A Experimental Study of the Irreversible and Reversible Thermodynamics of Magnetic Field Induced Superconducting to Mixed State Phase Transitions in Yttrium BARIUM(2) COPPER(3) Oxygen(x)

By using a temperature measurement, the magnetic field induced dissipation in polycrystalline YBa _2Cu_3O_ {x} at T << T _{c} has been measured for magnetic fields up to 2970 Oe applied at constant ramp rates, in the absence of a transport current. The B-H hysteretic behavior has been simultaneously measured so a comparison of the dissipation measured by magnetization to that of thermal dissipation could be determined. Results show that at low temperatures (4.2-8 K) the thermal measurement is very sensitive (dissipation energies of a fraction of an erg/cm^3 in a 60 hz bandwidth can be sensed) and may provide a superior method in determining dissipation state properties for high temperature superconductors. The dissipation and magnetization results have been modeled using a classical eddy current mechanism, a Rayleigh-Bean model, and Preisach's hysteresis model. No single model described all of the dissipation data. However, at low fields in nonvirgin runs the classical eddy current model roughly described the data, but for high fields in virgin runs the Rayleigh-Bean and Preisach model described the data. We also report the observation of magnetocaloric cooling in polycrystalline YBa_2Cu _3O_{x} at 6.5 and 7.1 K. By using a temperature measurement the field induced cooling in an adiabatic environment has been measured for magnetic fields up to 500 Oe applied at constant ramp rates. The cooling is proportional to the square of the applied field and independent of the ramp rate of the magnetic field. Using classical thermodynamic arguments we find that 0.001-0.0045% of the material has returned to the normal state in an applied field of 100 Oe at 7.1 K, and that the field dependence is consistent with a quantized flux lattice. This experiment is unique; it is the first thermal measurement of the magnetic field induced dissipation in YBa_2Cu_3O _{x} or any other high T_{c} compound.