Rheology and Particle Packing of Chem‐ and Phys‐Adsorbed, Alkylated Silicon Nitride Powders

Abstract

Three alcohols with extended carbon chain lengths between ∼1 and 2 nm were chem‐adsorbed on a Si3N4 powder by reacting with hydroxyl surface groups at temperatures 200°C. Slurry rheology, particle packing density, and body rheology were determined for toluene and dodecane slurries formed with these chem‐adsorbed powders. These same properties were determined for slurries where the alcohol was simply added, but not reacted with the powder (phys‐adsorbed powders). The viscosities of chem‐adsorbed slurries are shear‐thinning with longer chains producing lower viscosities at a given shear rate. The relative density of powder compacts produced by pressure filtration (10 MPa) was high (∼0.60) for octadecanol and dodecanol‐reacted powders, and lower (∼0.50) for the octanol‐reacted powder. When a sufficient amount > 10 times that required for chem‐adsorption) of the same alcohol was simply added to the unreacted Si3N4 slurry system, the phys adsorbed slurries exhibited similar rheological behavior as the chem‐dsorbed slurries, but, unlike chem‐adsorbed slurries, their packing densty was lower, and their slurries were destabilized by water vapor. Stress relaxation experiments showed that bodies formed with the octadecanol chem‐adsorbed powders were plastic after consolidation, whereas phys‐dsorbed bodies were brittle (fractured before flow). All evidence suggests that the short‐chained alkyl groups are steric “stabilizers” at small interparticle distances and thus prevent the particles from making surface‐surface contact in common organic liquids; i.e., they produce a short‐range interparticle, repulsive potential. Chem‐adsorbed molecules, but not phys‐adsorbed molecules, persist during particle packing and in moist environments.