Microstructural Evolution of Magnesia/Yttria Nanocomposites

Two-phase nanocomposite oxides have emerged as an alternative material used in the fabrication of optical windows for military applications due to their enhanced properties over their coarse-grained and single-phase counterparts. To retain their optical properties, the grains/phase domains must be stable under the operating conditions for the windows. Until now, there have been no systematic studies on the structural stability of these materials due to the difficulty in making thin enough samples from these ultra-hard ceramics. By using a novel foaming sol-gel route, stable homogeneous magnesia/yttria nanocomposites were produced in the form of flakes thin enough for direct observation with transmission electron microscopy (TEM). The evolution of nitrous oxides during the reaction causes the product to foam, and the calcination of this foam gives nanocomposite powders with extremely fine, uniform grains and phase domains. ^ Extensive microstructural characterization was performed to study the evolution of the grains/phase domains in this composite system under a systematic series of post-calcination heat treatments (PCHTs). TEM imaging of PCHT powders revealed that the grains/phase domains are very fine (\u3c 20 nm) below 1000°C and the microstructure reaches its pinned/stagnation point quickly (within 0.5 h). The composites exhibit transitory growth behavior between PCHTs of 1000 to 1100 °C where the microstructure undergoes time-dependent growth-to-stagnation. Even at these PCHT temperatures, the composite still retains its nanoscale phase domain structure. ^ These microstructures are remarkably stable in both its powder and consolidated form. The structural stability of these nanocomposites is attributed to the formation of a metastable amorphous/vitreous intermediate during calcination followed by concurrent crystallization and phase separation on the nanoscale.