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Viscous Sealing Glass Compositions for Solid Oxide Fuel Cells

The following invention is composed of viscous glass compositions (excluding any alkalis) that are stable against crystallization and can be used to fabricate reliable, thermochemically and thermomechanically stable, hermetic seals for solid oxide fuel cells.

Solid Oxide Fuel Cells (SOFCs) are multi-layered structures formed primarily of high-purity metal oxides, including an ionic conducting electrolyte, which generate electricity from the electrochemical oxidation of a fuel source. Planar SOFC configurations are relatively simple to manufacture and have greater power densities and efficiencies than other configurations, but require hermetic seals to prevent mixing of the fuel and oxidant streams within the cell stack and to seal the stack to the system manifold.

The seals must have a low electrical conductivity and must be chemically and mechanically stable in a high temperature reactive environment (moist reducing and/or oxidizing conditions). The seals should exhibit no deleterious inter-facial reactions with other cell components, should be created at a low enough temperature to avoid damaging cell components (under 900¡C for some materials), and should not migrate or flow from the designated sealing region during sealing or cell operation because of any applied load.

In addition, the sealing system should be able to withstand thermal cycling between the operational temperature and room temperature. That is, thermal stresses that develop because of mismatches in the thermal contraction characteristics of the different SOFC materials must either be reduced to well below the failure strengths of the materials or must be relieved in some fashion. Although it is possible to design rigid glass-ceramics with coefficient of thermal expansion (CTE) characteristics that are compatible with other SOFC materials (e.g., YSZ, ferritic stainless steels such as SS441, and alumina (as a coating material on ferritic steels)), and are stable over a long period of time at the operational temperature, stresses can still develop because of in-plane temperature gradients during operation and thermal cycling. If these stresses lead to cracks in the rigid glass seal or at one of the seal interfaces, the operational integrity of the SOFC will be compromised.

The use of viscous glass seals provides one means of reducing the risk that thermal stresses will result in catastrophic failures, and may provide a means for the seal to recover if cracks do form. The viscosity of the viscous glass seal at the operational temperature should be low enough (e.g., < 1012 Pa-s) for the seal to exhibit liquid-like properties, including viscous relaxation. This contrasts with rigid glass seals which either possess glass transition temperatures above the SOFC operational temperature, or are crystallized to the extent that they exhibit no viscous relaxation. On heating the seal above glass transition temperatures (Tg), the glass becomes viscous and any flaws within the seal (or at a seal interface) will heal because of viscous flow. In addition to providing a means to repair cracks in a seal caused by thermal stresses, the use of a viscous seal could also reduce the magnitude of those stresses compared with a rigid glass seal with the same thermal expansion characteristics since the stresses will be relieved at temperatures above Tg in the viscous glass seal, reducing the effective (Delta)T over which thermal stresses develop.

Additional Details

Owner

Missouri S&T

Intellectual Property Protection

Pending Patent



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