Improved microwave dielectric properties of MgAl ₂O ₄ spinel ceramics through (Li _1/3Ti _2/3) ³⁺ doping

A series of nominal compositions MgAl _{2− x} (Li _1/3Ti _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 _1/3Ti _2/3) _x O ₄ ceramics with a spinel structure was obtained at x ≤ 0.12, whereas the second phase of MgTi ₂O 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 _1/3Ti _2/3) ³⁺ for Al ³⁺. Results showed that (Li _1/3Ti _2/3) ³⁺ doping into MgAl ₂O ₄ spinel ceramics effectively reduced the sintering temperature and improved the quality factor ( Q _f) values. Good microwave dielectric properties were achieved for a sample at x = 0.20 sintering at 1500 °C in air for 4 h: dielectric constant ε _r = 8.78, temperature coefficient of resonant frequency τ _f = −85 ppm/°C, and Q _f = 62 300 GHz. The Q _f value of the x = 0.20 sample was about 2 times higher than that of pure MgAl ₂O ₄ ceramics (31600 GHz). Thus, MgAl _{2− x} (Li _1/3Ti _2/3) _x O ₄ ceramics with excellent microwave dielectric properties can be applied to 5G communications.