Casimir Effect for spherical boundaries: A thermofield dynamics approach
  The Casimir effect for spherical geometry is calculated using generalized Thermofield Dynamics for the case of scalar field. Casimir force and Casimir pressure are presented. It is found that for high temperatures the Casimir force does change sign. 
D.U. Matrasulov, Kh.T. Butanov, Kh.Yu. Rakhimov, F.C. Khanna, A.E.   Santana