On the intrinsic sinterability of MgAl2O4 nanopowders

Abstract

Magnesium aluminum spinel MgAl₂O₄ has found applications in transparent ceramics. It is known for poor sinterability, often attributed to slow mass transport kinetics in the literature. The situation can be improved with nanopowders, yet pressureless sintering to 95%–96% (desirable for the final-step hot isostatic pressing, HIP) still requires a homologous temperature T/T_m of about 0.75. Technological solutions to lower the sintering temperature and refine grain size are urgently needed. Here, we proposed that the poor sinterability is partially due to agglomerations of the nanopowders and sought to uncover the intrinsic sinterability. Using commercially available MgAl₂O₄ nanopowders, we demonstrated that the sinterability can be dramatically enhanced with proper deagglomeration treatment and colloidal centrifugal casting. Compared to the control group formed by dry pressing and cold isostatic press, the colloidal process can lower the sintering temperature for full densification by ∼200°C. > 99% relative density can now be achieved at 1350°C for 2 h, that is, at T/T_m = 0.67. > 97% relative density can now be achieved at 1275°C for 2 h, that is, at T/T_m = 0.64. Our work shows that commercial nano-sized MgAl₂O₄ powders with proper treatments and forming techniques can be readily sintered at low temperatures to reach closed porosity while maintaining fine grain size below 200 nm. The results reported here are encouraging for the development of ultrafine-grained MgAl₂O₄ transparent ceramics with lowered HIP temperature.