Interparticle potentials in nonaqueous silicon nitride suspensions

The rheological properties of nonaqueous silicon nitride suspensions are studied. Suspensions were prepared to volume fractions of solids of 0.21, 0.25, 0.29, and 0.33, and dispersed with phosphate ester in a mixture of solvents (methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone). Expanded viscosity curves were obtained by measuring under controlled rate and stress conditions, and the experimental data were fitted to the Cross model that provides the high shear limiting viscosity (eta(infinity)). The evolution of viscosity with volume fraction of solids was fitted to the Krieger-Dougherty equation, to predict the maximum packing fraction (phi(m)). The electrostatic pair potential was calculated based on the DLVO theory by evaluating the dielectric constant of the three-component solvent and the Hamaker constant of the Si3N4-solvent system. The surface potential was calculated by measuring the elastic modulus through dynamic rheological measurements. The steric potential was also evaluated from the available models. It has been observed that phosphate ester provides a purely steric stabilization at short separation distances (up to 9 nm), while electrostatic forces dominate at larger separation distances.