Aqueous gel-forming of silicon nitride using carrageenans

Much effort has been devoted recently to the development of near-net-shapin g processes in water, Agarose has been demonstrated to be a suitable gellin g agent for aqueous forming. However, its high cost and the difficulties in controlling the rheological properties have restricted large-scale applica tions. In this work a novel gelling binder, namely carrageenan, is proposed as a low-cost and high-gel-strength additive for gel-forming ceramic powde rs. The capability to obtain silicon nitride parts by carrageenan gelation is described. Aqueous silicon nitride slips are prepared at pH >11 to a sol ids content of 70 wt% using tetramethylammonium hydroxide as a dispersant, Dissolution and gelation of carrageenan are studied by continuous measureme nts of viscosity with temperature for solutions prepared at different pH va lues. The final injection slips prepared by mixing at <60<degrees>C the pre viously heated suspension and the carrageenan solution are theologically ch aracterized 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.