Abnormal Shrinkage upon Nickel Oxidation at Low Temperatures

Tuesday, 28 July 2015
Hall 2 (Scottish Exhibition and Conference Centre)
K. Yashiro, T. Nakamura (Tohoku University, Japan), K. Yamaji (National Institute of AIST), S. I. Hashimoto (Tohoku University), K. Amezawa (Tohoku University, Sendai,), and T. Kawada (Tohoku University, Japan)
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.