GLASS-CERAMIC MATERIAL AND METHOD OF MAKING
GLASS-CERAMIC MATERIAL AND METHOD OF MAKING This invention was made with Government support under Contract DE-AC0676RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. This application is a continuation-in-part of application serial number 09/562,583 filed May 1 , 2000 which is in turn a continuation-in-part of application serial number 09/365,343 filed July 30, 1999, now US Patent No. 6,430,966. FIELD OF THE INVENTION The' present invention is a glass ceramic material and method of making, specifically for use in electrochemical devices such as fuel cells, gas sensors, oxygen or hydrogen pumps/separators, or for sealing any material with a thermal expansion coefficient similar to the seal material. As used herein, the terms "solid electrolyte" or "solid oxide ion conducting electrolyte" are interchangable. As used herein, the term "joint" includes the term "seal" because, in this glass-ceramic field, the "seal" joins at least two parts. However, the "joint" may be intermittent thereby not serving as a "seal". BACKGROUND OF THE INVENTION Ceramic materials are being used more often from automobile turbochargers to experimental fuel cells. However, there remains the problem of joining and/or sealing ceramic components to other ceramic components, to metal components, or to combinations thereof (e.g., cermet components) such that the joint maintains integrity during operation. For example, solid oxide ion conducting electrolytes are useful for oxygen separation and high temperature fuel cells. Although many technical challenges of their development have been
overcome, there remains the problem of sealing. In a planar design, a gas-tight seal must bond the components together and prevent the mixing of the gas species on both sides of the solid oxide ion conducting electrolyte. A limited number of materials are suitable as a solid oxide ion conducting electrolyte. The most commonly used materials are yttria stabilized zirconia (YSZ), doped ceria, doped bismuth oxide and doped lanthanum gallate. The thermal expansion coefficient (TEC) of these materials can range from 10.1 x 10"6 to 14.3 x 10"6 °C"1 depending on the type of dopant and concentration. Of particular interest are materials having a TEC of 12 x 10"6 °C"1 or greater. The operating temperature can also range from 700°C to 1000°C depending upon which material is chosen as the electrolyte. Therefore, the seal material must be tailored to match the electrolyte thermal expansion, maintain a gas tight seal at temperatures ranging from 200 °C to 1200 °C, and not have detrimental chemical interactions with the fuel cell components. In addition, the seal material must also be stable at the operating temperature (800-1000°C) for extended periods of time (>9,000 hr) and be electrically insulating. For a solid oxide fuel cell, the seal must be able to survive extremely reducing environments. Various efforts to seal solid oxide ion conducting devices have been made with varying degrees of success. Silica, boron, and phosphate-based glasses and glass-ceramics have been evaluated as a sealing material1"4 for solid oxide fuel cells. Experiments conducted by P.H. Larsen et al1 have shown major problems with glasses purely based on phosphate as the glass former. At temperature, the phosphate volatilized and reacted with the anode to form nickel phosphide and zirconiumoxyphosphate. Additionally, these phosphate glasses usually crystallized to form meta- or pyrophosphates, which exhibited low stability in a humidified fuel gas at the operating temperature. Borosilicate glasses and glass ceramics have also been considered as potential seal materials. These glasses have been investigated by C. Gϋnther et al2 and K.L. Ley et al3 for use in solid oxide fuel cells. However, boron will react with a humidified hydrogen atmosphere to form the gaseous species B2(OH)2 and B2(OH)3 at the operating temperature2. Therefore, any high boron seal may corrode in a humidified hydrogen environment over time. Glasses with B2O3 as
the only glass former have showed up to a 20% weight loss in the humidified hydrogen environment and extensive interactions with fuel cell component materials both in air and wet fuel gas.1 Silica-based glasses and glass-ceramics offer the most promise. They typically have a higher chemical resistance and show minimal interaction with the fuel cell component materials.1 Unfortunately, these glasses tend to have thermal expansions below the range needed for a sealing material. At the operating temperature, most glasses will crystallize with time. Therefore, it is critical to have a glass composition in which the thermal expansion coefficient after crystallization is compatible with the solid oxide ion conducting electrolyte. Once the glass is fully crystallized, it is typically very stable over time. In addition, crystallized glasses tend to be stronger mechanically at operating temperature, improving-seal performance. One further difficulty that has been encountered by those having skill in the art is a tendency for the TEC of glasses to lessen over time. Hence, there is a need in the art for a sealing material that can operate at an operating temperature up to about 900°C, has a TEC of 12 x 10"6 °C"1 or greater in the crystalline phase which does not degrade over time, and has no detrimental chemical interactions with the components. BACKGROUND BIBLIOGRAPHY 1 . P.H. Larsen, C. Bagger, M. Mogensen and J.G. Larsen, Proc. 4th Int. Symp. Solid Oxide Fuel Cells, Volume 95-1 , 1995, pp.69-78. 2. C. Gϋnther, G. Hofer and W. Kleinlein, Proc. 5th Int. Symp. Solid Oxide Fuel Cells, Volume 97-18, 1997, pp.746-756. 3. K.L. Ley, M. Krumpelt, R. Kumar, J. H. Meiser, and I. Bloom, J. Mat. Res., Vol. 1 1 , No. 6, (1996) pp. 1489-1493. 4. Yoshinori Sakaki, Masatoshi Hattori, Yoshimi Esaki, Satoshi Ohara, Takehisa Fukui, Kaseki Kodera, Yukio Kubo, Proc. 5th Int. Symp. Solid Oxide Fuel Cells, Volume 97-18, 1997, pp.652-660.
SUMMARY OF THE INVENTION The present invention is a glass-ceramic compound and method of making that are useful in joining or sealing ceramic components to other ceramic components, to glass components, to metal components, or to combinations thereof (e.g., cermet components). More specifically, the present invention is useful for joining/sealing in an electrochemical cell having at least one solid electrolyte having a first and second side exposed to first and second gas species respectively. The seal is necessary for separating the first and second gas species. The glass-ceramic compound contains at least four metal oxides, M1-M2- M3-M4. M1 is BaO, SrO, CaO, MgO, or combinations thereof. M2 is AI2O3 and is present in the compound in an amount from 2 to 15 mol%. M3 is SiO2 with up to 50 mol% B2O3. M4 is either between 0.1 - 7.5 mol% a metal oxide selected from the group of La2O3, Y2O3, Nd2O3 or combinations thereof, or between 0.1 and 7.5 mol% K2O. In the case of a metal oxide from the group La O3ι Y2O3, Nd2O3 or combinations thereof, it is preferred that the composition contain an additional 0.1 to 3 mol %CuO as a wetting agent to assist the bonding of the glass. The compound substantially matches a coefficient of thermal expansion of the solid ceramic component and at least one other solid component that is either ceramic, metal, or a combination thereof, when those components are selected as having a coefficient of expansion in the crystalline phase of 12 or greater 12 x 10"6 °C"1 as measured from 25 °C to 1000 °C. According to the present invention, a series of glass ceramics in the M1- AI2O3-M3-M4 system can be used to join or seal both tubular and planar ceramic solid oxide fuel cells, oxygen electrolyzers, thermal barrier coatings, as a protective coating for metal substrates used as supports for catalytic particles used for high temperature catalytic reactions, for high temperature super-alloy applications, where it is desirable to coat metal parts with a ceramic material to improve their oxidation resistance,.and in membrane reactors for the production of syngas, commodity chemicals and other products.
For high temperature super-alloy applications, it is often necessary to coat metal parts with a ceramic material to improve their oxidation resistance. For example, thermal barrier coatings are used in the areospace industry for coating turbine blades and other components. Typically, multi-layered coatings are used to address issues of oxygen diffusion as well as thermal expansion mis-match. The present invention is well suited for this application. High temperature catalytic reactions also require the use of protective coatings for metal surfaces. Thin metal substrates are often used as supports for catalytic particles, however at elevated temperatures and pressures, they oxidize and crumble. A protective oxide coating applied to thin metal foils could be used to prolong their life, and could be cheaper and easier to manufacture than an all-ceramic part. These oxide surfaces then become the substrate that holds catalytic particles. The present invention is well suited to be used in this manner. The present invention is further well suited for sealing glass material surfaces also, such as those utilized in the lighting industry. High performance, special application light bulbs require glass to metal seals in order to provide a joint between the filament and the glass bulb. Some of the gasses in these light bulbs are highly corrosive, and since the bulbs get very hot, there are thermal expansion mis-match issues between the electrical contacts for the filament and the glass wall, creating a common failure site for these bulbs. The present invention is well suited to solve this problem. Applications of the present invention using metal oxide selected from the group of La2O3, Y2O3, Nd O3 or combinations thereof are particularly suited for uses where electrical resistance is desired, solid oxide fuel cells, while applications of the present invention using K2O are particularly suited for applications where electrical resistance is not paramount, such as oxygen sensors. It is an object of the present invention to provide a compound useful for joining or sealing a solid electrolyte or a solid oxide ion conducting electrolyte. An advantage of a joint/seal made with the compound of M1-AI2O3-M3- M4 is the maintaining of a substantially constant coefficient of thermal expansion from the glass to crystalline phase.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description. DESCRIPTION OF THE PREFERRED EMBODIMENT(S) The present invention is a glass-ceramic compound and method of making the glass-ceramic compound. The present invention is useful for joining or sealing between at least two solid ceramic parts, for example a seal in an electrochemical cell having at least one solid electrolyte having a first and second side exposed to first and second gas species respectively. The present invention is also useful for joining or sealing between a solid ceramic component and a metal component or a cermet component. The seal is necessary for separating the first and second gas species during operation, usually at elevated temperatures. The present invention includes a joint between a solid ceramic component and at least one other solid component that is preferably a solid ceramic component, a metal component, or a combination thereof such as a cermet component. The joint has at least four metal oxides of M1-M2-M3-M4. M1 is BaO, SrO, CaO, MgO, or combinations thereof. M2 is AI2O3. M3 is SiO with up to 50 mol% B2O3. M4 is either between 0.1 - 7.5 mol% a metal oxide selected from the group of La2O3, Y2O3, Nd2O3 or combinations thereof, or between 0.1 and 7.5 mol% K2O. In the case of a metal oxide from the group La2O3, Y2O3, Nd2O3 or combinations thereof, it is preferred that the composition contain an additional 0.1 to 3 mol %CuO, which has been shown to provide good wetting, and therefore assists with bonding, while improving, or at a minimum not degrading, the TEC over time. The joint substantially matches a coefficient of thermal expansion of the components comprising the joint. The coefficient of thermal expansion of the joint is 12 x 10"6 °C"1 or greater as measured from 25 °C to 1000 °C.
The composition of the joint/seal is preferably in the range wherein M1 is present in an amount from about 20 mol% to about 55 mol%, AI2O3 is present in an amount from about 2 mol% to about 15 mol%, and M3 is present in an amount from about 40 mol% to about 70 mol%. M4 is either between 0.1 - 7.5 mol% a metal oxide selected from the group of La2O3, Y2O3, Nd2O3 or combinations thereof, or between 0.1 and 7.5 mol% K2O. In the case of a metal oxide from the group La2O3, Y2O3, Nd2O3 or combinations thereof, it is preferred that the composition contain an additional 0.1 to 3 mol %CuO for its wetting properties. Substantially the same coefficient of thermal expansion is herein defined as the coefficient of thermal expansion of the seal material within about 30%, preferably within about 16%, more preferably within about 5% of the sealed material. The joint may be used in an electrochemical test cell to join an oxygen ion pump and a test material. In addition, the joint may be used in an oxygen generator or a fuel cell to join an oxygen ion conducting electrolyte, for example a zirconia electrolyte, and an interconnect, for example manganite, chromite, metal, and combinations thereof. For those aspects of the invention utilizing a metal oxide from the group of La2O3| Y2O3, Nd2O3 or combinations thereof, preferred applications are those where electrical resistance is desired, such as solid oxide fuel cells. For those aspects of the invention utilizing K2O, preferred applications are those where electrical resistance is not critical, such as oxygen generators. An experiment was conducted to demonstrate the glass-ceramic materials of the present invention. Glasses were fabricated having the compositions shown in table 1 :
Table 1 The glasses of Table 1 were then tested to determine their coefficient of thermal expansion both before and after crystallization, and before and after repeated thermal cycling. The results are given below in table 2, where TEC x 10"6 °C"1 as measured from 25 °C to 1000 °C. As shown in Table 2, all of the glasses met the criteria of having a TEC of 12 x 10"6 °C"1 or greater as measured from 25 °C to 1000 °C. Each of the glasses where then tested to determine whether the TEC after crystallization would decrease after repeated thermal cycling. It was determined that in each case, the TEC of the example glasses would withstand repeated thermal cycling and remain at or above 12 x 10"6 °C"1 as measured from 25 °C to 1000 °C. CLOSURE While a preferred embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes
and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.
The present invention is a glass-ceramic material and method of making useful for joining a solid ceramic component and at least one other solid component. The material is a blend of M1-M2-M3-M4, wherein M1 is BaO, SrO, CaO, MgO, or combinations thereof, M2 is Al2O3, present in the blend in an amount from 2 to 15 mol%, M3 is SiO2 with up to 50 mol% B203 and a metal oxide selected from the group of La2O3, Y2O3, Nd2O3 or combinations thereof, or between 0.1 and 7.5 mol% K2O. In the case of a metal oxide from the group La2O3, Y2O3, Nd2O3 or combinations thereof, it is preferred that the composition contain an additional 0.1 to 3 mol %CuO. In all cases, the glass ceramic material in a crystalline phase substantially matches a coefficient of thermal expansion of solid electrolytes having a thermal expansion coefficient of 12 x 10-6 °C-1 as measured from 25 °C to 1000 °C, and does not degrade with repeated heat cycling. According to the present invention, the series of glass ceramics in the M1-Al2O3-M3-M4 system can be used to join or seal both tubular and planar solid oxide fuel cells, oxygen electrolyzers, and membrane reactors for the production of syngas, commodity chemicals and other products. CLAIMS We claim: 1. A joint between a solid ceramic component and at least one other solid component, said joint comprising: at least four metal oxides of M1-M2-M3 combined together wherein M1 is selected from the group consisting of BaO, SrO, CaO, MgO, and combinations thereof, M2 is AI2O3 wherein M2 is present in an amount from 2 to 15 mol%- M3 is SiO2 with up to 50 mol % of B2O3, and M4 is K2O wherein M4 is present in an amount between 0.1 and 7.5 mol % said joint substantially matching a coefficient of thermal expansion of said solid ceramic component and said at least one other solid component. 2. The joint as recited in claim 1 , wherein said at least one other solid component is ceramic. 3. The joint as recited in claim 1, wherein said at least one other solid component is metal. 4. The joint as recited in claim 1, wherein said at least one other solid component is cermet. 4. The joint as recited in claim 1, wherein said at least one other solid component is glass. 6. The joint as recited in claim 1 that is a seal. 7. The joint as recited in claim 1 wherein said coefficient of thermal expansion after crystallization that is greater than 12 x 10"6 °C"1 as measured from 25 °C to 1000 °C. 8. A joint between a solid ceramic component and at least one other solid component, said joint comprising:
at least four metal oxides of M1-M2-M3 combined together wherein M1 is selected from the group consisting of BaO, SrO, CaO, MgO, and combinations thereof, M2 is AI2O3 wherein M2 is present in an amount from 2 to 15 mol%, M3 is SiO2 with up to 50 mol % of B2O3, and M4 is a metal oxide ' 5 selected from the group of La2O3, Y2O3, Nd2O3 or combinations thereof , said joint substantially matching a coefficient of thermal expansion of said solid ceramic component and said at least one other solid component. 9. The joint as recited in claim 8, wherein said at least one other solid 10 component is ceramic. 10. The joint as recited in claim 8, wherein said at least one other solid component is metal. 15 11. The joint as recited in claim 8, wherein said at least one other solid component is cermet. 12. The joint as recited in claim 8, wherein said at least one other solid component is glass. 0 13. The joint as recited in claim 8 that is a seal. 14. The joint as recited in claim 8 wherein said coefficient of thermal expansion after crystallization that is greater than 12 x 10"6 °C"1 as measured 25 from 25 °C to 1000 °C. 15. The method as recited in claim 8, wherein said solid ceramic component and said at least one other solid component are an oxygen ion conductor and an interconnect in a solid oxide fuel cell. 30 16. The joint as recited in claim 8, wherein said solid ceramic component and said at least one other solid component are an oxygen ion conductor and an interconnect in an oxygen generator.
17. The joint as recited in claim 8, wherein said joint further comprises between 0.1 and 3% CuO.