Enhancing the microwave performance of LiAlO2 ceramics with low dielectric constant via Mg substitution

LiAl 1 −  x Mg x O 2− x /2 ( x  = 0, 0.01, 0.02, 0.03, 0.04, 0.05) ceramic were prepared by conventional solid-phase reaction method. After X-ray diffraction analysis, a single-phase solid solution was formed in this system throughout the doping range. Through the oxygen vacancies formation by the replacement of Al 3+ with Mg 2+ , the relative density was increased from 94.33% to 96.1%. The dielectric constant ( ε r ) showed a trend of first enhancing and then decreasing with doping concentration. Their relationship mainly depended on the synergistic effect of relative density and dielectric polarizability. The improvement in the quality factor ( Q×f ) from 15,647 GHz to 34,507 GHz was attributed to the increased doping ratio and the strengthened covalent bonds. The change in the temperature coefficient of resonance frequency ( τ f ) was explained from the perspective of oxygen bonding valence. Excellent microwave properties were ε r  = 6.59, Q×f  = 34,507 GHz, and τ f = -115.8 ppm/°C for LiAl 0.97 Mg 0.03 O 1.985 ceramic at 1325 °C for 5 h.