Rheological Properties of Boron Nitride-Silica Composite Suspensions

In this paper, a novel colloidal forming process has been developed to prepare boron nitride-silica composites. The rheological properties of mixed boron nitride-silica powders in deionised water have been investigated by adjusting the concentration of dispersant, pH value of the solution and other process parameters. The isoelectric points for both boron nitride and silica powders are in the pH range of 2 to 3. Boron nitride doped with or without A-1 (an anionic dispersant) as a dispersant shows a similar electrophoretic behavior to silica powder, i.e. Zeta potential becomes more negative with the increase of pH value from 2 to 12. The sedimentation and rheological results of aqueous boron nitridesilica suspensions indicate that the optimized pH value is around 7.2. It depends upon the synthetic effects of dissociation of A-1 and sufficient positive adsorption sites on the particle surfaces in aqueous media. The optimum concentration of the dispersant at pH=6 is 0.6wt % (dry weight basis of composite powders). Boron nitride-silica composite suspensions with a solid content as high as 50vol% have also been achieved. A ball milling process can improve the fluidity of the suspensions.