PARTICLE-SIZE EFFECTS ON COLLOIDAL PROCESSING OF OXIDE POWDERS

Rheological behavior, properties of colloidal solids consolidated by filtration, and their structure change during drying were studied with aqueous suspensions of a mullite powder of nanometer size and two kinds of alumina powders of submicrometer sizes. The rheological properties of non-Newtonian flow suspensions were analyzed by a power law equation of S = K-gamma(n), where S is the shear stress, gamma the shear rate, and K and n (0 less-than-or-equal-to n less-than-or-equal-to 1) constants. The critical solids content (V(c)) at n = 0 (indicating colloidal solids) depended greatly on the zeta potential of particles in suspensions, and dominated the densities of dried green compacts (V(g)) of submicrometer sized powders. In a nanometer sized powder, the densities of dried green compacts were dependent on both V(c) value and the solid contents of suspensions. Phase diagrams of one-component colloidal systems were constructed by plotting the V(c) and V(g) values against the zeta potential of particles. These phase diagrams indicate that the colloidal solids range (surrounded by V(c) and a minimum V(g) lines) is narrow for nanometer sized powder and wide for submicrometer sized powder. The solids content range of dried green compacts was very narrow for submicrometer sized powder but relatively wide for nanometer sized powder due to the low flexibility of colloidal structure during drying.