BaZn2Si2O7 and the solid solution series BaZn2−xCoxSi2O7 (0<x≤2) as high temperature seals for solid oxide fuel cells studied by high-temperature X-ray diffraction and dilatometry

Abstract

Abstract For sealing of solid oxide fuel cells, glasses from which crystalline phases with high coefficient of thermal expansion (CTE) can be crystallized are required. In this paper, a new solid solution series BaZn 2− x Co x Si 2 O 7 (0 x ≤2) is introduced which exhibit extremely high CTE. The prepared powders were characterized by X-ray diffraction and high-temperature X-ray diffraction (BaZn 2 Si 2 O 7 ). Sintered specimens were characterized by dilatometry. The introduction of Co 2+ does not lead to a change in the space group. All compounds show a transition of a low to a high temperature modification. The attributed temperature increases from 300 °C for BaZn 2 Si 2 O 7 to 850 °C for BaCo 2 Si 2 O 7 . The volume expansion which runs parallel to the phase transition decreases with increasing cobalt concentration. The phase BaZn 2 Si 2 O 7 shows the largest CTE and a steep volume effect during phase transition. For the compound BaZn 0.25 Co 1.75 Si 2 O 7 the CTE is minimum (8.6×10 −6 K −1 (50–900 °C)) and increases again until for the compound BaCo 2 Si 2 O 7 a CTE of 16.6×10 −6 K −1 (50–900 °C) is reached. In the cobalt rich composition range, the CTEs are in the right range for high temperature fuel cells and can be adjusted by the composition.