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

Low permittivity LiAl 1- x Li x O 2- 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 Al 3+ generated oxygen vacancy O vac , and enhanced the relative density from 94.33% to 97.13%. The dielectric constant ε r 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 τ f was explained from the view of oxygen bond valence. When x  = 0.01, the ceramic exhibited the best microwave dielectric properties with ε r  = 5.37, Q×f  = 31,627 GHz and τ f  = −104.2 ppm/°C when sintered at 1300 °C for 5 h.