Processing of beta-silicon nitride from water-based alpha-silicon nitride,alumina, and yttria powder suspensions

A water-based route for processing beta-Si3N4 from alpha-Si3N4, Al2O3, and Y2O3 powder mixtures was established. The surface charges and isoelectric p oints of the three different powders were investigated within the pH range from pH 3 to pH 12, Citric acid diammonium salt was found to be an effectiv e deflocculant for shifting the isoelectric points to pH 3.5 for Al2O3 and to pH 6 for Y2O3, Aluminum hydroxide (AI(OH),) showed strong interaction wi th the Si3N4 powder, shifting the isoelectric point from pH 7 to pH 5.5, Lo w-viscosity, high-solids-loading suspensions (60-63 vol%) thus were possibl e at pH 9.7, The preparation of homogeneous and stable suspensions with a s olids content of less than or equal to 61 vol% and a viscosity <1 Pa.s was limited to a pH regime between pH 9 and pH 10.5 because of the high solubil ity of yttria, The homogeneous suspensions were easily solidified by direct coagulation casting (DCC), using the urease-catalyzed decomposition of ure a at pH 9 to pH 10, by forming salt. No shrinkage cracking, sedimentation, or phase separation was observed during coagulation or drying. The green-de nsity distribution was homogeneous throughout all bodies, even for complex geometries.