Compressive Seals Research Articles

A review of sealing technologies applicable to solid oxide electrolysis cells

This article reviews designs and materials investigated for various seals in high temperature solid oxide fuel cell “stacks” and how they might be implemented in solid oxide electrolysis cells that decompose steam into hydrogen and oxygen. Materials include metals, glasses, glass–ceramics, cements, and composites. Sealing designs include rigid seals, compressive seals, and compliant seals.

Compressive Mica Seals for Solid Oxide Fuel Cells

Sealing technology is currently considered a top-priority task for planar solid oxide fuel-cell stack development. Compressive mica seals are among the major candidates for sealing materials due to their thermal, chemical, and electrical properties. In this paper, a comprehensive study of mica seals is presented. Two natural micas, muscovite and phlogopite, were investigated in either a monolithic single-crystal sheet form or a paper form composed of discrete mica flakes. A “hybrid” mica seal, developed after identification of the major leak paths in compressive mica seals, demonstrated leak rates that were hundreds to thousands times lower than leak rates for conventional mica seals. The hybrid mica seals were further modified by infiltration with wetting materials; these “infiltrated” micas showed excellent thermal cycle stability with very low leak rates (10−3 sccm/cm). The micas were also subjected to studies to evaluate thermal stability in a reducing environment as well as the effect of compressive stresses on leak rates. In addition, long-term open circuit voltage measurements versus thermal cycling showed constant voltages over 1,000 cycles. The comprehensive study clearly demonstrated the potential of compressive mica seals as sealing candidates for solid oxide fuel cells.

Novel compressive seals for solid oxide fuel cells

Traditional seals for planar solid oxide fuel cells (pSOFCs) are rigid glass and glass–ceramic, which have caused the problem of being unable to replace malfunctioning components. Non-glass sealants have become a recent trend. In this paper, fumed silica-infiltrated alumina–silica fiber paper gaskets were investigated as a novel compressive seal for planar solid oxide fuel cells. The leak rates decreased with increase of the silica-infiltration amount and the compressive load. Samples pre-stressed at 10 MPa indicated far superior sealing characteristics, with leak rates as low as 0.04 sccm cm −1 at a 1 MPa compressive stress and a 10 kPa pressure gradient, and 0.05 sccm cm −1 for 0.05 MPa, and a 1.4 kPa pressure gradient.

Novel Compressive Seals for Solid Oxide Fuel Cells

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Long-term thermal cycling of Phlogopite mica-based compressive seals for solid oxide fuel cells

Planar solid oxide fuel cells (SOFCs) require sealants to function properly in harsh environments at elevated temperatures. The SOFC stacks are expected to experience multiple thermal cycles (perhaps thousands of cycles for some applications) during their lifetime service in stationary or transportation applications. As a result, thermal cycle stability is considered a top priority for SOFC sealant development. In previous work, we have developed a hybrid mica-based compressive seal with very low leak rates of 2–4 × 10 −2 to 10 −3 sccm cm −1 at 800 °C, and showed stable leak rates over limited thermal cycles. In this paper we present results of long-term thermal cycle testing (>1000 thermal cycles) of Phlogopite mica-based compressive seals. Open-circuit voltage (OCV) was measured on a 2 in. × 2 in. 8-YSZ plate with the hybrid Phlogopite mica seals during thermal cycling in a dual environment (2.75% H 2/Ar versus air). During two long-term cycling tests, the measured OCVs were found to be consistent with the calculated Nernst voltages. The hybrid mica seal showed excellent thermal cycle stability over 1000 thermal cycles and can be considered a strong candidate for SOFC applications.

Novel infiltrated Phlogopite mica compressive seals for solid oxide fuel cells

Abstract A novel compressive Phlogopite mica seal was developed that showed superior thermal cycle stability with very low leak rates at 800 °C. A commercially available Phlogopite mica paper was infiltrated with a wetting or melt-forming agent, and tested in a hybrid form during thermal cycling. The results of H 3 BO 3 -infiltrated mica showed a continued decrease in leak rates over thermal cycling, and very low rates ( −4 sccm/cm) were obtained after ∼15 thermal cycles. The results of Bi-nitrate infiltrated mica exhibited a constant leak rate of (1–4)×10 −3 sccm/cm over 36 thermal cycles. The leak rates of the infiltrated mica were one to two orders of magnitude lower than leak rates for the as-received micas. Open circuit voltage tests were also conducted using dense 8YSZ plates to assess the effectiveness of the mica seals. Open circuit voltages equivalent to, or close to, the theoretical (Nernst) values were obtained.

Phlogopite mica-based compressive seals for solid oxide fuel cells: effect of mica thickness

Abstract Commercially available Phlogopite mica papers of varying thickness, ∼0.1, ∼0.2, and ∼0.5 mm, were evaluated as potential solid oxide fuel cell (SPFC) seal materials. The micas were tested in two forms: plain and hybrid. The hybrid form involved the addition of glass interlayers between the mica and the adjacent components. For each sample, about 30 thermal cycles were conducted and the 800 °C leak rates were determined. The results showed an excellent thermal cycle stability of the Phlogopite micas in the hybrid design in that the leak rates remained almost constant after ∼10 cycles. In addition, the leak rate appeared to increase with increasing mica thickness in the hybrid design, but showed no thickness dependence for mica in the plain design. The Phlogopite micas also showed good mid-term (∼500 h) stability in both air and reducing environments. Microstructure characterization showed no distinct degradation such as fragmentation and particle formation after thermal cycle and the mid-term stability tests.

Novel silver/mica multilayer compressive seals for solid-oxide fuel cells: The effect of thermal cycling and material degradation on leak behavior

A novel Ag/mica compressive seal was thermally cycled between 100 °C and 800 °C in air to evaluate its stability. The novel Ag/mica compressive seal was composed of a naturally cleaved Muscovite mica sheet and two thin silver layers, and was reported in a previous study to have very low leak rates at 800 °C. In the present study, we examined the thermal cycle stability of the Ag/mica-based compressive seals pressed between mating couples with large and small mismatch in thermal expansion. For comparison, thermal cycling also was conducted on plain mica as well as plain silver only. In addition, the results were compared with published data of a similar mica seal using glass instead of Ag as the interlayers. For mating materials of large mismatch in thermal expansion coefficient (CTE; Inconel/alumina), the Ag/mica seal showed lower leak rates than the plain mica. For mating materials of small mismatch in CTE (SS430/alumina), the leak rates were similar for both the Ag/mica and the plain mica seal. Scanning electron microscopy was used to characterize the microstructure of the mica after thermal cycling. Microcracks, fragmentation, and wear-particle formation were observed on the mica and were correlated to the leak behavior. Overall, the novel Ag/mica seals present good thermal cycle stability for solid-oxide fuel cells, although the leak rates were greater than the corresponding mica seals with glass interlayers.

Novel Compressive Mica Seals with Metallic Interlayers for Solid Oxide Fuel Cell Applications

A novel hybrid compressive mica seal was developed that showed an exceptionally low leak rate of ∼8.9 × 10−4 sccm/cm (standard cubic centimeters per minute per unit leak length of seal) at 800°C and a compressive stress of 100 psi, about 740 times lower than that of the conventional compressive mica seals (6.6 × 10−1 sccm/cm), at a pressure gradient of 2 psi. The hybrid compressive mica seals were composed of two compliant metallic layers and a mica layer. Three commercially available micas were tested in this study. All showed substantial improvements in reducing the leak rates by using the hybrid design. The best results were obtained using muscovite single‐crystal mica and 125 μm silver layers. Using the paper form of muscovite and phlogopite mica, the leak rates were still far superior (∼1 × 10−1 sccm/cm) compared with mica without the compliant silver layer (about 6–9 sccm/cm). The microstructure of the mica was examined before and after the 800°C leak tests. These results are compared with results for hybrid seals using glass interlayers. In addition, an explanation for the substantial reduction of the leak rates is proposed, and the application of the hybrid compressive mica seals in planar SOFC stacks is briefly addressed.

Mid-term stability of novel mica-based compressive seals for solid oxide fuel cells

A novel mica-based compressive seal concept was examined at elevated temperatures under compressive stress to evaluate its stability. The “hybrid” mica compressive seals, composed of cleaved Muscovite mica and two compliant inter-layers, were reported earlier to have very low leak rates at 800 °C. In the present study, we examined the mid-term (∼350–700 h) stability of the mica-based compressive seals with three different inter-layers: a low melting borosilicate glass, a glass ceramic material, and a metallic material. The 800 °C leak test results showed excellent stability for the three different inter-layers in air at a compressive stress of 100 psi as the leak rates remained almost unchanged during the test. Microstructural characterization of the interfaces showed very limited interfacial reaction or glass penetration at the glass/8YSZ substrate interface. The results clearly demonstrate the applicability of the mica-based compressive seals for solid oxide fuel cells.

Thermal cycling and degradation mechanisms of compressive mica-based seals for solid oxide fuel cells

Thermal cycling was conducted on compressive mica seals at 800 °C in air. Thin (∼0.1 mm) Muscovite mica was pressed between a metal pipe and an alumina substrate and tested for leak rates at a stress of 100 psi in the plain (mica only) and the hybrid design. The hybrid design involves adding two glass interlayers and was found to greatly reduce the leak rates in an earlier paper. Two metals (Inconel #600 and SS430) with high and low coefficients of thermal expansion (CTE) were used to evaluate the effect of CTE mismatch on thermal cycling. The results showed that the leak rates were lower for the hybrid design than the plain micas. In addition, using the lower CTE (SS430) metal pipe resulted in lower leak rates as compared to Inconel #600 metal (high CTE). In general, the leak rates increased with the number of thermal cycles; however, it tended to level off after several tens of thermal cycles. Microstructure examination using scanning electron microscopy revealed steps, indents, fragmentation and particle formation on the mica after thermal cycling. CTE measurement of the heat-treated Muscovite mica showed a relatively low value of ∼7 ppm/°C. The cause for the degradation of the mica is discussed.

Ultra-low leak rate of hybrid compressive mica seals for solid oxide fuel cells

A novel hybrid compressive mica seal was developed that showed a reduction of leak rate by about 4300 times (compared to simple mica seals) at 800 °C. The hybrid compressive mica seal is composed of 2 compliant glass layers and a mica layer. Three commercially available micas were tested in hybrid compressive seals for solid oxide fuel cell applications. The best results were obtained using Muscovite single crystal mica. The normalized leak rate for this seal at 800 °C was only 1.55×10 −4 sccm/cm at a stress of 100 psi and a pressure gradient of 2 psi. Seals based on the other commercial micas (Muscovite and Phlogopite mica papers), also exhibited superior leak rates (∼0.011 sccm/cm) compared to simple mica seals without the compliant glass layer (about 6–9 sccm/cm). The microstructure of the mica was examined before and after the 800 °C leak tests using scanning electron microscopy. The cause for the substantial reduction of the leak rate was discussed. In addition, the effect of the compressive stresses was also investigated.

Compressive mica seals for SOFC applications

Muscovite and phlogopite micas have been assessed as SOFC seals at 800°C. Paper gaskets, composed of pressed mica platelets in an organic binder, proved ineffective seal materials predominantly because of their uneven surface. However, cleaved natural mica sheets (with no binder) indicated far superior sealing characteristics with leak rates lower than 0.1 sccm cm −1 at 800°C, and approximately 0.7 MPa (100 psi) compressive stress.