Aggregation effects on the compressive flow properties and drying behavior of colloidal silica suspensions

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous:silica (SiO2) suspensions of varying electrolyte (NH4Cl) Concentrations were studied. Compressive rheology measurements, including sedimentation and centrifugal consolidation, were first conducted to investigate consolidation behavior in the absence of solvent evaporation. The volume-fraction-dependent osmotic pressure and compressive yield stress were determined for dispersed and flocculated SiO2 suspensions, respectively. Consolidation behavior then was studied in situ by simultaneously measuring stress:evolution and solvent loss as a function of drying time, The observed drying stress histories of the films were complex, consisting of several characteristic regions. First, there was an initial period of stress rise to a maximum drying stress. These measured stress values exhibited good agreement with the osmotic pressure and compressive yield stress at equivalent SiO2 volume fractions for the dispersed and flocculated systems, respectively. Beyond the maximum drying stress there was a subsequent region of stress decay, which coincided with the draining of liquid-filled pores. No residual drying stress was detected for films prepared from salt-free SiO2 suspensions, whereas salt-containing films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO2 suspensions revealed nonuniformities in the spatial distribution of colloidal particles and precipitated salt, with the highest concentrations located at the outer edges of the films, Such features result:from capillary-induced transport of these species during drying, and they have important implications on colloidal processing of ceramic thick films and bulk forms.