  We compute the Casimir Energy of a spherical region using a Surface Impedance
approach. We characterize the Surface Impedance of the boundary using plasma
model. Exact analytical formulae are obtained by means of the zeta function
regularization method and all the divergencies are explicitly computed. We find
that it is possible to have negative Casimir energy for some range of values of
the relevant parameter. Limits of applicability of the model are discussed.
