Transport properties of sealants for high-temperature electrochemical applications: RO–BaO–SiO2 (R = Mg, Zn) glass–ceramics

The electrical properties and oxygen permeability of glass–ceramics 55SiO2–27BaO–18MgO, 55SiO2–27BaO–18ZnO and 50SiO2–30BaO–20ZnO (%mol), which possess thermal expansion compatible with that of yttria-stabilized zirconia (YSZ) solid electrolytes, were studied between 600 and 950 °C in various atmospheres. The ion transference numbers, determined by the modified electromotive force (e.m.f.) technique under oxygen partial pressure gradients of 21 kPa/(1–8) × 102 Pa and 21 kPa/(1 × 10−18–2 × 10−12) Pa, are close to unity both under oxidizing and reducing conditions. The electronic contribution to the total conductivity increases slightly on increasing temperature, but is lower than 2% and 7% for the Zn- and Mg-containing compositions, respectively. The conductivity values measured by impedance spectroscopy vary in the range (1.4–7.8) × 10−6 S/cm at 950 °C under both oxidizing and reducing conditions, with activation energies of 122–154 kJ/mol and a minor increase in H2-containing atmospheres, indicating possible proton intercalation. In agreement with the electrical measurements which indicate rather insulating properties of the glass–ceramics, the oxygen permeation fluxes through sintered sealants and through sealed YSZ/glass–ceramics/YSZ cells are very low, in spite of an increase of 15–40% during 200–230 h under a gradient of air/H2–H2O–N2 due to slow microstructural changes.