Effect of local oxygen activity on Ni–BaTiO3 interfacial reactions

This investigation discovers a discrete metallic alloy layer containing Ni, Ti, and Ba between Ni and BaTiO3 co-fired in reducing atmospheres at ∼1300 °C along with an oxygen-depleted zone in BaTiO3 adjacent to the metallic layer. Using high-resolution transmission electron microscopy and electron energy-loss spectroscopy, detailed structural and chemical analyses of the metallic layer are carried out. With complementary thermodynamic approximations, these observations render an assessment of the local effective oxygen partial pressure, which is considerably lower than the external, ambient processing environment. Through a set of model experiments on Ni–BaTiO3–carbon composites, we illustrate that the presence of carbon is sufficient to reduce locally BaTiO3 and form the observed metallic layer at ∼1300 °C. A phase stability analysis indicates that the formation of the Ba and Ni–Ti liquid phases provides a kinetic path for the interfacial reaction.