Preparation of BaTiO₃-BaFe₁₂O₁₉ Core-shell structure particles by homogeneous coprecipitation

BaTiO₃-BaFe₁₂O₁₉ core-shell structure particles were obtained using the homogeneous coprecipitation method. The effects of temperature, molar ratio of urea to metal ions (R), and BaTiO₃ concentration on the morphology and structure of the core-shell particles were investigated. The mechanism of formation for the BaTiO₃-BaFe₁₂O₁₉ core-shell structure particles and their magnetic property were also discussed. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the morphology and structure of the BaTiO₃-BaFe₁₂O₁₉ precursor core-shell structure particles and the BaTiO₃-BaFe₁₂O₁₉ core-shell structure particles. A vibrating sample magnetometer (VSM) was used to characterize the magnetic property. The results indicated that the obtained sample possessed an integrated and smooth shell of about 10 nm in size and the metal ions precipitated completely after homogeneous coprecipitation when the sample was synthesized under the following conditions: 100 °C, R=180, and a BaTiO₃ concentration of 2.5 g·L^-1. A large amount of free particle impurities appeared if the temperature and R value were too high. The shell thickness of the BaTiO₃-BaFe₁₂O₁₉ precursor core-shell structure particles tended to decrease as the BaTiO₃ concentration increased. The BaFe₁₂O₁₉ phase in the shell began to form when the calcination temperature reached 900 °C. The mechanism of formation included the formation of BaFe₂O₄ by the reaction of crystalline α-Fe₂O₃ and BaCO₃ initially, and this was followed by the reaction of BaFe₂O₄ and α-Fe₂O₃ to form the final BaFe₁₂O₁₉. As the temperature increased to 1000 °C, a complete BaFe₁₂O₁₉ shell was obtained. The saturation magnetization and coercivity of the BaTiO₃-BaFe₁₂O₁₉ core-shell structure particles increased and decreased from 16.5 to 39.5 A·m²·kg^-1 and from 340 to 316 kA·m^-1, respectively, as the calcination temperature increased from 900 to 1000 °C.