Influence of TMAH and NaOH on the stability of SiC aqueous suspensions

The study aimed at evaluation of the rheological properties of aqueous suspensions of SiC stabilized with either TMAH (tetramethyloammonium hydroxide) or NaOH in the amount of a given dispersant in the range of 0-1.0 wt% as calculated on the dry mass of SiC powder. The optimal amount of the dispersants was finally evaluated at 0.4-0.6 wt %, based on the results of rheological studies (flow curves and viscosity curves) and the sedimentation rate, which corresponded to the pH of the suspension within the range of 9.5-10.5. The amount of the dispersant here did not depend on the solid volume fraction in the suspension, but rather, according to the zeta potential measurement results, the pH range guaranteed the formation of a stable suspension. This effect allowed to propose a dispersant mechanism for stabilizing the suspension. The maximum amount of silicate anions formed, accordingly, in the pH range of 9.5-10.5 on the surface of SiC particles, which in turn could be able to adsorb Na+ or TMA(+ )cations and thus increase the thickness of the Stern layer, increasing electrostatic repulsion between the SiC particles and improving the stabilization of suspensions and their rheological properties. However, the increase in suspension pH deteriorated their stabilization - most likely due to the occurrence of polymerization of amorphous silica and the presence of additional cations in water increasing its ionic strength. These suggestions were confirmed by the results of XPS analysis of the chemical state of SiC particle surfaces.