Mica Seals Research Articles

Short Stack Kit for the Open Flange™ Set-up Using Soft Mica Seals

Fiaxell has been active since 2008 in the Solid Oxide Fuel Cells (SOFC) industry and has been developing components and products for SOFC research. Fiaxell is the inventor of the “2R-Cell”, an anode supported cell able to withstand full re-oxidation and thermal cycles. The 2R-Cell is robust and has demonstrated high fuel utilization (85-90%) and electrical efficiency up to 50% with pure hydrogen on button cells. Fiaxell has developed the Open Flange™ Set-Up in order to test SOFC and SOEC cells. The Set-Up allows researchers to do various experiments, such as current-voltage characteristics, polarization, impedance spectroscopy measurements, electrolysis, co-electrolysis and in situgas processing (POX, SR) thanks to its integrated steam generator. Fiaxell is now offering a complete short stack kit that can be directly mounted onto the Open Flange™ Set-Up platform. This kit allows researchers to perform a range of experiments and to study stack parameters such as flow distribution, electrical contact resistance, and to investigate protective coatings to avoid chromium evaporation, in situfuel processing and sealing gas tightness. Each parameter can be independently characterized and optimized by combining ideal and real stack conditions on both sides of the cell, for instance Crofer steel on the anode and gold mesh on the cathode. Fiaxell is proposing Crofer 22H interconnects with different flow pattern designs for 50 X 50 mm and 80 X 60 mm cells. Alternatively, customers can propose their own customized design which can then be chemically grooved and laser cut by Fiaxell. The short stack kit comes with soft mica sealing but can also be made gas tight with any glass paste or other sealants. The soft mica seals have been designs to be easily removed and replaced, contrary to the use of glass based sealants that require mechanical grinding to remove. Mechanical simulations and calculations have been made to ensure that the design of the end Flange™ is adequate to compress the entire stack without any distortion or stress concentration points, especially on the interconnect plates. In order to see the gas distribution in the interconnect pattern, Fiaxell has developed an alternative, and complementary method to simulation, which consists of direct visualization and image analysis of the real flow motion. Screen captures will be included and discussed. In this paper, we present the experimental results of the short stack tests using two different interconnect patterns and also discuss the effectiveness of the soft mica gasket seals. Image caption: Exploded view of two cells short stack showing the three bipolar plates (interconnect) and soft mica gaskets (in beige). The short stack has been designed as a kit to be integrated directly on the Open Flange™ Set-Up platform. Figure 1

Short Stack Kit for the Open Flange™ Set-up Using Soft Mica Seals

Results from Fiaxell’s SOFC/SOEC short stack kit designed for the Open Flanges Set-Up, using ASC (2R-Cell) cells with Crofer 22H interconnects are presented. The stack incorporates compressible mica seals that demonstrated high fuel utilization (till 82%). In parallel, a mechanical strength simulation was carried out. The latter showed that the design of the housing could be sufficient for stack testing at our operating temperature (750-800 °C). Secondly several Cell-Connex™ interconnect patterns that were made by a chemical etching technique and were mounted in a transparent housing which allowed flow patterns to be visualized and recorded with a high speed camera. A square flow-field pattern was used in this study. A short stack kit was tested with a single cell and current-voltage characteristics were measured at high and low fuel utilization.

Performance of Non‐Destructive Compressive Seals for Reversible Solid Oxide Cells

AbstractContrary to hardened glass ceramics, compressive non‐destructive seals for solid oxide cells (SOCs) could enable stack repairs without utilizing destructive separation methods. In light of increasing interest for high temperature steam electrolysis and energy storage, the performances of two commercially available compressive materials were investigated in humidified gas conditions: Thermiculite‐866 and AFS‐170 alumina felts. Mass loss and thermogravimetric (TGA) experiments carried out in inert, oxidizing, and reducing atmospheres at 800 °C showed no influence of the atmosphere on the alumina felts, and a reversible redox behavior for Thermiculite‐866. Thermiculite‐866 showed better cyclability compared to AFS‐170 during SOC thermal cycling experiments. Initial open circuit voltages in dry H2 were measured and compared to those obtained with commercial mica seals. The highest were recorded with Thermiculite‐866, followed by mica and AFS‐170. The species outgassed by Thermiculite‐866 were quantified, and fluorine was found to be the main component released at 800 °C.

Comments on “Ultra-low leak rate of hybrid compressive mica seals for solid oxide fuel cells”

Comments on “Ultra-low leak rate of hybrid compressive mica seals for solid oxide fuel cells”

Long-term ageing and materials degradation of hybrid mica compressive seals for solid oxide fuel cells

Hybrid phlogopite mica seals with silver interlayers were evaluated in long-term isothermal ageing tests in a dual environment consisting of dilute hydrogen versus air at 800 °C. High-temperature leak tests with helium showed very stable leakage of 0.01–0.02 sccm cm −1 for 28,366 h under a low applied compressive stress of 82 kPa (12 psi). Post-mortem SEM and EDS analyses of the mica showed minimum degradation in terms of changes in microstructure and chemical composition, although there appeared to be some Ag migration and segregation at interstices between mica flakes. Fluorine was also found to be released from mica. Overall, the low, constant leakage through the hybrid mica/Ag seals clearly demonstrated a very promising candidate for SOFC sealing.

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.

Material Degradation during Isothermal Aging and Thermal Cycling of Hybrid Mica Seal with Ag Interlayer under SOFC Exposure Conditions

Hybrid phlogopite mica seals with silver interlayers were evaluated in terms of material degradation in a combined isothermal aging and thermal cycling test. The hybrid mica was composed of a phlogopite mica paper sandwiched between two foils. The hybrid micas were first aged at for in a low hydrogen fuel , followed by short-term thermal cycling between and . The combined test was repeated three times for a total of aging at and 119 thermal cycles. The results of high-temperature leak rates showed very good thermal stability and thermal cycle stability with leak rates of . The hybrid mica was also tested in a high-water-content fuel and demonstrated similar leakage during the isothermal aging and the subsequent thermal cycles. Postmortem analyses showed no extensive reaction of Ag with phlogopite mica as well as the mica degradations. The issues of volatility of Ag and mica in the solid oxide fuel cell (SOFC) environments were discussed. Simple models were simulated to estimate the effect of leakage on the open-circuit voltage and the total fuel loss at various SOFC stack sizes. The results showed very minute fuel loss for the current hybrid micas and demonstrated it to be a good candidate for SOFC sealing.

Material degradation during isothermal ageing and thermal cycling of hybrid mica seals under solid oxide fuel cell exposure conditions

Hybrid phlogopite mica seals with glass interlayers were evaluated in terms of materials degradation in a combined ageing and thermal cycling test. Three glass interlayers were investigated: a standard Ba–Ca–Al silicate glass (G18), a modified Ba–Ca–Al silicate glass with a nucleation agent added (G18m), and a borosilicate glass (G6). The hybrid phlogopite mica seals were aged at 800 °C for ∼500 to ∼1000 h in moist, dilute hydrogen fuel, and then subjected to short-term thermal cycling between ∼100 and 800 °C. Seals with G18 and G18m glass interlayers showed very poor thermal cycle stability after isothermal ageing. Seals with borosilicate glass interlayers showed very good thermal cycle stability with constant, low leakage (<0.01 sccm cm −1 at 0.2 psi) after ageing for 508 h followed by 56 thermal cycles. Post-mortem analyses showed degradation of the mica due to interaction with the interlayer glasses. Leak paths were identified and correlated to the measured leak rates.

Thermal cycle stability of a novel glass–mica composite seal for solid oxide fuel cells: Effect of glass volume fraction and stresses

A novel glass–mica composite seal was developed based on a previously reported concept of “infiltrated” mica seals for solid oxide fuel cells. Ba–Al–Ca silicate sealing glass powder and Phlogopite mica flakes were mixed at glass volume fractions of 10–50 vol.% to make the glass–mica composite seals. The seals were leak tested for short-term thermal cycle stability as a function of glass volume fraction. Composite seals with 10 and 20 vol.% glass were also leak tested under compressive stresses from 3 to 100 psi and helium pressures of 0.2 or 2 psi. Post-mortem microstructure analyses were used to characterize the fracture (leak path) of the glass–mica composite seals and were related to the high temperature leakages. Open circuit voltage tests on dense 8YSZ electrolyte with the glass–mica composite seal showed very good thermal cycle stability over 250 cycles with minute (<1%) voltage drop.

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.

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.

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.