The effect of alcohol treatment on the rheology of Si3N4

Abstract

Abstract Slip casting of Si 3 N 4 requires well-dispersed low viscosity suspensions. Two alternative procedures have been applied: (a) premixing by attritor milling in isopropanol, drying followed by the final slip preparation with deflocculant and water in a ball mill (batch A) and (b) suspending the powder in water using a deflocculant and mixing milling in a continuous attritor-like mill (batch B). Both batches were identical with respect to the starting powder, additives and the solid content. Astonishingly, the rheology was found to be quite different. Batch A had a strong tendency to Bingham behaviour whereas batch B showed a clear Newtonian shear curve. The differences are attributed to a reaction of the alcohol used (isopropanol) in batch A with the powder surfaces forming stable surface groups. The most probable configuration is a silyl ester Si-O-C-R, shielding the active sites against the coupling of the deflocculants. Burn-out experiments revealed that temperatures of 500°C are necessary to remove these surface groups. After this treatment the powders could be suspended giving the same shear curve as batch B.