Comparison between barium and strontium-glass composites for sealing SOFCs

The present study demonstrates the feasibility of adding micron-scale Y₂O₃-stabilized ZrO₂ (YSZ) powders to modify the properties of two borate glasses used for sealing electrolyte supported SOFCs. The crystallization of the composite made with a Ba-containing glass was found to be independent of the volume fraction of YSZ, as opposed to the situation for Sr-glass composites where the crystallization temperature decreased with the volume fraction of YSZ. The variation of the flow properties of both glass composites was measured using a wettability test, and an increase of the contact angle was measured when the volume fraction of additives was increased. Examining the microstructure showed that initially the Ba-containing glass reacted with YSZ to form a BaZrO₃ compound. Long time exposure at 800 °C caused a large reduction of the coefficient of thermal expansion (CTE), which is explained by increased formation of BaZrO₃ and further change in glass composition. On the other hand, the reaction involving the Sr-containing glass with the YSZ additive shows the initial formation of calcium zirconate (Ca is an ingredient in both glasses) followed by appearance of strontium zirconate with further heating. For this Sr glass composition, the observed reduction of CTE was associated with the change in composition of the remaining glassy phase since the CTEs of the reaction products are close to the CTE of the YSZ additives.