Aggregation Effects on the Compressive Flow Properties and Drying Behavior of Colloidal Silica Suspensions

Abstract

The influence of aggregation phenomena on the compressive flow properties and drying behavior of nonaqueous and aqueous silica (SiO 2 ) suspensions of varying electrolyte (NH 4 Cl) 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 SiO 2 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 SiO 2 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 liquidfilled pores. No residual drying stress was detected for films prepared from salt-free SiO 2 suspensions, whereas saltcontaining films exhibited residual drying stresses likely due to salt-bridging effects. Microstructural characterization of dried films prepared from aqueous SiO 2 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.

A new model for the drying of droplets containing suspended solids after shell formation

“…Rapid evaporation resulting from drying in air at 178 • C also favours the formation of a thicker crust, (17). The pressure predicted across the shell at the end of thickening is broadly inline with maximum stresses in silica films measured by Guo and Lewis [23].…”

“…Griffith [20] used 23 Na and 1 H NMR to observe the drying of a vial of crutcher mix over an extended period. 23 Na NMR could track the amount of sodium sulphate in solution as the vial dried. This was found to decrease in step with the free water content, indicating its concentration was approximately constant.…”

Colloidal Processing of Ceramics

“…Furthermore, they did not yield ceramic bodies with a monomodal pore-size distribution, even when crystallization occurred during consolidation. 2,30 Despite these limitations, monodisperse colloidal systems (e.g., those based on silica spheres) 11 have served as excellent model systems for the study of aggregation, [31][32][33][34][35] rheological, 36 -40 sedimentation, 40 -42 and drying phenomena 3,43 Lange and co-workers 44,45 proposed a new paradigm for powder processing in the early 1990s. Their approach embodied many principles that guided earlier efforts, including use of powders with controlled size, morphology, and purity; no added binder; and dispersion control.…”

“…92 and centrifugation. 3,[92][93][94] In centrifugation, this property can be assessed by measurement of either sediment height at multiple spinning speeds, volume fraction profile during centrifugation by destructive sectioning, or ␥-ray densitometry. A schematic illustration of the coordinate system for the centrifugation method is shown in Fig.…”

“…100 Guo and Lewis 3 found that the osmotic pressure (⌸) dependence on colloid volume fraction can be modeled using a modified Carnahan-Starling equation:…”