Effect of Polyethylenimine on Hydrolysis and Dispersion Properties of Aqueous Si ₃ N ₄ Suspensions

The roles of polyethylenimine (PEI) in the hydrolysis and dispersion properties of aqueous Si 3 N 4 suspensions were studied in terms of the hydrolysis, adsorption, electrokinetic, and rheological measurements. It was found that the pH change of the suspensions in the acidic environment could be minimized in the presence of ≥0.5 dwb% PEI. The ammonia and oxygen measurements suggest that this phenomenon is primarily attributed to the buffer mechanism generated by the ionized PEI, instead of the protection mechanism. The constant pH enables the suspensions to retain a better stability with time at acidic pH. The adsorption of PEI on Si 3 N 4 is a high‐affinity type at highly basic pH, but is a low‐affinity type at acidic pH. As the PEI amount increases, the adsorption shifts the isoelectric point (IEP) of Si 3 N 4 from pH 5.9 to pH ∼11 until complete coverage is attained. The stability of Si 3 N 4 suspensions is found to depend strongly on the saturated adsorption of PEI, which is as a function of the pH and PEI amount. Once the saturated adsorption limit is reached, the excess free PEI molecules become more detrimental to the stability with increased solid loading. The stabilization mechanisms of Si 3 N 4 suspensions by PEI were discussed in detail.