| Over last several decades, great progresses have been made in technology of preparing superfine powder, and the study on the application of superfine powder has received increasing attention. For most of the superfine powder applications, complete dispersion of particles in media is prerequisite. However, superfine particles are prone to form aggregates or agglomerates in the media due to their high activity and large surface area. Therefore, the key is to control stability of superfine powder during its application.The application of superfine powder in the field of ceramic materials has strongly promoted the development of advance ceramics. Processing through colloidal route is most commonly used in high-tech ceramics for its advantages in improving the materials performance. The stability of slurry is the main issue in colloidal processing. It not only influences greatly on the green compacts and the sintering performance but also on the quality of the final products.There are various methods to controlling the stability of suspensions. One way is using polymer or polyelectrolyte to modify the particle surface state based on electrostatics and steric force repulsion mechanism. The systems containing polyelectrolyte dispersant play an especially important role in colloidal processing. Polyelectrolytes exhibit several advantages over inorganic dispersants, including greater stability, greater control of thixtropy or flocculation state, and higher consolidated density.The suspension with polyelectrolyte as dispersants is a complex multiphase system. Some important parameters such as pH value, temperature, ionic strength as well as the routes of preparing suspension, all of those have great influence on the suspension stability. It is important to fully understand the relationship between stability of suspensions and these parameters for tailoring suspensions properties. Thus, this investigation was designed to elucidate the influence of some important parameters on suspension stability by the means of rheology and sedimentation. An aqueous α-Al2O3 system with polyacrylic acid was chosen as the model. Someimportant conclusions were obtained as follow:Both zeta potential and rheological properties of suspension are strongly influenced by the amount of the dispersant. The zero point of charge of the alumina powder used is at pH =8.5, but the addition of a type of cationic polyelectrolyte (PAA) shifted the zero point of charge to lower pH values because of the increase of negatively surface charged sites. The zero point of charge (zpc) was shifted to pH=3.5 after addition of PAA.There was an optimal dispersant content used for preparation of suspension with much lower viscosity. In 20 vol% (X-AI2O3 suspensions at pH=6.5, all of the stabilized systems had very low viscosities and the difference between suspensions stabilized at 1.6wt% and 4.0wt% of PAA was very small. As the solids loading increased, the magnitude of the viscosity also increased, whereas the effective content of dispersant range narrowed. If the dispersants added was higher or lower than the optimal dosages, the viscosity suspension increased, indicating that the stability declined.For a given amount of PAA. the rheological properties of the suspensions were improved dramatically as pH increased. The PAA amount needed to stabilize the suspensions decreased as pH increased. The viscosity of the suspensions with solids loading over 50% varied greatly with pH even at optimal PAA addition. The optimal pH value for preparing highly concentrated aqueous 01-AI2O3 suspensions with polyacrylic acid was at pH 8.5.The experiment results showed that the stability of suspensions of a-AhOjwas dependent on the adsorption, or processing routes. The stability of the suspension with a final pH 6.5 was improved considerably if it experienced an adsorption stage at pH above the zpc of the alumina. The pH values of the first adsorption stage had a great impact on the stability of slurries. When the pH value in the first stage of adsorption increased from 8.0 to 10.0. the final viscosity of the slurry decreased and the stability was enhanced. But when the pH in the first stage reached 11.0, the final viscosity of the slurry increased and the stability declined. Increasing the time of the first adsorption stage was helpful to the stabilization of suspensions, due toimproved homogeneity of the distribution of PAA on the particle surface. This evidently shows that homogeneity of the distribution of dispersant on the particle surfaces is critical for such system. For an unsaturated adsorption suspension, the stability was mainly determined by PAA distribution state on the surface of particles rather than by its average surface density of adsorption. The suspension with unsaturated adsorption limit could be well stabilized under the condition of homogeneous while heterogeneous one would cause flocculation.The ultrasonic dispersion was an effective way to improves heterogeneity of adsorption. The ultrasonic dispersion that effectively broke down the particles aggregates or agglomerates before adsorption could obviously improve PAA distribution state on surface of the particles, thus lead to enhancing flow properties of suspensions.The optimal dosages of PAA were influenced by ionic strength. But at the same time this is a pH dependent factor. Compared with acidic suspensions, an alkalic suspension was more sensitive to ionic strength with respect to optimal dosage. The similar phenomenon exists in acidic media.The experiments also showed that among various electrolytes, including NaCl Na2SC>4 and CaCb, CaCh exhibited most strong influence on the stability of suspensions, while Na2SO4 had weakest effects.It is also found that temperature had a great impact on the stability of the system. As the dosage of dispersant added is lower than that of saturation adsorption limit, the suspension shifted from a stable state to unstable one with increase temperature, but the viscosity of suspension dropped dramatically for the one of the higher solids loading if sufficient amount of PAA was added. |
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