Processing of concentrated aqueous Si3N4 slips stabilized with tetramethylammonium hydroxide

The optimization of the dispersion of 43 vol% aqueous Si3N4/sintering aid s lurries with tetramethylammonium hydroxide to achieve high parking slipcast compacts and subsequent high sintered densities was investigated, Slips co ntaining 43 vol% solids were obtained by dispersing most of the solid (35 v ol%) at high pH values (9.7 to 12.3) with different amounts of tetramethyla mmonium hydroxide and with subsequent addition of the remaining solid up to 43 vol%. The influence of pH on the viscosity and on the amount of tetrame thylammonium ions adsorbed, and in solution, of 43 vol% slips was studied, The minimum viscosity was observed in the 9.7 to 10.2 pH range. This pH ran ge seemed to be an intermediate situation between high ionic strength (pH h igher than 10.2) and low electrostatic repulsion (pH Lower than 9.7), The m aximum sintered density at 1650 degrees C, 97.5% of theoretical density (TD ), was obtained from bars of green density, 55.6% TD, prepared from slips w ith the minimum viscosity (44 mPa/s).