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Investigation of Mechanical Properties of Glass-Ceramic Sealant for Solid Oxide Fuel/Electrolysis Cells

Fakouri Hassanabadi, Masood | 2018

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51418 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Faghihi-Sani, Mohammad Ali; Abdoli, Hamid
  7. Abstract:
  8. The structural integrity of the sealing material is critical for the reliability of solid oxide fuel/electrolysis stacks. In this study, the mechanical properties of a rapid crystallizing glass of BaO-CaO-SiO2 system were evaluated. The crystallization as a determining factor in the mechanical behavior of the glass-ceramic sealant was investigated via high-temperature X-Ray diffraction spectrometer. Three- and four-point bending tests were carried out to measure the flexural strength (σ) of sealant material bars and head-to-head joined specimens at room- and high-temperatures. In addition, the elastic modulus (E) was measured by impulse excitation test up to 900 ºC. Also, the indentation tests were carried out to measure the fracture toughness (KIC), elastic modulus (E) and hardness (H) at room temperature. Tests were done for both as-sintered (as-joined) and 500 h annealed samples. A torsion test was implemented to evaluate the shear strength (τ) of used glass-ceramic and to compare it with that of a partially crystallized glass sealant at room- and operation relevant high-temperatures. Hourglass shaped specimens with different configurations of hollow- and full-halves were utilized for testing. The fracture surfaces were visualized via optical microscopy and complementary scanning electron microscopy. The main crystallization happened during the initial sealing step (heating at 850 ºC for 10 h). This glass-ceramic with mechanical properties of KIC = 1.2 ± 0.2 MPa∙m0.5, σbulk = 72 ± 4 MPa and σjoint = 26 ± 2 MPa at room temperature and τjoint = 21 ± 3 MPa at 800 ºC can mechanically compete with reinforced sealing materials. The annealing led to a slight decrease in E and σ, but didn’t change the KIC significantly. Contrary to flexural strength, the torsional shear strength decreased with increasing temperature. The fractography of torsion specimens revealed that the fracture mode changes from brittle at room temperature and 600 °C to ductile at 800 °C. The observed self-healing behavior of glass-ceramic justified the viscous flow capability of residual glassy phases at high temperature. The implemented finite element simulation of the torsion test indicated the presence of a non-shear stress at the sealant which can decrease by using hollow-full specimens with a thicker square end. Also, the simulation of joint behavior in bending and torsion tests showed that the stress distribution is different with that is calculated by simple static equations, and it depends on mechanical properties of the joint component at the test temperature. The cyclic loading condition indicates subcritical crack growth in the sealant at 600 °C and creep associated damage at 800 °C
  9. Keywords:
  10. Solid Oxide Fuel Cell (SOFC) ; Sealant ; Glass Ceramic ; Finite Element Simulation ; Crystallization ; Mechanical Properties

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