(Sr _{1‐ x} Ca _x ) ₂ TiO ₄ microwave dielectric ceramics with R‐P structure ( x = 0~0.15)

Abstract (Sr _{1‐ x} Ca _x ) ₂ TiO ₄ ceramics (0 ≤ x ≤ 0.15) were synthesized by a standard solid state reaction route. Tetragonal Ruddlesden‐Popper (R‐P) solid solutions (Sr _{1‐ x} Ca _x ) ₂ TiO ₄ with space group I 4/mmm were obtained in the composition range of x = 0~0.15, while Sr ₃ Ti ₂ O ₇ has been observed as secondary phase in x ≤ 0.10. In the present ceramics, both the dielectric constant ɛ _r and temperature coefficient of resonant frequency τ _f decreased at first and turned to increase again with increasing x , while the quality factor Qf decreased slightly at first and turned to decrease sharply after x = 0.1. The improved microwave dielectric properties with the best combination were obtained in (Sr _{1‐ x} Ca _x ) ₂ TiO ₄ ceramics at x = 0.075: ɛ _r = 39.3, Qf = 93 550 GHz, τ _f = 119 ppm/°C The obvious improvement was achieved in temperature coefficient of resonant frequency (from 140 to 119 ppm/°C) with Ca ²⁺ ‐substitution for Sr ²⁺ in Sr ₂ TiO ₄ ceramics due to the increased B‐site bond valance.