Fine Zirconia Powders by Hydrothermal Processing
Fine zirconia powders could be prepared by two hydrothermal techniques, (a) Hydrothermal crystallization of amorphous zirconia, and (b) Hydrothermal oxidation of Zr metal, both recently developed in our laboratory.
In the hydrothermal crystallization, ultrafine monoelinic ZrO2 crystals with 16nm of particle size were prepared using KF solutions at 200°C under 100 MPa for 24 h, whereas ultrafine tetragonal ZrO2 (15 nm) and monoelinic ZrO2 (19 nm) crystals were obtained with LiCl solutions at 300°C under 100 MPa for 24 h. The ZrO2 particles were well shaped and isolated from each other. Metastable tetragonal ZrO2 was formed only from amorphous phase.
Hydrothermal oxidation of Zr metal by pure water yielded only monoelinic ZrO2 with 24 nm of average crystallite size above 400°C under 100 MPa for 3h. When Ca solutions were used instead of H2O, fine cubic zirconia grains were partially formed, in addition to monoelinic zirconia particularly in the case of Ca(NO3)2 solutions with excess an amount of Zr metal at high temperatures, 700°C. This cubic zirconia seemed to be stabilized both by calcium and by nitrogen. Y(NO3)3 or NH4NO3 solution also yielded a small amount of cubic zirconia phase in the products.
KeywordsCrystallite Size Average Crystallite Size Monoclinic Zirconia Zirconia Phase Stabilizer Agent
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