Colloidal Forming Techniques

Contact: Elena Tervoort

Eq. 1: Enzymatic catalyzed hydrolysis of urea.

Fig. 1: Destabilization mechanisms that can be used in Direct Coagulation Casting: (a) increase of the solution ionic strength (I) and (b) shift of the suspension pH.

Fig. 2: Schematic representation of the Direct Coagulation Casting process (in this case with the pH shift mechanism).

Fig. 3: Complex-shaped ceramic parts that have been produced using the Direct Coagulation Casting process.

New colloidal forming techniques allow the formation of complex ceramic parts of extremely high reliability, but at a lower cost. Our work concentrates on the development of a new forming technique called Direct Coagulation Casting (DCC), a method where well stabilized ceramic suspensions are consolidated by changing pH or ionic strength catalyzed by enzymatic reactions.
An enzymatic catalyzed reaction that has been extensively applied in our group to destabilize initially dispersed suspensions is the hydrolysis of urea (CO(NH₂)₂) in the presence of urease, as shown in Equation 1.

The weak acid (H₂CO₃) and strong base (NH₃) produced through this reaction lead to a buffer pH of 9, which corresponds to the isoelectric point (IEP) of alumina. Therefore, alumina suspensions initially dispersed at acidic pHs (e.g. 4) can be coagulated into homogenous stiff green bodies using this in-situ enzymatic catalyzed reaction (pH shift mechanism). Alternatively, one can first shift the particle IEP to acidic values (e.g. 3-5) and then coagulate the suspensions at pH 9 through an in-situ increase of ionic strength (ionic strength mechanism). Besides its simplicity and low cost, the method has as additional advantage the non-toxicity of the reactants involved (as opposed to other colloidal processing methods) and the negligible amount of organic additives required for consolidation.
The DCC method has allowed us to prevent the formation of large agglomerates and heterogeneities in the material microstructure, increasing the strength of high-purity alumina by over 60% and the reliability by a factor of 3. By this means, one can produce ceramic parts with reliability comparable with cast iron.

For further information, consult the papers from Graule, Baader, Yang, Balzer, Hruschka, Tervoort, Will, Studart and Si in our literature database.