Delay of tetragonal-to-monoclinic transition in water vapour due to nanostructural effect

The ageing behaviour of ultrafine yttria-doped single-phased tetragonal zirconia was investigated at 400 °C in dry air and in water vapour by complex impedance analysis and Raman spectroscopy. Pellets sintered at 1500 °C for 2 h in air were characterized by a 60 nm average grain size, a unimodal distribution of 300 nm average aggregate size, a density of 97% of theoretical one and pore sizes lower than 20 nm. No degradation of the electrical properties of dense pellets was detected in both gaseous environments during 1000 h at 400 °C. Simultaneously, no monoclinic signature was detected by Raman spectroscopy. Furthermore, tests were investigated in conditions close to those of IT-SOFC: 700 °C in water vapour during 1000 h. In these conditions, a conductivity decrease of 5% was recorded and less than 2 mol% of monoclinic zirconia was observed by Raman spectroscopy.