@article{GUO2025, 
author = {Mengyuan GUO and Yunfei WU and Jie LI and Minghao YU and Ben GE and Xingyu XIONG and Zhibin YANG},
title = {Sealing Performance of Vermiculite–Glass Composite Sealing Materials for Solid Oxide Fuel Cell},
year = {2025},
journal = {Journal of the Chinese Ceramic Society},
volume = {53},
number = {10},
pages = {3003-3012},
keywords = {solid oxide fuel cell, sealing materials, silicate glass, vermiculite, interface compatibility},
url = {https://www.sciopen.com/article/10.14062/j.issn.0454-5648.20250198},
doi = {10.14062/j.issn.0454-5648.20250198},
abstract = {IntroductionSolid oxide fuel cells (SOFCs) can directly convert chemical energy into electrical energy with high efficiency and environmental friendliness. In the operation of SOFCs (at 600–800 ℃), hydrogen and air are supplied to the fuel and oxygen electrodes, respectively. Gas-tight sealing components are critical for isolating these electrodes to prevent gas crossover and efficiency loss, ensuring safe operation. Ceramic cell-metal interconnect sealing represents a bottleneck technology for SOFCs. Glass is widely used as a sealing material due to its scalability, operational simplicity, and cost-effectiveness. However, monolithic glasses exhibit a narrow functional temperature range and a poor thermal gradient resistance, often developing cracks due to the uneven thermal stress distribution. Glass-matrix composite seals offer enhanced thermal stability and hermeticity. Also, stack operation faces some challenges during startup/shutdown cycles. While mica-based seals demonstrate a superior thermal cycling resistance, their interlaminar bonding strength degrades a post-annealing, leading to delamination and compromised gas tightness. Vermiculite as a another layered silicate clay mineral with a structure similar to mica exhibits a better airtightness rather than mica in high-temperature sealing processes. This is because vermiculite contains an interlayer water, which turns into expanded vermiculite after high-temperature heating. The flexural interlayer structure blocks gas leakage channels, making the layers of vermiculite have greater constraint and bonding properties, thus having a potential for high-temperature sealing.MethodsA homogeneous mixture of SiO2, Na2CO3, K2CO3, KNO3, BaCO3, and minor TiO2 was loaded into an alumina crucible, calcined at 1450 ℃ for 2 h at 5 ℃·min-1, melted at a high temperature, and quenched in deionized water. The quenched glass block was washed, crushed, ground, dried, and sieved with a 200 mesh sieve to obtain a glass powder (labeled G0). A natural vermiculite powder heat-treated at 900 ℃ for 20 h was blended with G0 at 5%, 10%, and 15% to prepare vermiculite–glass composite sealing materials (i.e., designated as GV5, GV10, and GV15), respectively. Sealing material powder and organic binder (prepared by uniformly mixing ethyl cellulose and terpineol in a mass ratio of 4:96) were homogenized in a mass ratio of 2.5:1.0 to form a composite seal paste. This paste was applied between polished SUS430 stainless steel plates to prepare the specimens for high-temperature interfacial compatibility studies, and was also used in a custom-built high-temperature hermeticity testing apparatus.Results and discussionThe XRD patterns of the sealing material show that the crystallization degree of the composite sealing material increases with increasing vermiculite content in sample G0. The overall phase structure changes of the composite sealing material are affected due to the glass structure changes, and vermiculite as an additive exhibits stable high-temperature thermal properties.  The results of a further open circuit voltage (OCV) test at 750 ℃show significant OCV fluctuations in sample GV5 possibly due to its low softening temperature (Ts), causing excessive softening of the glass matrix at 750 ℃ and forming gas leakage channels. However, the self-healing property of glass heals microcracks and other leakage channels during softening, leading to OCV fluctuations in sample GV5. Sample G0 has a Ts of 25 ℃, which is higher than sample GV5, showing little OCV fluctuations at 750 ℃ with a good stability. The OCVs of samples GV10 and GV15 remain stable for 900 min, and sample GV10 stabilizes at 1.1 V and sample GV15 does at 1.06 V. The lower OCV of sample GV15 is presumably due to the excessive amount of vermiculite, where the glass matrix cannot fully wet and encapsulate vermiculite particles after high -temperature softening, creating potential leakage channels and affecting gas tightness, while sample GV10 exhibits an excellent gas tightness. The results of further gas tightness tests show that for sample GV10 at an inlet pressure of 7 kPa, gas leakage rates at 700, 750 and 800 ℃ are 0.0019, 0.0031 and 0.0073 sccm·cm–1, respectively.  Increasing vermiculite content from 0 to 15% increases porosity and pore coarsening in the sealing material, possibly because vermiculite particles reabsorb water during green body preparation. At high temperatures, some water vapor enters the lamellar structure of vermiculite, and vermiculite particle expansion under a water vapor pressure causes pore coarsening and increases porosity. Based on the analysis of energy dispersive spectroscopy (EDS), vermiculite particles embed in glass blocked diffusion of Cr, Fe and other elements from the interconnect into the sealing material. More importantly, lamellar vermiculite with a reversible interlayer expansion potentially adjusts a thermal stress distribution in glass, bridges crack, slows crack propagation and growth, making it a promising sealing material for SOFCs.ConclusionsThree vermiculite–glass composite sealing materials were developed via incorporating 5%, 10%, and 15% vermiculite powder into a K–Na–Ca–Ba–Si silicate glass matrix and applied to SOFCs. These composites demonstrated excellent gas–tightness and chromium resistance. Among them, the composite with sample GV10 was tested in a button cell configuration, having a stable OCV of 1.1 V at 750 ℃. The results of gas leakage tests confirmed its superior sealing performance, with a leakage rate of 0.0031 sccm·cm–1 under an input gas pressure of 7 kPa. The gas tightness improved at lower operating temperatures. At 700 ℃ under the same pressure, the leakage rate decreased to 0.0019 sccm·cm–1, which was attributed to a lower Ts (538 ℃) of sample GV10. Reducing the operating temperature effectively mitigated excessive glass softening, thereby enhancing a sealing reliability. The further microstructural characterization by scanning electron microscopy and EDS revealed that the lamellar vermiculite particles embedded in the glass matrix significantly suppressed a chromium diffusion into the sealing material. This unique microstructure also contributed to reduced gas leakage rates and improved fracture toughness. Therefore, vermiculite–glass composite sealing materials could have a promising potential used as high-performance sealing candidates for SOFCs.}
}