Enzyme catalysis of ceramic forming: alumina and silicon carbide

ABSTRACT In this work we describe how stable ceramic suspensions can be destabilized by time delayed reactions. We also describe how this method can be used in case of high solids loaded suspensions for forming complex and reliable ceramic components. The method uses an aqueous, electrostatically stabilized ceramic suspension with high solids loadings free of agglomerates and of low viscosity. It is cast into a mold and then coagulates forming a stiff, wet green body. Coagulation is performed by changing the pH of the suspension and/or by creating salt directly inside the suspension using a controlled time-delayed reaction. Enzyme catalyzed reactions that are decomposing a substrate, or self-propagating decomposition reactions of a substrate, can be used. After the coagulation reaction in the suspension, the wet green body shows good mechanical properties and can be demolded, dried and sintered. Polymers show also sol-gel transitions upon changing pH, ionic strength or temperature. In case such polymer sols are combined with particles in a suspension, we obtain very rigid ceramic green bodies. Since the forming process takes place without pressure and at ambient temperatures, inexpensive molds and tools can be used. Ceramic bodies with homogeneous, defect poor microstructures can be cast using almost any ceramic powder or even powder combinations with different IEP behavior. Complex shaped components with thin and thick cross sections can be cast as there is no gradient in density and/or temperature during consolidation of the green body. High strength (680 MPa) alumina parts of high reliability (Weibull-module m = 40) have been demonstrated.