Microstructure and flux distribution in both pure and carbon-nanotube-embedded Bi2Sr2CaCu2O8+delta superconductors

In order to improve pinning properties of bulk Bi2Sr2CaCu2O8+d (Bi-2212) ma terials, samples of both pure Bi-2212 and Bi-2212 with carbon nanotubes emb edded (CNTE Bi-2212) have been prepared by partial-melting processing. The preparation conditions are chosen so that a significant fraction of carbon nanotubes can be successfully embedded in the material, as indicated by the rmogravimetric analysis. The microstructure and composition of non-supercon ducting second phases are found to be different in these two types of sampl es. By means of magneto-optical (MO) imaging, flux distributions in both ty pes of samples are investigated up to T = 77 K. The MO investigation reveal s the propagation of a flux front in both pure and CNTE Bi-2212, showing th at there is a strong coupling between grains (clusters) which enables the f low of inter-granular currents. This grain coupling persists in our field r ange of +/- 180 mT. In bulk non-textured ceramic high-T-c superconductors, the flux fronts caused by currents flowing through the entire sample are ob served for the first time. Intra-granular current densities are obtained fr om the images as a function of grain size. The MO investigations have revea led the differences in the current densities caused by the presence of carb on nanotubes, showing that the carbon nanotubes are indeed functioning like columnar defects produced by heavy-ion irradiation. The increase of the fl ux penetration field is also a manifestation of the increase of the transpo rt current density in the CNTE Bi-2212. The superconducting properties in o ur samples are very well reproducible. (C) 1999 Published by Elsevier Scien ce B.V. All rights reserved.