  We consider the problem of electron transport in segregated
conductor-insulator composites in which the conducting particles are connected
to all others via tunneling conductances, thus forming a global
tunnelingconnected resistor network. Segregation is induced by the presence of
large insulating particles, which forbid the much smaller conducting fillers
from occupying uniformly the three-dimensional volume of the composite. By
considering both colloidal-like and granular-like dispersions of the conducting
phase, modeled respectively by dispersions in the continuum and in the lattice,
we evaluate by Monte Carlo simulations the effect of segregation on the
composite conductivity {\sigma}, and show that an effective-medium theory
applied to the tunneling network reproduces accurately the Monte Carlo results.
The theory clarifies that the main effect of segregation in the continuum is
that of reducing the mean interparticle distances, leading to a strong
enhancement of the conductivity. In the lattice-segregation case the
conductivity enhancement is instead given by the lowering of the percolation
thresholds for first and beyond-first nearest neighbors. Our results generalize
to segregated composites the tunneling-based description of both the
percolation and hopping regimes introduced previously for homogeneous
disordered systems.
