Mikko Pihlatie

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 p H2O /p H2 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.

Curvature and Strength of Ni-YSZ Solid Oxide Half-cells after RedOx Treatments

One of the main drawbacks of anode-supported solid oxide fuel cell technology is the limited capability to withstand reduction and oxidation (“RedOx”) of the Ni phase. This study compares the effect of RedOx cycles on curvature and strength of half-cells, composed of a Ni-YSZ support, a Ni- YSZ anode and an 8YSZ electrolyte. Five different treatments were studied: (i) reduction at 600°C, (ii) reduction at 1000°C, (iii) 1 RedOx cycle at 750°C, (iv) 5 RedOx cycles at 750°C and (v) 5 RedOx cycles at 600°C. The strength was measured by the ball-on-ring method, where it is calculated analytically from the force. In this calculation the thermal stresses have been estimated from the curvature of the half-cell. For each treatment, more than 30 samples were tested. About 20 ball-on-ring samples were laser cut from one original 12x12 cm2 half-cell. Curvature and porosity were measured for each sample before and after RedOx treatments. The first observations show that increasing the reduction temperature enhance strength but does not influence the curvature, whereas 1 RedOx cycle at 750°C increases the curvature without changing the strength. Consecutive RedOx cycles seem to decrease anode-supported cell strength, but this is coupled to lower porosity of the sample.

Curvature and Strength of Ni-YSZ Solid Oxide Half-Cells After RedOx Treatments

One of the main drawbacks of anode-supported solid oxide fuel cell technology is the limited capability to withstand reduction and oxidation (“RedOx”) of the Ni phase. This study compares the effect of RedOx cycles on curvature and strength of half-cells, composed of a Ni-YSZ support, a Ni-YSZ anode and an 8YSZ electrolyte. Five different treatments were studied: (i) reduction at 600°C, (ii) reduction at 1000°C, (iii) 1 RedOx cycle at 750°C, (iv) 5 RedOx cycles at 750°C and (v) 5 RedOx cycles at 600°C. The strength was measured by the ball-on-ring method, where it is calculated analytically from the force. In this calculation the thermal stresses have been estimated from the curvature of the half-cell. For each treatment, more than 30 samples were tested. About 20 ball-on-ring samples were laser cut from one original 12×12 cm2 half-cell. Curvature and porosity were measured for each sample before and after RedOx treatments. The first observations show that increasing the reduction temperature enhance strength but does not influence the curvature, whereas 1 RedOx cycle at 750°C increases the curvature without changing the strength. Consecutive RedOx cycles seem to decrease anode-supported cell strength, but this is coupled to lower porosity of the sample.Copyright