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We have measured the current-voltage (I-V} characteristics of several high-temperaturesuperconducting materials with widely difkrent morphologies {bulk Ag/Pb-Bi-Sr-Ca-Cu-0 tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-0 samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current I"the normal-state resistance R",and y, which is proportional to the pinning potential U(H, T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines I"we conclude that its use puts this parameter on a solid physical basis.

The properties of a small pancake coil made with a 10mm wide copper-stabilized YBa2Cu3O7−x (YBCO) coated conductor were investigated. The radial component of the magnetic field was mapped at the coil edge in both the dc and ac regimes and differs significantly from that calculated assuming a uniform current distribution. The observed hysteresis indicates the strong influence of the ferromagnetic properties of the substrate. The ac losses of the coil were measured for ac frequencies between 60 and 1000Hz. The differences in properties of the YBCO coil and a similarly prepared copper coil are discussed.

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