EFFECTS OF CHEMICAL-SPECIES ON THE CRYSTALLIZATION BEHAVIOR OF A SOL-DERIVED ZIRCONIA PRECURSOR

The thermal and crystallization behaviors of a sol-derived zirconia pr ecursor are affected by the chemical species (such as chloride, acetat e, and hydroxyl; i.e., Cl-, CH3COO-, and OH-) present in the solution in which they are aged. The crystallization temperature of zirconia pr ecipitate increases steadily with decreasing pH when hydrochloric acid is added to the aging mother liquor. A more dramatic increase in crys tallization temperature is observed when acetic acid is used to reduce the pH of the aging solution, The precursors aged in the solutions of intermediate pH range demonstrate a typical two-step crystallization behavior, due to the transition from a OH--dominated surface structure to a CH3COO--dominated one. These results show that a higher crystall ization temperature results when the OH- groups attached to the surfac e of precursor particles are replaced by the CH3COO- groups. Only a sl ight decrease in the crystallization temperature of aged precipitates occurs when free species, such as CH3COO- and OH-, are being removed f rom the aging solution.