Patent Application
20140356681
This invention generally relates to brazing structures and compositions. In some particular embodiments, it relates to energy storage devices, such as batteries, that include brazing structures.
Metal chloride batteries, especially sodium-metal chloride batteries with a molten sodium negative electrode (usually referred to as the anode) and a beta-alumina solid electrolyte, are of considerable interest for energy storage applications. In addition to the anode, the batteries include a positive electrode (usually referred to as the cathode) that supplies/receives electrons during the charge/discharge of the battery. The solid electrolyte functions as the membrane or “separator” between the anode and the cathode.
When these metal chloride batteries are employed in mobile applications like hybrid locomotives or plug-in electric vehicles (PHEV), the batteries are often capable of providing power surges (high currents) during the discharge cycle. In an ideal situation, the battery power can be achieved without a significant loss in the working capacity and the cycle life of the battery. The advantageous features of these types of batteries provide opportunities for applications in a number of other areas as well. Examples include their incorporation into uninterruptable power supply (UPS) devices; or as part of a battery backup system for a telecommunications (“telecom”) device, sometimes referred to as a telecommunication battery backup system (TBS).
A typical, general design for metal chloride cells and other types of thermal batteries is depicted in
Many types of seal materials and sealing systems 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 usually 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 formation of strong, leak-proof, reliable seals between metal and ceramic parts can be a very challenging prospect.
“Active brazing” techniques have been developed to improve the joining of ceramics to metals, as well as ceramic materials to other ceramic materials. Active brazing uses an active metal element that promotes wetting of a ceramic surface, enhancing the capability of providing a hermetic seal, e.g., one capable of containing battery constituents within interior regions of the device. 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 decomposition 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 or bond, which may also be referred to as an “active braze seal.”
The type of active metal element (usually one of titanium (Ti), zirconium (Zr), hafnium (Hf), or vanadium (V)) in the braze alloy for a particular type of braze bond can be important. The active metal must form a continuous reaction layer, to ensure a strong, hermetic joint. Moreover, the composition of the components being joined together is also an important consideration in forming a desired hermetic joint.
The joining of aluminum oxide (alumina) components to nickel metal components by active brazing techniques can sometimes present some difficulties. During the brazing process, at elevated temperatures, active elements like titanium sometimes diffuse in the molten state toward nickel, at a rate faster than diffusion toward the alumina component. Diffusion in this manner effectively deprives the alumina of the necessary titanium content. In some cases, this can lead to an inferior brazed joint, which can, in turn, lead to failure of the particular seal. Various other metals may be as susceptible to this diffusion problem, as in the case of nickel. However, the predominant use of nickel in various electrochemical devices like thermal batteries demonstrates that the problem can be a serious one.
Thus, new methods for ensuring that very strong and hermetic braze seals can be formed between various metal and ceramic components, would be welcome in the art. The methods and related brazed structures should be effective for providing strong seals between various ceramic components and nickel components, e.g., when those components are used for sealing structures in high-performance, thermal batteries. Moreover, the methods should not adversely affect the performance of devices which include the sealing structures, in any significant way.
An embodiment of the invention is directed to a brazing structure for a thermal battery, comprising:
a) a nickel or nickel alloy component;
b) a ceramic component;
c) a braze alloy layer disposed between components (a) and (b), and formed of a composition comprising an active metal element; and
d) a barrier layer disposed between component (a) and the braze alloy layer, capable of preventing or minimizing the diffusion of the active metal element into the nickel or nickel alloy component.
Another embodiment relates to an electrochemical cell that includes at least one ceramic component and at least one metallic component that comprises nickel or a nickel alloy. The components are joined to each other by a braze alloy composition that comprises an active metal element. A barrier layer is disposed between the metallic component and the braze alloy composition, said barrier layer being capable of preventing or minimizing the diffusion of the active metal element into the metallic component.
A method of joining a nickel or nickel alloy (metallic) component in a thermal battery to a ceramic component in the battery, constitutes another embodiment of the invention. The method includes the steps of:
(I) applying a barrier layer to at least a portion of the surface of the metallic component;
(II) introducing an active metal-containing braze alloy composition between the barrier layer and the ceramic component to be joined; and
(III) heating the components and the braze alloy composition to form an active braze seal (joint) between the metallic component and the ceramic component.
The barrier layer is capable of preventing or minimizing the diffusion of the active metal into the metallic component.
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.
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. Moreover, 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. Furthermore, 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 described previously, the brazing structure for embodiments of this invention includes a metal component like nickel (or nickel alloys), joined to a ceramic component, by the use of a braze alloy material disposed between the two components. The ceramic component is often alumina, but other ceramics are possible as well, such as zirconia, titania, silicon carbide, spinel, magnesium oxide, yttria, and yttria aluminum garnet (YAG). As further described elsewhere herein, the ceramic and metal components are often (but not always) in the form of parts used in electrochemical devices, such as batteries.
A wide variety of brazing compositions may be used for the present invention. Some are described, for example, in pending U.S. application Ser. No. 13/407,870 (Docket 254898), filed on Feb. 29, 2012; and in U.S. application Ser. No. 13/538,203 (Docket 256606-2), filed on Jun. 29, 2012, both of which are incorporated herein by reference. The braze alloy compositions often comprise nickel, the active metal element discussed below, and at least one element selected from the group consisting of germanium, copper, niobium, chromium, cobalt, iron, molybdenum, tungsten, and palladium. In some embodiments, the braze alloy composition further comprises at least one of silicon and boron, often used to suppress the melting temperature or liquidus temperature of the alloy.
Some of the general terminology related to this disclosure and brazing technology can be reviewed briefly. 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 solidus 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 melts at a lower temperature than the base materials; or melts at a very specific temperature.
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.
As mentioned above, the braze composition that will form the braze alloy layer includes an active metal element that is critical for 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-metal joint/bond, which may also be referred to as an “active braze seal.”
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 of forming an alloy with a base alloy (e.g. Cu—Ni alloy). In some preferred embodiments for the present invention, the active metal element is 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.
As described in U.S. application Ser. No. 13/538,203, for example, the presence and the amount of the active metal may influence the thickness and the quality of the thin reactive 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. In the case of electrochemical cells of the type described herein, 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 improving the wetting of the ceramic surface, and forming the thin reaction layer, for example, less than about 10 microns. In the case of sodium-metal halide batteries, an excessive amount of an active metal in the brazing composition may sometimes cause or accelerate halide corrosion.
As alluded to previously, 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° C., and in some instances, from about 850° C. to about 1300° C. In some specific embodiments, the liquidus temperature is from about 950° C. to about 1250° C.
However, in the embodiment of
The composition of the barrier layer will depend on a number of factors, some of which are described more specifically below. In general, the factors include, for example, the specific types of materials being brazed; the other constituents that are present in the braze composition, e.g., the type of active metal; as well as the type of device or other article in which the brazing structure is to be incorporated. In some embodiments, the barrier layer comprises molybdenum (Mo), chromium (Cr), tungsten (W), or rhenium (Re). Combinations of two or more of these metals may also be used. As discussed below, in other embodiments, the barrier layer comprises molybdenum, chromium, tungsten, rhenium, niobium (Nb), tantalum (Ta), manganese (Mn), or combinations thereof. In the case of braze compositions containing titanium (or mainly titanium) as the active metal, the barrier layer often comprises molybdenum or chromium, or combinations thereof.
Moreover, in the case of braze structures for sodium/metal halide electrochemical cells, the choice of a barrier layer is greatly influenced by the material's stability, relative to the chemistry of the cell, e.g., its anti-corrosive character. The barrier layer material should not be one that will dissolve into the active metal, so as to form brittle intermetallic phases in the braze joint. The barrier layer material should also not have a tendency to form intermetallics with nickel or nickel alloys (when the metal halide is based on nickel), or to form intermetallics with the braze elements. These intermetallics can also result in poor joint strength.
Thus, the most suitable composition for the barrier layer will depend in part on the location of the braze structure, relative to an anode or cathode. With these considerations in mind, it has been determined that the barrier layer can comprise Mo, Cr, W, Re, or combinations thereof, for the cathode region of the cell, since the other elements described above may induce corrosion or other undesirable effects, as explained previously. However, in the case of the anode region of the cell, the barrier layer can comprise any of the elements (Mo, Cr, W, Re, Nb, Ta, Mn), or combinations thereof.
The barrier layer can be positioned between the nickel component and the braze alloy layer by a number of techniques. For example, the barrier layer could be deposited by conventional techniques such as chemical vapor deposition (CVD). Alternatively, sputtering techniques could be used, as well as traditional plating techniques (electroplating or electroless plating), or physical deposition techniques. The most suitable technique will depend on the particular barrier material employed; the desired thickness for the layer; and several of the other factors set out previously.
The thickness of the barrier layer will also depend on some of the considerations noted above, including, for example, the thickness of the adjacent braze alloy layer and the nickel layer; as well as the composition of the barrier layer. In general, the barrier layer usually has a thickness in the range of about 0.1 micron to about 1,000 microns; and more often, in the range of about 10 microns to about 500 microns. In the case of a chromium or molybdenum barrier layer, used in conjunction with a titanium-based active braze material, the thickness may often be about 20 microns to about 250 microns.
As described previously, the brazing structure can be incorporated into 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 part of a thermal battery, e.g., a sodium-sulfur cell or a sodium-metal halide cell. As discussed above, the braze alloy composition may provide an active braze seal to join the components. In one embodiment, the first component of the cell comprises a metal, and the second component comprises a ceramic. The metal component can be a ring that is formed of nickel or a nickel alloy. The ceramic component can be a collar that includes an electrically insulating material, such as alpha-alumina.
An electrically insulating ceramic collar 60, which may be made of alpha-alumina, is situated at a top end 70 of the separator tube 32. 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 32, as illustrated in
In order to seal the cell 30 at the top end (i.e., its upper region), and protect the alumina collar 60 in the corrosive environment, a metal ring 110 is sometimes disposed, covering the alpha alumina collar 60, and joining the collar with the current collector assembly 80, underneath the cap structure 90. The metal ring 110 usually 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 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 34; 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
In general, braze alloys (e.g., the braze alloy layer) and the active braze seal formed thereof, generally have good stability and chemical resistance within determined parameters at a determined temperature. In the case of electrochemical cells, 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-metal joint, and during operation of the cell.
In some instances, optimum performance of the electrochemical cell is generally obtained at a temperature greater than about 300° C. In one embodiment, the operating temperature may be in a range from about 270° C. to about 450° C. In one embodiment, the glass-seal process is carried out at a temperature of at least about 1000° C. In some other embodiments, the glass-seal process is carried out in a range of from about 1000° C. to about 1200° C. 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 metal and the ceramic, and the surface properties of the ceramic. The use of the barrier layer within the brazing structure is an important feature in enhancing the strength and integrity of the seal(s) within the cells.
According to some embodiments of this invention, an energy storage device is provided. The device includes a plurality of the electrochemical cells as described herein. 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.
As mentioned previously, another embodiment of the invention is directed to a method for joining a metallic component to a ceramic component in a device or other type of article. The method includes the step of applying a barrier layer to at least a portion of the surface of the metallic component (often made of nickel or a nickel alloy)—a portion sufficient to allow the formation of the desired seal for containing or isolating various solids, liquids, or gases. An active metal-containing braze alloy composition can then be introduced between the barrier layer and the ceramic component to be joined, forming a layer of the braze. The components are then heated with the braze material to form an active braze seal (i.e., a joint) between them. The use of the barrier layer in this process to restrict the migration of the active metal often leads to the high-quality seal described previously.
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, Mass.), Sigma Aldrich (St. Louis, Mo.), Spectrum Chemical Mfg. Corp. (Gardena, Calif.), and the like.
A braze alloy composition was prepared, by weighing out the individual constituents according to the selected proportions, to form a paste-composition, as follows:
Cu-3Si-2Al-2.25 Ti, in the shape of pre-forms; and Cu-3Si-2Al-13Ti, in the form of powder. The composition had a liquidus temperature of approximately 1024° C.
The braze composition was used to join a nickel ring to an alumina collar, i.e., as depicted in simple form, in
The quality of the braze structure was then evaluated. The test-piece was cross-sectioned; and the interface between the alumina and nickel plates was examined, using scanning electron microscopy (SEM).
The same type of braze composition was then used to form the Sample 2 braze structure, depicted in the scanning electron micrograph of
The quality of this braze structure was then evaluated. The test-piece for Sample 2 was cross-sectioned; and the interface between the alumina and nickel plates was examined, using scanning electron microscopy (SEM).
The present invention has been described in terms of some specific embodiments. They are intended for illustration only, and should not be construed as being limiting in any way. Thus, it should be understood that modifications can be made thereto, which are within the scope of the invention and the appended claims. Furthermore, all of the patents, patent applications, articles, and texts which are mentioned above are incorporated herein by reference.
