Patent Application
20080090715
As used herein, all compositions are given in bulk weight percent (wt. %), including specified crystalline component or phase and glass component percentages. For clarity, the term “glass-ceramic” as understood in the art means that the material has a glass phase or component and a crystalline phase or component that is dispersed uniformly throughout the glass. It is to be understood that the compositions of the invention can contain trace elements, herein meaning <0.4 wt. % and preferably <0.1 wt. %.
As to be understood herein and by way of example, reference to “>50% of the crystals of the crystalline component of the glass-ceramic having a structure selected from the group represented by walstromite, cyclowollastonite and μ-(Ca,Sr)SiO3” is directed to the structure and not to the formula of the crystalline material represented by the “names” walstromite, cyclowollastonite and μ-(Ca,Sr)SiO3. For example, the classical (end member) formulas for walstromite and cyclowollastonite are Ca2BaSi3O9 and α-CaSiO3, respectively. The seal materials described herein may contain additional elements in the structures in solid solution (see Examples 1-7 in Table 1), but >50% of the crystals in the crystalline phase will have the walstromite and cyclowollastonite structure and may include such additional elements. The remainder (that is, the <50%) of the crystals in the crystalline component will constitute a secondary phase if such phase is present. The classical (end member) formulas for various structures mentioned herein are: walstromite [Ca2BaSi3O9], cyclowollastonite [α-CaSiO3], wollastonite (μ-CaSiO3), diopside (CaMgSi2O6), akermanite (Ca2MgSi2O7), hardystonite (Ca2MgSi2O7), enstatite (MgSiO3), forsterite (Mg2SiO4), gehlenite [Ca2Al2SiO7], anorthite [Ca2Al2Si2O8], kilchoanite [Ca6(SiO4)(Si3O10)], and corundum [Al2O3].
The invention is directed to boron-free glass-ceramic seals for solid oxide fuel cells, said seal comprising a glass-ceramic having a glass component and a crystalline component. In one embodiment, the crystalline component of the glass-ceramic is >50 wt. % and the glass component <50%. In another embodiment, the crystalline component of the glass-ceramic is >75 wt. % and the glass component is <25 wt. %. In another embodiment the crystalline component of the glass-ceramic is >90% and the glass component is <10%. In a further embodiment the invention is directed to boron-free, zinc-free glass-ceramic seals for solid oxide fuel cells, said seals comprising a glass-ceramic having a glass component and a crystalline component, the percentage each component, glass or ceramic, being as given above in this paragraph in the embodiments of the boron-free, glass-ceramic seals.
Regarding only the crystalline component of the glass-ceramic, ≧50 wt. % of the crystalline component, which hereafter may be referred to as the primary crystalline phase, has a structure selected from the group consisting of walstromite, cyclowollastonite and μ-(Ca,Sr)SiO3), and solid solutions and mixtures thereof of the foregoing crystalline structures, or of the potassium aluminosilicates kalsilite and kaliophilite [KAlSiO4], and wollastonite, and mixtures thereof. In addition to the above primary crystalline phases, one or more additional or secondary crystalline phases (the remaining <50% of the crystalline component) may also be present in the crystalline component of the glass-ceramic, such phases constituting the remainder of the crystalline component of the glass ceramic. Example of such secondary crystalline phases, without limitation, can include akermanite, hardystonite, wollastonite, diopside, enstatite, and forsterite. For example, if the primary crystalline phase in the crystalline component is 75% cyclowollastonite, the remainder, the secondary crystalline phase, can be 25% akermanite if magnesium is present or hardystonite is zinc is present. Those skilled in the art will understand that the exact nature and amount of the secondary phase will depend on the composition of the glass.
Seals are an integral part of solid oxide fuel cell (SOFC) planar design; they prevent the mixing of fuel and air, and also keep the fuel from leaking out of the stack or individual cells. The requirements of the seals (and thus the sealing agents or materials used to form them) are severe. For example, the seals must be capable of withstanding exposure to high temperatures of up to 1000° C., and exposure to both oxidizing and reducing environments. In addition, the seals must have a low vapor pressure, and must remain leak-tight and insulating over the lifetime of the stack which may exceed 50,000 hrs. Further, the seals must not degrade due to thermal cycling of the stack or due to changes in viscosity and chemical composition over time. These latter changes can result from volatility of certain species as well as from reaction with other fuel cell components such as electrodes and stainless steel interconnects. Finally, the seals must not themselves be a source of contamination that adversely affects the operation of other stack components, especially the cell electrodes.
The most commonly used sealing agents are cements, glasses, and glass-ceramics. Cement seals usually do not form leak-tight seals, and while glass seals can provide the required hermeticity, the upper temperatures at which they can be used are generally limited. The use of glass-ceramics as sealing agents enables one to avoid most, if not all, of these issues.
Powder-processed (frit-sintered) glass-ceramics are well known as metal-to-metal, metal-to-ceramic, and ceramic-to-ceramic sealing materials as well as high-performance coatings for metals and ceramics. In comparison with glasses, glass-ceramics offer higher use temperatures, superior mechanical properties and corrosion resistance, and a very wide range of thermal expansion coefficients (CTEs), which allow them to be used as expansion-matched seals for many different alloys. The ability to fill re-entrant angles and complex internal shapes by viscous flow of the molten glass during the crystallization that forms the glass-ceramic makes glass-ceramic materials particularly suited to applications where high strength of the system is important.
Nevertheless, even many glass-ceramic seals, particularly those containing a significant glassy component and/or readily-diffusing cations such as small alkali ions, can be unduly susceptible to reaction with the SOFC components and subsequent degradation of the device can occur over time. In one embodiment the present invention is directed to highly crystalline glass-ceramic seals, with less than 25% residual glass (crystalline component to glass component ratio of >75/<25), which are particularly well suited for the SOFC application. In another embodiment the present invention is directed to highly crystalline glass-ceramic seals, with less than 20% residual glass (crystalline component to glass component ratio of >80/<20), which are particularly well suited for the SOFC application. In a further embodiment the invention is directed to highly crystalline glass-ceramic seals with less than 10% residual glass (crystalline component to glass component ratio of >90/<10). The overall glass-ceramic seal has a thermal expansion closely matched to that of the fuel cell electrolyte and interconnect, and the glassy component that remains in the final microstructure is confined to interstices and some grain boundaries, and does not form a continuous path through the seal.
Advantages of highly crystalline glass-ceramic seal materials of the invention include:
A requirement for highly crystalline glass-ceramic SOFC seals is that they have near-stoichiometric compositions such that they undergo near-complete crystallization and thus provide the high thermal expansions required for the SOFC application. Glass-ceramics which meet these criteria include those in which the crystals in the crystalline phase has a structure selected from the structures represented by the walstromite, cyclowollastonite and μ-(Ca,Sr)SiO3 structures, with secondary phases whose structures are based on structures including but not limited to the structures of wollastonite, diopside, akermanite, hardystonite, enstatite, and forsterite.
In another aspect of the invention, >50 wt. % of the crystalline component of the glass-ceramic is selected from the group consisting of potassium aluminosilicates kalsilite and kaliophilite, with secondary phases including but not limited to wollastonite, gehlenite, anorthite, kilchoanite and corundum. The CTE of these glass-ceramics is in the range 70-130×10−7/° C., preferably 85-115×10−7/° C.
Tables 1 and 2 below exemplify some of the compositions, in weight percent, that can be used as seals for SOFC applications. Seals made from the glass-ceramic materials exemplified in Table 1 contain SiO2, Al2O3 and MO, where MO is an alkaline earth oxide of Mg, Ca, Ba and Sr. The primary (>50%) crystalline phase of the glass-ceramics exemplified in Table 1 is at least one of walstromite, cyclowollastonite and μ-(Ca,Sr)SiO3), with secondary (<50%) phases of wollastonite, diopside, akermanite, enstatite, forsterite, and hardystonite, and solid solutions or mixtures thereof.
The composition of the glass-ceramic material useful as SOCF seals that are exemplified in Table 1 are glass-ceramic containing SiO2, Al2O3, and MO, where MO is an alkaline earth oxide of Mg, Ca, Ba and Sr, and wherein the sum the alkaline earth oxides (ΣMO), where M is two or more of Mg, Ca, Ba and Sr, is in the range of 40-60 wt. %; and Al2O3 is in the range of 2-4 wt. %; and SiO2 is in the range of 36-58 wt. %.
Boron-free, Zinc-free seals made from the glass-ceramic materials exemplified in Table 2 contain SiO2, Al2O3 and CaO, and R2O, where R is an alkali ion, preferably potassium (K). The crystalline component of the glass-ceramics exemplified in Table 2 has a primary crystalline phase whose structure is selected group consisting of kaliophilite and kalsilite structures, and can further have a secondary phases including, but not limited to, wollastonite, gehlenite, and corundum. The boron-free, zinc-free glass-ceramic seals of Table 2 have a compositional range of 5-25 wt. % Al2O3, 25-45 wt. % CaO, 25-45 wt. % SiO2, 1-10 wt. % K2O and 0-25 wt. % GeO2. In a preferred embodiment the compositional range of the boron-free, zinc-free glass-ceramic seals have a composition of 10-20 wt. % Al2O3, 30-40 wt. % CaO, 30-40 wt. % SiO2, 2-8 wt. % K2O and 5-20 wt. % GeO2.
Glass compositions used for preparing the glass-ceramics according to the invention were prepared by melting the component materials in vessel, for example, a platinum crucible, at a temperature in the range of 1450-1650° C. for a time in the range of 2-5 hours. The starting materials may be the oxides, carbonates, nitrates, hydroxides and form a of the metals described herein that are known in the art to be useful in the preparation of glasses. In some embodiments, the melts were carried out at a temperature of 1600±50° C. for a time in the range of 2.5-4 hours. For each composition, a small, approximately 5 cm piece was formed from the molten glass composition and was annealed at a temperature of 750±40° C. These samples served as visual indicators of the overall glass stability. The remainder of the glass in each crucible was drigaged into water and milled to a mean particle size in the range of 10-20 μm (325 mesh). The resulting frit (frit=powdered glass) powder was formed into an article (pellets, bars, rods, etc,) using techniques known in the art. For example, for the testing purposes described herein the frit was dry-pressed into 12.76 cm diameter (0.5 inch) pellets and/or 10×0.6×0.6 cm CTE bars (4×0.25×0.25 inches), and then fired (sintered) at temperatures in the range of 850° C. to 1000° C. for a time in the range of 1-2 hours.
The glass-ceramic compositions of the invention have a coefficient of thermal expansion in the range of 70-130×10−7/° C., preferably 85-115×10−7/° C. For use as SOFC seals, in one preferred embodiment the highly crystalline glass-ceramic seals have a crystalline phase of >75 wt. % and a glass phase of <25 wt. %. In a further embodiment the crystalline phase is >90 wt. % and the glass phase is <10 wt. %.
Phase and structural information for the crystalline forms mentioned herein can be obtained from Phase Diagrams for Ceramists and other sources known to those skilled in the art; for example, XRD information can be found in JCPDS databases and used to identify crystalline forms present in the glass-ceramics.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
This application is related to co-pending U.S. patent application Ser. No. 11/402,746 titled “HIGH THERMAL EXPANSION CYCLOSILICATE GLASS-CERAMICS” filed Apr. 11, 2006. Co-pending application Ser. No. 11/402,746 and the present application have a common co-inventor, Linda R. Pinckney and are commonly assigned to Corning Incorporated.
