THE EFFECT OF ALCOHOL TREATMENT ON THE RHEOLOGY OF SI3N4

Slip casting of Si3N4 requires well-dispersed low viscosity suspension s. Two alternative procedures have been applied: (a) premixing by attr itor milling in isopropanol, drying followed by the final slip prepara tion with deflocculant and water in a ball mill (batch A) and (b) susp ending 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 cl ear Newtonian shear curve. The differences are attributed to a reactio n of the alcohol used (isopropanol) in batch A with the powder surface s 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-degrees-C are necessary to remove these surface groups. After t his treatment the powders could be suspended giving the same shear cur ve as batch B.