Structures of nanocrystalline MgO, ZnO and WO3 prepared by gas evaporation and in situ compaction

Nanocrystalline magnesium oxide (n-MgO), zinc oxide (n-ZnO) and tungsten oxide (n-WO3) were prepared by gas evaporation and successive in situ compacting. Nanometer-sized amorphous aluminum oxide particles were also prepared by gas evaporation. The structures of the nanometer-sized particles and the nanocrystalline ceramics, and their sintering behaviors were investigated by high-resolution transmission electron microscopy. It was found that the structural features of the as-compacted n-MgO, n-ZnO and n-WO3 are the same; they have similar porous structures. These nanocrystalline ceramics were prepared by the bonding of nanometer-sized grains at room temperature without neck growth. The sintering behavior in these ceramics is, however, different with respect to densification and neck growth. Substantial densification can be realized in n-ZnO by increasing the annealing temperature up to 800 degrees C. On the other hand, voids remained in n-MgO and n-WO3 after annealing at the same temperature. The process of neck growth in n-MgO and n-ZnO is different from that in n-WO3. No amorphous layer forms during the annealing in n-MgO and n-ZnO, while amorphous necks form at 600 degrees C and then crystallize at 800 degrees C in n-WO3.