In this study, $ZrO_2$ powders were prepared by hydrothermally crystallizing two $ZrO_2$ gel powders which had been synthesized by precipitaion reactions. One with nano-scaled particle size had been synthesized by neutral precipitaion reaction and the other with micron-scaled particle size and spherical shape had been synthesized by thermal hydrolysis reaction. These two kinds of $ZrO_2$ gel powders were dispersed in pure water and 4.0M of KOH solution and then hydrothermally crystallized at 220℃ under the saturated water vapor pressure.
On the hydrothermal treatment in pure water, the nano-scaled gel powder was crystallized to be nano-scaled and equiaxed, and the micron-scaled, spherical gel powder nonaggregated and spherical. The powder obtained by hydrothermally crystallizing the micron-scaled, spherical gel powder had 0.4㎛ of particle size and was composed of nano-scaled, primary particles. On the other hand, the powders obtained by hydrothermally crystallizing two gel powders in 4.0M of KOH solution did not sustain its original shape and became micron-scaled elongated monoclinic partieles, because the solubility of the gel in the KOH solution is so much higher than in pure water that the disssolution-precipitation process occures much faster in the KOH solution.
$Y_2O_3$ and CaO-doped $ZrO_2$ powders were also prepared by hydrothermally crystallizing the micron-scaled, spherical gel powder with the addition of $YCl_3-8H_2O$ and $Ca(OH)_2$ in the course of its redispersion in pure water, respectively. Their mean particle size was 0.4㎛ and they were composed of nano-scaled primary particles.
The powder obtained by hydrothermally crystallizing the micron-scaled, spherical gel powder in pure water was uniaxially pressed to compacts. The relative density of compact were about 53%. On the uniaxial pressing, the micron-scaled, spherical secondary particle was crushed to be nano-scaled primary or nano-scaled secondary particles. Therefore, it was known that the agglomeration between primary particles is very weak.
