Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl2O4) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl2O4 spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl2O4 spinel ceramics have been successfully fabricated by co-doping CeO2 and La2O3 via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO2 and La2O3 on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl2O4 spinel ceramics were systematically investigated. The results show that by co-doping CeO2 and La2O3 can increase the defect concentration due to the lattice distortion. This could promote the movement of Al3+ and Mg2+ at high temperature, which is beneficial to the formation of more secondary MgAl2O4 spinel. t-ZrO2 with more Ce4+ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO3 that was mainly distributed along the grain boundary of the MgAl2O4 phase, both of which were favorable for the formation of dense microstructure of MgAl2O4 spinel materials. At the same time, the formation of more secondary MgAl2O4 spinel and sintering densification also improve the mechanical properties of spinel ceramics. La3+ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO2 and 3 wt% La2O3 co-doping, the bulk density of the sample increased from 3.02 g∙cm−3 to 3.55 g∙cm−3; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.