Abstract: Influences of acid leaching and thermal oxidation on aqueous dispersibility were investigated for four types of commercial silicon nitride powders, in conjunction with their DRIFT spectra and soluble counter-ions. Three of the powders, referred to FD1, FD2 and M11, were produced by the direct nitridation of silicon powder with different pulverization processes, while the UBE powder was made by calcination of the silicon diimide precipitate. Particle surface groups and soluble counter-ions (especially high-valency counter-ions) are proved to be the crucial factors that determine the aqueous dispersibility of Si3N4 powders. The study indicates that the high-valency counter-ions are the crucial factor limiting the dispersibility of powder FD1, which can be eliminated by acid leaching. For powder FD2, the barrier to preparing highly concentrated suspension is the large number of hydrophobic Si-O-C-R groups on particle surfaces, and those groups can be removed by thermal oxidation. For powder M11, the high concentration of soluble NH⁴⁺ restricts the aqueous dispersibility of the as-received sample, which can be removed by thermal oxidation. For the as-received powder UBE, however, hydrophobic Si-O-Si surface groups limit the preparation of highly concentrated slurries. After particle surface modification, the four types of silicon nitride powders show excellent aqueous dispersibilities.

Key words: Si₃N₄ powder, dispersibility, acid-leaching, thermal oxidation