The surface chemistry and suspension behavior of aluminum nitride was analyzed using sedimentation and electrophoretic techniques. Aluminum nitride shows an isoelectric point near pH 8.5, very similar to that observed for aluminum oxide powders in the literature. This is attributed to the instability of the nitride in water which results in the formation of the oxide at the surface and is accompanied by the release of ammonia. Nonaqueous electrophoretic measurements indicate that the surface of A1N is heterogeneously composed of both acidic and basic sites. Large positive potentials are achieved in chloroform, an acidic medium and large negative potentials result in dimethyl formamide (DMF), a basic medium. The basic sites appear to predominate on the A1N particles since in the amphoteric liquid,acetonitrile, they exhibit negative potentials as well. Calcining the aluminum nitride powders results in a reduction of the zeta potential in acidic solvents with no observed potential changes in the basic media. This indicates a loss on calcining of surface aluminum hydroxyl groups which make the surfaces more basic. Despite the high zeta potentials for A1N dispersions in the acidic chloroform and in the basic dimethyl formamide, these media have significantly different settling behaviors in 5 volume % solids suspensions. The acidic chloroform causes formation of loose powder flocs and voluminous sediments, while the basic DMF provides excellent dispersibility and dense sediment. This is attributed to the lower dielectric constant of chloroform which results in lower electric fields and therefore smaller repulsion forces between particles. In addition, A1N dispersions in the low dielectric constant pyridine solvent have low zeta potentials in dilute suspensions. However, in more concentrated suspensions, pyridine yields sediments as dense as those in dimethyl formamide.

Keywords: Aluminum Nitride, Surface Chemistry, Sedimentation, Dispersion