Hydrogen permeability of YSZ single crystals at high temperatures

The atomic hydrogen (H) permeability, JH, of 10YSZ single crystal (fluorite-type) was measured at 1800–1050 K using the Nigara-method. Two tubular specimens (φ 1.30×0.93 cm; length: 3.35 cm (Specimen(L)) and 1.00 cm (Specimen(S))), annealed at 1570 K for 6 h in air, were used to eliminate the permeations of supporting materials (Pt rings, alumina tube, and alumina disk). Measurement was carried out after the specimen had been kept at 1800 K for 20 h. The JH was 3.97×10−7–2.79×10−9/mol h−1 cm−1, when it was assumed that PH2=7.36×103 Pa and PH2O=2.30×103 Pa outside of both specimens and that the Ar flow rates to the inside of Specimen(L) and Specimen(S) were 21.5 and 16.9–19.9 cm3 min−1, respectively. Log JH decreased with the decrease of temperature and was proportional to the inverse temperature. The activation energies were 0.61±0.09, 1.60±0.05, and 0.79±0.02 eV at 1800–1680, 1680–1330, and 1330–1050 K, respectively. The protonic conductivity, which was roughly calculated from the JH and the hydrogen partial pressures inside and outside of the specimens, was 2.4×10−6–1.5×10−8/S cm−1 at 1800–1050 K. The JH of other 10YSZ single crystal specimens, which were not annealed and which were measured after being kept at 1800 K for 1 h, was lower than that of the above specimens, especially at high temperatures, and the activation energies were 1.27±0.03, 0.15±0.03, and 1.49±0.09 eV at 1800–1425, 1425–1150, and 1150–1050 K, respectively. The 10YSZ single crystal would be an electron–proton mixed conductor under H2–H2O atmosphere at high temperatures. Since the JH of 12YSZ single crystal specimens, which were annealed at 1870 K for 24 h in air and were measured after being kept at 1800 K for 18 h, was less than 1×10−9/mol h−1 cm−1 at 1800 K, it was difficult to determine the relation between its hydrogen permeability and temperature.