Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part I: Development of glass‐ceramic microstructure and thermomechanical properties

Abstract

Sealing performance in solid oxide cell (SOC) stacks and the devitrification process of commercially available alkaline earth boroaluminosilicate glasses containing 48-61 mol% SiO 2 , 18-28 mol% CaO, 1-7 mol% MgO, 7-10 mol% Al 2 O 3 , 1-11 mol% B 2 O 3 plus minor amounts of Na 2 O, K 2 O, FeO, and TiO 2 were investigated and quantified through analysis of phase assemblages as function of heat treatments above the glass transition temperatures using the electron microprobe and powder X-ray diffraction. For two of these glasses devitrification behavior was compared to the devitrification behavior of similar glasses produced in the laboratory. Glasses were characterized after annealing in air at 800°C and 850°C for up to 6 weeks. Even though the glasses lie within a relatively narrow compositional range, sealing performance and the resulting microstructures differed significantly. Best thermomechanical properties was developed in one of the laboratory-produced glasses, MCAS, which may be applied in SOC-stacks by allowing for a slow solidification in the range 750-800°C followed by crystallization at or slightly above 800°C. The relatively high thermal expansion coefficient (CTE) from RT-800°C, 11 9 10 À6 K À1 , which was developed over~1000 hours at 800°C, depends mainly on the formation of cristobalite and quartz as well as the presence of a residual glass phase. The glass ceramic sealant appears relatively stable over time, except for a slow transition of cristobalite to quartz, and can possibly show self-healing behavior if later brought close to 850°C. Devitrification led to increases of the thermal expansion coefficients in all other glasses tested, but did not reach levels interesting for SOC-stack sealing. K E Y W O R D Sborosilicate glass, glass-ceramics, microstructure, solid oxide cell stack sealing

Review on silicate and borosilicate‐based glass sealants and their interaction with components of solid oxide fuel cell

“…Many researchers 160–176 have investigated the crystallization of CAS (CaO‐Al 2 O 3 ‐SiO 2 ) glasses. CaO‐based borosilicate glasses have high CTE than BaO‐ and SrO‐based glasses 169 .…”

“…The study indicates that a good interface formation with interconnect as well as YSZ in reducing and oxidizing atmosphere. Agersted et al 170 investigated a SiO 2 ‐CaO‐MgO‐Al 2 O 3 ‐B 2 O 3 glass with few various additives. The glass sealant having almost no additives and 18 mol% of CaO proved to be best among all for its sealing properties with the most adequate thermomechanical properties.…”

“…31 However, the BaO-containing glass sealant required high temperature for sealing to achieve proper gas tightness due to its higher crystallization rate with high viscosity and poor wettability to adhere with interconnecting materials. 31 CaO-containing glasses Many researchers [160][161][162][163][164][165][166][167][168][169][170][171][172][173][174][175][176] have investigated the crystallization of CAS (CaO-Al 2 O 3 -SiO 2 ) glasses. CaO-based borosilicate glasses have high CTE than BaO-and SrO-based glasses.…”