DEVELOPMENT OF SHORT-RANGE REPULSIVE POTENTIALS IN AQUEOUS, SILICON-NITRIDE SLURRIES

The properties of aqueous, dispersed, silicon nitride slurries, with an isoelectric point of pH 5.5, can be changed with additions of NH4Cl. At pH 10 the effect of adding NH4Cl is similar to that suggested by DLVO theory; namely, for concentrations greater-than-or-equal-to 0.5M, the viscosity vs shear rate behavior, the elastic modulus, and the relative packing density are identical to those for slurries prepared at the isoelectric point. On the other hand, the effect of salt on dispersed slurries prepared at pH 2 differs from the behavior implied by classic DLVO theory; i.e., measurement of the same properties showed that the attractive interparticle potential was much weaker relative to that produced at the isoelectric point. As previously reported for alumina slurries, the results suggest that a short-range, repulsive interparticle potential is developed in salt-added slurries prepared at pH 2 which prevents attractive particles in the slurry from touching and aids particle packing. The same short-range potential apparently is not developed with salt additions at pH 10. The difference between silicon nitride and alumina slurries is apparent when the slurries are consolidated. Bodies consolidated from any silicon nitride slurry are elastic (i.e., they fracture before they flow) unlike salt-added alumina slurries, which are plastic.