Wang Zheng,Ma Juncheng,Xiong Zhe,Yang Hongyu,Zhang Xing,Tang Bin
In this paper, the strategy of substituting Ti 4+ for Nb 5+ in Mg 4 Nb 2 O 9 microwave dielectric ceramic was employed to improve the microwave dielectric properties. Specifically, a maximum Q × f value of 137,462 GHz was obtained at x = 0.15, resulting in a nearly 30% increase in the quality factor ( Q × f ) for the Mg 4 Nb 2− x Ti x O 9− x /2 sample. Moreover, we investigated the phase composition, micromorphology, and the relationship between structural characterization and dielectric properties. The X-ray diffraction (XRD) patterns indicated that only a single Mg 4 Nb 2 O 9 phase with a trigonal corundum-type structure was formed in all the samples. The ɛ r was related to the ionic polarizability, and the τ f value was dependent on the B–O bond valence ( V B–O ) and BO 6 octahedral distortions. Ultimately, the Mg 4 Nb 1.85 Ti 0.15 O 8.925 ceramic sample exhibited microwave dielectric properties with ɛ r = 13.89, Q × f = 137,462 GHz, and τ f = − 55.1 ppm/°C.
Substitution of Ga 3+ by [Zn 0.5 Ti 0.5 ] 3+ in MgGa 2 O 4 improved the comprehensive properties of the resultant spinel. A continuous solid solution with a spinel structure was formed in Mg(Zn 0.5 Ti 0.5 ) 2 x Ga (2−2 x ) O 4 (0 ≤ x ≤ 1); here, Zn 2+ occupied the tetrahedral 8 a site while Ti 4+ occupied the octahedral 16 d site, resulting in a decrease in the degree of inversion λ from 0.86 to 0. The optimal sintering temperature decreased from 1460 °C to 1220 °C, and the dielectric constant ( ε r ) increased from 8.8 to 14.9; these changes could be ascribed to the greater polarizability of [Zn 0.5 Ti 0.5 ] 3+ relative to that of Ga 3+ . The temperature coefficient of resonant frequency ( τ f ) of the material remarkably increased from −64.1 ppm/°C to −16.8 ppm/°C, which may be attributed to the increase in dielectric polarizability/molecular volume, bond energy, and bond valence in B-sites. The high-quality factor ( Q × f ) slightly decreased from 89,253 GHz to 73,432 GHz with decreasing λ and packing fraction. Optimal microwave dielectric properties were obtained from the sample with x = 1 and sample sintered at 1220 °C; in this case, ε r = 14.9, Q × f = 73,432 GHz, and τ f = −16.8 ppm/°C.
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