A new method was developed which produced the monodisperse and spherical zirconia precursor particles through heating of the zirconyl oxychloride solution with the solvent of alcohol-water mixture. The precipitation took place when the dielectric constant of solvent was decreased below a certain value of about 25 by heating of the starting solution. It was thought that the precipitation occurred because of the decrease of solubility which was caused by the decrease of the dielectric constant of the solvent.
The large secondary particles were composed of smaller primary particles and the specific surface area of the particles was as large as about 63.27 ㎡/g. These meant that the secondary particles were porous and the aggregation was a major growth pathway in this work.
The kind of alcohol and the volume ratio of alcohol to water (R/H ratio) in the solvent of alcohol-water mixture affected the morphologies of resulting particles. The differences in morphologies could be understood by considering parameters which affect the colloidal stability. In this case, the magnitude of energy barrier between particles was mainly controlled by the surface potential of particles and the dielectric constant of solvent. When 1-PrOH or 2-PrOH among several alcohols was used as an alcohol in the solvent mixture, the resulting particles had a spherical shape and narrow size distribution. In those case, the surface potential of particles and the dielectric constant of the solvent were optimum values, so the controlled aggregation of primary particles could occur. The mean particle size was decreased with the increase of R/H ratio because of the decrease of energy barrier between particles which was caused by the negative increase of the surface potential.
The mean particle size increased as the zirconyl chloride concentration increased. This suggested that larger particles were formed by a number of nuclei which was increased with the zirconyl chloride concentration. As the zirconyl chloride concentration increased up to 0.4M, the particles began to deviate from a spherical shape and form the agglomerates such as doublets and multiplets. These agglomerates in the high concentration could be accounted by the increase of the collosion frequency and the reduction of the colloidal stability which was attributed to the high number density of nuclei and secondary particles.
To prevent the formation of the agglomerates between secondary particles, hydroxypropyl cellulose (HPC) was added to the starting salt solution. The mean particle size and the particle agglomeration decreased with the increase of the HPC concentration. The effect of HPC could be explained by the steric stabilization which came from the adsorption of macromolecules on the surface of the particles.
In this work, it was found that the formation of zirconia precursor particles was significantly influenced by the experimental conditions such as stirring speed, temperature uniformity, and heating rate. To obtain monodisperse and spherical particles, the uniform temperature distribution, the lower stirring speed and the rapid heating rate were required. But these conditions could not be satisfied by conventional heating method. Monosize and spherical particles could be prepared by the microwave heating which made the solution heated uniformly and rapidly without stirring.
The FTIR spectrum, EDS spectrum, and TG analysis showed that the composition of the precipitate was similar to $Zr(OH)_4$. As-prepared amorphous particles were crystallized to the mixture of metastable tetragonal phase and monoclinic phase at about 450℃. The metastable tetragonal phase was converted into the monoclinic phase when the calcination temperature increased. After the calcination, the spherical shape was retained and the particle size was slightly decreased because of dehydration and crystallization. Compared with the powders obtained from nutral precipitation method, the powder obtained from this work was crystallized at a higher temperature and the metastable tetragonal phase was more stable at a higher temperature. These results could be explained by the remaining $Cl^-$ ion.
After the calcination at 600℃ for 4h, the specific surface area of particles was remarkably decreased from 63.27 to 7.72 ㎡/g. But, the crystalline zirconia powder with high specific surface area of 18 ㎡/g could be obtained by the hydrothermal crystallization at 150℃ for 30h.
