关键词: microwave dielectric ceramics, MgAl₂O₄ ceramic co-substitution, MgTi₂O₅, solid solubility limit

Abstract: A series of nominal compositions MgAl$_{2-x}$(Li$_{1/3}$Ti$_{2/3}$)$_{x}$O$_{4}$ ($x = 0$, 0.04, 0.08, 0.12, 0.16, and 0.20) ceramics were successfully prepared via the conventional solid-state reaction route. The phase compositions, microstructures, and microwave dielectric properties were investigated. The results of x-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that a single phase of MgAl$_{2-x}$(Li$_{1/3}$Ti$_{2/3}$)$_{x}$O$_{4}$ ceramics with a spinel structure was obtained at $x \le 0.12$, whereas the second phase of MgTi$_{2}$O$_{5}$ appeared when $x > 0.12$. The cell parameters were obtained by XRD refinement. As the $x$ values increased, the unit cell volume kept expanding. This phenomenon could be attributed to the partial substitution of (Li$_{1/3}$Ti$_{2/3}$)$^{3+}$ for Al$^{3+}$. Results showed that (Li$_{1/3}$Ti$_{2/3}$)$^{3+}$ doping into MgAl$_{2}$O$_{4}$ spinel ceramics effectively reduced the sintering temperature and improved the quality factor ($Q_{\rm f}$) values. Good microwave dielectric properties were achieved for a sample at $x = 0.20$ sintering at 1500 ${^\circ}$C in air for 4 h: dielectric constant $\varepsilon_{\rm r} =8.78$, temperature coefficient of resonant frequency $\tau_{\rm f} = -85 $ ppm/${^\circ}$C, and ${Q_{\rm f}} = 62 300 $ GHz. The $Q_{\rm f}$ value of the $x = 0.20$ sample was about 2 times higher than that of pure MgAl$_{2}$O$_{4}$ ceramics (31600 GHz). Thus, MgAl$_{2-x}$(Li$_{1/3}$Ti$_{2/3}$)$_{x}$O$_{4}$ ceramics with excellent microwave dielectric properties can be applied to 5G communications.

Key words: microwave dielectric ceramics, MgAl₂O₄ ceramic co-substitution, MgTi₂O₅, solid solubility limit