Evidence of ZrO₂ sol–gel transition by gelation time and viscosity

Sol–gel transition is a stage of the sol that evolves towards the formation of a gel-like network and that exhibits gradually a mobility loss. Hence, the measurement of both viscosity and gelation time is expected to evidence the sol–gel transition occurrence. ZrO₂ sols were prepared using zirconium oxychloride octahydrated as precursor material. The structures of the sols before and after gelation were characterized by TEM. Gelation time depends on the concentrations of the reactants and the aging temperature. Viscosity increases linearly with the gelation time firstly, and then increases exponentially nearby the sol–gel transition point. TEM results show the gel is granular before the sol–gel transition point, and appears as brain-like structure after this point. The determination of the sol–gel transition point is to enable the optimum aging time of a sol and to perform the next operations which determine the desired shape of gel (bulk, film, powder, etc.).