Abstract: The nucleating mechanism of crystallite was investigated from the microcosmic kinetics. It was proposed that the idealized
nucleation mechanism consists of the formation of growth unit, the oxolation reaction, and the transform of O bridge into OH bridge. The main factors affecting the particle
size of powders were analyzed from nucleating rate. It was concluded that the interior factors affecting the particle size of powders are the formation energy of growth unit
and lattice energy of crystal. The relative particle size of various oxide powders as a function of structure type was summed up from this. The particle size of powders with
the structure type of CaF₂ or TiO₂ is smaller than that of powders with the structure type of α-Al₂O₃; the particle size of powders with the structure type of
α-Al₂O₃ is smaller than that of powders with the structure type of ZnO. According to above analysis, the difference in particle size of various oxide
powders prepared by hydrothermal method, and the effect of the reaction medium and the reaction temperature on the particle size were reasonably explained.

Key words: growth mechanism, particle size, hydrothermal method