CROSS REFERENCE TO RELATED APPLICATIONS
This application relates to, and claims priority from, the provisionally filed
U.S. patent application having serial number 61/651,817, entitled "COMPOSITIONS FOR
BRAZING, AND RELATED METHODS AND DEVICES", filed on May 25'^ 2012, which
application is hereby incorporated by reference.
TECHNICAL FIELD
This invention generally relates to a braze composition. In some specific
embodiments, the invention relates to a braze composition that provides corrosion-resistant
sealing and other benefits to high temperature rechargeable batteries.
BACKGROUND OF THE INVENTION
Many types of seal materials have been considered for use in high-temperature
rechargeable batteries/cells for joining different components. Sodium/sulfur or sodium/metal
halide cells generally include several ceramic and metal components. The ceramic
components include an electrically insulating alpha-alumina collar and an ion-conductive
electrolyte beta-alumina tube, and are generally joined or bonded via a sealing glass. The
metal components include a metallic casing, current collector components, and other metallic
components which are often joined by welding or thermal compression bonding (TCB).
However, metal-to-ceramic bonding can sometimes present some difficulty, mainly due to
thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic
and metal components.
The metal-to ceramic bonding is most critical for the reliability and safety of
the cell. Many types of seal materials and sealing processes have been considered for joining
metal to ceramic components, including ceramic adhesives, brazing, and sintering. However,
most of the seals may not be able to withstand high temperatures and corrosive environments.
A common bonding technique involves muhiple steps of metalizing the
ceramic component, followed by bonding the metallized ceramic component to the metal
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component using a thermal compression bond (TCB). The bond strength of such metal-toceramic
joints is controlled by a wide range of variables, for example, the microstructure of
the ceramic component, the metallization of the ceramic component, and various TCB
process parameters. In order to ensure good bond strength, the process requires close control
of several parameters involved in various process steps. In short, the method is relatively
expensive, and complicated, in view of the multiple processing steps, and the difficulty in
controlling the processing steps.
Brazing is another potential technique for making the ceramic-to-metal joints.
A braze material is heated above its melting point, and distributed between two or more
close-fitting parts by capillary action. However, most of the brazing materials (or braze
materials) have limitations that prevent them from fulfilling all of the necessary requirements
of high temperature batteries. Moreover, some of the commercial braze materials can be
quite expensive themselves; and using them efficiently in various processes can also be
costly.
It may be desirable to develop new braze alloy compositions that have
properties and characteristics that meet performance requirements for high temperature
rechargeable batteries, and are less complicated and less expensive to process, as compared to
the existing sealing methods.
BRIEF DESCRIPTION
Various embodiments of the present invention may provide braze alloy
compositions for sealing a ceramic to a metal, to form a seal that can withstand corrosive
environments.
In accordance with an embodiment of the invention, a braze alloy composition
is disclosed, comprising nickel, silicon, boron, and an active metal element. The braze alloy
includes nickel in an amount that is usually greater than about 50 weight percent, and the
active metal element in an amount up to about 10 weight percent.
In one embodiment, an electrochemical cell incorporating the braze alloy
composition is disclosed. The braze alloy includes an active metal element that forms a
ceramic-to-metal joint, and has good sodium- and halide-resistance at operating temperatures,
along with other complimentary mechanical properties; stability at high temperatures; good
3
thermal expansion properties, and the like. In one embodiment, an energy storage device is
also disclosed.
BRIEF DESCRIPTION OF DRAWINGS
These and other features, aspects, and advantages of the present invention will
become better understood when the following detailed description is read with reference to
the accompanying drawings, wherein:
Fig. 1 is a schematic view showing a cross-section of an electrochemical cell,
according to an embodiment;
Fig. 2 depicts X-ray diffraction patterns for two alloy samples; and
[0014] Fig. 3 shows scanning electron micrographs of cross-sections of a joint between a
ceramic component and a metal component.
DETAILED DESCRIPTION
The invention includes embodiments that relate to a braze alloy composition for
sealing an electrochemical cell, for example, a sodium/sulfur or a sodium metal halide
battery. The invention also includes embodiments that relate to an electrochemical cell made
by using the braze composition. As discussed in detail below, some of the embodiments of
the present invention provide a braze alloy for sealing a ceramic component to a metal
component, e.g., in an electrochemical cell; along with a metal halide battery formed thereof.
These embodiments advantageously provide an improved seal and method for the sealing.
Though the present discussion provides examples in the context of a metal halide battery,
these processes can be applied to any other application, including ceramic-to-metal or
ceramic-to-ceramic joining.
When introducing elements of various embodiments of the present invention, the
articles "a," "an," "the," and "said" are intended to mean that there are one or more of the
elements, unless otherwise indicated. The terms "comprising," "including," and "having" are
intended to be inclusive, and mean that there may be additional elements other than the listed
elements. As used herein, the term "and/or" includes any and all combinations of one or
more of the associated listed items. Unless otherwise indicated herein, the terms "disposed
^
on", "deposited on" or "disposed between" refer to both direct contact between layers,
objects, and the like, or indirect contact, e.g., having intervening layers therebetween.
Approximating language, as used herein throughout the specification and claims,
may be applied to modify any quantitative representation that could permissibly vary without
resulting in a change in the basic function to which it may be related. Accordingly, a value
modified by a term such as "about" is not limited to the precise value specified. In some
instances, the approximating language may correspond to the precision of an instrument for
measuring the value.
As used herein, the term "liquidus temperature" generally refers to a temperature
at which an alloy is transformed from a solid into a molten or viscous state. The liquidus
temperature specifies the maximum temperature at which crystals can co-exist with the melt
in thermodynamic equilibrium. Above the liquidus temperature, the alloy is homogeneous,
and below the liquidus temperature, an increasing number of crystals begin to form in the
melt with time, depending on the particular alloy. Generally, an alloy, at its liquidus
temperature, melts and forms a seal between two components to be joined.
The liquidus temperature can be contrasted with a "solidus temperature". The
solldus temperature quantifies the point at which a material completely solidifies
(crystallizes). The liquidus and solidus temperatures do not necessarily align or overlap. If a
gap exists between the liquidus and solidus temperatures, then within that gap, the material
consists of solid and liquid phases simultaneously (like a "slurry").
"Sealing" is a function performed by a structure that joins other structures
together, to reduce or prevent leakage through the joint between the other structures. The
seal structure may also be referred to as a "seal" herein, for the sake of simplicity.
Typically, "brazing" uses a braze material (usually an alloy) having a lower
liquidus temperature than the melting points of the components (i.e. their materials) to be
joined. The braze material is brought slightly above its melting (or liquidus) temperature
while protected by a suitable atmosphere. The braze material then flows over the
components (known as wetting), and is then cooled to join the components together. As used
herein, "braze alloy composition" or "braze alloy", "braze material" or "brazing alloy", refers
to a composition that has the ability to wet the components to be joined, and to seal them. A
braze alloy, for a particular application, should withstand the service conditions required, and
5
should melt at a lower temperature than the base materials; or should melt at a very specific
temperature. Conventional braze alloys usually do not wet ceramic surfaces sufficiently to
form a strong bond at the interface of a joint. In addition, the alloys may be prone to sodium
and halide corrosion.
As used herein, the term "brazing temperature" refers to a temperature to which a
brazing structure is heated to enable a braze alloy to wet the components to be joined, and to
form a braze joint or seal. The brazing temperature is often higher than or equal to the
liquidus temperature of the braze alloy. In addition, the brazing temperature should be lower
than the temperature at which the components to be joined may become chemically,
compositionally, and mechanically unstable. There may be several other factors that
influence the brazing temperature selection, as those skilled in the art understand.
Embodiments of the present invention provide a braze alloy composition capable
of forming a joint by "active brazing" (described below). In some specific embodiments, the
composition also has high resistance to sodium and halide corrosion. The braze alloy
composition includes nickel, silicon, boron, and an active metal element, as described herein.
Each of the elements of the alloy usually contributes and optimizes at least one property of
the overall braze composition. These properties may include liquidus temperature,
coefficient of thermal expansion, flowability or wettability of the braze alloy with a ceramic;
corrosion resistance, and ease-of-processing. Some of the properties are described below.
According to most of the embodiments of the invention, the braze alloy
composition is a nickel-based alloy. In other words, the alloy contains a relatively high
amount of nickel, as compared to the amount of other elements in the alloy. Nickel is
relatively inert in a corrosive environment, as compared to other known base metals, e.g.
copper, iron, chromium, cobalt etc. Additionally, it is observed that nickel may enhance
other properties of the braze alloy, such as the thermal expansion coefficient, and the phase
stability. In general, the amount of nickel balances the alloy based on the amounts of the
other constituents. In some embodiments of this invention, a suitable level for the amount of
nickel may be at least about 20 weight percent, based on the total weight of the braze alloy.
In some embodiments, nickel is present in an amount greater than about 50 weight percent.
In some embodiments that are preferred for selective end-use applications, the nickel is
present from about 60 weight percent to about 90 weight percent, and in some specific
embodiments, from about 70 weight percent to about 80 weight percent, based on the total
weight of the braze alloy.
In spite of above discussed properties, nickel-based alloys may have an
undesirably high liquidus temperature, i.e., above the required brazing temperature. In order
to reduce the liquidus temperature, a melting point depressant may be chosen to form an alloy
with nickel, which reduces the melting point of the overall alloy composition. As used
herein, the term "melting point depressant" refers to an element or compound which may
depress the melting point of the resulting alloy, when added to another element or an alloy.
The melting point depressant element may decrease the viscosity and, in turn, increase the
flowability (also referred to as wettability) of the braze alloy, at an elevated temperature.
Suitable examples of the melting point depressant include, but are not limited to,
silicon, boron, niobium, palladium, or a combination thereof According to some of the
embodiments of the invention, the braze alloy composition includes silicon and boron, and
forms a ternary base alloy, i.e., a Ni-Si-B alloy. Any additional melting point depressant may
be added to further adjust the liquidus temperature and/or viscosity of the alloy.
The present inventors conceived of a balance of nickel and the melting point
depressant (silicon and boron) levels that optimized the liquidus temperature requirements
and the requirements for corrosion resistance. A suitable, total amount of the melting point
depressant may be up to about 20 weight percent, based on the total weight of the braze alloy.
In some specific embodiments, the braze alloy includes from about 1 weight percent to about
10 weight percent silicon, based on the total weight of the braze alloy. In some
embodiments, the braze alloy includes up to about 10 weight percent boron, based on the
total weight of the braze alloy. A suitable range for each of silicon and boron is often from
about 2 weight percent to about 10 weight percent. In some embodiments, a small amount of
each of silicon or boron (e.g., less than about 5 weight percent) is used, as each of these may
be reactive with the active metal element (e.g. titanium), if present in an amount more than
the solubility limit of the element in the alloy.
The braze alloy may include additional melting point depressants, as mentioned
above. Examples include niobium and/or palladium. In addition, palladium and niobium
may provide good corrosion resistance in a sodium-containing environment. In some
embodiments, the braze alloy includes up to about 10 weight percent palladium (e.g., about
1
0.1 weight percent to about 10 weight percent), based on the total weight of the braze alloy.
In some embodiments, the braze alloy includes up to about 5 weight percent niobium (e.g.,
about 0.1 to about 5 weight percent), based on the total weight of the braze alloy.
As mentioned above, the concept of "active brazing" is important for
embodiments of this invention. Active brazing is a technique often used to join a ceramic to
a metal, or a ceramic to a ceramic. Active brazing uses an active metal element that promotes
wetting of a ceramic surface, enhancing the capability of providing a hermetic seal. An
"active metal element", as used herein, refers to a reactive metal that has high affinity to the
oxygen within the ceramic, and thereby reacts with the ceramic. A braze alloy containing an
active metal element can also be referred to as an "active braze alloy." The active metal
element undergoes a reaction with the ceramic, when the braze alloy is in a molten state, and
leads to the formation of a thin reaction layer on the interface of the ceramic and the braze
alloy. The thin reaction layer allows the braze alloy to wet the ceramic surface, resulting in
the formation of a ceramic-ceramic or a ceramic-metal joint/bond, which may also be referred
to as an "active braze seal."
Thus, an active metal element is an essential constituent of a braze alloy for
employing active brazing. A variety of suitable active metal elements may be used to form
the active braze alloy. The selection of a suitable active metal element mainly depends on the
chemical reaction with the ceramic (e.g., alumina) to form a uniform and continuous reaction
layer, and the capability of the active metal element to form an alloy with a base alloy (e.g. a
Ni-Si-B alloy). The active metal element for embodiments herein is often titanium. Other
suitable examples of the active metal element include, but are not limited to, zirconium,
hafnium, and vanadium. A combination of two or more active metal elements may also be
used.
The presence and the amount of the active metal may influence the thickness and
the quality of the thin reaction layer, which contributes to the wettability or flowability of the
braze alloy, and therefore, the bond strength of the resulting joint. In some embodiments, the
active metal is present in an amount less than about 10 weight percent, based on the total
weight of the braze alloy. A suitable range is often from about 0.5 weight percent to about 5
weight percent. In some specific embodiments, the active metal is present in an amount
ranging from about 1 weight percent to about 3 weight percent, based on the total weight of
the braze alloy. The active metal element is generally present in small amounts suitable for
t
improving the wetting of the ceramic surface, and forming the thin reaction layer, for
example, less than about 10 microns. A high amount of the active metal layer may cause or
accelerate halide corrosion.
The braze alloy composition may further include at least one additional element.
The additional element may provide further adjustments in several required properties of the
braze alloy, for example, the coefficient of thermal expansion, liquidus temperature, brazing
temperature, corrosion resistance, and the strength of the braze alloy. In one embodiment, the
additional element (some of which were mentioned above) can include, but is not limited to,
iron, chromium, cobalt, niobium, molybdenum, tungsten, or a combination thereof. In some
specific embodiments, the braze alloy includes chromium and iron.
With respect to the amount of the additional element(s), the braze alloy includes
up to about 10 weight percent (e.g., about 0.1%-10%) of the additional elements, based on the
total weight of the braze alloy. In some embodiments, the braze alloy includes up to about 10
weight percent chromium (e.g., about 0.1 to about 10 weight percent), based on the total
weight of the braze alloy. In some embodiments, the braze alloy includes from about 0.1
weight percent to about 10 weight percent iron, and in some specific embodiments, up to
about 5 weight percent iron, based on the total weight of the braze alloy. In some
embodiments, the braze alloy includes from about 0.1 weight percent to about 5 weight
percent of molybdenum, based on the total weight of the braze alloy.
In some embodiments, any of the braze alloys described herein may also include
cobalt. The addition of cobalt improves the corrosion resistance of the overall composition.
The braze alloy may include from about 1 weight percent to about 50 weight percent cobalt,
based on the total weight of the braze alloy. In some specific embodiments, the braze alloy
includes up to about 10 weight percent of cobalt, based on the total weight of the braze alloy.
In some other embodiments, cobalt may replace nickel, and the alloy may be cobalt-based. In
these instances, the alloy may include a higher level (from about 40 weight percent to about
80 weight percent) of cobalt, and from about 1 weight percent to about 20 weight percent of
nickel.
As mentioned previously, the active metal element usually exhibits relatively
high reactivity with each of boron and silicon. The addition of an active metal element like
titanium in a braze alloy has been thought to be technically challenging, due to the possibility
^
of titanium boride and titanium silicide formation, which may be undesirable. The formation
of these borides and/or silicides may prevent sufficient amounts of titanium from being
available for use as the active element. However, for compositions of the present invention,
it was observed that the titanium was not "captured" in titanium boride and/or titanium
silicide form.
The presence of other elements like nickel, chromium and iron, may avoid the
formation of the borides/silicides of the active metal element (i.e., titanium) because of higher
reactivity of these elements with boron and/or silicon, than with titanium. Furthermore, the
possibility of the formation of titanium boride and/or silicide can also be sometimes avoided
by controlled processing of the alloy. For example, during material processing of the alloy
into a desired form or shape (e.g., rolling into sheets), when the molten alloy was rapidly
quenched, for example by melt spinning, it was observed that the alloy was in the glassy
amorphous state, and did not include any boride and/or silicide. Fig. 2 shows X-ray
diffraction (XRD) images of two samples formed of alloy compositions, samples 2 and 3, by
rapidly quenching during melt spinning. It is clear from the XRDs that the alloys are in an
amorphous phase, in both the sheets.
These amorphous alloy samples or "sheets" meh during brazing, and may
provide available (free) active metal to react with the ceramic component. Thus, controlled
processing of the alloy and brazing process, allows the formation and stabilization of such
alloys containing boron, silicon, and titanium.
[0038] It may also be possible during the brazing process that the ceramic may react
with boron and/or silicon present in the braze alloy, and form an amorphous phase bond.
This bond across the ceramic component and the braze alloy may further enhance the strength
of the joint.
In one embodiment, the braze alloy includes greater than about 50 weight percent
nickel, and between about 1 weight percent and about 10 weight percent of each of silicon,
boron, chromium, iron, and an active metal element, based on the total weight of the alloy. In
some embodiments for selected end-uses, the braze alloy consists essentially of nickel,
chromium, iron, at least one of silicon or boron, and an active metal element. In some
preferred embodiments, the active metal element comprises titanium, which may be present
V ^
in an amount between about 1 weight percent and about 5 weight percent, based on the total
weight of the alloy.
As discussed above, the braze alloy has a liquidus temperature lower than the
melting temperatures of the components to be joined. In one embodiment, the braze alloy has
a liquidus temperature of at least about 850 degrees Celsius. In one embodiment, the braze
alloy has a liquidus temperature from about 850 degrees Celsius to about 1300 degrees
Celsius, and in some specific embodiments, from about 950 degrees Celsius to about 1250
degrees Celsius.
Some embodiments of the invention provide an electrochemical cell that
comprises a first component and a second component joined to each other by a braze alloy
composition. The cell may be a sodium-sulfur cell or a sodium-metal halide cell, for
example. As described previously, the braze alloy composition includes nickel, silicon,
boron, and an active metal element. At least one additional element, such as chromium, iron,
niobium, molybdenum, and/or tungsten may further be added. The constituents of the alloy
and their respective amounts are described above.
As discussed above, the braze alloy composition may provide an active braze seal
to join components in the cell. In one embodiment, the first component of the cell comprises
a metal or a metal alloy, and the second component comprises a ceramic. The metal
component can be a ring that includes nickel. The ceramic component can be a collar that
includes an electrically insulating material, for example alumina.
For example, sodium-sulfur or sodium-metal halide cells may contain the braze
alloy composition that forms an active braze seal to form metal-to-ceramic joints. The active
braze seal secures an alpha-alumina collar and a nickel ring. Fig. 1 is a schematic diagram
depicting an exemplary embodiment of a sodium-metal halide battery cell 10. The cell 10
has an ion-conductive separator tube 20 disposed in a cell case 30. The separator tube 20 is
usually made of P-alumina or P"-alumina. The tube 20 defines an anodic chamber 40
between the cell case 30 and the tube 20, and a cathodic chamber 50, inside the tube 30. The
anodic chamber 40 is usually filled with an anodic material 45, e.g. sodium. The cathodic
chamber 50 contains a cathode material 55 (e.g. nickel and sodium chloride), and a molten
electrolyte, usually sodium chloroaluminate (NaAlCU).
w
An electrically insulating ceramic collar 60, which may be made of alphaalumina,
is situated at a top end 70 of the tube 20. A cathode current collector assembly 80 is
disposed in the cathode chamber 50, with a cap structure 90, in the top region of the cell. The
ceramic collar 60 is fitted onto the top end 70 of the separator tube 20, and is sealed by a
glass seal 100. In one embodiment, the collar 60 includes an upper portion 62, and a lower
inner portion 64 that abuts against an inner wall of the tube 20, as illustrated in Fig. 1.
In order to seal the cell 10 at the top end (i.e., its upper region), a metal ring 110
is sometimes disposed. The metal ring 110 has two portions; an outer metal ring 120 and an
inner metal ring 130, which are joined, respectively, with the upper portion 62 and the lower
portion 64 of the ceramic collar 60, by means of the active braze seals 140 and 150. The
active braze seal 140, the seal 150, or both may be formed by using a suitable braze alloy
composition described above. The collar 60 and the metal ring 110 may be temporarily held
together with an assembly (e.g., a clamp), or by other techniques, until sealing is complete.
The outer metal ring 120 and the inner metal ring 130 are usually welded shut to
seal the cell, after joining with the ceramic collar 60 is completed. The outer metal ring 120
can be welded to the cell case 30; and the inner metal ring 130 can be welded to the current
collector assembly 80.
The shape and size of the several components discussed above with reference to
FIG. 1 are only illustrative for the understanding of the cell structure; and are not meant to
limit the scope of the invention. The exact position of the seals and the joined components
can vary to some degree. Moreover, each of the terms "collar" and "ring" is meant to
comprise metal or ceramic parts of circular or polygonal shape, and in general, all shapes that
are compatible with a particular cell design.
The braze alloys and the active braze seal formed thereof, generally have good
stability and chemical resistance within determined parameters at a determined temperature.
It is desirable (and in some cases, critical) that the braze seal retains its integrity and
properties during several processing steps while manufacturing and using the cell, for
example, during a glass-seal process for a ceramic-to-ceramic joint, and during operation of
the cell. In some instances, optimum performance of the cell is generally obtained at a
temperature greater than about 300 degrees Celsius. In one embodiment, the operating
temperature may be in a range from about 270 degrees Celsius to about 450 degrees Celsius.
12^
In one embodiment, the glass-seal process is carried out at a temperature of at least about
1000 degrees Celsius. In some other embodiments, the glass-seal process is carried out in a
range of from about 1000 degrees Celsius to about 1200 degrees Celsius. Moreover, the
bond strength and hermeticity of the seal may depend on several parameters, such as the
composition of the braze alloy, the thickness of the thin reaction layer, the composition of the
ceramic, and the surface properties of the ceramic.
In accordance with some embodiments of this invention, an energy storage
device includes a plurality of the electrochemical cells as disclosed in previous embodiments.
The cells are, directly or indirectly, in thermal and/or electrical communication with each
other. Those of ordinary skill in the art are familiar with the general principles of such
devices. For example, U.S. Patent 8,110,301 is illustrative, and incorporated by reference
herein. However, there are many other references which generally describe various types of
energy storage devices, and their construction.
Some embodiments provide a method for joining a first component to a second
component by using a braze alloy composition. The method includes the steps of introducing
the braze alloy between the first component and the second component to form a brazing
structure. (The alloy could be deposited on one or both of the mating surfaces, for example,
as also described below). The brazing structure can then be heated to form an active braze
seal between the first component and the second component. In one embodiment, the first
component includes a ceramic; and the second component includes a metal. The braze alloy
composition includes nickel, silicon, boron, and an active metal element. At least one
additional alloying element, such as chromium, palladium, niobium, molybdenum, iron,
and/or tungsten, may further be added. The constituents of the braze alloy and their
respective amounts (and proportions) are described above.
In the general preparation of the braze alloy, a desired alloy powder mixture may
be obtained by combining (e.g., mixing and/or milling) commercial metal powders of the
constituents in their respective amounts. In some embodiments, the braze alloy may be
employed as a foil, a sheet, a ribbon, a preform, or a wire, or may be formulated into a paste
containing water and/or organic fluids. In some embodiments, the precursor metals or metal
alloys may be melted to form homogeneous melts, before being formed and shaped into
particles. In some cases, the molten material can be directly shaped into foils, preforms or
wires. Forming the materials into particles, initially, may comprise spraying the alloy melt
\3
into a vacuum, or into an inert gas, to obtain a pre-alloyed powder of the braze alloy. In other
cases, pellets of the materials may be milled into a desired particle shape and size.
In one embodiment, a layer of the braze alloy is disposed on at least one surface
of the first component or the second component to be joined by brazing. The layer of the
braze alloy, in a specific embodiment, is disposed on a surface of the ceramic component.
The thickness of the alloy layer may be in a range between about 5 microns and about 300
microns. In some specific embodiments, the thickness of the layer ranges from about 10
microns to about 100 microns. The layer may be deposited or applied on one or both of the
surfaces to be joined, by any suitable technique, e.g. by a printing process or other dispensing
processes. In some instances, the foil, wire, or the preform may be suitably positioned for
bonding the surfaces to be joined.
In some specific embodiments, a sheet or foil of the braze alloy may be desirable.
The thickness of the sheets or foils may usually vary between about 20 microns and about
200 microns. The alloys can be rolled into sheets or foils by a suitable technique, for
example melt spinning. In one embodiment, the alloy may be melt spun into a sheet or a foil,
along with rapid quenching during the spinning.
In a typical embodiment, the method further includes the step of heating the
brazing structure at the brazing temperature. When the brazing structure is heated at the
brazing temperature, the braze alloy melts and flows over the surfaces. The heating can be
undertaken in a controlled atmosphere, such as ultra-high pure argon, hydrogen and argon,
ultra-high pure helium; or in a vacuum. To achieve good flow and wetting of the braze alloy,
the brazing structure is held at the brazing temperature for a few minutes after melting of the
braze alloy, and this period may be referred to as the "brazing time". During the brazing
process, a load can also be applied on the samples.
The brazing temperature and the brazing time may influence the quality of the
active braze seal. The brazing temperature is generally less than the melting temperatures of
the components to be joined, and higher than the liquidus temperature of the braze alloy. In
one embodiment, the brazing temperature ranges from about 900 degrees Celsius to about
1500 degrees Celsius, for a time period of about 1 minute to about 30 minutes. In a specific
embodiment, the heating is carried out at the brazing temperature from about 1000 degrees
Celsius to about 1300 degrees Celsius, for about 5 minutes to about 15 minutes.
Ah
During brazing, the alloy melts, and the active metal element (or elements)
present in the melt react with the ceramic and form a thin reaction layer at the interface of the
ceramic surface and the braze alloy, as described previously. The thickness of the reaction
layer may range from about 0.1 micron to about 2 microns, depending on the amount of the
active metal element available to react with the ceramic, and depending on the surface
properties of the ceramic component. In a typical sequence, the brazing structure is then
subsequently cooled to room temperature; with a resulting, active braze seal between the two
components. In some instances, rapid cooling of the brazing structure is permitted.
In some embodiments, an additional layer containing the active metal element
may be first applied to the ceramic component. The additional layer may have a high amount
of the active metal element, for example more than about 70 weight percent. Suitable
examples may include nanoparticles of the active metal element, or a hydride of the active
metal element, e.g., titanium hydride.
Some of the embodiments of the present invention advantageously provide braze
alloys, which are compositionally stable, and chemically stable in the corrosive environment
relative to known braze alloys, and are capable of forming an active braze seal for a ceramicto-
metal joint. These braze alloys have high sodium corrosion resistance, and halide
corrosion resistance for many end uses. The formation of ceramic-to-metal seals for high
temperature cells (as discussed above) by active brazing simplifies the overall cell-assembly
process, and improves the reliability and performance of the cell. The present invention
provides advantages to leverage a relatively inexpensive, simple, and rapid process to seal the
cell or battery, as compared to currently available methods.
EXAMPLES
The examples that follow are merely illustrative, and should not be construed to
be any sort of limitation on the scope of the claimed invention. Unless specified otherwise,
all ingredients may be commercially available from such common chemical suppliers as
Alpha Aesar, Inc. (Ward Hill, Massachusetts), Sigma Aldrich (St. Louis, Missouri), Spectrum
Chemical Mfg. Corp. (Gardena, California), and the like.
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Example 1
3 braze alloy compositions (samples 1-3) were prepared. For each braze sample,
as shown in Table 1, individual elements were weighed according to the desired composition.
These elements were arc-melted to provide an ingot for each composition. To ensure
homogeneity of the compositions, the ingots of the samples were triple-melted. The Hquidus
temperatures of the 3 samples (sample I, 2, and 3) were measured using Differential
Scanning Calorimeter (DSC).
Table 1.
Braze
Samples
Sample 1
Sample 2
Sample 3
Braze alloy composition (weight
percent)
Ni-7Cr-4.5Fe-4.5Si-3.2B-l.25Ti
Ni-7Cr-4.5Fe-4.5Si-3.2B-2Ti
Ni-7Cr-4.5Fe-4.5Si-3.2B-3Ti
Liquidus
temperature
1069
1090
1104
Each ingot of samples 2 and 3 was melt-spun into approximately a 75 micronthick
sheet, and rapidly quenched during spinning. Fig. 2 shows XRD images of the sample
sheets 2, 200 and the sample sheet 3, 202. The XRD images show the presence of the
amorphous phase of the alloys in both the sample sheets 2 and 3. These sheets were further
measured for elemental analysis by Electron probe micro-analysis (EPMA). The EPMA
study confirmed the absence of titanium boride and titanium silicide phases, and indicated the
presence of minor amounts of borides and silicides of nickel and chromium.
The sample sheet 2 was then placed between the surfaces of an alpha alumina
piece and a nickel piece (parts) to be joined. This assembly was then heated up to about 1200
degrees Celsius for about 10 minutes, and then cooled to room temperature, to form a joint.
Fig. 3 shows cross-sectional SEM images 300 and 400 of the joint in low and
high magnification, respectively. SEM image 300 shows a good interface between the
alumina piece and the nickel piece. SEM image 400 shows an interface between the alumina
piece 402 and braze sample 2, 404, at the joint. A reaction layer 406 was observed between
K
the braze sample 2 and alumina at the braze-ceramic interface, which indicates a reaction
between the braze alloy and the ceramic, and the formation of an active braze seal.
While only certain features of the invention have been illustrated and described
herein, many modifications and changes will occur to those skilled in the art. It is, therefore,
to be understood that the appended claims are intended to cover all such modifications and
changes as fall within the true spirit of the invention.
n

1. A braze alloy composition, comprising nickel, silicon, boron, and an active metal
element, wherein nickel is present in an amount greater than about 50 weight percent, and the
active metal element is present in an amount up to about 10 weight percent, based on the total
weight of the braze alloy composition.
2. The braze alloy composition of claim 1, comprising from about 60 weight percent to
about 90 weight percent nickel.
3. The braze alloy composition of claim 1, comprising from about 1 weight percent to
about 10 weight percent boron.
4. The braze alloy composition of claim 1, comprising from about 1 weight percent to
about 10 weight percent of silicon.
5. The braze alloy composition of claim 1, comprising from about 0.5 weight percent to
about 5 weight percent of the active metal element.
6. The braze alloy composition of claim 5, comprising from about 1 weight percent to
about 3 weight percent of the active metal element.
7. The braze alloy composition of claim 1, wherein the active metal element comprises
titanium, zirconium, hafnium, vanadium, or a combination thereof.
8. The braze alloy composition of claim 1, wherein the active metal element comprises
titanium.
\^
9. The braze alloy composition of claim 1, wherein the brazing alloy comprises at least
one additional element.
10. The braze alloy composition of claim 9, wherein the additional element comprises
chromium, niobium, cobalt, iron, molybdenum, tungsten, tantalum, or a combination thereof.
11. The braze alloy composition of claim 10, comprising from about 1 weight percent to
about 50 weight percent cobalt.
12. The braze alloy composition of claim 10, comprising from about 0.1 weight percent to
about 10 weight percent chromium.
13. The braze alloy composition of claim 10, comprising from about 0.1 weight percent to
about 10 weight percent iron.
14. The braze alloy composition of claim 10, comprising from about 0.1 weight percent to
about 5 weight percent molybdenum.
15. The braze alloy composition of claim 1, having a liquidus temperature of at least
about 850 degrees Celsius.
16. The braze alloy composition of claim 15, having a liquidus temperature in a range
from about 850 degrees Celsius to about 1250 degrees Celsius.
17. A braze alloy composition, comprising greater than about 50 weight percent nickel,
and from about 1 weight percent to about 10 weight percent each of chromium, iron, silicon,
boron, and an active metal element, based on the total weight of the braze alloy composition.
l1
18. An electrochemical cell, comprising a first component and a second component joined
to each other by the braze alloy composition comprising nickel, silicon, boron, and an active
metal element, wherein nickel is present in an amount greater than about 50 weight percent,
and the active metal element is present in an amount up to about 10 weight percent, based on
the total weight of the braze alloy composition.
19. The electrochemical cell of claim 18, wherein the braze alloy composition provides an
active braze seal that joins the first component to the second component.
20. The electrochemical cell of claim 18, wherein the first component comprises a metal,
and the second component comprises a ceramic.
21. The electrochemical cell of claim 18, wherein the first component comprises nickel.
22. The electrochemical cell of claim 18, wherein the second component comprises
alumina.
23. An energy storage device comprising a plurality of electrochemical cells as defined in
claim 18.