No abstract

The shear modulus and yield stress of attractive alumina particle networks in aqueous slurries was determined as a function of volume fraction (0.1 to 0.5), pH (2, 4, 5, 6, and 9), and salt (NH4l) concentration (0.25M to 2.34) using both vane and couette rheological tools. Consistent with previous observations concerning the relative strength of attractive particle networks, the shear modulus increased to a plateau value with salt concentration. In this work we have shown that the salt concentration at which this plateau value is achieved is a function of the pH, and thus, the surface charge density. The values of the shear modulus [G′], yield stress [τy], and yield strain [γy] of the attractive networks can be described with power law functions for particle volume fraction [φ] (G′∝φ4.75, τy∝φ3.6, and γy∝φ−1.1) and salt concentration [c] (G′∝ [c]2.0, τ, ∝ [c]1.15, and γy∝ [c]−0.85).

Direct measurements of forces between silicon nitride surfaces in the presence of poly(acrylic acid) (PAA) are presented. The force-distance curves were obtained at pH > pH iep with an atomic force microscopy (AFM) colloidal-probe technique using a novel spherical silicon nitride probe attached to the AFM cantilever. We found that PAA adsorbs onto the negatively charged silicon nitride surface, which results in an increased repulsive surface potential. The steric contribution to the interparticle repulsion is small and the layer conformation remains flat even at high surface potentials or high ionic strength. The general features of the stabilization of ceramic powders with PAA are discussed; we suggest that PAA adsorbs onto silicon nitride by sequential adsorption of neighboring segments ("zipping"), which results in a flat conformation. In contrast, the long-range steric force found in the ZrO 2 /PAA system at pH > pH iep arises because the stretched equilibrium bulk conformation of the highly charged polymer is preserved via the formation of strong, irreversible surface-segment bonds on adsorption.

No abstract