New high-Tc oxide compounds, whose superconducting transition Tc's exceed 100K, were discovered in Bi-Sr-Ca-Cu-O (BSCCO) and Tl-Ba-Ca-Cu-O (TBCCO) systems. The stoichiometric composition of the high-Tc phase was found to be Bi2Sr2Ca2Cu3Ox and Tl2Ba2Ca2Cu3Ox. Upper critical field Hc2's of these compounds were evaluated for bulk sintered specimens. The Hc2 (T=0K) defined at the midpoint of the resistive transitions are 160T and 220T for BSCCO and TBCCO high-Tc phases, respectively. High values of critical current density Jc at 77K have been already attained for sputtered BSCCO and TBCCO, while the Jc of the bulk specimens still remain less than 104A/cm2 at 77K and 0T. The parameters restricted the Jc in the bulk polycrystalline materials are considered to be (1) anisotropy in Hc2 and Jc, (2) weak link between grains, (3) weak pinning force, and (4) flux creep. The effect of each parameters is discussed with the experimental results of Jc and magnetic measurements.
Experimental and theoretical investigations of time dependent heat transfer in pressurized He II channel terminated by a copper rod have been carried out. Heat flux is applied in the form of a sinusoidal wave. As the frequency of heat flux increases, the change of copper and He II temperatures converge for the case of constant application of the effective value of heat flux. And no dispersion of Kapitza conductance is observed within the frequency range from 0 to 40Hz. On the basis of the mutual friction counterflow combining with the heat conduction in copper and Kapitza conductance, a one-dimensional heat transfer model has been developed, which can well predict the experimentally obtained time dependent heat transfer characteristics.
He II under pressures from 1atm to saturated vapor pressure and constant bath temperature was investigated as a possible coolant for superconducting magnets. The cooling strength is described in terms of a critical frequency fc at which the peak transport currents corresponding to successive constant voltage pulses decrease abruptly due to a breakdown of superfluidity. Maximum fc was found under the pressure at the lower λ-point (He IIλ) i.e. with a hydrostatic pressure head of about 2m. This allows the use of ordinary cryostats while retaining the efficiency of saturated He II systems. He IIλ does not give full stabilization in the classical sense, but is a promising coolant for pulse magnets or meta-stabilized DC magnets.