RHEOLOGY AND PARTICLE PACKING OF CHEM-ADSORBED AND PHYS-ADSORBED, ALKYLATED SILICON-NITRIDE POWDERS

Three alcohols with extended carbon chain lengths between almost-equal-to 1 and 2 nm were chem-adsorbed on a Si3N, powder by reacting with hydroxyl surface groups at temperatures less-than-or-equal-to 200-degrees-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 (approximately 0.60) for octadecanol and dodecanol-reacted powders, and lower (approximately 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 chemdsorbed slurries, but, unlike chem-adsorbed slurries, their packing density 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.