Viscoelastic properties of concentrated silicon nitride slurries

The colloidal behavior, rheological behavior, and viscoelastic behavior of concentrated Si3N4 slurries are investigated in detail by electrophoresis and rheological measurements in order to prepare a suspension with low viscosity and high solid content. The X-ray photoelectron spectroscopy reveals a decrease of the surface density of acidic silanol group after that Si3N4 powder is surface modified. The isoelectric; point of the modified Si3N4 particle shifts to basic region gently and the zeta potential in alkaline region evidently increases in magnitude. Attempts have been made to apply rheological models to the suspensions with various solid volume fractions (phi). For the as-received suspensions, the Sisco model provides the best fit in the range of phi 0.30 while the Casson model in 0.35 phi 0.45. The shear behavior of modified suspensions fits to Sisco model in the range of phi 0.40 and Casson model in 0.45 phi 0.54. The rheological behavior of modified suspensions is improved efficiently. The critical strain decreases and the linear viscoelastic regime narrows continuously with increasing solid concentration. With increasing solid concentration, the magnitude of the viscoelastic response increases, the suspension transforms from more viscous to more elastic progressively and the characteristic frequency shifts toward lower frequencies. The viscoelastic behaviors can be understood in terms of the structural relaxation time of the suspension.