Ambient air partial internal reduction of NiO in a mixed ionic-electronic conducting ceramic

Partial internal reduction is observed of NiO precipitates in a matrix of a Ruddlesden-Popper type mixed ionic-electronic conducting ceramic with composition of (Pr 0.9 La 0.1 ) 2.0 Ni 0.74 Cu 0.21 Ga 0.05 O 4+δ during the cooling from sintering temperature to room temperature in ambient air. A metallic Ni interlayer between NiO and the oxide matrix is demonstrated using scanning transmission electron microscopy combined with electron energy-loss spectroscopy. The possible formation mechanism for the metallic Ni is discussed in light of the partial internal reduction during the cooling process in ambient air. The drop of temperature during cooling process in ambient air causes a drop of the oxygen chemical potential in the Ruddlesden-Popper type oxide ceramic, which acts as an oxygen absorber. Thus, the local oxygen chemical potential at the internal interface between Ruddlesden-Popper type matrix and NiO precipitate can fall below the stability limit of the later. As consequence, NiO is partially reduced to metallic Ni at the interface.