Correlation of Phase Structure, Defect Relaxation, and Microwave Dielectric Properties in Low-Loss Mg n+1Ti n O3n+1 Ceramic Systems

Low-loss microwave dielectrics are of significant importance for the miniaturization and integration of microwave devices. In this paper, the ceramics of nominal composition Mg n+1Ti n O3n+1 (n = 3-6) are synthesized, and the correlations among their phase compositions, defect behaviors, and microwave dielectric properties are systematically investigated. The analyses indicate that the Mg n+1Ti n O3n+1 ceramics are a biphasic system consisting of hexagonal ilmenite-structured MgTiO3 and cubic spinel-structured Mg2TiO4. The Rietveld refinement results demonstrate that as the n value increases, the content of the MgTiO3 phase gradually increases, while that of the Mg2TiO4 phase decreases. The results of the thermally stimulated depolarization current (TSDC) indicate that the oxygen vacancy-related defects are considered to be the main types of defects in Mg n+1Ti n O3n+1. As the value of n increases, the increase of the MgTiO3 phases with a more stable crystal structure leads to a gradual decrease in the concentration of oxygen vacancies. The Mg n+1Ti n O3n+1 ceramics exhibit excellent microwave dielectric properties at n = 6: epsilon r = 17.55, Q x f = 284,600 GHz, and tau f = -46.39 ppm/degrees C. In addition, the relationship between the dielectric constant and the electric field distribution is solved utilizing high-frequency structure simulator (HFSS) software, and the Q x f values are also predicted. The low-frequency dielectric temperature spectra and direct current pulse charge/discharge tests show that the low-loss Mg n+1Ti n O3n+1 ceramics can be good candidates for microwave capacitors even when operated under high-temperature conditions.