Morphology of Zirconia Nanopowders Crystallised under Hydrothermal Conditions

Zirconia nanopowders stabilised with 6 mol% calcium oxide were crystallised under hydrothermal conditions. The effect of pH on chemical composition, phase composition and morphology of the resultant nanopowders was studied. Behaviour of the nanopowders during uniaxial pressing and pressureless sintering was characterized. It was found that the solution pH higher than 10 is required to retain CaO totally in the nanopowder during hydrothermal crystallisation. Oval nanocrystallites of tetragonal symmetry were obtained as the main phase. An increase of the solution pH increased both the content and aspect ratio of needle-shaped crystallites of monoclinic symmetry being the secondary phase. This improved compressibility of the nanopowders but deteriorated their sinterability and the phase composition of the resultant polycrystals.