Effects of heating rate on microstructures and microwave dielectric properties of (1 − x)ZnAl2O4–xTiO2 (x = 0.21) ceramics

(1 − x)ZnAl2O4–xTiO2 (x = 0.21) ceramics were synthesized at 1500 °C for 3 h using the solid-state reaction at a heating rate from 1 to 7 °C/min. The effects of heating rate on the microstructure, phase composition and oxidation state of titanium in the ceramics were investigated. The XRD results show that this system is composed of two phases, i.e. ZnAl2O4 spinel and rutile. The “black core” phenomenon resulting from reduction of Ti4+ ion valence appears after the ceramics are sintered at the speed of 1 and 3 °C/min. As the heating rate increases, the density and quality factor (Q·f) increase initially and reach the maximum value when the heating rate is 5 °C/min, and then reduce quickly to the minimum, while the dielectric constant (ɛr) and temperature coefficient of resonator frequency (τf) nearly do not change. The optimal microwave dielectric properties can be achieved in (1 − x)ZnAl2O4–xTiO2 (x = 0.21) ceramics sintered at a heating rate of 5 °C/min with an ɛr value of 11.6, a Q·f value of 74,000 GHz (at about 6.5 GHz), and a τf value of −0.4 ppm/°C.