Thermally induced flux motion in grain aligned Y-Ba-Cu-O

The flux-flow-induced Nernst effect was studied in grain aligned polycrystalline Y-Ba-Cu-O. Nernst voltages have been measured for magnetic fields up to 6 T and temperatures ranging from 75 K to 100 K. The observed behavior is similar to that seen in low-temperature superconductors. The linear temperature dependence of the Nernst voltage on the applied temperature gradient has been verified. The flux-flow resistivity has been measured in magnetic fields up to 6 T. The broadening of the transition curve with increasing applied fields is evident. The resistivity is approximately five times higher than that measured in a high-quality single crystal. The increase may be attributable to intergranular effects. The measured Nernst voltages and flux-flow resistivities have been used to calculate the flux-line transport entropy. A comparison with the transport entropy derived from single-crystal data indicates that the polycrystalline transport entropy is a factor of 5 smaller. The discrepancy may be related to the intergranular resistivities which are not flux-flow-induced, leading to an artificial suppression of the flux-flow viscosity and the calculated transport entropy.