| The colloidal molding process can not only prepare ceramic material parts with complex shapes,but also effectively inhibit particle agglomeration and reduce embryonic defects.The preparation of the stable suspension with high solid content and low viscosity is the basis of the colloidal molding process.The suspension stability affects compactness and sintering performance of the embryo,and it also has an influence on final performance of the ceramic material.However,types of dispersant,adsorption capacity and adsorption mechanism of dispersants,powder surface modification and suspension rheology still need to be further analyzed.High-purity spherical and platelet ?-Al2O3 particles are used and the research methods such as ChemiBio 3D,Cryo-SEM,TEM,FT-IR,XPS,AES,Mathematica are applied in this research.The regularities among surface potential,particle aggregation and the stability of the suspension without dispersant are explored.The method of adding heteropolyacid salt,adding polyelectrolyte,physical shearing fragmentation are systematically studied to improve the stability of high-purity ?-Al2O3 suspension,at the same time Mathematica is used to model and calculate the fragmentation kinetics of destabilized flocs in this research,the purpose is to find out the rules and mechanisms about effects of different methods on the stability of the ?-Al2O3 suspension.It provides a theoretical basis for the colloidal forming process of new ceramic materials.The study shows that there are certain regularities among particle surface potential,particle aggregation,and the stability of the suspension.The fractal dimension of spherical and platelet ?-Al2O3 particle aggregate are 3.2 and 2.4 at isoelectric point,the microstructure of ?-Al2O3 particles is analyzed by non-lattice variable-Df model and the fractal dimension values are the same as the experiment.The yield stress of ?-Al2O3 suspensions and solid volume fraction have a linear relationship.The IEP of the spherical ?-Al2O3 particle is 9.5.When pH is greater than 11 or less than 7,the system stability of spherical ?-Al2O3 suspension behaves a better performance.The IEP of the platelet ?-Al2O3 particle is 9.2.When pH is less than 3,the stability of platelet ?-Al2O3 suspension has a better performance.However,the corrosion of the mold needs to be considered under higher or lower pH conditions.Under the condition of different suspension concentration,yield stress has a linear relationship with the square of zeta potential.Heteropolyacid salt dispersants have a significant influence on the stability of the ?-Al2O3 suspension system.The patch charge attraction force is produced between heteropolyacid ions and positively charged ions under low pH conditions which decreases the stability of the suspension.The surface coverage of heteropolyacid ions on the particle surface is decisive factor for the stability of the spherical ?-Al2O3 suspenison.The platelet ?-Al2O3 particles have a smaller specific surface area and the active adsorption sites on different surfaces are different,the maximum yield stress of suspension is nearly the same at different heteropolyacid ions surface coverage.Lithium heteropolytungstate is chemically adsorbed on the surface of ?-Al2O3 particles and the better steric repulsive effect is obtained.The optimal dosage of heteropolyacid salt is lower at alkaline conditions.When the concentration of suspension is 55wt%,the optimal dosage of LST is 0.6dwb%and the optimal pH of the suspension is 9.5.The platelet ?-Al2O3 suspension appeares shear thickening at high shear rate.Polymer dispersant is an important factor to improve the stability of ?-Al2O3 suspension.With the increase of pH,the saturated adsorption capacity of the anionic polyelectrolyte dispersant on the particle surface decreases.The isoelectric point moves to a lower pH value after the adsorption of anionic dispersant.It's easy to obtain a stable ?-Al2O3 suspension under alkaline conditions.PssNa obtained a good dispersion effect under basic conditions,and its saturation adsorption capacity on the particle surface is three times than CA and DIBMA.PssNa chemically adsorbs on the surface of the ?-Al2O3 particles,and stabilizes the suspension through the electrostatic steric mechanism.The adsorption of PEI moves IEP to a higher pH.The amino groups in PEI chemically adsorbed on ?-Al2O3 particles and PEI with higher molecular weight has a better dispersion effect.The sequential addition of 0.3dwb%DIBMA and 0.1 dwb%PEI to 50wt%?-Al2O3 suspension can greatly reduce the viscosity of the ?-Al2O3 suspension,and obtain a lower viscosity than single anionic or cationic polyelectrolyte dispersant,the viscosity is only 0.016 Pa·S.At the same time,it obtains a better steric repulsive effect and the maximum yield stress is reduced by 75%compared with no additive.The ?-Al2O3 floc fragmentation kinetics analysis shows that high speed shearing fragmentation process decreases the floc size and improves the stability of the suspension system.A simulation method for the particle size of ?-Al2O3 flocs after fragmentation is proposed.Mathematica was used to solve the population balance equation and fragmentation kinetics equation,the simulated particle size distribution data were well fitted with the experimental results. |
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