Direct Coagulation Casting of Alumina Suspension by High Valence Counter Ions Using Ca(IO₃)₂ as Coagulating Agent

Abstract

A new direct coagulation casting of aqueous alumina suspension was developed via controlled release of high valence counter ions from calcium iodate with increase in the temperature from 55°C to 70°C. The influence of calcium iodate on the rheology of alumina suspension was investigated. A small amount of calcium iodate increased the viscosity of the concentrated alumina suspension at high temperature and finally transformed it into a wet‐coagulated body. The mechanism of coagulation is proposed such as that the solubility of calcium iodate increases with increase in temperature. The high valence Ca2+ ions diffuse into the double electrical layer of alumina particles surface through electrostatic attraction, reduces the zeta potential, hence decreases repulsive force between particles. Also reaction between Ca2+ and citrate leads to insufficient dispersant coverage on the particle surface. Both factors contribute to the coagulation of the suspension. The coagulation time was from 1 to 4 h by maintaining the temperature in the range of 55°C–70°C. The wet‐coagulated bodies prepared from 50 vol% alumina suspension showed a high compressive strength of 2.6–3.2 MPa with uniform microstructure. The relative density of sintered sample is 99.4% at 1550 °C for 2 h with perfect microstructure.

Properties of Alumina Coagulated Bodies Prepared by Direct Coagulation Casting via High Valence Counter Ions (DCC‐HVCI)

“…A low viscosity and high solid loading suspension is a prerequisite to ensure low shrinkage rate during the direct coagulation casting process and uniform microstructure of green body . Stable suspension of 50 vol% alumina can be prepared with 0.3 wt% triammonium citrate in previous report . CCC is the minimum concentration of counter ions to coagulate the suspension, which can be reflected by a rapid increase in the viscosity of the suspension.…”

“…Ca 2+ ions are high valence counter ions in this system. The electrical double layer on the surface of alumina particles is strongly compressed by Ca 2+ , and zeta potential of the particle surfaces decreased . The interparticle repulsion potential energy reduces and the dispersed particles come to close contact, leading to increase in viscosity.…”

“…In this process, high valence counter ions (Ca 2+ ) were controlled release from calcium iodate and calcium phosphate by increasing temperature which increased the viscosity of the concentrated alumina suspension. The change in solubility played an important role in the procedure . Shaped ceramics parts have been successfully produced by this method with high compressive strength and uniform microstructure.…”

Gel Casting of Free‐Shapeable Ceramic Membranes with Adjustable Pore Size for Ultra‐ and Microfiltration

“…1, for the slurry preparation 0.6 g of each alginate, were dissolved in 50 mL of double deionized water at room temperature (RT) using a dispermat (IKA Rw20.n, Staufen, Germany) for 30 min at 300 rpm. 28 The added amount of TCA depends on the specific surface area of the used alumina particles and was 0.08 wt% m À ². On the one hand, TCA acts as a disperging agent to stabilize the alumina particles in the suspension, 27 on the other hand it reduces later coagulation effects of the calcium iodate.…”

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

“…Based on this concept, we proposed a novel ceramic forming method called direct coagulation casting via controlled release of high valence counter ions (DCC-HVCI) with a combination of the DLVO theory and the Schulze-Hardy rule. High valence counter ions (Ca 2þ , Mg 2þ , SO 4 controlled release via increasing temperature which can destabilize high solid loading alumina suspension [23][24][25][26][27]. Complex ceramic components have been manufactured with high sintered density and uniform microstructure via the above methods.…”