  In this paper we find that the second law of thermodynamics requires an upper
limit of the conductivity. To begin with we present an ideal model, the cavity
with a mobile plate, for studying the thermodynamic properties of radiation
field. It is shown that the pressure fluctuation of thermal radiation field in
the cavity leads to the random motion of the plate and photons would be
generated by dynamical Casimir effect. Meanwhile, such photons obey a
non-thermal distribution. Then, to ensure the second law of thermodynamics,
there must be a upper limit of the conductivity.
