Structure and microwave dielectric properties of Ca_1-xY_xTi_1-xAl_xO₃ (CYTA) ceramics

The structure and dielectric properties of Ca_1-xY_x Ti_1-xAl_xO₃ (CYTA) ceramics prepared by the mixed-oxide route have been investigated. CYTA forms a complete solid solution with an orthorhombic perovskite structure. Residual Y₄Al₂O₂ and Y₃Al₅O₁₂ resulting from incomplete reaction are observed for ≥ 0.9. Scanning electron microscopy shows that CYTA ceramics exhibit uniform microstructures, with an average grain size that decreases from ∼200 μm at x = 0 to ∼10 μm at x = 1.0. Transmission electron microscopy of CYTA (x = 0.3) ceramics reveals the presence of ferroelastic domains, and electron-diffraction patterns are indexed on the Pnma space group, consistent with an a⁺b^-b^- octahedral tilted structure. The relative permittivity, ε_r, decreases continuously from 170 to 12, while the microwave quality factor, Q·f_r increases from 10,000 to 12,000 GHz, for x = 0 and 1, respectively. CYTA (x = ceramics exhibit ε_r ∼ 38, Q·f_r of ∼14,212 GHz, and a temperature coefficient of resonance frequency, τ_f, of -14 ppm/°C. Small additions of acceptor (0.3 wt% ZnO) or donor (1 wt% Nb₂O₅) dopants decrease Q·f_r by ∼20-30%, respectively. 0.30).