Casimir energy in a spherical surface within surface impedance approach:   the Drude model
  The Casimir Energy of a spherical surface characterized by its surface impedance is calculated. The material properties of the boundary are described by means of the Drude model, so that a generalization of previous results is obtained. The limits of the proposed approach are analyzed and a solution is suggested. The possibility of modulating the sign of the Casimir force from positive (repulsion) to negative (attraction) is studied. 
Luigi Rosa and Lucia Trozzo
hep-th