Determination of iso-electric point of silicon nitride by adhesion method

The iso-electric point (i.e.p.) of silicone nitride is an important paramet er in the fine ceramics production technology. The determination of its val ue may present a problem, primarily because of surface reactivity of silico ne nitride. For this purpose the adhesion method for i.e.p. determination w as developed. In this paper the batch method was demonstrated. The kinetics of attachment of silicone nitride colloid particles on titanium collector beads was followed. The closed cylinders containing colloidal suspension of silicone nitride together with certain amount of titanium beads were rotat ed, and the particle number concentration of silicone nitride was measured as a function of time. From the experimental data the rate constants of att achment were determined by considering the reverse detachment process. The i.e.p. of titanium is at pH 2, so that titanium surface was negatively char ged in the whole examined pH region. In the region of pH < 8 silicone nitri de was positively charged which promoted the attachment. At pH > 8, both si licone nitride and titanium surface are negatively charged which significan tly reduces the attachment rate. The theoretical rate constant in absence o f interaction forces (k(diff)) was calculated by considering the convection diffusion of colloid particles towards the moving beads. The velocity of t itanium beads, with respect to liquid suspension, was calculated, taking in to account gravitation, bouyancy and friction forces. The pH at which the e xperimental rate constant coincides with the theoretical k(diff) value was taken as the iso-electric point. The results were confirmed by independent electrokinetic measurements. Preliminary experiments in non-aqueous medium indicated the applicability of adhesion method also to these systems. (C) 2 000 Elsevier Science B.V. All rights reserved.