The effect of different cooling procedures, after sintering, on the microstructural and superconducting properties of Bi2Sr2CaCu2Ox (B-2212) screen-printed tapes has been investigated. It was found that the amorphous phase originally present in grain boundaries at the sintering temperature undergoes a conversion to the Bi-2212 phase during slow cooling. However, the excess oxygen absorbed during slow cooling (in 21% oxygen) disturbs the order and ordering process of Bi-2212 crystals, but if the slow cooling process is conducted under monotonically reduced oxygen partial pressure, to ensure that the optimum oxidation state at sintering temperature of the Bi-2212 phase can be retained during the cooling procedure, tapes are then characterized by narrow grain boundaries and the highest Jc value ≳104 A/cm2 at 77 K and 0 T.

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Ag-sheathed Bi,Sr,CaCu,O, tape was fabricated by a combination of partial melting, subsequent cooling, annealing and quenching. The partial-melting temperature T, affects t h e growth of the 2212 grains. The promotion of t h e growth at T , 1 8 8 5 °C enhances the J, value at 77 K and 0 T, until the appearance of microcracks at T H 2 895°C. The annealing temperature T, also affects J, and yields a maximum between 840°C 5 T, 5 860°C. Needle-shaped precipitates appeared at T , 5 860°C and porous microstructures at T, 5 840°C cause t h e weak links at the grain boundaries and reduce J,. The optimum cooling period ts from T , to T L is around 10 h, which corresponds to a cooling rate of 3.8% h-'. The prolongation of t h e period t, during annealing at T, enhances J, by the improvement of the weak links and reaches the highest value ofJ, = 6.5 x IO' A cm-2 at 77 K and 0 T fort, = 150 h.

Changes in the energy dissipation of the tape form Ag-(Bi,Pb)2Sr2Ca2Cu3Ox conductor have been interpreted in terms of magnetic flux movement from, or into, the intergrain regions of the highly textured plate like grain structure. The time dependent increase or decrease of the transport critical current (Ic), originates from time dependent angular superposition of the current induced self field (HSF), and external magnetic field [field cooling (HFC), and zero-field cooling (HZFC)] at the intergrain regions. The existence of time dependent changes of Ic can be explained in terms of the time dependent flux redistribution in grains and grain boundaries caused by thermal relaxation processes.

We investigate the AC loss of a high-T c superconducting power-cable conductor with four layers and interlayer insulation. The electric-circuit model is applied for the calculation of the AC loss of the conductor. The result of a calculation for the AC loss shows that the calculated value is nearly equal to the experimental value; therefore this model explains the electromagnetic property of the conductor well. Moreover the result of a calculation for the current distribution of the conductor indicates that almost all the current passes through the outer layer when the transport current is less than the I c of the conductor. Finally we calculate the relation between AC loss and the spiral pitch of the high-T c superconducting wires which make up the conductor. It is found that the AC loss is markedly decreased when the spiral pitch is less than 0.5 m.