Superconducting Properties of High- J c MgB 2 Coatings
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Superconducting Properties of High-Jc MgB2 Coatings D.K. Christen, C. Cantoni, J.R. Thompson, M. Paranthaman, M.F. Chisholm, H.R. Kerchner, and H.M. Christen, Oak Ridge National Laboratory, Oak Ridge, TN 37831
ABSTRACT We report the fabrication and superconducting properties of ~0.5 µm thick, fine-grained polycrystalline coatings of MgB2 on single-crystal substrate surfaces. The films exhibit large critical current densities, implying little effect from the grain boundaries. Analyses for thermal activation effects are inconclusive, and evidence is presented that the irreversibility line is dominated by the combined influences of Hc2 anisotropy and polycrystallinity. Comparative studies of the magnetic persistent currents and electrical transport properties reveal excellent agreement over a wide range of temperature and magnetic field. This result is contrary to similar comparisons on high-temperature cuprates, where disparities arise from the effects of large flux creep and the diverse electric field regimes probed by the two techniques. The MgB2 films exhibit extremely sharp voltage-current relations away from the irreversibility line, in qualitative agreement with observed large Jc values and low rates of magnetic flux creep.
INTRODUCTION The discovery of superconductivity at 40 K in MgB2 has created interest for use of the material in the field of superconducting power applications, where, due to the simplicity and low fabrication cost, this compound might substitute for the more complex high Tc cuprates.[1] For electric power applications, the advantages of MgB2 as compared to YBCO or BSCCO for fabricating low-losses superconducting wires are still controversial. The lower critical temperature of MgB2 represents a serious challenge for the engineering and economics of cryogenic refrigeration systems. Moreover, the critical current behavior in magnetic field, as reported for bulk samples and polycrystalline or uniaxial textured films, is still weaker than that of BSCCO tapes in large magnetic fields. On the other hand, advantages of MgB2 include the smaller anisotropy of the electronic structure and strong-linked properties of the grain boundaries[2]; the latter act as a major obstacles to current flow and are problematic for scale-up of YBCO-based high-Jc coated conductors[3]. The investigation of the H-T phase diagram and electrical transport properties of MgB2 in forms that have potential practical application is very important in order to fully understand this material, particularly the features that limit the current conduction in high magnetic fields and the mechanisms that can expand the operating field. Here we report electrical transport and magnetization measurements of polycrystalline MgB2 films grown on Al2O3 substrates. The transport voltage-current characteristics were measured in a temperature range from 5 to 40 K, up to magnetic fields of 15 T and electric fields of 0.25 V/cm. From these measurements we deduced an upper critical field line Hc2(T), and identified important considerations regardin
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