Structure and microwave dielectric properties of novel walstromite-type MCa2Si3O9 (M = Ba, Sr) ceramics

Novel walstromite-type MCa2Si3O9 (M = Ba, Sr) ceramics, with triclinic space group P-1, were prepared through a solid-state reaction method. The P–V-L theory proves that the lattice energy and bond energy of the Si–O bond play a leading role in the quality factor and the dielectric constant is mainly determined by the ionic polarization. Excellent microwave dielectric properties of BaCa2Si3O9 and SrCa2Si3O9 ceramics could be obtained: εr = 8.99 ± 0.23, Q × f = 44,542 ± 500 GHz, and τf = −25.9 ± 3.0 ppm/°C and εr = 7.39 ± 0.23, Q × f = 48,772 ± 500 GHz, and τf = −27.5 ± 3.0 ppm/°C, when sintered at 1240/1280 °C for 4 h. Then SrCa2Si3O9 ceramic is applied to a new microstrip bandpass filter, because of its high microwave dielectric properties and low thermal expansion coefficient. With reduced dimension, the filtering performance of the circuit is also highly improved, including reduced capacitor parasitic effect and the optimized stopband insertion loss. Accordingly, the SrCa2Si3O9 ceramic is a promising candidate for sub-6 GHz a filter of microstrip bandpass applications.