  The dissipative mechanism at low current density is compared in three
different classes of superconductors. This is achieved by measurement of
resistance as a function of temperature and magnetic field in clean
polycrystalline samples of NbSe2, MgB2 and Bi2Sr2Ca2Cu3O10 superconductors.
Thermally activated flux flow behavior is clearly identified in bulk MgB2.
While the activation energy at low fields for MgB2 is comparable to
Bi2Sr2Ca2Cu3O10, its field dependence follows a parabolic behavior unlike a
power law dependence seen in Bi2Sr2Ca2Cu3O10. We analyze our results based on
the Kramer's scaling for grain boundary pinning in MgB2and NbSe2.
