In Situ Rheological Investigation of the Coagulation in Aqueous Alumina Suspensions

Abstract

Electrostatically stabilized alumina suspensions can be destabilized by the enzyme-catalyzed decomposition of urea (direct coagulation casting). Depending on the conditions, this reaction can shift the pH of a suspension to the buffer pH of the reaction products or increase the ionic strength at the buffer pH. The coagulation for both mechanisms was investigated using in situ rheological measurements. Using a vane tool in oscillation mode, the measuring conditions were optimized to find a reasonable method for time-dependent measurements. Constant parameters (stress or strain) proved to be unsuitable, because the linear viscoelastic region shifted considerably during the coagulation. Furthermore, the gel structure produced on coagulation via increase of ionic strength (⌬I) was very sensitive to the oscillation. Therefore, for long-time experiments, a short continuous measurement with a low strain was followed by amplitude sweeps with increased intervals to determine the linear values of G and G. In this way, the increase of the moduli G and G could be followed for longer times, and it was possible to demonstrate two results. First, the final G of the network was about 10 times higher for ⌬I-coagulated material than for suspensions coagulated via pH shift (⌬pH). Second, particle rearrangement processes took place in ⌬I-coagulated networks even after the chemical changes were finished, whereas ⌬pH-coagulated samples were "frozen-in" when approaching the isoelectric point and showed no further physical changes afterward.

Relation between microstructure and mechanical behavior of concentrated silica gels

“…This process leads to gelation by either a shift of the pH toward the isoelectric point (IEP) of the particles ( pH method), or by an increase of ionic strength at the buffer pH of the reaction products ( I method). By using these two different methods for the destabilization, we are able to form gels that show significantly different properties both in terms of rheology [12,13] and in terms of microstructure [14].…”

Rheological Properties of Alumina Slurries: Effect of Deflocculant Addition

“…In this article, mention is made of some of most recent articles: Briscoe et al (1998), Davies and Binner (2000), Kim et al (2000), Balzer et al (2001), Papo and Piani (2003), Tomasik et al (2003), and Pradhan and Bhargava (2005).…”

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

“…Direct coagulation casting is a novel method for near-net-shape preparation of ceramic parts from concentrated aqueous ceramic powder suspensions by introducing the DLVO theory into ceramic colloidal forming. The concentrated suspension is destabilized by the decomposition producing acid, base or electrolyte in situ from water soluble precursor molecules present in the suspension to either shift the pH of the suspension to the isoelectric point (IEP) of the ceramic powder system or increase the ionic strength of the suspension [12][13][14][15]. However, there are some drawbacks such low wet strength ($ 100 kPa), long coagulation time (1-3 days) and easy cracking which have hindered DCC as a promising colloidal forming method in industrial application [16,17].…”