The monodisperse and spherical $ZrO_2$ gel powders were synthesized by thermal hydrolysis and condensation reactions through microwave heating. The size of the gel powders was about 0.23±0.4㎛. The $ZrO_2$ powders crystallized by calcination and hydrothermal treatment were compared in terms of their properties - phase composition, primary particle size, morphology, and agglomeration degree of primary particles. The mechanism of hydrothermal crystallization was confirmed by using various solvents with different solubility of $ZrO_2$ gel powders in hydrothermal process.
In the first part of thesis, the crystallization behavior of $ZrO_2$ gel powders was studied. The $ZrO_2$ powder crystallized by calcination was composed of primary particles with 36±8.2nm size and kept the spherical shape of $ZrO_2$ gel powders. Even though the phase stabilizer was doped in powders, the monoclinic phase fraction was over 20% in the clacination condition of 900℃ for 1hour. When the powders were sintered at the 1400℃ for 2hours, the monoclinic phase was transformed into the tetragonal phase. These phenomena were caused by non-uniform distribution of phase stabilizer. The $ZrO_2$ powders crystallized by hydrothermal treatment was composed of primary particles with 19±3nm size and kept the spherical shape of $ZrO_2$ gel powders. In case hydrothermal crystallization, all the powders with phase stabilizer showed the tetragonal phase only.
Because hydrothermal crystallization rate in KOH 2M solutions was faster than Mixed solvent composed of 90vol.% EtOH and 10vol.% distilled water, the mechanism of hydrothermal crystallization was more of the dissolution-reprecipitation than the In-situ dehydration. KOH 2M solution could promote the driving force of dissolution-reprecipitation mechanism, while mixed solvent could promote the driving force of In-situ dehydration.
In the second part of thesis, the compaction and sintering behavior of $ZrO_2$ powders were studied. The $ZrO_2$ powders crystallized by calcination kept the spherical shape under 150MPa pressure and the $ZrO_2$ powders crystallized by hydrothermal treatment was collapsed to fine particle over 30MPa pressure. From these observations, the latter was considered to be weakly agglomerated enough to apply conventional forming processing.
The hydrothermally crystallized powder sintered at 1250℃ for 2 hours showed same high density, 98% as the commercial powders which was sintered at 1350℃ for 2 hours. In those sintering condition, the sintered specimen started from hydrothermally synthesized powders had the grain size of 147±60nm, and the sintered specimen started from commercial powders had the grain size of 174±52nm.
