Dielectric Behavior and Second Phases in X7R‐Formulated BaTiO 3 Sintered in Low‐Oxygen Partial Pressures

Multi‐layer ceramic capacitor chips prepared from an X7R‐formulated BaTiO 3 powder and nickel base‐metal electrodes were sintered at 1200°C and re‐oxidized at 1000°C in low‐oxygen partial pressures ( p O 2 ). While chips A and B, sintered in p O 2 ≈10 −9 and 10 −11 atm, respectively, exhibited a typical temperature coefficient of capacitance resembling the X7R characteristics, normal dielectric behavior was retained in chip C sintered at the lowest p O 2 of ∼10 −13 atm 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 Ca 2 Y 8 Si 6 O 26 . Tetragonal Ba 2 TiSi 2 O 8 was another crystalline second phase specific to chip C. Eutectic liquids have also formed principally among BaO, SiO 2 , and solid‐state additives of CaO and Y 2 O 3 below or at 1200°C to aid the densification of BaTiO 3 dielectrics. They were solidified upon cooling to a residual glassy second phase in the ceramics. Sintered BaTiO 3 grains 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 BaTiO 3 ceramics sintered at low temperatures.