AC Loss and Thickness Dependence of Critical Currents in Coated Conductors

This program of research is directed toward understanding the physical properties of certain materials in superconductive “coated conductors.” Specifically investigated were Ni1−xWx alloys for use as substrate and thin films of YBa 2Cu3O7, a high- Tc superconductor with many attractive features. A study has been conducted on the magnetic properties of a series of biaxially textured Ni1−xWx materials with compositions x = 0, 3, 5, 6, and 9 at.% W. These materials are important as substrates for “RABiTS”-type coated conductors that incorporate high temperature superconductors for current transport. The quasi-static dc and ac hysteretic loss W was determined to support estimates of the ferromagnetic contribution to the overall ac loss in potential ac applications. The alloys were prepared by either vacuum casting or powder metallurgy methods, and the hysteretic loss tended to be lower in materials that were recrystallized at higher temperatures. Some samples were progressively deformed to simulate winding operations; this increased the hysteretic loss, as did sample cutting operations that create localized damage. In ac magnetization measurements, the effects of ac frequency and dc bias field on the ferromagnetic loss were determined.

Furthermore, in order to better understand the complex problem of vortex pinning and the identification of defects that support the critical current density Jc in these “RABiTS”-type coated conductors, we have made magnetometric studies of the Jc flowing in thin YBa2Cu3O7−d (YBCO) films of various thicknesses d, both as a function of applied field and temperature T. The films, grown by a BaF2 ex - situ process and deposited on buffered“RABiTS” substrates of Ni-5%W, have thicknesses d ranging from 28 nm to1.5μm.

Isothermal magnetization loops M(H; T) and remanent magnetization Mrem(T) in zero applied field H = 0, were measured with H c-axis (i.e., normal to film plane). The Jc(d) values, which were obtained from a modified critical state model, increase with thickness, peak near a particular thickness, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find a power law falloff Jc ∝ H−β with β ∼ (0.56 - 0.69) for all materials. This feature compares well with the power-law exponent β = 5/8 obtained theoretically by Ovchinnikov and Ivlev for pinning by large random defects, as are observed by TEM. Comparison of the theoretical predictions with experimental Jc(H, T, d) yields a mostly consistent picture, using values for the size and density of defects that are comparable with those deduced from TEM images.

Finally, for higher temperatures approaching the irreversibility line, we find J(T, sf) ∝ (1 − T/Tc)n with n ∼ 1.1 - 1.3. This points to “δTc inning” (pinning that suppresses Tc locally) in all of these YBCO materials, as expected for the observed large, non-superconducting defects.