Study of Seal Glass for Solid Oxide Fuel/Electrolyzer Cells
Seal glass is essential and plays a crucial role in solid oxide fuel/electrolyzer cell performance and durability. A seal glass should have a combination of thermal, chemical, mechanical, and electrical properties in order to seal different cell components and stacks and prevent gas leakage. All the desired properties can simultaneously be obtained in a seal glass by suitable compositional design. In this dissertation, SrO-La₂O₃-A₂O₃-B₂O₃3-SiO₂ based seal glasses have been developed and composition-structure-property relationships have been investigated. B₂O₃ free SrO-La₂O₃-Al₂O₃-SiO₂ based seal glass is the most suitable and its compatibility with the metallic interconnects and sealing performances have been evaluated.
A seal glass should be stable for 5,000-40,000 hrs in the oxidizing and reducing atmospheres at 600-900°C but both the thermal and chemical stability is a persistent problem. The effect of Al₂O₃ on a SrO-La₂O₃-Al₂O₃-B₂O₃-SiO₂ based seal glass has been studied to improve the thermal properties, such as glass transition temperature, softening temperature and thermal expansion coefficient, and the thermal stability. Al₂O₃ improves the thermal stability but does not significantly affect the thermal properties of the seal glass.
Comprehensive understanding of composition-structure-property relationships is needed to design a suitable seal glass. The thermal properties and stability of a borosilicate seal glass depend on the B2O3:SiO2 ratio in the composition. The role of B₂O₃:SiO₂ ratio on the glass network structure of the SrO-La₂O₃-Al₂O₃-B₂O₃-SiO₂ based seal glasses has been studied using Raman spectroscopy and nuclear magneto resonance spectroscopy. The thermal properties and thermal stability were correlated with the glass network structure and the calculated network connectivity. This study shows that the thermal properties degrade with increasing B₂O₃:SiO₂ ratio due to increase in the non-bridging oxygen and decrease in the network connectivity. High B₂O₃:SiO₂ ratio induces BO4 and SiO4 structural unit ordering, increases micro-heterogeneity, and subsequently degrades thermal stability. B₂O₃ free SrO-La₂O₃-Al₂O₃-SiO₂ seal glass shows the best combination of the thermal properties and thermal stability among the studied glasses.
Nickel or nickel oxide is added into a seal glass to modify the thermal properties depending on the specific composition. The role of nickel as a network former or modifier and its effect on the thermal properties and thermal stability of the SrO-La₂O₃-Al₂O₃-SiO₂ based seal glasses have been investigated. Nickel is a modifier in this glass system and does not improve the thermal properties but degrades thermal stability by decreasing network connectivity and inducing micro-heterogeneity.
The interconnect-seal glass interface stability is the most crucial for solid oxide fuel/electrolyzer cell. Crofer 22 APU and AISI 441 alloys are the preferred interconnects. The interfacial stability of the SrO-La₂O₃-Al₂O₃-SiO₂ based seal glass with these alloys have been studied as a function of time (0-1000 hrs), temperature (700-850°C), atmospheres (air, argon, and H₂O/H₂) using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction analysis (XRD). Complementary analytical techniques such as wave length dispersive spectroscopy (WDS) and SEM of thin samples were also carried out for selected samples. This study shows good interfacial stability of the SrO-La₂O₃-Al₂O₃-SiO₂ based seal glass with these alloys for the studied conditions.
A suitable seal glass should be hermetic and withstand 100-1000 thermal cycles for practical application. Sealing performances of the SrO-La2O3-Al2O3-SiO2 based seal glass have been evaluated by pressure-leakage method. The seal glass is hermetic for at least 2000 hrs and withstands 100 thermal cycles. Overall, present work shows that the SrO-La₂O₃-Al₂O₃-SiO₂ based glass has all the desired properties and suitable for solid oxide fuel/electrolyzer cell seal.