Title of Invention

"A METHOD OF MANUFACTURING GLASS TAPE FOR SEALING IN A SOLID OXIDE FUEL CELL AS A GASKET"

Abstract

This invention relates to a m&thod of manufacturing glass tape for use as a gaset for sealing in a solid oxide fuel cell comprising the following method steps: i) preparing the tape casting paste (l) composed of components based on SiO2-Al2O3-CaO-MgO-B2O3 such as glass powder, solvents, dispersant, tinders, plasticizers and other additives such as homogenizers by dry grinding the powders for 24 hours followed by wet grinding in acetone medium for 48 hours, ii) spreading said tape casting pastes compound into a flat surface like tempered glass bed lined with a carrier film to a controlled thickness with the doctor blades, iii) solvents are allowed to evaporate from said tape casting slurries spread on said glass bed by moving to a dryer.

Full Text

The present invention relates to a method of manufacturing glass tape for use as a gasket for sealing in a solid oxide fuel cell in a stack assembly of planner design and ensuring leak free operation at 1000°c in solid oxide Fuel cell (SOFC). PRIOR ART There have been a few references indicating the successful demonstrations of tape casting of glass ceramics for multi layer dielectric and substrates. The glass composition incorporated into a cashable dielectric composition is used in a method of forming 3 low loss green tape by casting as disclosed in US Patent No.6,147,019. Low firing glass ceramic green sheets useful in the production of glass ceramic substrates have flexibility and the green sheets can be punched to form fine through holes with a small pitch as disclosed in US Patent No.5,660,781. In SOFC sealing situations, the seal material, glass and ceramic fiber or glass ceramic is generally applied in the form of paste as disclosed in Science and Technology of Ceramic fuel cell, Nguyen Quang Minh and Takehiko Takahashi; Elsever publication 1995 Pp.291. Glass ceramic as a sealant has been used in SOFC operating at 1000°C with limited success. Alternate sealing methods such as metallic foils have also been tried out as disclosed in US Patent No. 6,106 967. These processes sometimes lack flexibility as the entire assembly needs to be heat treated in a single step, making it difficult to control defects during processing.However the inventor has successfully developed stable Glass seal for high temperature operation by using a system based on Si02 -Al- 0.,-CaO-MgO ~B203 glass as disclosed in a copending application. In glass system, the original material soften and join the electrolyte and separator and single glassy phase is formed at the interface. This enables the glass to seal during SOFC operation. Glass sealant with its matching thermal expansion characteristics is most preferred because it is a single component system and purely amorphous and non reactive. Development of glass in the form of tape or gasket simplifies fuel stack assembly. However tape casting has been difficult due to the non wetting characteristics of the amorphous glass. This prevents thorough mixing of the glass powder with other liquid additives to form a homogenous slip. BACKGROUND OF THE INVENTION A Solid Oxide Fuel Cell (SOFC) operates in a temperature range of 650-1000°C. Seals for a fuel cell has a critical function of arresting leakage of gases. This disclosure sets forth a method of manufacture of glass seal in the form of a gasket for easy application during SOFC assembly. The glass seal gasket can be cut into required shape for easy application as well as can be tailor made to required thickness. The system based on Si02 -Al20,-CaO-MgO ~B2°3 £lass were successfully cast into the tape as it was found to be stable against thermal cycling and verified for sealing effecftiveness for prolonged operation.SUMMARY OF THE INVENTION Therefore the main object of the present invention is to develop a method of manufacture of glass tape and gasket for use in sealing of solid oxide fuel cell. Another object of the present invention is to provide a combination of binders etc to enable to form a consistent slip of glass based on Si02-Al203-CaO-MgO-B203 Yet another object of the present invention is to use the slip to form flexible, pore free, wrinkle free uniform thickness tape using doctor blade. Still further object of the present invention is to form a gasket which can be located precisely for easy assembly. According to the present invention there is provided a method of manufacturing glass tape for use as a gasket for sealing in a solid oxide fuel cell comprising the following method steps:5. i) preparing the tape casting paste (1) composed of components based on SiO^-Al-O.-CaO-MgO-B-O., such as ceramic powder, solvents, dispersant, binders, plasticizers and other additives such as homogenizers by dry grinding the powders for 24 hours followed by wet grinding in acetone medium for 48 hours, ii) spreading said tape casting pastes compound into a flat surface like tempered glass bed lined with a carrier film to a controlled thickness with the doctor blades, iii) solvents are allowed to evaporate from said tape casting slurries spread on said glass bed by moving to a dryer. The nature of the invention, its objective and further advantages residing in the same wil be appearent from the following description made with reference to the non limiting examplary embodiments of the invention represented in the accompanying drawings. Figure 1 schematically shows the principle of a solid oxide fuel cell, Figure 2 shows schematics of planar solid oxide fuel call stack showing gas sealing area. Figure 3 shows the tape casting process using doctor blades, Figure 4 shows a photograph of a glass tape and a gasket.6. DESCRIPTION OF THE INVENTION Principle And Features Of SOFC The fuel and the oxidant in a SOFC react electrochemically to produce electricity through an oxygen-ion conducting solid state device. Figure 1 shows the SOFC operating principles. The basic element consists of solid oxide electrolyte layer in contact with a porous air electrode and fuel electrode on either side. The solid electrolyte (9) is non porous, dense oxygen-ion conducting material like Yttria stabilized zirconia. Air electrode (10) is the cathode. The cathode material is usually Strontium doped lanthanum manganite. The anode is denoted as fuel electrode (11). The material is usually Nickel/yttria stablized zirconia. The fuel (12) and oxidant (13) gases flow past the reverse sides of the air electrode (10) and fuel electrode (11), respectively and generate electrical energy by means of electro chemical oxidation of the fuel and the electrochemical reduction oxygen. The transport rate of oxygen ions in the solid electrolyte (9) is adequate for practical applications at high temperature of 1000°C using presently available electrolyte materials and design.7. The inlets (12,13) for fuel and air is denoted by H2/CO and 1/2 02 respectively and outlets (14,15) as H20/C02-Since it operates around 1000°C, it becomes essential that high temperature seal is effective and prevents leakage of gases. Figure 2 shows the sealing area in a typical stack. It should also be stable and should not crack due to thermal cyling. It should be applied easily during assembly of the SOFC stack. The seal material is usually glass ceramic/glass are generally proprietary in nature. The characteristics of the seal material should have to be matched to that of solid electrolyte used in specific system. S_eal material The system is based on Si02-Al20--CaO-MgO ~B20- glass which functions as an effective seal during operation of the SOFC at 1000°C. The sealing system is chemically stable to oxidizing and reducing atmosphere, chemically stable both to electrolyte and interconnect, e.g. YSZ, La CrO~ and thermal expansion closely matching to suit zirconia electrolyte and thereby ensuring gas tightness. The optimized glass system was tested in fuel cell assemblies extending upto a maximum period of 750 hrs. The location of the Glass seal in a Solid Oxide Fuel Cell assembly is shown in Figure 2._Tape casting Tape casting has long been used as a fabrication technique for producing large - area, thin, flat ceramic plates and laminated, multi layer ceramic. The tape - casting process is shown schematically in Figure 3. Tape casting involves spreading slurry (1) of ceramic powders and organic ingredients onto a flat surface where the solvents are allowed to evaporate. After drying, the resulting tape develops a leather - like consistency and can be stripped from the casting surface. The optimal amount and type of organics has mainly been based on empirical studies because little is known about the interaction of the different components in a ceramic slurry. The slip is spread to a controlled thickness with the doctor blade (3,4) of a batch. The casting thickness can be controlled by the blade (3,4) gaP (Figure 3). The dried green tape thickness depends on slip viscosity, casting rate, blade gap setting and reservoir depth behind the doctor blade (3). All of these conditions must be controlled to obtain uniform tapes reproducibly. After the tape is cast, it is moved to a dryer to remove the solvent. The drying conditions depend upon the casting machine, the type of solvent to the thickness of the tape. These conditions are carefully controlled to minimize curling, cracking and trapping of gas bubbles. For example, too rapid drying of the wet slurry may seal the surface against further removal of solvent (skinning), resulting in trapped gas bubbles in the drying tape. Use of humidity drying limits evaporation of solvent from the slurry while permitting migration of the solvent surface as disclosed into US Patent No.6,147,019. However, presently glass has always been made as a paste and applied in the required locations unlike ceramics due to amorphous nature of glass which does not wet in liquids, if these glass seal material can be formed into a gasket/tape it will have the following advantages. 1. The seal can be formed to required thickness as warranted by the application. 2. The tapes/gasket can be cut to any odd shapes and precisely located for easy assembly. The present invention, as embodied and broadly described herein is to achieve the specific need of easy applicability of a seal based on Si02-Al203-CaO-MgO-B203 glass in the form of gasket/tape during assembly of an SOFC stack and provide effective sealing in a SOFC operating at 1000°C. DESCRIPTION OF THE PRESENT EMBODIMENT Though the glass powder can be used to seal by first forming a paste with organic solvent and applying the paste in the required area in a device and heat treated to form an integral seal. In the present development, the glass system has been successfully cast into the tape to required thickness and applied as a gasket prior to heat treatment. Application in the tape form has significant advantage, the glass compositions were tape cast using doctor blade techniques. Particle size of the powder is one of the important parameters. Initially dry grinding of the powders were carried out for 24 hours and followed by wet grinding in acetone medium for 48 hrs. This process yield glass powder of 4.15 micron median size. Tape casting slurries are composed of several components such as ceramic powder, solvents, dispersant, binders, plasticizers other additives such as homogenizers. The tape casting process was optimized for glass compositions based on fish oil as a dispersant. The process and constituents such as binder etc were optimized to get tape of uniform thickness, easily removable from the base plate after drying. The resultant tapes were free of tiny holes and free of any grain. The tape casting formation is given in Table I. Tapes with thickness of 500 - ISOOum were cast and several gaskets were made and characterized before using them in the cell experiment. Table 1 Constituents and their composition for tape casting of glass powder(Table Removed)The assembly was observed under microscope and also evaluated for following properties, adhesion, cracks both at glass surface an YSZ plate, continuity and shrinkage. Number of cells were assembled using tape as described in this invention. The cells were tested for a period of lOOhrs and in some cases even upto 750hrs to evaluate the performance. The cell tests confirmed the reliability of glass tape seal with constant flow of oxygen or hydrogen. The tapes formed with the above process has the following characteristics : 1. Glass can be cast into a tape or gasket which is pore free, flexible and which can be easily case and removed on drying. 2. Glass tape can be formed with the use of fish oil as a dispersant. 3. Tape can be cast over a wide range of thickness, 200µm - 1500µm. 5. Glass tape can be easily made into a gasket of any size and shape for easy application and precise location. The invention described hereinabove is in relation to the non-limiting embodiments and as defined by the accompanying claims. WE CLAIM; 1. A method of manufacturing glass tape for use as a gasket for sealing in a solid oxide fuel cell comprising the following method steps: i) preparing a tape casting paste (1) composed of components based on Si02-Al20_-CaO-MgO-B203 such as glass powder, solvents, dispersant, binders, plasticizers and other additives such as homogenizers; by dry grinding the powders for 24 hours followed by wet grinding in acetone medium for 48hrs; ii) spreading said tape casting/pastes (1) compound into a flat surface like tempered glass bed (5) lined with a carrier film (2) to a controlled thickness with the doctor blades (3,4), iii) solvents are allowed to evaporate from said tape casting slurries (1) spread on said glass bed (5) by moving to a dryer. 2. The method of manufacturing glass tape as claimed in claim 1 wherein the range of thickness of tapes or gasket is 500-1500um. 3. The method of manufacturing glass tape as claimed in claim 1 wherein the constituents and their composition for tape casting of glass powder is as given in Table 1. 4. The method of manufacturing glass tape as claimed in claim 1 wherein the process steps of dry an wet grinding yields glass powder of 4.15 micron medium size. 5. The method of manufacturing glass tape as claimed in claim 1 wherein said glass compositions is optimized based on fish oil as a dispersant. 6. The method of manufacturing glass tape as claimed in claim 1 wherein the glass powder is G13 and the solvent iused is toluene/ethanol. 7. The method of manufacturing glass tape as claimed in claim 1 wherein the binder used is poly ethylene glycol 400 for uniform tape thickness which is easily removable from the base plate after drying. 8. The method of manufacturing glass tape as claimed in claim 1 wherein said ^lasticizers are poly ethylene glycol 400 and Benzyl butyl phthaiate and said homogenizer is cyclohexanone. * 9. The method of manufacturing glass tape as claimed in claims 1 to 8 wherein the tape or gasket for cast from glass is pore free and flexible. 10. A method of manufacturing glass tape for use as a gasket for seaing in a solid oxide fuel cell as herein described and illustrated in the accompanying drawings.

Documents:

12-del-2002-abstract.pdf

12-del-2002-claims.pdf

12-del-2002-correspondence-others.pdf

12-del-2002-correspondence-po.pdf

12-del-2002-description (complete).pdf

12-del-2002-drawings.pdf

12-del-2002-form-1.pdf

12-del-2002-form-19.pdf

12-del-2002-form-2.pdf

12-del-2002-form-3.pdf

12-del-2002-gpa.pdf