Colloidal stability for concentrated zirconia aqueous suspensions

Abstract

This work started as part of an investigation into the mechanisms by which fine zirconia aqueous dispersions can be processed for ceramic materials engineering. Aqueous dispersions of TZ3Y fine zirconia particles obtained by dispersion of dry powder in acidic solutions (pH 3) have been subjected to compression through osmotic experiments. The results show a behavior that is unusual when compared with the classical behavior of colloidal dispersions. Indeed, the 50 nm particles are well dispersed and protected from aggregation by electrical double layers, with a high zeta potential (60–80 mV). Yet, during osmotic compression, the dispersion goes from a liquid state to a gel state at a rather low volume fraction, φ=0.2, whereas the liquid–solid transition for repelling particles is expected to occur only at φ=0.5. This early transition to a state in which the dispersion does not flow may be a severe drawback in some uses of these dispersions, and thus it is important to understand its causes. A possible cause of this early aggregation is the presence of a population of very small particles, which are seen in osmotic stress experiments and in light scattering. We propose that aggregation could result from the compression of this population, through either of the following mechanisms: (a) An increase in pressure causes the small particles to aggregate with each other and with the larger ones or (b) An increase in pressure induces a depletion flocculation phenomenon, in which the large particles are pushed together by the smaller ones.