Rheological Behaviors Of Ceramic Suspensions And Inhibition Of Flawsin Colloidal Formed Green Bodies

This research was deveolped aiming to eliminate or inhibit the flaws suchas pores and cracks in ceramic green bodies formed by colloidal processing,and further to obtain perfect green bodies with high strength. In particular,rheological behaviors of alumina suspensions, a novel method of preparingconcentrated silicon nitride suspension, and the controllable and uniformsolidification of silicon nitride suspension were studied. The reasons for thegeneration of pores and cracks in colloidal formed green bodies were foundout, and some applicable solutions to suppress these flaws were broughtforward.The rheological behaviors of alumina suspension were studied. Thestructural reasons for the shear thinning and shear thickening behaviors ofelectro-stabilized suspensions were analyzed from the view of the change ofinteractive potential between particles in the suspension. The effects oftri-ammonium citrate ratio, the pH and the solid volume fraction of thesuspension and the diameter of suspended particles on the rheologicalbehaviors of alumina suspension were studied. It is believed that shearthinning is the result of decomposition of thermodynamic particle clusters,while shear thickening can be attributed to the formation of hydrodynamicparticle clusters in the suspension. The viscosities of alumina suspension athigh and low shear rate are respectively determined by Stern and Zetapotential near the particle surface, and the critical shear rates (the onset ofshear thickening) dependence on the particle size was quantified with a simpleshear model.A surface modification method for Si3N4 particles, by the adsorption ofgrowing polymer chains on the particles during the polymerization of acrylicacid in thin Si3N4 suspension, was proposed to increase the solid volumefraction of Si3N4 suspension. The suspension, containing more than 50vol%surface modified Si3N4 powder, was solidified by gelcasting controllably anduniformly under a ultrasonic wave aid, which accelerated the solidification ofSi3N4 suspension because it destroyed the cage effect of free radicals, and thestrength of green body was increased obviously.The genesis of the pores in colloidal formed green bodies was alsostudied in this research, and a method of degassing the suspension withmagnetic stirring under vacuum was put forward. As a result, the degassingeffect of the suspension is improved, then the pores volume and diameter inthe solidified green body are reduced, resulting in the increment of thestrength and density of the sintered body.Further, the debinding of gelcasting green bodies was comprehensivelystudied with TG/DTA and FTIR methods. The fact that polyacrylamide inceramic green bodies decomposed and oxidized in a narrow temperature rangewas found to be the major reason for the occurring of cracks during greenbody debinding. A systematic approach was put forward to avoid suchcracks during debinding, which could guarantees the strength of green bodywith less organic content.Additionally, pure calcium aluminate cement was applied to solidifyAl2O3 suspension for the first time. During hydrolysis, the cement yields Ca2+,then the molecular chains of sodium alginate in Al2O3 suspension arecrosslinked, and the Zeta potential near Al2O3 surface is reduced because ofthe increased ionic strength in the suspension. As a result, the strength of thegreen body from the solidified suspension is increased.