Surface and rheological properties of zirconia suspensions

The importance of surface chemistry on the rheological properties of concentrated zirconia suspension is investigated. The pH and nature of anions were found to have a very significant effect on the rheological properties of ZrO₂ suspensions. The variation in rheological properties with pH and anions correlated well with the change in surface properties of the ZrO₂ with pH and anion type and concentration. The isoelectric point (IEP) for monoclinic ZrO₂ determined in 0.001 M sodium chloride solution was 6.7. At and near the IEP, ZrO₂ suspensions are highly flocculated and exhibited pronounced yield stress behaviour. At pH a few units either side of the IEP, the magnitude of the mobility is high and a low viscosity dispersed suspension is obtained. The ZrO₂ suspension was dispersed with KOH at high pH while hydrochloric or nitric acid can be used for dispersion at low pH. Sulphuric acid however does not disperse the ZrO₂ suspension at low pH because of significant sulphate adsorption on the zirconia particle. At low pH, ZrO₂ is positively charged, sulphate adsorption reduces the magnitude of the surface charge which is observed as a low positive mobility. The results clearly show how the surface chemistry of ZrO₂ can be regulated to control the rheology.