Phase composition and microwave dielectric properties of NaSrB5+5O9+7.5 composite ceramics

Microwave dielectric ceramics based on NaSrB5+5xO9+7.5x (0 ≤ x ≤ 0.11) were synthesized following a conventional solid-state reaction route at temperatures below 850 ℃. The crystal structure, phase composition, chemical bond characteristics, and microwave dielectric performance of NaSrB5+5xO9+7.5x (0 ≤ x ≤ 0.11) ceramics were systematically investigated. The X-ray diffraction technique was used for sample analysis, and the results revealed that the NaSrB5+5xO9+7.5x ceramics consisted of NaSrB5O9 and SrB2O4 as the phases. The content of the non-stoichiometric B element in the NaSrB5+5xO9+7.5x ceramics exerted a significant effect on the SrB2O4 second phase content. The mass percentage of SrB2O4 decreased from 18.62% to 6.98% when the samples were sintered at 825 ℃ with x = 0.07. The εr value was primarily affected by the relative density. The Q×f value was significantly influenced by the packing fraction, the total lattice energy of the NaSrB5+5xO9+7.5x ceramics, and the FWHM values of the most intense peak at approximately 738 cm-1 in the Raman spectral profile. The optimal microwave dielectric properties were achieved for x = 0.07 sintered at 825 ℃, εr = 5.68 ± 0.03, Q×f=57,173±1100 GHz, τf=−14.83 ± 1.45 ppm/°C).