The purpose of this study is to investigate the synthesis of monodisperse, spherical $ZrTiO_4$ particles by the thermal hydrolysis of metal salts in alcohol-water mixed solvent. In conventional thermal hydrolysis process, water have been used as solvent of metal salts. This process required high reaction temperature of 90℃ and long period of reaction time of several weeks to obtain ceramic particles. In the present work, the mixed solution of 1-propanol (1-PrOH) and water was used as solvent to obtain $TiO_2$ and $ZrTiO_4$ particles by the thermal hydrolysis and the effects of 1-PrOH on paricle formation and growth in the mixed solution were studied. The chemical composition and the thermal behavior of $ZrTiO_4$ particles were also invesitigated.
In chapter 4-1, titania particles were synthesized from the mixed solution of $TiCl_4$ by adjusting the volume ratio of 1-PrOH to water and the particle formation and the growth mechanisms were discussed. Precipitation temperature of titania particles was decreased with the increase of the volume ratio of mixed solvent. At the higher volume ratio than 3, the precipitation temperature increased as the volume ratio increased. The particle morphology also depended on the volume ratio of the mixed solvent. As the volume ratio increased, the particle size increased and the particle morphology became uniform and spherical. At the volume ratio of 3.0, monodisperse spherical particles were obtaineds.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations showed that the secondary spherical particles were composed of nanometer-scaled primary particles. The result of FTIR analysis showed that the particles in mixed solvent interacted with 1-PrOH, resulting in the decrease of the particle zeta potential. As the volume ratio increased, the zeta potential of particles and the dielectric constant of the mixed solvent decreased. Considering colloidal stability of the particles interacting van der waals potential and electrostatic repulsive potential, the energy barrier, inhibiting the agglomeration of particles, depended on the dielectric constant and particle zeta potential under the constant ionic strength. In this work, external electrolytes were not added except the HCl added to avoid the formation of $Ti(OH)_4$ in dissolving $TiCl_4$ into water. Under these conditions, the potential energy barrier depends on the dielectric constant and zeta potential. Therefore, the large size of the particles obtained with the high volume ratio of 3 was due to the low energy barrier which was caused by the lower dielectric constant of solvent and the lower zeta potential of particles.
Hydroxypropyl cellulose (HPC) was used as a steric stabilizer to control the particle size. As the concentration of HPC increased, the average particle size and the size distribution decreased. At the critical amount of HPC, however, the particle size distribution increased with HPC concentration increasing. SEM analysis of the particles showed that HPC provided the heterogeneous nucleation site above the critical concentration.
The spherical particles were also synthesized from $Ti(SO_4)_2$ in mixed solvent of 1-PrOH and water. The particles obtained at the volume ratio of 0 were highly aggregated. As the volume ratio was increased, the particles became spherical. At the volume ratio of 1.0, spherical $TiO_2$ particles with narrow size distribution were obtained. Further increase of the volume ratio produced the aggregated particles with irregular shape. The energy barriers calculated by DLVO theory increased with the the volume ratio. At the volume ratio of 1.0, the highest value were obtained. The particle size was also controlled by the HPC concentration.
In chapter 4-2, $ZrTiO_4$ particles were synthesized from the mixed salts of $TiCl_4$ and $ZrOCl_2ㆍ8H_2O$ in the mixed solvent by the thermal hydrolysis. EDS, WDS, and ICP elemental analyses showed that the composition of the precipitated particles could be controlled by the composition of Zr and Ti in starting materials and the volume ratio of the mixed solvent. At higher volume ratio above 3, the composition of each particle only depended on the composition of Zr and Ti ratio in starting solution. In the low volume ratio of mixed solvent, however, the composition of particle depended on the solubility of Zr and Ti in the mixed solvent. These results showed that the solubility of metal salts, depending on the pH of aqueous solution, was determined by the dielectric constant of the mixed solvent having high volume ratio of 1-PrOH.
The thermal behavior of the $ZrTiO_4$ particles was also investigated. The precipitated powders obtained by the thermal hydrolysis were compacted into disc shape and sintered at various temperature for 3 h in air. When the green bodies were sintered at 1500℃ for 3 h, the relative density of about 90% was obtained. Comparing the particles obtained by the conventional solid state reaction, the particles obtained by the thermal hydrolysis provided the higher sintering density at the same sintering temperature. It is believed that the high sinterability of the powders is due to the controlled spherical morphology and the atomic scale mixing of Zr and Ti element in the $ZrTiO_4$ particles.
