Enhanced Superconducting Properties in Bi2Sr2Ca1Cu1.75Na0.25Oy Ceramics Prepared by Hot-Pressing Under Different Pressures and Temperatures

Bi2Sr2Ca1Cu1.75Na0.25Oy ceramics have been prepared by the classical solid state reaction method followed by hot-pressing process under several applied pressures and temperatures: 22 and 33 MPa at 750 degrees C, and 22 MPa at 800 degrees C. X-ray diffraction data indicate that the major phase in all samples is the Bi2Sr2Ca1Cu1.75Na0.25Oy phase, accompanied by small amounts of Bi2CaO4 and Bi3Sr4CaCu3O4 secondary phases. Microstructural results clearly showed that the best grain orientation has been obtained under 33 MPa at 750 degrees C. Critical temperatures, T-C (onset) and T-C (offset), of these samples were significantly increased to 96.6 and 85.6 K, respectively, pointing out to a good intergranular coupling. M-H hysteresis curves for all samples display a very large area due to the microstructural characteristics obtained in these samples. The highest critical current density, calculated from M-H measurements according to Bean's model, has been obtained in samples prepared under 33 MPa at 750 degrees C (201.5 x 10(4) A/cm(2) at 1.9 T and 15 K), which are much larger than those reported for samples with the same composition prepared through the classical solid state method.