The agglomerat-of-spheres model. Theoretical considerations

Abstract

The ‘agglomerate-of-spheres’ (AOS) model associates the resistivity and the mechanical strength of PbO 2 with the narrow zones between the sphere-like agglomerates. Some fundamentals of the electric contact theory are reported. The apparent conductivity of the electrocrystalline network and its relaxation are effected by the mechanical stress in the ‘neck’ zones. To complement this hypothesis, we have considered the surface tension to be effective when a new particle is growing from a planar nucleus, thus forming the ‘neck’. The surface tension acts to minimize the surface of the particle and casts it into a sphere as long as this force, which is proportional to the reciprocal of the diameter, is large enough to overcome the internal friction or strength of the solid body inside its surface. Therefore, during the formation of the PbO 2 active material a so-called electroformative force is observed. It represents the force, which is needed to create the new order of the agglomerate-of-spheres. The nature of this force is similar to that one, which in winter time cracks freezing water pipes. Our theoretical considerations do create the expectation, that at the beginning of discharge, the apparent specific resistance of the PbO 2 network should decrease, but sharply increase at the end of discharge. A further expectation states a shrinking during discharge and a re-expansion during recharge. This expectations are based on the existence of a coherent network with stable necks as long as the necks are not cracked by the discharge process itself or by deposition of too much of lead sulfate within the pores.