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Ceramic composites [ x Ni 0.15 Cu 0.30 Zn 0.55 Fe 2 O 4 –(1- x )BaTiO 3 ] were successfully prepared by a direct solid-state reaction of raw materials (BaCO 3 , CuO, α-Fe 2 O 3 , NiO, TiO 2 , and ZnO). The composites are so homogeneous that the ferrite and BaTiO 3 grains do not react with each other. The x -dependent permeability and permittivity are found to obey Maxwell–Garnett effective medium theory, which suggests that the composites consist of ferrite particles with barium titanate medium. This model, however, starts to deviate from the experimental data at x = 0.75, and the roles of medium and inclusions seem to be exchanged. It can be qualitatively explained by the fact that geometrical close-packing of spheres is limited up to about 74 vol % (Kepler conjecture).

Ceramic composites (2Ni 0.41 Zn 0.41 Cu 0.18 Fe 2 O 4 –BaTiO 3 ) were successfully prepared by a direct solid-state reaction of raw materials (BaCO 3 , CuO, α-Fe 2 O 3 , NiO, TiO 2 , and ZnO). X-ray diffraction (XRD) and electron probe micro analysis (EPMA) measurements were performed for these samples and it is confirmed that the composites consist of spinel ferrite and BaTiO 3 phases. The composites are so homogeneous that the ferrite and BaTiO 3 grains do not react with each other and have radii in the range of 1–5 µm. Hexagonal BaTiO 3 ( h -BaTiO 3 ) can be made in this composite form with a sintering temperature of 1200 °C, although h -BaTiO 3 can be usually synthesized above 1460 °C. The freezing-point depression of BaTiO 3 takes place due to the mixing with the spinel ferrite, which may result in the formation of h -BaTiO 3 at a low temperature of 1200 °C.