CERAMIC METAL JOINING WITH ACTIVE BRAZE ALLOY AND METALLIC INTERLAYER; AND RELATED DEVICES

TECHNICAL FIELD

[0001] This invention generally relates to the joining of a ceramic and a metal part using active braze alloys. In some specific embodiments, the invention relates to using an interlayer along with an active braze layer to provide strong sealing between components used at high temperatures, e.g., thermal rechargeable batteries.

BACKGROUND OF THE INVENTION

[0002] A variety of electrochemical devices require processes and compositions for providing seals on or within the devices. The seals may be used to encapsulate the entire device, or they may separate various chambers within the device. As an example, many types of seal materials have been considered for use in high-temperature rechargeable batteries/cells for joining different components.

[0003] Sodium/sulfur or sodium/metal halide are good examples of high- temperature batteries that may include a variety of ceramic and metal components. The ceramic components often 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 usually include a metallic casing, current collector components, and other metallic components which are often joined by welding or thermal compression bonding (TCB). While mechanisms for sealing these components are currently available, their use can sometimes present some difficulty. For example, metal-to-ceramic bonding can be challenging, due to thermal stress caused by a mismatch in the coefficient of thermal expansion for the ceramic and metal components.

[0004] The metal-to ceramic bonding is most important for the reliability and safety of the high-temperature cells. 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.

[0005] A common bonding technique for joining ceramic and metal components involves multiple steps of metalizing the ceramic component, followed by bonding the metallized ceramic component to the metal component using a thermal compression bond (TCB). The bond strength of such metal-to-ceramic joints is controlled by a wide range of variables. Some of the variables include 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.

[0006] 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. Nonetheless, brazing techniques remain of considerable interest for joining ceramic and metallic parts in various high-temperature devices.

[0007] In view of some of these challenges, it may be desirable to develop new compositions and methods that have 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

[0008] In one embodiment of the invention, an electrochemical cell is presented. The cell includes a first component, a second component, and an intermediate structure disposed between the first component and the second component. The first component includes alpha alumina and the second component includes nickel. The intermediate structure includes a first layer adjacent to the first component, a second layer adjacent to the second component, and a reaction layer. The first layer includes an active braze alloy that has greater than about 50 weight percent nickel, less than about 10 weight percent of an active metal, and has a liquidus temperature in a range from about 1000°C to about 1250°C. The second layer includes an interlayer metal having a coefficient of thermal expansion (CTE) value less than about 5.5 ppm/°C. The reaction layer is disposed between the first component and the first layer.

[0009] One embodiment of the invention is related to a battery that includes a plurality of interconnected electrochemical cells. Each of the electrochemical cells includes a ceramic collar having alpha alumina, a metal ring having nickel, and a sealing structure in between the ceramic collar and the metal ring. The sealing structure includes a first layer adjacent to the ceramic collar, a second layer adjacent to the metal ring, and a reaction layer. The first layer includes an active braze alloy that has greater than about 50 weight percent nickel, less than about 10 weight percent of an active metal, and has a liquidus temperature in a range from about 1000°C to about 1250°C. The second layer includes an interlayer metal having a coefficient of thermal expansion (CTE) value less than about 5.5 ppm/°C. The reaction layer is disposed between the ceramic collar and the first layer.

[0010] Another embodiment of the invention is related to an electrochemical cell having the structure of above defined electrochemical cell, further including a first intermediate alloy layer and a second intermediate alloy layer, both layers including nickel. The first intermediate alloy layer is disposed between the first layer and the second layer, and the second intermediate alloy layer is disposed between the second layer and the second component.

BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a schematic view showing a cross-section of an electrochemical cell, according to some embodiments of this invention.

[0012] FIG. 2 is a diagrammatical representation of a layered structure of the joint between the ceramic component and a metal component of an electrochemical cell, according to some embodiments of this invention.

[0013] FIG. 3 is a diagrammatical representation of an initial layered structure of the joint between the ceramic component and a metal component of an electrochemical cell, according to some embodiments of this invention.

[0014] FIG. 4 is a diagrammatical representation of a layered structure of the joint between the ceramic component and a metal component of an electrochemical cell, according to a specific embodiment of this invention.

[0015] FIG. 5 is a diagrammatical representation of an initial layered structure of the joint between the ceramic component and a metal component of an electrochemical cell, according to a specific embodiment of this invention.

[0016] FIG. 6 is a diagrammatical representation of a layered structure of the joint between the ceramic component and a metal component of an electrochemical cell, according to a specific embodiment of this invention.

[0017] FIG. 7 is a depiction of a scanning electron micrograph of cross-sections of two different brazed joints between a ceramic component and a metal component; and

[0018] FIG. 8 is a Weibull probability plot of a braze alloy joined structure, without and with a metallic interlayer.

DETAILED DESCRIPTION

[0019] The invention includes embodiments that relate to a structure including an active braze alloy layer and an interlayer for providing various types of seals. Non-limiting examples of the applications that require these kinds of seals include various electrochemical cells, e.g., those in a sodium/sulfur or a sodium metal halide battery. The invention also includes embodiments that relate to devices made by using the active braze alloy compositions along with a metallic interlayer.

[0020] As discussed in detail below, some of the embodiments of the present invention include a metallic interlayer along with an active 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. Although 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.
[0021] 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. 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 there between

[0022] 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.

[0023] 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 |3-alumina or [3"-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).

[0024] An electrically insulating ceramic collar 60, which may be made of alpha-alumina, 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.

[0025] 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 may have 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.

[0026] The active braze seal 140, the seal 150, or both may be formed by using a suitable braze alloy composition, such as those described in pending Patent Applications 13/407870, filed on February 29, 2012; 13/538203, filed on June 29, 2012; 13/600333, filed on August 31, 2012; and 13/628548, filed on September 27, 2012, the entire contents of which are incorporated herein by reference. There can sometimes be a large mismatch in the coefficient of thermal expansion (CTE) between an alpha alumina ceramic part and the metal rings. This large CTE mismatch leads to large undesired tensile stresses in the ceramic, resulting in low joint strength. Achieving high bond strength between the ceramic parts, active braze alloys, and the metallic part is desirable. A metallic interlayer in-between the active braze alloy and the metal collar is hereby disclosed to be very effective in reducing the CTE mismatch and strengthening the overall bonding of the ceramic-metal joints.

[0027] Accordingly, in one embodiment, an electrochemical cell is disclosed that has a first component 200 and a second component 400, and an intermediate structure 300 disposed between the first component and the second component, as shown in FIG. 2. In one embodiment, the first component 200 of the cell comprises a ceramic, and the second component 400 comprises a metal. The (first) ceramic component 200 can be a collar that includes an electrically insulating material, such as alpha-alumina. The (second) metal component 400 can be formed of a variety of metals and alloys. In a particular embodiment, the second component is made up of nickel metal, i.e., the component contains at least about 95 % of nickel metal.

[0028] The intermediate structure 300 includes a first layer 310, a second layer 320 and a reaction layer 330, as shown in FIG. 2. The first layer 310 of the intermediate structure comprises an active braze alloy and is disposed adjacent to the first component 200. In one embodiment, the first layer is formed of the active braze alloy. An "active braze alloy" is an alloy composition including an active metal element that promotes wetting of a ceramic surface, enhancing the capability of providing a hermetic seal with the first component 200. As used herein the terms "adjacent" refers to both direct contact between layers, objects, and the like, or indirect contact, e.g., having intervening layers there between. Therefore, the first layer 310 may be adjacent t the first component 200, without or with an intervening layer.

[0029] The second layer 320 is adjacent to the second component and includes an interlayer metal. As used herein, the "interlayer metal" represents a metal being present in the second layer 320 that is an interlayer between the active braze alloy layer and the second component. In one embodiment, the second layer 320 having an interlayer metal is a metallic interlayer having one or more metals. The second layer 320 may be adjacent to the second component 400, without or with an intervening layer. The reaction layer 330 of the intermediate structure 300 is disposed between the first component 200 and the first layer 310. The reaction layer 330 generally forms as a result of the reaction between the first layer 310 and the first component 200, during the process of active brazing.

[0030] The first component 200 used herein is an alpha alumina containing ceramic component having oxygen as part of the composition of the component. The "active metal element" of the active braze alloy refers to a reactive metal that has high affinity to the oxygen within the ceramic, and thereby reacts with the first component. 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 the reaction layer 330 at the interface of the first component 200 and the braze alloy of the first layer 310.

[0031] The 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) of the first component 200 to form the reaction layer 330. The reaction layer 330 formed may be of uneven thickness and discontinuous in some embodiments, but may be uniform and continuous in some other embodiments. 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.

[0032] In one embodiment, the reaction layer 330 includes a metallic element of the first component 200 and a sub-oxide of the active metal element. For example, in an embodiment where the first component is of alpha alumina, and the active metal element is titanium, the reaction layer 330 may have a composition comprising aluminum and a sub-oxide of the active metal element. A sub-oxide of titanium may be defined as a titanium oxide with the titanium to oxygen ratio less than 1:2. Thus a titanium sub-oxide is TiOx with x < 2. Ti20, TiO, and TiOi 2 can be the examples of a titanium sub-oxide.

[0033] The presence and the amount of the active metal in the first layer 310 may influence the thickness and the quality of the reactive layer 330, which contributes to the wettability or flowability of the braze alloy of the first layer 310 with the first component 200, and therefore, the bond strength of the resulting joint. In some embodiments, the active metal is present in a concentration less than about 10 weight percent, based on the total weight of the active braze alloy. A suitable range is often from about 0.1 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 active braze alloy of the first layer 310. The active metal element is generally present in small amounts suitable for improving wetting of the ceramic surface, and forming the reaction layer 330. A high amount of the active metal, e.g., above about 10 weight percent, may cause or accelerate halide corrosion.

[0034] The reaction layer 330 is often a very thin layer that allows the
active braze alloy of the first layer 310 to wet the surface of the first component 200. In one embodiment, the reaction layer 330 formed in the intermediate structure 300 is continuous and has an average thickness less than about 10% of the intermediate structure 300. In one embodiment, the reaction layer thickness is less than about 5% of the intermediate structure 300. In some preferred embodiments, the reaction layer thickness is less than about 10 micrometers, and specifically less than about 5 micrometers.

[0035] In order to address some of the problems associated with corrosion, the present inventors discovered that nickel could be used as a component of the active braze alloy of the first layer 310. Nickel provides a degree of chemical inertness in a corrosive environment. Further, the use of nickel in the active braze alloy of the first layer 310 may promote the integration of joints with the nickel-containing second component 400. Additionally, nickel can also increase the liquidus temperature of the alloy composition. 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 when heated. However, a high amount of nickel may undesirably raise the liquidus temperature of the alloy composition, i.e., above the required brazing temperature.

[0036] In some embodiments of this invention, a suitable level for the amount of nickel in the braze alloy composition may be at least about 30%, based on the total weight of the braze alloy. Very often, nickel is present in an amount of at least about 45%. In some embodiments that are preferred for selective end-use applications, the nickel is present from about 50% to about 70%, based on the total weight of the braze alloy, and more often, from about 50% to about 65%.

[0037] The active braze alloy composition of the first layer 310 may further comprise an element selected from chromium, niobium, tantalum, cobalt, and combinations thereof. These elements are especially useful for providing strength, high-temperature resistance, and corrosion-resistance in a sodium-containing environment.

[0038] In one embodiment, the braze alloy composition is based on a nickel-germanium (Ni-Ge) binary alloy. Nickel is a base metal for the braze alloy, which is relatively inert in corrosive environments, as compared to other known base metals, e.g. copper, iron, chromium, etc. Germanium is a melting point depressant, the addition of which reduces the melting point of the overall composition. As used herein, the term "melting point depressant" refers to an element 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.

[0039] In order to reduce the liquidus temperature of the first layer 310, additional melting point depressants may be added. Suitable examples of the additional melting point depressant include, but are not limited to, silicon, boron, manganese, or iron.

[0040] In some specific embodiments, the active braze alloy is substantially free of some commonly used melting-point depressant materials. For example, in one embodiment the active braze alloys are free of copper, silver, gold, and platinum. In another example, the active braze alloys are free of zinc, rhenium, and phosphorous. In a specific embodiment, the active braze alloy is free of melting point depressant materials such as copper, silver, gold, zinc, platinum, rhenium, and phosphorous. In some particular embodiments, these elements may only be present as impurities, not being added purposefully to optimize a particular characteristic of the first layer 310. The addition of some these elements may be helpful for decreasing the liquidus temperature of the first layer 310, but can sometimes be problematic in the case of sodium metal halide electrochemical cells. For example, gold and silver tend to form various intermetallics with sodium, at the operating temperature of the cells, and this can promote corrosion when the cell is in operation. Thus, it is often preferred that the active braze alloy composition be free of these metals.

[0041] Generally, the active braze alloy of the first layer 310 has a liquidus temperature lower than the melting temperatures of the first and second components. In one embodiment, the braze alloy of the first layer 310 has a liquidus temperature of at least about 850 degrees Celsius. In one embodiment, the braze alloy has a liquidus temperature from about 950 degrees Celsius to about 1300 degrees Celsius, and in some specific embodiments, from about 1000 degrees Celsius to about 1250 degrees Celsius.

[0042] In some specific embodiments, a sheet or foil of the active braze alloy may be desirable. The thickness of the sheets or foils may usually be less than about 75% of the thickness of intermediate structure 300. The thickness of the first layer 310 in an intermediate structure may generally be in a range from about 20 microns to about 200 microns. In specific embodiments, the thickness of the first layer 310 may be between 50 micrometers and 100 micrometers.

[0043] The second layer 320 adjacent to the second component 400 includes an interlayer metal that aids in reducing the effect of thermal expansion mismatch between the first component 200 and the second component 400. The second layer 320 may include the interlayer metal in a concentration greater than about 50 weight percent, based on the weight of the second layer. In a specific embodiment, the second layer 320 consists essentially of a metal or an alloy. The second layer 320 may also be termed as an interlayer. In one embodiment, the coefficient of thermal expansion (CTE) value of the metal or alloy is less than the CTE value of the first component. This low CTE value may have a CTE mismatch with the first component 200, and may exert a desirable compressive stress on the first component 200 and the first layer 310 during cooling of the electrochemical cell from a higher temperature. In one embodiment, the CTE value of the second layer 320 is less than both the CTE values of the first component 200 and the second component 400. Further, in one embodiment, the CTE value of the second layer 320 is less than the CTE value of the first layer 310. In a specific embodiment, the CTE value of the second layer 320 is less than about 5.5 ppm/°C.

[0044] The second layer (interlayer) 320 may have a small thickness, yet thick enough to have sufficient bonding strength with the second component 400. In one embodiment, the interlayer has a thickness greater than about 25% of the intermediate structure 300. In one embodiment, the thickness of the interlayer 320 is in a range of about 5 micrometers to about 100 micrometers. In another embodiment, the interlayer has a thickness in a range of about 10 micrometers to about 50 micrometers. In yet another embodiment, the interlayer has a thickness in a range of about 15 micrometers to about 40 micrometers, and in embodiments especially preferred in some instances, from about 20 micrometers to about 30 micrometers.

[0045] The metal or alloy of the interlayer may be used to improve the diffusion bond with the metallic part of the second component 400. In one embodiment, the metal may include a refractory metal. In one embodiment, the refractory metal of the second layer 320 may include niobium, tungsten, molybdenum, or any combinations of these elements. In one embodiment, the second layer 320 comprises one or more of molybdenum, tungsten, niobium, and tantalum. In one specific embodiment, the metal is molybdenum.

[0046] Some embodiments of this invention provide a method for joining the first component 200 to the second component 400 by using the first and second layers. As illustrated in FIG. 3, the method includes the steps of introducing an initial first layer 302, including the previously described active braze alloy, and an initial second layer 304, comprising the previously described interlayer metal, between the first component 200 and the second component 400. The layers 302 and 304 may be obtained by combining (e.g., mixing and/or milling) commercial powders of the constituents in their respective amounts. In some embodiments, these 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 layers. In some cases, the molten material can be directly shaped into foils, preforms or wires.

[0047] In a typical embodiment, the method further includes the step of heating the structure to form an intermediate structure 300 between the first component 200 and the second component 400, thus forming a joint between the first and second components, as previously described in FIG. 2. When the initial structure is heated to the required temperature, some components of the initial first layer 302 and the initial second layer 304 melt and flow over the surfaces to be joined. 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.

[0048] In general, the composition and microstructure of the first component 200 remains unchanged after the joining step with component 400, except for surface changes due to the interaction with the layer 302 during heating. Similarly the second component 400 may undergo surface changes after interaction, during heating, with the interlayer metal of the initial second layer 304.

[0049] Depending on the heating conditions and the reactivity of the compositions of the initial first layer 302, initial second layer 304, and the second component 400, one or more additional layers, other than the reaction layer 330, may also be formed as part of the intermediate structure 300 (FIG. 3). For example, the elements of the initial first layer 302 and the initial second layer 304 may react and form a first intermediate alloy layer 340 comprising nickel and the interlayer metal, disposed between the first layer 310 and the second layer 320, as shown in FIG. 4. In one embodiment, a concentration of nickel in the first intermediate alloy layer 340 is greater than about 20 weight percent.

[0050] Similarly, the elements of the initial second layer 304 and the second component 400 may react to form a second intermediate alloy layer 350 comprising nickel and the interlayer metal, disposed between the second layer 320 and the second component 400, as shown in FIG. 4. In one embodiment, a concentration of nickel in the second intermediate alloy layer 350 is greater than about 20 weight percent. Thus, in one embodiment, the intermediate structure 300 may include the first layer 310, second layer 320, reaction layer 330, a first intermediate alloy layer 340, and the second intermediate alloy layer 350, as shown in FIG. 4.

[0051] In an additional embodiment of the invention, the initial setup may include an initial third layer 306 (FIG. 5) disposed between the initial second layer 304 and the second component 400. The initial third layer 306 may include an active braze alloy composition that it similar to or different from the active braze alloy composition of the initial first layer 302. In another embodiment, the initial third layer 306 can be a conventional braze alloy without any active metal being present. On heating, the initial third layer 306 may react with the initial second layer 304 and the second component 400. A third intermediate alloy layer 370 (FIG. 6) may be formed that joins the second layer 320 and the third layer 360. Furthermore, a second reaction layer 380 may be formed in between the third layer 360 and the second component 400, as shown in FIG. 6. In one embodiment, the second reaction layer 380 is different from the first reaction layer 330 that is formed between the first intermediate layer 310 and the first component 200.

[0052] As one skilled in the art would appreciate, the layers such as reaction layer 330, first intermediate alloy layer 340, the second intermediate alloy layer 350 (FIG. 4), the third intermediate alloy layer 370, and the second reaction layer 380 (FIG. 6), etc., may be present as single layers or a combination

of different layers. Further, these layers may have a uniform composition and microstructure throughout the layer thickness, or may have gradational differences within the layer, in terms of composition, structure, microstructure and /or density.

[0053] Some of the other inventive embodiments of this invention are directed to an energy storage device (battery) that 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.

EXAMPLE

[0054] 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.

[0055] A braze alloy composition having 2 wt% aluminum, 3 wt% silicon, 2.25 wt% titanium, with the remainder being copper, was prepared by weighing individual elements and arc-melting, and then melt spinning into a 75 micron-thick sheet, and used as an initial first (braze) layer between alpha alumina ceramic and a nickel metal components, in samples 1 and 2. Sample 1 did not have any second layer having an interlayer metal. Sample 2 was prepared by using molybdenum foil as a second layer between the first layer and the nickel component.

[0056] Low magnification cross-sectional SEM images of sample 1 and 2 after heat-treatment are shown in FIG. 7. The SEM micrograph of sample 1 (500) shows the existence of microcracks 560 in the alpha alumina component 510 after joining the alpha alumina component 510 and the nickel component 520, using the braze alloy layer 530, with the formation of reaction layer 550. For sample 2, the alpha alumina component 610 and the nickel component 620 are joined by using the first (braze) layer 630; the second layer (having molybdenum) 640, and the third (braze) layer 632. The first layer 630 was the braze alloy composition that is used in sample 500. A reaction layer 650 is formed at the interface of the alpha alumina component 610 and the first braze layer 630. The third layer 632 is of different composition than the first layer 630. A reaction layer 670 is formed in between the third layer 632 and the nickel component 620. In contrast to sample 1, the structure of sample 2 does not display any microcracks.

[0057] FIG. 8 shows a Weibull probability plot of the sample 1 (710) and sample 2 (720), showing the probability of failure for each sample. . The Weibull probability plot is a statistical plot showing the strength of the metal-ceramic joints. A failure probability F(Vo) of the joints is given by the formula where P(Vo) is the probability of survival, a is the applied stress, and Vo is the material volume being tested. While a is the stress applied at which the survival or failure probability is being evaluated, the constants η and β are material constants of interest for a constant-flaw population. For a brittle ceramic material, r| is the characteristic strength of the material that is approximately close to the mean strength, but mathematically defined as the stress corresponding to 1/e or 37% survival probability. The constant β is the Weibull modulus, and is a measure of the variability (similar to standard deviation) in material strength. Therefore, a high value of η and low value of β indicates a good joint, while the joints having low η and highβ is expected to have more probability of failure at lower loads.

[0058] In FIG. 8, the Weibull plot 710 of sample 1 corresponding to the metal-ceramic joint without a metallic interlayer was drawn using 8 data points measured at different load values. Weibull plot 720 of sample 2 corresponding to the metal-ceramic joint with the molybdenum interlayer 640 was drawn using 9 data points measured at different load values. The Weibull plot shows that the probability of failure is high for the sample 1 (710) at lower loads, as compared to the probability of failure of sample 2 (720). The determined η and β values of the plots 710 and 720 are as follows.

Therefore, it is clearly seen that inclusion of molybdenum interlayer 640 increases the metal-ceramic joint strength when applied along with an active braze layer.

[0059] 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.

CLAIMS
1. An electrochemical cell, comprising:

a first component comprising alpha alumina;

a second component comprising nickel; and

an intermediate structure disposed between the first component and the second component, comprising

a first layer adjacent to the first component and comprising an active braze alloy having nickel at a concentration greater than about 50 weight percent,

an active metal element at a concentration less than about 10 weight percent, and

a liquidus temperature in a range from about 1000°C to about 1250°C;

a second layer adjacent to the second component and comprising an interlayer metal having a coefficient of thermal expansion (CTE) value less than about 5.5 ppm/°C; and

a reaction layer disposed between the first component and the first layer.

2. The electrochemical cell of claim 1, wherein the concentration of the active metal element in the first layer is in a range from about 0.1 weight percent to about 5 weight percent.

3. The electrochemical cell of claim 1, wherein the active metal element comprises titanium, zirconium, hafnium, vanadium, or a combination thereof.

4. The electrochemical cell of claim 1, wherein the active braze alloy further comprises germanium.

5. The electrochemical cell of claim 1, wherein the active braze alloy further comprises chromium and iron.

6. The electrochemical cell of claim 1, wherein the active braze alloy further comprises chromium, niobium, and cobalt.

7. The electrochemical cell of claim 1, wherein a thickness of the first layer is less than about 75% of the thickness of the intermediate structure.

8. The electrochemical cell of claim 1, wherein the interlayer metal of the second layer comprises niobium, chromium, tungsten, silicon, germanium, zirconium, molybdenum, or a combination thereof.

9. The electrochemical cell of claim 1, wherein a thickness of the second layer is greater than about 25% of the thickness of the intermediate structure.

10. The electrochemical cell of claim 1, wherein the reaction layer comprises aluminum and a sub-oxide of the active metal element.

11. The electrochemical cell of claim 10, wherein a thickness of the reaction layer is less than about 10% of the thickness of the intermediate structure.

12. The electrochemical cell of claim 1, wherein the intermediate structure further comprises a first intermediate alloy layer comprising nickel and the interlayer metal, disposed between the first layer and the second layer.

13. The electrochemical cell of claim 12, wherein a concentration of nickel in the first intermediate alloy layer is greater than about 20 weight percent.

14. The electrochemical cell of claim 1, wherein the intermediate structure further comprises a second intermediate alloy layer comprising nickel and the interlayer metal, disposed between the second layer and the second component.

15. The electrochemical cell of claim 14, wherein a concentration of nickel in the second intermediate layer is greater than about 20 weight percent.

16. The electrochemical cell of claim 1, wherein the active braze alloy is substantially free of copper, silver, gold, and platinum.

17. The electrochemical cell of claim 1, wherein the active braze alloy is substantially free of zinc, rhenium, and phosphorous.

18. A battery that comprises a plurality of interconnected electrochemical cells, wherein each cell comprises:

a ceramic collar comprising alpha alumina;

a metal ring comprising nickel; and

a sealing structure disposed between the ceramic collar and the metal ring, comprising

a first layer adjacent to the ceramic collar and comprising an active braze alloy having

nickel at a concentration greater than about 50 weight percent,

an active metal element at a concentration less than about 10 weight percent, and

a liquidus temperature in a range from about 1000 °C to about 1250 °C; and

a second layer adjacent to the metal ring and comprising an interlayer metal having a coefficient of thermal expansion (CTE) value less than about 5.5 ppm/°C; and

a reaction layer disposed between the ceramic collar and the first layer.

19. An electrochemical cell, comprising:

a first component comprising alpha alumina;

a second component comprising nickel; and

an intermediate structure disposed between the first component and the second component, comprising

a first layer adjacent to the first component and comprising an active braze alloy having

nickel at a concentration greater than about 50 weight percent,

an active metal element at a concentration less than about 10 weight percent, and

a liquidus temperature in a range from about 1000 °C to about 1250 °C;

a second layer adjacent to the second component and comprising a metal having a coefficient of thermal expansion (CTE) value less than about 5.5 ppm/°C;

a reaction layer comprising aluminum disposed between the first component and the first layer;

a first intermediate alloy layer comprising nickel, disposed between the first layer and the second layer; and

a second intermediate alloy layer comprising nickel, disposed between the second layer and the second component.