 1.0 wt % to 0.5 wt %. If the at
least one element selected from the group consisting of Mn, P, S,
and O is greater than 0.5 wt %, it is difficult to form an
amorphous phase.
[0069] Furthermore, in the present invention, the Ni-based
amorphous alloy may be used in the bipolar plate. The Ni-based
amorphous alloy has similar corrosion characteristics to those of
the Fe-based amorphous alloys in corrosion current density of the
fuel cell. However, the Ni-based amorphous alloy is superior to the
Fe-based amorphous alloy in terms of passivation electric current.
In addition, the Ni-based amorphous alloy has a supercooling region
that is larger than that of the Fe-based amorphous alloy.
[0070] The Ni-based amorphous alloy has a large passivation region
of up to 1.6V. Theoretically, combination of one hydrogen ion, two
oxygen ions, and one electron generates an electric force of 1.23V.
As the electric force increases, the potential of the fuel cell is
reduced to 0.7V which is applied to the bipolar plate. However, if
the fuel cell operates under a loaded power, the electric power is
increased to a theoretical value of 1.23V. The value exceeds the
potential of passivation areas of the Fe-based amorphous alloy and
stainless steel (SS316L). However, the value is less than a
potential of a passivation region of the Ni-based amorphous alloy.
Therefore, the bipolar plate made of the Ni-based amorphous alloy
has good corrosion resistance under an operating condition of the
fuel cell having a large passivation area.
[0071] A Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy,
a Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy, a
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy, and a
Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy, etc., may be
used as the Ni-based amorphous alloy.
[0072] Corrosion resistance of the Ni-based amorphous alloy depends
on concentration of passivation elements such as Zr included in a
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy, Nb
included in an Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy,
Ti included in a
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy, Ti
included in a Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy,
etc. Concentrations of Nb oxide and Zr oxide on a surface of the
Ni-based amorphous alloy are greater than those of Nb and Zr,
respectively. Ti is distributed on a surface of the Ni-based
amorphous alloy, and its concentration is similar to that of Ti
concentration of the Ni-based amorphous alloy.
[0073] Since a concentration of the Ni is relatively low in the
passive film, the Ni-based amorphous alloy can show good corrosion
resistance under a condition that an oxide film including Nb.sup.-
ions, Zr.sup.- ions, and Ti ions are sufficiently formed.
[0074] The Ni-based amorphous alloy according to the present
invention may include niobium at 10.0 wt % to 25.0 wt %. If the
content of niobium is less than 10.0 wt % or greater than 25.0 wt
%, it is difficult to form an amorphous phase.
[0075] In addition, the Ni-based amorphous alloy may include
zirconium at 5.0 wt % to 25.0 wt %. If the amount of zirconium is
less than 5.0 wt % or greater than 25.0 wt %, it is difficult to
form an amorphous phase.
[0076] Furthermore, the Ni-based amorphous alloy may include
titanium at 5.0 wt % to 10.0 wt %. If the amount of titanium is
less than 5.0 wt % or greater than 10.0 wt %, it is difficult to
form an amorphous phase.
[0077] The Ni-based amorphous alloy may include tantalum at 25.0 wt
% or less. If the amount of tantalum is greater than 25.0 wt %, it
is difficult to form an amorphous phase.
[0078] The Ni-based amorphous alloy may include at least one
element selected from a group consisting of Sn and Si. If the
amount of the at least one element selected from the group
consisting of Sn and Si is greater than 6.5 wt %, it is difficult
to form a bulk amorphous phase with a thickness of about 5 mm.
[0079] The Ni-based amorphous alloy may include at least one
element selected from a group consisting of C and O. If the amount
of the at least one element selected from the group consisting of C
and O is greater than 0.5 wt %, it is difficult to form an
amorphous phase.
[0080] Ta or Ti as an early transition metal may be added to the
Ni-based amorphous alloy. Ta may be added to the Ni-based amorphous
alloy if the second element thereof is Nb while Ti may be added
thereto if the second element thereof is Zr. Therefore, amorphous
forming ability is improved a little.
[0081] X-ray diffraction analysis is conducted on the Fe-based
amorphous alloy or the Ni-based amorphous alloy manufactured by the
aforementioned method in order to determine whether they have a
halo pattern, which is a unique pattern of the amorphous alloy. In
addition, glass transition temperature (T.sub.g) and
crystallization peak temperature (T.sub.x) are measured by using
DSC (differential scanning calorimetry). Next, corrosion
resistance, contact resistance, and strength of the bipolar plate
are estimated.
[0082] Amorphous Structure
[0083] Amorphous structures of the Fe-based and Ni-based amorphous
alloys are explained below.
[0084] FIG. 1 illustrates XRD traces of the Fe-based amorphous
alloy whose chemical composition is
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 and the
Ni-based amorphous alloy whose chemical composition is
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si. As illustrated in
the left side of FIG. 1, a halo peak appears in each of the
Fe-based and Ni-based amorphous alloys. The halo peak means that
the alloy has an amorphous phase when the alloys undergo rapid
solidification.
[0085] FIG. 2 illustrates DSC analysis curves of the Fe-based
amorphous alloy whose chemical composition is
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 and the
Ni-based amorphous alloy whose chemical composition is
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2. The heating
rate was 20.degree. C./min. The glass transition temperature
(T.sub.g) and the crystallization peak temperature (T.sub.x) of the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 alloy and
the Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy can
be measured from FIG. 1 by using the DSC analysis. The glass
transition temperature of the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 alloy is
580.degree. C. and crystallization peak temperatures thereof are
605.degree. C. and 685.degree. C. On the other hand, the glass
transition temperature (T.sub.g) of the
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy is
555.degree. C. and the crystallization peak temperature (T.sub.x)
thereof is 607.degree. C. Therefore, the glass transition
temperature (T.sub.g) of the above two alloys is in a range from
550.degree. C. to 610.degree. C., and the crystallization peak
temperature (T.sub.x) thereof is in a range from 600.degree. C. to
700.degree. C.
[0086] Consequently, if the amorphous alloy is used in the bipolar
plate, because of the glass transition temperature (T.sub.g) and
crystallization peak temperature (T.sub.x) of the amorphous alloy,
it can be used without controlling a microstructure thereof.
[0087] Mechanical Properties
[0088] Amorphous materials have high microhardness and high
strength. In particular, Vickers microhardness of the Fe-based and
Ni-based amorphous alloys is 1000 kgf/mm.sup.2 (10 GPa) or more and
strength thereof is substantially 3000 MPa (3 GPa). The thickness
of the bipolar plate can be greatly reduced if an amorphous
material with a high strength is used therein, so weight and volume
of the fuel cell can be significantly reduced. Therefore, amorphous
materials with the aforementioned Vickers microhardness and
strength are preferable for use in the bipolar plate.
[0089] Corrosion Resistance
[0090] Amorphous materials have good corrosion resistance. FIGS. 3
to 6 illustrate potentio-dynamic curves from which the corrosion
resistance of the amorphous materials can be analyzed under
conditions that are similar to those of the PEMFC. The process for
measuring the potentio-dynamic is explained below.
[0091] Hydrogen bubbling or air bubbling is supplied under a 1M
H.sub.2SO.sub.4+2 ppm F.sup.- environment at 75.degree. C. and
80.degree. C. in order to realize an environment of the anode and
cathode, respectively. The materials form a layer until it reaches
an equilibrium state during corrosion. When the layer is completely
formed, the corrosion reaction is stopped. However, it is
absolutely impossible to reach an equilibrium state in an
environment where a reaction is active. In this case, an oxide
layer is partly dissolved and then a new surface of the material is
exposed. The dissolved oxide layer contaminates an aqueous solution
with by-products, and the by-products reduce electrode efficiency,
particularly in the PEMFC. Therefore, it is important to minimize
contamination occurring in the materials used in the bipolar
plate.
[0092] FIG. 3 illustrates potentio-dynamic curves of bipolar plates
made of Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2
and Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2 glassy alloys
and stainless steel (SS316L) under a 1M H.sub.2SO.sub.4+2 ppm
F.sup.- environment at 75.degree. C. with hydrogen bubbling.
[0093] The corrosion resistance can be expected by the
potentio-dynamic curves. However, other factors are relatively
important since other factors are applied in a potential that is
fixed in a very narrow range and materials are varied depending on
a peripheral environment that influences a surface thereof during a
corrosion test. The surface modification cannot be instantly done
but requires a suitable time. Therefore, the corrosion rate
measured on a firstly exposed surface is much greater than that
measured on an exposed sample that is in a passivation state for a
few hours. The value obtained from a potentio-dynamic experiment is
generally greater than the corrosion current density.
[0094] Since the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 glassy
alloy illustrated in FIG. 3 includes rich Cr at about 18 wt %, it
shows good passivation behavior with good corrosion resistance if
this glassy alloy is used in the bipolar plate. However, since the
Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2 glassy alloy
includes poor Cr of abut 15 wt %, a passivation region is not
distinctly shown.
[0095] An Fe-based amorphous alloy with rich Cr such as glassy
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 is
passivated with a current density of 0.75 mA/cm.sup.2 at -0.1V
which is a potential value of the PEMFC anode. However, the
stainless steel (SS316L) containing the same amount of Cr is in an
active passivation transition region showing a current density of
1.1 mA/cm.sup.2. This means that stability of the passivation layer
of the Fe-based amorphous alloy is greater than that of the
stainless steel.
[0096] FIG. 4 illustrates potentio-dynamic curves of bipolar plates
made of the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 and
Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2 glass transition
alloys and a stainless steel (SS316L) under 1M H.sub.2SO.sub.4 and
2 ppm F.sup.- at 75.degree. C. with air bubbling, respectively.
[0097] The Fe-based amorphous alloy including rich Cr such as the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2C.sub.14B.sub.6Y.sub.2 glass
transition shown in FIG. 4 shows a low passivation current density
of 78.4 A/cm.sup.2 in spite of a distinct passivation region under
an air bubbling condition at 75.degree. C. The passivation current
density, which is 0.91 mA/cm.sup.2, is rather high in the Fe-based
amorphous alloy including poor Cr.
[0098] The Fe-based amorphous alloy shows higher corrosion
resistance under an air supply than under a hydrogen supply. This
means that it is advantageous for air including oxygen to form a
passivation layer. Inactivity of the corrosion potential of the
amorphous alloy under air supply is higher than that under hydrogen
supply. That is, the Fe-based amorphous alloy including rich Cr has
a corrosion potential of -0.268V under hydrogen supply and 0.062V
under air supply. In addition, the Fe-based amorphous alloy
including poor Cr has a corrosion potential of -0.201V under
hydrogen supply and 0.097V under air supply. This means that
corrosion of the amorphous alloy does not often occur under air
supply as opposed to under hydrogen supply.
[0099] As illustrated in FIGS. 5 and 6, the Ni-based amorphous
alloy shows a similar behavior to the Fe-based amorphous alloy
since the current density is low when the Ni-based amorphous alloy
is used in the PEMFC. The Ni-based amorphous alloy has an advantage
that the passivation current density is low and the passivation
region is large. Therefore, the Ni-based amorphous alloy can
maintain high corrosion resistance regardless of the applied
potential.
[0100] The aforementioned corrosion resistance deeply relates to
ions with passivation of the bipolar plate. The variation of the
current density indicates that the oxide layer is passivated. When
an X-ray diffraction method (XPS) is carried out, a composition
according to the thickness of the oxide film is shown to depend on
the environment.
[0101] FIGS. 7 to 9 illustrate the XPS depth profile of the passive
film. FIG. 7 illustrates a passive film before the experiment, FIG.
8 illustrates a passive film during a potentiostatic polarization
under a PEMFC anode environment, and FIG. 9 illustrates a passive
film during a potentiostatic polarization under a PEMFC cathode
environment. In the XPS depth profile of the passive film
illustrated in FIGS. 7 to 9, the passive film formed during air
supply includes an oxide layer containing Fe at about 18 at % and
an oxide layer containing Cr at about 5 at %.
[0102] A passivation layer is formed on the SiO.sub.2 layer with a
sputtering rate of 3.5 nm/min. The thickness of the passivation
layer formed before experiment, during a potentiostatic
polarization under a PEMFC anode environment, and during a
potentiostatic polarization under a PEMFC cathode environment is
about 2.1 nm, 3.8 nm, and 5.6 nm, respectively. In this case, there
is a large difference between the passivation layer formed in the
PEMFC environment and the passivation layer under air supply. That
is, when an oxide layer including rich Cr is formed with the iron
oxide layer, a concentration of Cr oxide increases to two and four
times in comparison with the passivation layer under air supply. On
the contrary, a concentration of the iron oxide is reduced by two
times or there is little change.
[0103] As can be known from FIGS. 8 and 9, the thin iron oxide is
firstly formed. This is caused by the fact that oxygen diffuses
toward the inside and Cr diffuses toward the outside, and thereby a
Cr.sub.2O.sub.3 protective oxide layer is formed since the iron
oxide is formed with a plurality of pores.
[0104] The concentration of Mo is constantly maintained at about
4.0 at % in FIGS. 8 and 9. Therefore, the
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2Y.sub.2C.sub.14B.sub.6 amorphous
alloy shows high corrosion resistance due to the passive film
including Cr in the PEMFC environment by the XPS diffraction
analysis. Minor elements such as Al, Co, N, Ni Ti, and V also
contribute to formation of the oxide.
[0105] Contact Resistance
[0106] In a fuel cell, total electric resistance includes the bulk
resistances of the bipolar plate and the carbon cloth as well as
the interfacial resistance between the bipolar plate and the carbon
cloth. Consequently, the contact resistance is more important than
the bulk resistance. Contact resistance means a resistance
generated in a contact plane that is higher than that of other
portions when electric current passes through the contact planes of
conductors contacting each other. The contact resistance is varied
depending on kinds of conductors, pressure, current density,
whether the oxide film exists, etc.
[0107] The interfacial contact resistance can be measured by using
a set-up condition modified from Davies' method. In the set-up
condition, two pieces of conductive carbon paper are sandwiched
between the amorphous sample and two copper plates. An electric
current is supplied via the two copper plates. The total resistance
can be calculated by measuring the total voltage drop as a function
of the applied force by using the following equation.
R = VA s I Equation 1 ##EQU00001##
[0108] Here, R is electric contact resistance, V is the voltage
drop during the set-up, I is a supplied current, and A.sub.s is a
surface area.
[0109] The measured total resistance is a sum of four interfacial
components including two resistances at an interface between a
carbon paper and a copper plate (R.sub.C/Cu), the resistance of the
carbon paper, and the resistance of the boundary film of the sample
interface. Only one layer of the carbon papers sandwiched between
the two copper plates is measured in order to correct the
interfacial contact resistance (R.sub.C/Cu) between the copper
plate and carbon paper. The measured total resistance can be
controlled by forming and using a boundary between the two carbon
papers and the copper plate. After the correction, assuming that
the surfaces are uniform if a result is divided in half, the
interfacial contact resistance (R.sub.C/SS) between the carbon
paper and the stainless steel can be obtained.
[0110] As illustrate in (A) and (B) of FIG. 10, as a compaction
pressure is rapidly increased, the contact resistances of the
materials are rapidly reduced at low compaction pressures while
they are gradually reduced at high compaction pressures. At a given
compaction pressure, the contact resistance increases in an order
of graphite.ltoreq.stainless steel.ltoreq.Fe-based amorphous
alloys.ltoreq.Ni-based amorphous alloys. The contact resistances of
graphite, stainless steel (SS316L), an
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 alloy, an
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 alloy, a
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy, and a
Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 alloy are measured to
be 5.9 cm.sup.2, 8.3 cm.sup.2, 10.9 cm.sup.2, 12.3 cm.sup.2, 15.7
cm.sup.2, and 24.9 cm.sup.2 under a pressure of 180N/cm.sup.2,
respectively. The result shows that, even though contact resistance
of metallic alloys is higher than that of graphite, the difference
between the contact resistance of the Fe-based amorphous alloys and
stainless steel is not very large. Therefore, ohmic resistances of
a unit cell are similar to each other in the stainless steel and
the bipolar plates made of the Fe-based amorphous alloy. However,
the Ni-based alloy shows contact resistance that is rather higher
than those of the rest of the materials.
[0111] Viscosity
[0112] FIG. 11 illustrates viscosity data of the
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 alloy and
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy. The
viscosity is measured by heating the alloys at a rate of 30K/min
and applying a load of 250 mN.
[0113] As illustrated FIG. 11, although viscosity of the alloy is
reduced near the glass transition temperature and in the
supercooled liquid area, it increases from the crystallization
area. The difference between the glass transition temperature and
the crystallization temperature given in Table 1 depends on the
difference of heating rate.
[0114] The minimum viscosity of the
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 alloy,
the Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 alloy,
and the Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 alloy
is 3.times.10.sup.-8 Pa, 5.5.times.10.sup.-8 Pa, and
1.2.times.10.sup.-8 Pa, respectively. Consequently, it is
preferable to form an amorphous alloy at a temperature in which the
viscosity is low. However, the higher the temperature, the shorter
the time for nucleation of the crystal. Therefore, the time and
temperature has to be optimized in order to generate an accurate
pattern and avoid nucleation. In addition, the data means that
Fe-based and Ni-based amorphous alloys can easily form an amorphous
phase due to the large supercooled liquid area.
[0115] FIG. 12 schematically illustrates a stack 100 of the fuel
cell provided with a bipolar plate 25 made of an amorphous metal
according to an embodiment of the present invention. The stack 100
of the fuel cell is included in the fuel cell. FIG. 12 illustrates
a bipolar plate to which the amorphous alloy of the present
invention is applied as an example. However, this is merely to
illustrate the present invention and the present invention is not
limited thereto. Therefore, the amorphous alloy can be used for
other uses.
[0116] In the stack 100 of the fuel cell according to an embodiment
of the present invention shown in FIG. 12, the MEA 21 is arranged
at a center position and bipolar plates 23 and 25 are arranged at
both sides of the MEA 21. Therefore, a minimum unit of electricity
generating part 19 is formed and electric energy is generated by
reaction of hydrogen and oxygen. A plurality of electricity
generating parts 19 are continuously arranged, and the stack 100 in
which a plurality of electricity generating parts 19 are assembled,
can be formed. An anode is formed on one side of the MEA 21 located
between the bipolar plates 23 and 25 while a cathode is formed on
the other side thereof. An electrolyte membrane is formed between
the above two electrodes.
[0117] The anode separates hydrogen into hydrogen ions and
electrons, and the electrolyte membrane transfers the hydrogen ions
to the anode. The electrons and hydrogen ions received from the
cathode are reacted with oxygen included in air by the cathode and
then water is generated.
[0118] The bipolar plates 23 and 25 closely adhere to both sides of
the MEA 21. The bipolar plates 23 and 25 act as a conductor that
serially connects the anodes and cathodes of the MEA 21. In
addition, the bipolar plates 23 and 25 act as a hydrogen passage
for supplying hydrogen gas to the anode of the MEA 21 and act as an
air passage for supplying air to the anode.
[0119] For this, one bipolar plate 23 forms a hydrogen passage for
supplying hydrogen to the anode while being closely arranged on the
anode of the MEA 21. In addition, the other bipolar plate 25 forms
an air passage for supplying air to the cathode of the MEA 21 while
being closely arranged on the cathode of the MEA 21. The hydrogen
passage is formed to include a hydrogen transferring channel 23c
that is formed on a closely adhering side of one bipolar plate 23
facing the MEA 21. In addition, an air passage is formed to include
an air transferring channel (not shown) that is formed on a closely
adhering side of the other bipolar plate 25 facing the MEA 21.
[0120] The bipolar plates 23 and 25 are manufactured using
amorphous alloys according to an embodiment of the present
invention. As described above, since the bipolar plates 23 and 25
act as hydrogen and air passages, the bipolar plates 23 and 25
should have good corrosion resistance if they are made of a metal.
According to the present invention, since the bipolar plates 23 and
25 are manufactured by using the amorphous alloy, there is no
possibility that the bipolar plates 23 and 25 will be corroded. The
amorphous alloy will be explained in detail below.
[0121] The present invention will be explained with reference to
the experimental examples of the present invention below. The
experimental examples are merely to illustrate the present
invention and the present invention is not limited thereto.
[0122] Test of Amorphous Structure and Mechanical Properties of
Amorphous Alloys
[0123] Physical properties of the bipolar plates made of the
Fe-based and Ni-based amorphous alloys were measured. The test was
performed on six Fe-based amorphous alloys and two Ni-based
amorphous alloys. The glass transition temperature (T.sub.g),
crystallization temperature (T.sub.x), Vickers microhardness (Hv),
and compression strength (.sigma..sub.f) are described in Table
1.
TABLE-US-00001 TABLE 1 H.sub.v T.sub.g T.sub.x (kgf/ Alloy
composition (.degree. C.) (.degree. C.) mm.sup.2) .sigma..sub.f
(MPa) Fe-based Fe.sub.48Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2
575 612 -- -- Fe.sub.45Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2 599
625 1132 2781
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 626 640
1192 -- Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Co.sub.2
596 626 1024 3684
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 590 624
1187 3919 Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Ni.sub.2
596 620 1121 2972 Ni-based
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 553 607 3015
Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 581 623 -- --
[0124] As described in Table 1, both the Fe-based and Ni-based
amorphous alloys have T.sub.g and T.sub.x. This means that the
Fe-based and Ni-based amorphous alloys can be used at a high
temperature without modifying microstructures. In addition, these
amorphous alloys have microhardness of over 1000 kgf/mm.sup.2 and
high strength in a range from 2500 MPa and 4000 MPa. Materials with
the above microhardness and strength are suitable for using in an
operation environment of bipolar plates.
[0125] Potentio-Dynamic Experiments of the Amorphous Alloy
[0126] Potentio-dynamic experiments were carried out on five
Fe-based amorphous alloys, two Ni-based amorphous alloys, and
stainless steel (SS316L) under 1M H.sub.2SO.sub.4+2 ppm F.sup.- at
80.degree. C. with hydrogen bubbling and air bubbling. Corrosion
potentials and corrosion currents according to the potentio-dynamic
experiments are described in Table 2. FIG. 13 illustrates
Fe.sub.45Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2,
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Ni.sub.2, and
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 included
in the Fe-based amorphous alloy under a hydrogen supplying
condition while FIG. 14 illustrates them under an air supplying
condition.
TABLE-US-00002 TABLE 2 Corrosion current Corrosion Passivation
density (mA) potential (V) current (mA) Alloy composition H.sub.2
air H.sub.2 air H.sub.2 air Fe-based
Fe.sub.45Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2 0.68 0.38 0.08
0.094 0.76 4.14
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 0.11 0.09
-0.232 0.048 0.79 2.13
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Co.sub.2 1.34 0.53
0.072 0.085 0.79 1.3
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 0.43 0.016
0.046 0.011 0.79 1.0
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Ni.sub.2 0.57
0.024 0.05 0.013 0.42 1.4 Ni-based
Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 0.112 0.009
-0.231 -0.143 0.68 0.657
Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 0.235 0.036 -0.113
-0.115 0.0697 0.066 SUS-316L
Fe--16.8Cr--10.3Ni--2.25Mo--1.95Mn--0.65Si--0.04N--0.5Cu--0.5Co
1.51 3.26 -0.242 -0.221 0.99 1.4 (in wt %)
[0127] As illustrated in FIG. 13, when a constant voltage of -0.1V
was applied to the amorphous alloy, the current density was
negative and increased rapidly at the first step of polarization
and then stabilized to reach very low values after about 30
minutes. The negative current indicates that the passive layer is
protected by the cathode, which means that there is no active
dissolution at the anode of polymer electrolytic fuel cells.
[0128] Similar behavior occurred when a constant voltage of 0.6V
was applied to the amorphous alloy based on a cathode condition
made of the amorphous alloy. In this case, the current density was
positive and quickly decreased to a stabilized value in a short
time. The current drop from 5.times.10.sup.-2 A/cm.sup.2 at the
beginning of the experiment to 1 mA/cm.sup.2 after 1 hour of the
polarization test was caused by formation of a passive layer. As
the passive layer is formed on the entire surface, the current
required for maintaining the passive layer is reduced. The fact
that the current becomes constant after one hour means that the
passive film formed on the Fe-based amorphous alloys is stable.
[0129] The potentio-dynamic curves shown in FIGS. 13 and 14
relatively illustrate a potential of -0.1V with hydrogen bubbling
and a potential of 0.6V with air bubbling, respectively. The
initially negative current density at the anode increased to reach
almost zero with hydrogen bubbling. On the other hand, the current
density at the cathode quickly decreased from the initial value
indicating formation of a passive layer on a surface of the
amorphous alloy with air bubbling.
[0130] Referring to FIGS. 13 and 14, Ni shows good corrosion
resistance with air bubbling while N shows good corrosion
resistance with hydrogen bubbling. In addition, if the corrosion
current density and corrosion potential of the amorphous alloy is
compared with stainless steel (SS316L), it can be seen that
corrosion resistance of the Fe-based amorphous alloy is high. The
Ni-based amorphous alloy shows the same result.
[0131] In addition, referring to FIGS. 13 and 14, among the
Fe-based amorphous alloy, the
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 alloy has
the most excellent corrosion resistance and contact resistance.
Therefore, since characteristics of the amorphous alloy are
improved when N is added to the amorphous alloy, it is suitable for
use in the bipolar plate. There is an advantage that current flows
well if N is mixed in an oxide layer formed on a surface of the
amorphous alloy.
[0132] Meanwhile, as illustrated in Table 3 below, the initial and
final values of the current density were varied depending on other
characteristics and thickness of the oxide layer.
TABLE-US-00003 TABLE 3 Charge density (Q) coul/cm.sup.2 Alloy
composition H.sub.2 air Fe-based
Fe.sub.45Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2 -5.148 15.07
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 -0.22
13.64 Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2
-0.0687 12.35
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Ni.sub.2 -1.66
8.43
[0133] Corrosion Tests
[0134] A solution was analyzed after the corrosion test was
performed to the alloy. An ICP-AA method, which is more direct and
reliable than a potentio-static method or a potentio-dynamic
method, was used to evaluate corrosion solutions of the
Fe--Cr--Mo--Al--C--B--Y amorphous alloy. As described in Table 4
below, the amount of Cr, which is an element for dissolving under a
hydrogen atmosphere and forming passivation, is greater than that
contained in the PEMFC including air bubbles. This means that a
reaction is very active during hydrogen supply rather than during
air supply, which corresponded to a result of the potentio-dynamic
experiment.
[0135] The corrosion resistance of the material was directly
measured by using corrosion current density. The equivalent
corrosion current density (I.sub.corr-S) was calculated by
averaging the charge density obtained from a solution analysis
(Q.sub.S) described in Table 4 below. The corrosion current density
(I.sub.corr-P=0.197 mA/cm.sup.2) was measured by applying the Tafel
slope to the potentio-dynamic curves for comparison.
[0136] The corrosion current density (I.sub.corr-S) by the
electrolysis was very low under hydrogen bubbling. In addition, a
slightly higher corrosion rate was observed at the potentio-dynamic
test (I.sub.corr-P). In fact, the potentio-dynamic tests were
performed for a shorter time. This means that sufficient time was
not given for forming a passive film. However, these corrosion data
already provide useful information regarding corrosion behavior.
I.sub.corr-S described in Table 4 are the same as I.sub.corr-P
(=0.015 mA/cm.sup.2) under air bubbling. This means that the
passive layer was rapidly stabilized. By comparing the I.sub.corr-S
with the I.sub.corr-P, it could be observed that the amorphous
alloy had a lower corrosion current density than the stainless
steel. This means that the amorphous alloy has better corrosion
resistance than that of stainless steel. This result means that the
Fe-based amorphous alloy according to the present invention can be
used as a material of the bipolar plate of the PEMFC.
TABLE-US-00004 TABLE 4 Dissolved species analyzed by ICP-AA (mg/l)
Alloy constitutive elements H.sub.2 bubbled Air bubbled
Fe.sub.50Cr.sub.18Mo.sub.8C.sub.14B.sub.6Y.sub.2Al.sub.2 Fe 1.09
2.72 Cr 0.6 0.29 Mo 0.05 0.38 B 0.091 0.12 Y 2.16 0.66 Al 0.23 0.25
Charge density (Q.sub.S) C/cm.sup.2 0.179 0.242 Corrosion current
mA/cm.sup.2 0.00995 0.01344 density (I.sub.corr-S)
[0137] Contact Resistance Test
[0138] Contact resistance tests were performed on the amorphous
alloys. The interfacial contact resistance of amorphous alloys was
measured using a set-up modification from Davies' method while
applying forces having different values and was compared with
values obtained from graphite and stainless steel (SS316L). Two
pieces of carbon paper were sandwiched between the amorphous alloy
specimen and two copper plates. The interfacial contact resistance
between fresh samples of the different plates made of an amorphous
alloy, stainless steel, and carbon paper was measured depending on
different applied forces. With increasing compaction pressure, the
contact resistance of the materials rapidly decreased at low
compaction pressure and then gradually decreased at high compaction
pressure.
[0139] For example, the contact resistance increased in an order of
graphite.ltoreq.stainless steel.ltoreq.Fe-based amorphous
alloy.ltoreq.Ni-based amorphous alloy when compaction pressure of
180N/cm.sup.2 was applied as in Table 5 below.
TABLE-US-00005 TABLE 5 Contact resistance (m/cm.sup.2) at 180
N/cm.sup.2 compaction Alloy composition force Fe-based
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2Al.sub.2 11
Fe.sub.43Cr.sub.18Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.2 9.5
Fe.sub.44Cr.sub.15Mo.sub.14C.sub.15B.sub.6Y.sub.2N.sub.4 8.0
Ni-based Ni.sub.59Zr.sub.16Ti.sub.13Nb.sub.7Sn.sub.3Si.sub.2 15.5
Ni.sub.60Nb.sub.20Ti.sub.10Zr.sub.5Ta.sub.5 25
[0140] As shown in Table 5, the contact resistance of the Fe-based
amorphous alloy was almost equal to that of the stainless steel. On
the contrary, the Ni-based amorphous alloy had a slightly higher
contact resistance. Therefore, the amorphous alloy according to the
present invention can also be used in an ocean environment as well
as in the bipolar plate.
[0141] Surface Energy Experiment of the Amorphous Alloy
[0142] Surface energies of the amorphous alloys were measured. The
material of the bipolar plate should have characteristics of low
water absorption, sufficient mechanical strength, low weight, and
chemical stability in the electrolyte membrane fuel cell
environment as well as the above characteristics. The water
absorption at a cathode of the bipolar plate depends on the
wettability of the material. In addition, it affects the
performance of the cell. The surface energy of the amorphous alloy
including Al and N can be obtained by measuring the wetting angle
(.theta.) and is given in Table 6. The wetting angle was higher
than that of stainless steel (76.degree.), and was lower than that
of graphite (104.degree.). That is, wetting angle is in a range
from 80.degree. to 100.degree.. This means that the surface energy
of the amorphous alloy to water is lower than that of the stainless
steel (SS316L). Therefore, it can be predicted that water generated
in the PEMFC can flow and be removed when it contacts the amorphous
alloy.
TABLE-US-00006 TABLE 6 NO Materials Wetting angle
(.theta.)(.degree.) 1 Graphite 104 2 stainless steel (SS316L) 76 3
Fe.sub.50Cr.sub.18Mo.sub.8Al.sub.2Y.sub.2C.sub.14B.sub.6 92 4
Fe.sub.41Cr.sub.18Mo.sub.14Y.sub.2C.sub.14N.sub.6 88
[0143] Although exemplary embodiments of the present invention have
been described in detail hereinabove, it should be clearly
understood that many variations and/or modifications of the basic
inventive concept herein taught, which may appear to those skilled
in the art, will still fall within the spirit and scope of the
present invention as defined in the appended claims.
* * * * *