No abstract

Electrostatically stabilized aqueous dispersions of nanometric-sized silica particles have been characterized by light scattering, small angle X-ray scattering, and osmotic pressure measurements. All three studies yield similar values for the particle size, molecular weight, and particle surface charge. In addition, good agreement is found between directly measured osmotic pressure values and those calculated from scattering studies. Using the particle properties as determined from the three experiments, the osmotic pressure as a function of volume fraction is compared to liquid state theory models with no adjustable parameters. Finally, small angle X-ray scattering studies indicating the presence of long-range order in moderately to highly concentrated dispersions are reported.

Viscosity and yield stress measurements of A120, slurries containing high concentrations of electrolyte are reported. Contrary to what is expected from DLVO theory, the particles in coagulated slurries produced in this way are held together by weaker forces than particles in slurries brought to the isoelectric point by changing the pH. In both cases an attractive, connective particle network is present. However, the network at the isoelectric point is stronger, as indicated by its much higher yield stress and by its higher viscosity at stresses above the yield point. An additional short-range repulsive force that appears at low pH and high electrolyte concentrations is believed to be responsible. This force prevents the particles from reaching the primary minimum associated with particle contact. The effects of solids loading, electrolyte concentration and type, and particle size have been investigated.

Dispersed ceramic slurries with high concentrations of indifferent electrolyte have been shown to effectively eliminate mass segregation effects in composite materials. Furthermore, high packing efficiencies have been achieved with these systems. It has been suggested that the addition of high concentrations of indifferent electrolytes to an otherwise dispersed ceramic slurry modifies the interaction potential so that the short-range adhesive attraction is diminished by additional repulsive forces. This lower attractive force is still high enough to cause coagulation which raises the viscosity, but small enough to allow easy particle rearrangement during filtration or centrifugation. This rearrangement leads to high packing densities. The effectiveness of this behavior on the improvement of ceramic powder processing of composites is demonstrated with Al2O3 and ZrOz.[

The relation between relative density and applied network pressure has been determined for aqueous A1,0, slurries prepared with different interparticle potentials and consolidated by centrifugation. Attractive interparticle potentials were obtained by either changing the pH to the isoelectric point or adding an excess electrolyte. A range of centrifugal speeds produced consolidation pressures between and 10 MPa. At lower speeds, the density gradient was determined with an X-ray absorption technique. The maximum packing density (0.62 & 0.02) was achieved for both dispersed and coagulated slurries at network pressures >0.5 MPa. At lower pressures, the packing density of these slurries was pressure-dependent, where the coagulated (salt added) slurries had a relative density between that of the dispersed and flocced slurries. Slurries flocculated by adjusting the pH to the isoelectric point never reached the highest packing density at the largest pressure.