Study of the magnetic properties of a BSCCO superconducting film thermally treated in a domestic microwave oven

Full text: Since the discovery of high temperature superconductors (HTSs), several materials with T_c between 20 and 160 K have appeared. It is well established that the use of precisely controlled thermal treatments is critical to achieve the desired phases as well as the improvement of structural, electrical and magnetic properties of a superconducting material. In the 80s, D. R. Baghurst et. al., proposed the use of a domestic microwave oven (2450 MHz) in the synthesis of superconducting materials. Such proposal was based in the discovery of that CuO is a good absorber of the referred microwave frequency. In the present work we report the preparation of a superconducting film using a good quality fine powder (less than 38 μm) of the BSCCO- 2212 system. Such power was obtained commercially and presented a nominal composition of Bi_1.8Pb_0.4Sr₂CaCu₂O_x. The film was grown on a LaAlO₃ (100) crystalline substrate and was thermally treated at 800°C for 10 min using a domestic microwave (Panasonic model NN– S46BK, 800W and 2.45 GHz) suitably adapted for this purpose. The structural characterization was performed by X-ray diffraction (XRD) in a Shimadzu diffractometer model XRD-6000 with copper Kα filter, indicated the preferential formation of the 2212 phase, however, the thermal treatment favored the appearance of the 2201 phase and a secondary phase of Bi₂CaO₄. Magnetic measurements were carried out in a Quantum Design magnetometer model PPMS-6000. Analysis of AC-susceptibility for excitations field of H_AC=0.1 Oe and H_AC=1.0 Oe with frequency ν = 1 kHz shown some superconducting transitions which the higher temperature of a transition was T_c=79 K. A significant difference between those curves is related to the inhomogeneity of the weak-links (WLs) caused, e.g., by non-superconducting materials formed in the sintering process as well as the wide distribution of grains size. Such behavior is also present in magnetization (normalized by H_DC) vs. temperature curves for H_DC=100 Oe and H_DC=1 kOe. Since WLs limit the shielding and/or the transport current and this limitation is strongly dependent of H_DC, even for the film cooled down up to 10 K, only a single superconducting-normal estate transition takes place. Such behavior means that, for these values of the H_DC, there are not intergranular currents (WLs are not working) and the response is due to the intragranular portion. The magnetization hysteresis loops as a function of magnetic field, allied with the Bean model, was used to determine the critical current density. Thus we obtained for zero magnetic field and at T=10 K J⁰_c=8.64 x 10⁴ Acm^-2 and at 65 K, J⁰_c=2.50x10⁴ Acm^-2. Reference: [1] Patel, R. H.; Nabialek , A.; Niewczas, M. Characterization of superconducting properties of BSCCO powder prepared by attrition milling, Supercond. Sc. Tec., 18 (2005) 317–324. (author)