PACKING OF BIMODAL MIXTURES OF COLLOIDAL SILICA

The aim of this work was to determine the influence of the size distribution of silica powders on the characteristics of the resultant green bodies obtained by colloidal filtration. In order properly to investigate the scientific relationships between the forming parameters and the nature of the particle packing, a model ceramic material was chosen, i.e. monosized (100 and 10 nm) spherical powders of SiO2 and colloidal filtration under 3.45 MPa was used to cast monomodal and bimodal mixtures of silica sols. The electrophoretic and rheological behaviour of those systems have been investigated. The permeability as well as the pore size distribution was found to decrease as the volume faction of fines increased (to 30%). The evolution of the density as a function of the volume fraction of fines follows the behaviour observed for the packing of bimodal mixtures of hard spheres. The systems with only one size particle were found to pack with densities close to that of random close packing. Two dimensional computer simulations and correlation to the experimental results showed that a model of "triangular" pores is preferred to a model of "square" pores.