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Parameters that promote rapid formation of aligned high-T, phase in the high-temperature superconductor (Bi, Pb)2Sr2Ca2Cu30x (Bi-2223) powder in tube superconducting tapes have been studied. A rapid phase formation was achieved by two means. Firstly, frequent mechanical pressings during the early stages of the heat treatment were applied. The pressings counteract the expansion that occurs during sintering, increase grain connectivity and reduce diffusion distances. Secondly, the fraction of process time within the thermodynamically stable region of the Bi-2223 phase was maximized by ramping of the oxygen partial pressure during heating and cooling. In addition, the starting phase composition of the superconductor powder was found to have a critical influence on the reaction speed. A complex phase composition gave the fastest formation rate of 81-2223 phase. Within 30 h of processing time, critical current densities exceeding 12 kA c w 2 (1 WV cm-', 77 K, 0 T) were obtained compared to 2 4 kA cm-> without the above measures. Also, the effect of variations in oxygen paltial pressure, pressing schedules, paltial melting and a two-component starting powder based on (Bi, Pb)2Sr2CaCu20, were investigated for this process.

A new calorimetric method has been developed to measure ac losses in superconductors. Calibration of the losses requires a reference heater of the same size and shape as the superconductor mounted in a symmetric environment. Its main advantage is the direct determination of the total losses. Variations of the method include a null technique and averaging of the main Fourier component to reduce noise effects. The method has been tested on Bi-2223 tapes and gives the expected hysteresis losses as well as a frequency independent loss which can be attributed to flux creep. The measured losses are similar to those obtained by other groups with other methods.

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The ac loss of a superconducting cable conductor carrying an ac current is small. Therefore the ratio between the inductive (out-of-phase) and the resistive (in-phase) voltages over the conductor is correspondingly high. In vectorial representations this results in phase angles between the current and the voltage over the cable close to 90 degrees. This has the effect that the loss cannot be derived directly using most commercial lock-in amplifiers due to their limited absolute accuracy. However, by using two lock-in amplifiers and an appropriate correction scheme the high relative accuracy of such lock-in amplifiers can be exploited. In this paper we present the results from ac-loss measurements on a low loss 10 metre long high temperature superconducting cable conductor using such a correction scheme. Measurements were carried out with and without a compensation circuit that could reduce the inductive voltage. The 1 µV cm-1 critical current of the conductor was 3240 A at 77 K. At an rms current of 2 kA (50 Hz) the ac loss was derived to be 0.6±0.15 W m-1. This is, to the best of our knowledge, the lowest value of ac loss of a high temperature superconducting cable conductor reported so far at these high currents.