Abnormal Shrinkage upon Nickel Oxidation at Low Temperatures
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.