Abnormal Shrinkage upon Nickel Oxidation at Low Temperatures

Chemical- and mechanical stability of Ni-YSZ cermet is an important factor for long-term operation of SOFCs, because the cermet is often used as not only an anode but a support of the cell.  It has been reported that microstructure of the cermet changes upon reduction and oxidation of nickel; and that nickel sintering takes place during long-term operation.  These phenomena lead to deterioration of cell performance. 

It is well known that oxidation of Ni causes increase in volume.  However we recently found that Ni-YSZ cermet was unexpectedly shrunk during the oxidation in the temperature range less than 500 °C.  To clarify this phenomenon, thermal expansion behavior of sintered nickel metal was measured in oxidizing atmosphere by a dilatometry.  Oxygen partial pressure and temperature varied from 10^-3 to 0.2 bar and room temperature to 700 °C, respectively.  The extraordinary shrinkage behavior was observed in temperature range between 300-500 °C.  The amount of shrinkage depended on oxygen partial pressure and temperature ramp.  Maximum shrinkage of 1% was observed in 20%O₂-N₂at heating rate of 1 °C/min.  

Microstructure change of nickel was also observed by a scanning electron microscope before and after oxidation treatment at 400 °C in order to clarify the causes of volume increase.  Comparing the microstructure before and after oxidation, it seemed that nickel sintering was accelerated by oxidation of nickel.