PLASTIC-TO-BRITTLE TRANSITION OF CONSOLIDATED BODIES - EFFECT OF COUNTERION SIZE

The effect of ion size on the force necessary to remove counterions fr om the surface when particles are forced into contact (primary minimum ) was investigated. Alumina slurries dispersed at pH 12 and then coagu lated with 0.5 M Li+, Cs+, and tetraethylammonium (TEA(+)) chlorides w ere consolidated by pressure filtration at different applied pressures . Uniaxial compression tests were performed to determine the plastic-t o-brittle transition pressure for bodies formulated with each of the d ifferent counterions. Brittle bodies are produced when particles are p ushed together to form a strong touching network. The force required t o push particles together is related to the slope of the repulsive pot ential barrier, with steeper slopes requiring greater force. The conso lidation pressure necessary for brittle behavior was greater for slurr ies with small counterions such as Li+ as compared with Cs+ and TEA(+) . The results presented here show that the resistance to pushing parti cles into contact is related to the size of the bare counterion in wea kly attractive (salt-coagulated) slurries. That is, the slope of the p otential barrier produced when the slurry is formulated with smaller i ons is steeper. This result is consistent with the idea that smaller c ounterions are more strongly bound to the surface, as well as the fact that the repulsion due to the finite size of the ion occurs at smalle r interparticle separation distances for the Smaller counterions. (C) 1997 Academic Press.