In Situ Surface Modification of Silicon Carbide Particles Using Al³⁺ Complexes and Polyelectrolytes in Aqueous Suspensions

Abstract

Silicon carbide (SiC) particles were modified in situ, using Al 3؉ complexes in aqueous media, via control of Al(NO 3 ) 3 concentration and pH. The Al 3؉ formed hydrolyzed complexes that adsorbed onto the charged sites on the SiC particle surfaces. As a result, the surface-modified SiC particles behaved in an alumina-like manner in the approximate pH range 5-8. The modified SiC particles were further treated with two types of polyelectrolytes that were sequentially adsorbed onto the particle surface, to give a maximum surface charge. As a result of this surface-modification process, the SiC could be dispersed with Al 2 O 3 in aqueous media without heteroaggregation.

Effect of dispersants on the rheology of aqueous silicon carbide suspensions

“…The effect of varying the levels of the A40 and KA11 dispersants on the dispersion of the SiC particles, and hence on the viscosity, is shown in figure 6. When the A40 concentration was as low as 2 mg g -1 the SiC particle surfaces will have only been partially covered by A40 molecules and thus the KA11 will have been able to adsorb onto the remaining uncovered sites on the particles [24]. Although the total adsorption of the two polyelectrolytes was probably relatively low, yielding weak repulsion and hence high viscosities, as expected the viscosity decreased as more of the KA11 adsorbed.…”

“…Overall this resulted in a viscosity that was low but changed little with increasing KA11 concentration. At the intermediate A40 concentration of 6.7 mg g -1 , the particle surfaces will have been only moderately covered and so the long chain KA11 polymer could still adsorb [24]. The long tail of the latter would be free to extend out into the liquid phase yielding a higher steric force, and hence a lower viscosity, than present with either 2 or 10 mg g -1 of A40, at higher KA11 concentrations.…”

“…At higher concentrations of A40, more of the particle surfaces will have been covered by a layer of short chain ammonium polyacrylate, increasing the degree of negative charge on the surface and so restricting further adsorption of the longer chain KA11 [24]. At the highest A40 level, 10 mg g -1 , most of the particle surfaces will have been covered by a layer of the polymer preventing much KA11 from adsorbing.…”

“…The surface modification procedure for the SiC particles has been described in detail elsewhere [24]. Briefly, it involved mixing Al(NO 3 ) 3 solution into deionised water followed by addition of the SiC powder and pH adjustment to 5.8.…”

“…A similar method has been studied by Luther [23] to coat Si 3 N 4 powder. In previous work by the present authors [24], SiC particles have been modified in-situ using Al 3+ complexes in aqueous suspension via additions of Al(NO 3 ) 3 and control of the pH. The Al 3+ formed hydrolyzed complexes that adsorbed onto the charged sites on the SiC particle surfaces, as a result the SiC particles behaved in an alumina-like manner in the approximate pH range 5 -8.…”