Structure, Raman spectra and properties of two low-εr microwave dielectric ceramics Ca3B2Ge3O12 (B = Al, Ga)

Ca3B2Ge3O12 (B = Al, Ga) cubic garnet ceramics with space group Ia-3d were synthesized by solid-state reaction. Ca3Al2Ge3O12 ceramic sintered at 1340 degrees C showed the optimal microwave dielectric performances with epsilon(r) = 8.97, Q x f = 114,871 GHz and tau(f) = -54.6 ppm/degrees C. The trace amount of secondary phase Ca2Ga2GeO7 was observed by XRD in Ca3Ga2Ge3O12 ceramic. The pure phase Ca3Ga2Ge3O12 could be obtained at 1320 degrees C by adding an excess of 3.3 mol% GeO2, which exhibited epsilon(r)similar to 9.79, Q x f similar to 129,356 GHz and tau(f)similar to-50.19 ppm/degrees C. The differences in microwave dielectric properties of Ca3B2Ge3O12 (B = Al, Ga) ceramics were discussed by Raman spectra, ionic polarizability, packing fraction and bond valence theories. Additionally, CaTiO3 was used to adjust the largely negative if of Ca3B2Ge3O12 (B = Al, Ga) ceramics to near zero, and optimized performances of epsilon(r) = 10.18, Q x f = 59,350 GHz and tau(f) = + 2.53 ppm/degrees C were obtained for the composite 0.88Ca(3)Al(2)Ge(3)O(12)-0.12CaTiO(3) ceramics at 1360 degrees C for 8 h, with epsilon(r) = 11.23, Q x f = 64,542 GHz and tau(f) = -3.26 ppm/degrees C for 0.9Ca(3)Ga(2)Ge(3)O(12)-0.1CaTiO(3) ceramics at 1320 degrees C.