Hysteresis losses and the behavior of the magnetic flux threading superconducting disks ofNb oscillating slowly over various angular displacements in static magnetic fields Ho directed perpendicular to the axis of rotation have been investigated by continuously monitoring (B z ) and (By), the average components of the magnetic induction, in the plane of the disk, parallel and perpendicular to Ho. A simple model proposed previously satisfactorily accounts for the initial changes, starting from various initial flux configurations, the subsequent cyclical behavior, and the hysteresis losses. Expulsion of flux from the disk occurs during part of the first oscillation, then the amount of flux threading the disk remains constant during the subsequent oscillations, although the orientation distribution of the flux profile undergoes spectacular periodic variations. The analysis shows that dI/J1 dx = ± k dB I dx describes the orientation of the sheets offlux lines with respect to Ho with distance from the flat surfaces. dB Idx is the critical gradient of flux density and k (T) is the shearing parameter of the flux lattice for the sample. The surface step, ignored in the previous work, is introduced in the model using simple approximations.
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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