Dimensional behavior of Ni-YSZ composites during redox cycling

The dimensional behavior of Ni–yttria-stabilized zirconia (YSZ) cermets during redox cycling was tested in dilatometry within the temperature range 600–1000°C. The effect of humidity on redox stability was investigated at intermediate and low temperatures. We show that both the sintering of nickel depending on temperature of the initial reduction and the operating conditions, and the temperature of reoxidation are very important for the size of the dimensional change. Cumulative redox strain (CRS) is shown to be correlated with temperature. Measured maximum CRS after three redox cycles varies within 0.25–3.2% dL/L in dry gas and respective temperature range of 600–1000°C . A high degree of redox reversibility was reached at low temperature, however, reversibility is lost at elevated temperatures. We found that at 850°C, 6% steam and a very high pH₂O/pH₂ ratio is detrimental for redox stability, whereas at 600°C no negative effect was observed. Pre-reduction at 1100 instead of 800°C more than doubled redox strain on reoxidation at 800°C. For samples similarly pre-reduced at 1000°C, lowering the reoxidation temperature from 1000 to 750°C or below reduces the redox strain to less than half.