Patent Application
20080217382
This invention relates to electrochemical devices, including, without limitation, solid oxide fuel cells, oxygen separators, and hydrogen separators. More specifically, the present invention relates to an improved braze used for forming joints utilized in electrochemical devices.
The publications J. S. Hardy, J. Y. Kim, K. S. Weil, “Joining Mixed Conducting Oxides Using An Air-Fired Electrically Conductive Braze,” J Electrochem. Soc. Vol. 151, No. 8, pp. j43-j49 and U.S. patent application Ser. No. 10/334,346, recently described a method of ceramic-to-metal brazing useful for fabricating high temperature solid-state devices such as oxygen generators. Referred to as “air brazing,” the technique differs from traditional active metal brazing in two important ways: (1) it utilizes liquid-phase oxide-noble metal melt as the basis for joining and therefore exhibits high-temperature oxidation resistance and (2) the process is conducted directly in air without the use of fluxes and/or inert cover gases. In fact, the strength of the bond formed during air brazing relies on the formation of a thin, adherent oxide scale on the specifically stated to the contrary in the claims. Further, when the language “at least a portion” and/or “a portion” is used, the claims may include a portion and/or the entire items unless specifically stated to the contrary. Likewise, where the term “input” or “output” is used in connection with an electric device or fluid processing unit, it should be understood to comprehend singular or plural and one or more signal channels or fluid lines as appropriate in the context. Finally, all publications, patents, and patent applications cited in this specification are herein incorporated by reference to the extent not inconsistent with the present disclosure as if each were specifically and individually indicated to be incorporated by reference and set forth in its entirety herein. metal substrate. The technique employs a molten oxide that is at least partially soluble in a noble metal solvent to pre-wet the oxide faying surfaces, forming a new surface that the remaining molten braze material easily wets. As user herein the term “braze” means a metal, or a metal containing mixture, used to form a hermetic joint between two parts, which parts may be metal, ceramic, or combinations thereof. “Braze” is often referred to as “filler material” or “filler metal” in the scientific and engineering literature. A number of metal oxide-noble metal systems are suitable, including Ag—CuO, Ag—V2O5, and Pt—Nb2O5 as described in Z. B. Shao, K. R. Liu, L. Q. Liu, H. K. Liu, S. Dou (1993) “Equilibrium Phase Diagrams In The Systems PbO—Ag and CuO—Ag,”J. Am. Cer. Soc., 76 (10) 2663-4; in A. M. Meier, P. R. Chidambaram, G. R. Edwards (1995), “A Comparison Of The Wettability Of Copper-Copper Oxide And Silver-Copper Oxide On Polycrystalline Alumina,” J. Mater. Sci., 30 (19) 4781-6; and in R. S. Roth, J. R. Dennis, H. F. McMurdie, eds. (1987) Phase Diagrams for Ceramists, Volume VI, The American Ceramic Society, Westerville, Ohio.
Ag—CuO air brazing thus does not require the use of an inert environment or a flux. Since silver is used as a matrix phase, brazing can be conducted directly in air and the resulting joint has good oxidation resistance at high temperature. The addition of copper oxide, which dissolves in a molten silver, improves the wettability of molten silver on both ceramic and metal faying surfaces, resulting in good bonding between silver and substrate surfaces.
Despite these advantages, the low viscosity of molten Ag—CuO causes the braze to squeeze-out (beading). To maintain good contact between the braze and the faying surfaces, a weight is typically used to squeeze them together. However, the molten braze is often squeezed out during processing, causing beads of braze to form outside of joined surfaces as shown in
The low viscosity of a molten air braze also causes formation of air pockets, especially when the wettability of the braze on a substrate is poor. Also, when CuO is dissolved as a molten liquid, it precipitates or reacts with the ceramic substrates to form brittle phases at the substrate/braze interface. While the wettability of the molten liquid improves with an increase in CuO content, due to the formation of these brittle interfacial phases, increases in flexural strength with additional CuO content is limited. The formation of continuous brittle interfacial phases is often observed in the braze with high CuO content, and is a crucial factor which limits flexural strength of joints since these brittle interfacial phases provide an easy path for crack propagation.
While advances in sealing techniques such as the brazing technique described above have improved the performance of ceramic to metal and/or ceramic to ceramic joints in high temperature environments typical of electrochemical devices such as solid oxide fuel cells, there still exists a need for further improvements in these joint that allow them to operate over multiple cycles while maintaining a hermetic seal between the metal and ceramic parts and while minimizing the formation of air pockets and continuous brittle interfacial phases. The present invention addresses that need.
Accordingly, present invention is a method for joining two ceramic parts, and/or a metal part to a ceramic part, and/or two metal parts, and the braze mixture used in that method. Briefly, the present invention joins together two or more parts by providing a braze consisting of a mixture of copper oxide, silver, and ceramic particulate. As used herein, the ceramic particulate thus becomes a part of the braze. The braze is placed upon the surfaces of the ceramic and/or metal parts, which are then held together for sufficient time and at a sufficient temperature to cause the braze to form a bond between the ceramic and/or metal parts. Preferably, although not meant to be limiting, a force is applied to the ceramic and/or metal parts to hold them in tight proximity with one another.
In addition to the mixture of copper oxide, silver, and ceramic particulate, the braze may further incorporate titanium oxide. Titanium oxide acts as a wetting modifier, assisting in the formation of a uniform, well spread layer of the braze between the ceramic and/or metal parts. Preferably, and not meant to be limiting, the titanium oxide comprises between about 0.1 mol % and 5 mol % of the silver in the mixture.
The braze may also further incorporate Pt, Pd and combinations thereof, as additives to modify the melting temperature of the braze. Preferably, and not meant to be limiting, the Pt, Pd and combinations thereof comprise between about 0.1 mol % and about 25 mol % of the silver in the mixture.
As used herein, the term “ceramic” means metal oxides, metal nitrides and metal carbides. The ceramics suitable for use in the present invention include, but are not limited to, perovskites, alumina, YSZ, NiO, CeO2, silicon carbide, silica, magnesia and combinations thereof.
Preferably, but not meant to be limiting, the copper oxide is between about 1 mol % and about 70 mol % of the silver. Also, preferably, but not meant to be limiting, the ceramic particulate is between about 1% and about 50% of the total volume of the mixture of copper oxide, silver, and ceramic particulate. While the present invention should be understood to encompass ceramic particulate of any size, ceramic particulate smaller than about 200 μm are generally preferred. These ceramic particulates may be or they still in the form of short fibers, long fibers, powders, flakes, and combinations thereof.
The following detailed description of the embodiments of the invention will be more readily understood when taken in conjunction with the following drawing, wherein:
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitations of the inventive scope is thereby intended, as the scope of this invention should be evaluated with reference to the claims appended hereto. Alterations and further modifications in the illustrated devices, and such further applications of the principles of the invention as illustrated herein are contemplated as would normally occur to one skilled in the art to which the invention relates.
A series of experiments were undertaken to demonstrate a preferred embodiment of the present invention. Using the copper oxide (CuO)-silver (Ag) phase diagram, four basic binary braze compositions (referred to as “CA” braze) were selected with the CuO concentration ranging from 0 to 20 mol %. The respective combinations are shown in Table 1. To identify joining characteristics of metal-ceramic composite braze, alumina powder (Al2O3) of between 0 vol % and 20 vol % was added to four basic binary braze compositions.
Each of these braze compositions were formulated by ball-milling appropriate amounts of copper powder (99%, 8˜11 μm, Alfa Aesar), silver powder (99.9%, 0.5˜1.0 μm, Alfa Aesar) and alumina power (99.9%, 0.35˜0.49 μm, Alfa Aesar). Results from X-ray diffraction indicate that the copper powder fully oxidized in-situ during a typical air brazing heating schedule to form CuO. The braze pastes used in these joining experiments were prepared by mixing the mixture of CuO, silver and alumina with a 15 wt % polymer binder (V006, Heraeus Inc.)
The ceramic parts joined with the braze were prepared from polycrystalline alumina plates (A123, Alfa Aesar) measuring 100 mm×25 mm×41 nm. Two alumina plates were joined along the long edge using one of the braze pastes to form a 100 mm×50 mm×4 mm plate from which specimens were cut. To maintain the uniform initial thickness of the braze layer, the paste was applied on each faying surface of the alumina plates using a stencil-printing technique and then dried at 80° C. for 30 min. Alter drying, the thickness of a braze layer between two alumina plates was ˜200 μm. Spring steel side clips and an appropriately positioned refractory brick (at weight of 200 g, providing a pressure of ˜5 kPa on the faying surfaces) were used to hold the specimens together during brazing and to keep them from slipping during the heating cycle. Brazing was conducted by heating in static air at 3° C./min to 1000° C., holding at 1000° C. for 1 h, and cooling to room temperature at 3° C./min.
Once joined, each sample was cut into rectangular bend bars, each measuring 4 mm×3 mm×50 mm with the joints located midway along their lengths. The edges to be placed under tension during bending were chamfered to remove machining flaws that could cause premature failure. Four-point bending tests following the ASTM standard test method (C1161-02c) were then conducted to measure the flexual strength of each joint. Microstructural analysis was conducted on polished cross-sectioned surfaces of the bending bars using a JEOL JSM-5900LV scanning electron microscope (SEM) equipped with an Oxford energy dispersive X-ray analysis (EDX) system. SEM analysis was also conducted on the fracture surfaces of the broken bend bars.
The addition of ceramic powders as described herein increased the viscosity of the molten braze, and improved the resistance against compression.
Increased viscosity also improves the resistance to the formation of air pockets, since the molten braze with high viscosity cannot freely move around even when the wettability between a substrate and a molten braze is not good.
Dispersion of ceramic particles also provides nucleation sites for brittle phases so that brittle phases form as discrete particles throughout the entire braze matrix rather than as a continuous layer at the substrate/braze interface.
The removal of air pockets and minimization of brittle interfacial phases increases the flexural strength of the alumina joint brazed with a variety of different Ag—CuO brazes. As shown in
The fracture surfaces of bend test specimens are shown in
While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. Only certain embodiments have been shown and described, and all changes, equivalents, and modifications that come within the spirit of the invention described herein are desired to be protected. Any experiments, experimental examples, or experimental results provided herein are intended to be illustrative of the present invention and should not be considered limiting or restrictive with regard to the invention scope. Further, any theory, mechanism of operation, proof, or finding stated herein is meant to further enhance understanding of the present invention and is not intended to limit the present invention in any way to such theory, mechanism of operation, proof, or finding.
Thus, the specifics of this description and the attached drawings should not be interpreted to limit the scope of this invention to the specifics thereof. Rather, the scope of this invention should be evaluated with reference to the claims appended hereto. In reading the claims it is intended that when words such as “a”, “an”, “at least one”, and “at least a portion” are used there is no intention to limit the claims to only one item unless
The invention was made with Government support under Contract DE-AC0676RLO 1830, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
