Barium-calcium aluminosilicate glass/mica composite seals for intermediate solid oxide fuel cells

Abstract

In this paper, the effects of phlogopite mica addition on the properties of barium-calcium-aluminosilicate (BCAS) glass seals are studied. Accordingly, BCAS glass powder is mixed with up to 30 wt% of phlogopite mica; the resulting mixture is formed and sintered at different temperatures. The microstructure and phase content of the samples are then analyzed by scanning electron microscope (SEM) and X-ray diffraction method. Mechanical properties including, hardness, compression strength, fracture toughness and Young's modulus are investigated. The coefficient of thermal expansion (CTE) and softening temperature of the seals are measured. The electrical conductivity of the selected specimens is studied by applying the electrochemical impedance spectroscopy method. The performance of the composite seals is analyzed under fuel cell working conditions by the leak test. It is found that the samples with the mica content of more than 10 wt% cannot be sintered to near full relative density. The sintering temperature above 750°C raises the risk of unfavorable reactions between mica and the BCAS glass matrix. It is also revealed that the addition of 5 wt% phlogopite increases Young's modulus and fracture toughness of the BCAS glass seals by 13% and ~633%, respectively, without impairing other mechanical and thermal properties. The composites seals with more than 10 wt% of mica content don't meet the required mechanical properties demands for SOFC seals. It is found that the composite seal with 5 wt% of phlogopite mica has very low electrical conductivity and leak rate of less than 1.2×10−4sccm/cm. This composite seal shows high thermal stability with ~5% reduction in CTE after 10 cycles under SOFC working conditions and consequently can be considered as a promising seal for SOFCs.