Synergistically Engineered Co1.1Zn0.9−xCaxTiO4 LTCC Ceramics via H3BO3-Assisted Sintering: Toward Enhanced Microwave Dielectric Performance

Ca 2+ -doped Co 1.1 ZnTiO 4 ceramics (0 ≤  x  ≤ 0.08) were successfully synthesized via the conventional solid-state reaction route to investigate the doping effects of Ca 2+ on their microwave dielectric properties. X-ray diffraction (XRD) results revealed a distinct crystal structure evolution: a single-phase spinel structure with the Fd-3m space group was maintained at x  ≤ 0.04, while the CaTiO 3 secondary phase precipitated at x  ≥ 0.06 due to the finite solid solubility of Ca 2+ in the matrix lattice. The optimal comprehensive dielectric performance was achieved at x  = 0.04 sintered at 1175°C, achieving Q  ×  f  = 73,300 GHz, ε r  = 21.46, and τ f  =  −17 ppm/°C. Notably, H 3 BO 3 addition enabled a 275°C reduction in sintering temperature (down to 900°C) while retaining phase purity, which was fully confirmed by XRD analysis. Scanning electron microscopy (SEM) results verified improved bulk densification (relative density > 93%) and uniform microstructures with a homogeneous grain size of approximately 2.5 μm. Raman spectroscopy further indicated obvious [TiO 6 ] octahedral distortion, and a redshift of the A 1g mode from 720 to 715 cm −1 directly demonstrated lattice expansion after cation doping. The low-temperature-sintered samples at 900°C still exhibited competitive microwave dielectric properties ( Q  ×  f  = 30,900 GHz, ε r  = 20.21, τ f  =  −18 ppm/°C), which strongly confirms that H 3 BO 3 serves as an efficient low-temperature sintering aid for the as-prepared titanate ceramics.