Optimized phase compositions and microwave dielectric properties of low loss Ca2Sn2Al2O9-based ceramics by M4+ substitution

The traditional solid-state reaction method was used to prepare Ca2Sn2-xMxAl2O9 (M = Ti, Zr, and Hf) ceramics. Then, the impact of an M4+ substitution of Sn4+ on the phase transition, crystal structural parameter, and microwave dielectric properties of Ca2Sn2-xMxAl2O9 (0 <= x <= 0.4) ceramics were investigated. Ti4+ could not replace the Sn4+ of Ca2Sn2Al2O9 due to its small ionic radius, and the Al-based second phases of Ca2Sn2-xTixAl2O9 ceramics were confirmed by the X-ray diffractometer and EDS map scanning results. With the Zr4+ and Hf4+ substitutions of Sn4+, the SnO2 and CaSnO3 second phases of Ca2Sn2Al2O9 ceramic were inhibited, and the Ca2Sn2-xMxAl2O9 (M = Zr and Hf) (0.05 <= x <= 0.2) single-phase ceramics with orthorhombic structure (Pbcn space group) were obtained. New MO2 (M = Zr and Hf) and CaAl2O4 second phases appeared in the Ca2Sn2-xMxAl2O9 (M = Zr and Hf) (0.3 <= x <= 0.4) ceramics, and their contents increased gradually with the increase in x. The Ca2Sn2-xMxAl2O9 (M = Zr and Hf) (0.05 <= x <= 0.2) ceramics exhibited high Q x f because of their pure phase compositions, and the Q x f of Ca2Sn2Al2O9 ceramic was improved to 77 800 GHz (12.6 GHz) in the Ca2Sn1.9Zr0.1Al2O9 ceramic. The Q x f values of Ca2Sn2-xMxAl2O9 single-phase ceramics were mainly controlled by r(c) (Sn/M-O) and r(c) (Al-O). The tau(f) values of single-phase Ca2Sn2-xMxAl2O9 ceramics were related to octahedral distortions. The Zr4+ and Hf4+ substitution of Sn4+ optimized the phase compositions and microwave dielectric properties of the Ca2Sn2-xMxAl2O9 ceramics, and the Ca2Sn1.9Zr0.1Al2O9 ceramic sintered at optimal temperature exhibited excellent microwave dielectric properties (epsilon(r) = 8.67, Q x f = 77 800 GHz at 12.6 GHz and tau(f) = -69.8 ppm/degrees C).