Aqueous Gel‐Forming of Silicon Nitride Using Carrageenans

Abstract

Much effort has been devoted recently to the development of near‐net‐shaping processes in water. Agarose has been demonstrated to be a suitable gelling agent for aqueous forming. However, its high cost and the difficulties in controlling the rheological properties have restricted large‐scale applications. In this work a novel gelling binder, namely carrageenan, is proposed as a low‐cost and high‐gel‐strength additive for gel‐forming ceramic powders. The capability to obtain silicon nitride parts by carrageenan gelation is described. Aqueous silicon nitride slips are prepared at pH >11 to a solids content of 70 wt% using tetramethylammonium hydroxide as a dispersant. Dissolution and gelation of carrageenan are studied by continuous measurements of viscosity with temperature for solutions prepared at different pH values. The final injection slips prepared by mixing at <60°C the previously heated suspension and the carrageenan solution are rheologically characterized also. After the blend is injected into cooled nonporous molds, gelling occurs in a few seconds and samples can be dried in air for 24–48 h. Green densities of 52% of theoretical are obtained.