Cation substitution for BaTiO₃phase in low-fired glass-ceramics for tunable microwave applications

Cation substitution for the Ba²⁺ and Ti⁴⁺ sites in the BaTiO₃ structure was attempted in low-fired glass-ceramics derived from glass powders that have perovskite phase compositions corresponding to (BaTiO₃ : REAlO₃) = 9:1 and 8:2 (RE = La, Nd and Sm) to control the Curie temperature (7_C) of the ferroelectric glass-ceramics. The La³⁺-substituted (9:1) sample heated at 950°C for 24 h showed a broadened ε_r-temperature relationship with substantial lowering in T_c to ∼-55°C. The Ba:La ratio in the perovskite particles was estimated to be in the range of 4:1-6.7:1, whereas very little A1³⁺ was incorporated into the same particles. It was found that the T_c of the glass-ceramics fabricated in our serial study could be adjusted by A-site cation substitution in BaTiO₃. Tailored glass-ceramic samples with room temperature ε_r of 200-300 and T_c of around 0&C had good microwave tunability of 30-40% under dc bias voltage of 10-50kV/cm. The high tuning sensitivity of the low-fired glassceramics was confirmed by comparative examination of the dielectric properties of well-crystallized ceramics and thin films.