Dielectric Behavior and Second Phases in X7R‐Formulated BaTiO3Sintered in Low‐Oxygen Partial Pressures

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Multi‐layer ceramic capacitor chips prepared from an X7R‐formulated BaTiO3powder and nickel base‐metal electrodes were sintered at 1200°C and re‐oxidized at 1000°C in low‐oxygen partial pressures (pO2). While chips A and B, sintered inpO2≈10−9and 10−11atm, respectively, exhibited a typical temperature coefficient of capacitance resembling the X7R characteristics, normal dielectric behavior was retained in chip C sintered at the lowestpO2of ∼10−13atm with the Curie point resurged at ∼120°C. The chips were analyzed using X‐ray diffractometry, transmission electron microscopy, and energy‐dispersive X‐ray spectroscopy. The dielectric layer contains a siliceous residual glassy second phase in grain boundaries, triple‐grain junctions, and quadruple‐grain corners and crystalline second phases in locations scattered inhomogeneously. A crystalline second phase, common to all chips, was determined to hexagonal silicate oxyapatite Ca2Y8Si6O26. Tetragonal Ba2TiSi2O8was another crystalline second phase specific to chip C. Eutectic liquids have also formed principally among BaO, SiO2, and solid‐state additives of CaO and Y2O3below or at 1200°C to aid the densification of BaTiO3dielectrics. They were solidified upon cooling to a residual glassy second phase in the ceramics. Sintered BaTiO3grains of 250–400 nm in both chips A and B contained the characteristic X7R core–shell structure. Those in chip C have grew significantly to 5–8 μm but lost the core–shell completely. With almost all additives in chip C reacting to form second phases, the microstructure is represented by the {111} single and double twins resembling that of undoped BaTiO3ceramics sintered at low temperatures.