Enhanced microwave dielectric properties in LiAlO2 ceramics by Li+ substitution for Al3,+

Low permittivity LiAl1-xLixO2-x (x = 0–0.02) ceramics were prepared by conventional solid-state reaction method. XRD analysis indicated that the system remained single phase solid solution in the whole range of x examined. Li + heterovalent substitution for Al3+ generated oxygen vacancy Ovac, and enhanced the relative density from 94.33% to 97.13%. The dielectric constant εr showed near linear variations versus sintering temperature and x value. Their relations mainly depended on the combined effects of relative density and dielectric polarizability. The quality factor Q×f was more than doubled from 15,647 GHz to 31,627 GHz, due to the packing fraction enlargement and covalent bond strengthening. The variation of temperature coefficient of resonance frequency τf was explained from the view of oxygen bond valence. When x = 0.01, the ceramic exhibited the best microwave dielectric properties with εr = 5.37, Q×f = 31,627 GHz and τf = −104.2 ppm/°C when sintered at 1300 °C for 5 h.