Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO, Al2O3 and MnO2

The anti-reduction of Ti4+ ions in Ba4.2Sm9.2Ti18O54 (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al2O3 and MnO2 were added separately to suppress the reduction of Ti4+ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al2O3, when acting as an acceptor, could effectively suppress the reduction of Ti4+ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO2 as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba4.2Sm9.2Ti18O54 ceramics doped with 0.2 wt.% Al2O3 sintered at 1340 °C for 3 h: ɛr = 76.9, Q × f = 10,120 GHz and τf = −22.7 ppm/°C.