High critical currents in powder in tube MgB2 wires: influence of microstructure and heat treatments

L. CIVALE; A. SERQUIS; D. L. HAMMON; X. Z. LIAO; J. Y. COULTER; Y. T. ZHU; D. E. PETERSON; F. M. MUELLER

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY

Año: 2003 vol. 13 p. 3347 - 3350

1051-8223

MgB2 wires were produced by the powder-in-tubemethod, using commercial MgB2 powder with 5%at Mg powderadded as an additional source of magnesium, and stainless steelas sheath material. Depending on the parameters of the heattreatments, the dc transport critical current can be increased ordecreased by more than one order of magnitude as comparedwith the as-drawn wire. Analysis of the zero-field-cooling dcmagnetization demonstrates that these large variations are due tochanges in the connectivity of the wires. We discuss theoptimization of the annealing conditions in terms of the changesin the microstructure, as determined from TEM and SEManalysis. We show that annealing can strongly improve theconnectivity by eliminating most of the micro-cracks present inthe un-annealed wires. In contrast, inappropriate annealingconditions result in a deterioration of the connectivity due to theloss of Mg.