F. Lera

Polymer solution processing of (Bi, Pb)SrCaCuO

Abstract A simple polymer processing route to the 110 K phase (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor has been developed. The influence of the polymer to metal starting ratio, as well as the sintering temperature and time on the quality of the resulting superconducting powders has been studied by XRD, a.c. susceptibility and resistivity measurements. Microstructure of ceramic compacts has been studied by SEM, and qualitatively analysed by EDS for compositional homogeneity. The results indicate that the polymer synthesis route described here may offer a good alternative to the conventional solid state preparation methods towards attaining homogeneous 110 K superconductor powder within a reasonable processing time.

(Bi,Pb)2Sr2Ca2Cu3O10+δ superconductor composites: Ceramics vs. fibers

Abstract Well characterized (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O 10+δ superconductor powder has been used to prepare superconductor-glass, -metal and -alloy composites through solid state reaction method. A recently developed Laser Floating Zone (LFZ) apparatus has been used to transform the ceramic precursors into oriented fibers. The diamagnetic properties have been studied by a.c. susceptibility. The microstructure of fibers has been studied by SEM and compared with that of the original ceramic precursors. XRD has been used to study phase composition on representative composite samples and fibers. The results indicate some potential for the 2223-Ag composite, which displays improved diamagnetic properties and grain orientation (fiber).

Oxygen stoichiometry, critical temperature and pinning mechanisms in the 2212 BSCCO superconductor

Abstract Accurate chemical and structural characterizations, critical temperatures ( T c ) derived from magnetic measurements and magnetization analysis of the pinning force density ( F p ) on a series of BSCCO samples have been used to model the effect of the oxygen content ( y ). The variations of c -axis, T c ( y ) and F p ( y ) are related to those of the irreversibility fields ( B irr ) through the change in the number of oxygen vacancies, which are considered as effective pinning sites. The scaling of F p ( B ) using B irr is analyzed.

Influence of oxygen stoichiometry on Tc and pinning force of Bi2Sr2CaCu2O8+δ

Abstract The influence of oxygen stoichiometry in T c and F p of Bi 2 Sr 2 CaCu 2 O 8+δ has been investigated by means of X-ray and electron diffraction, a.c. susceptibility and d.c. magnetization. Through proper oxygen control, T c = 94 K and an increase of one order of magnitude in F p with respect to the started material has been achieved.

Diamagnetism and critical currents of BiCaSrCuO samples

Abstract Three different samples of Biue5f8Caue5f8Srue5f8Cuue5f8O have been obtained. The solid state reaction ceramic method, followed by heat treatments at different temperatures, has been used. The samples have been microscopically characterized by X-ray, neutron diffraction and electron microscopy. From electrical and magnetic measurements the existence of a majority 85 K granular superconducting phase is determined. In comparison to the YBa 2 Cu 3 O 7−δ compound, the inductive intragrain critical current is one order of magnitude smaller, whereas the intergrain current is of the same order of magnitude.

Diamagnetism and electrical connectivity in an inhomogeneous Ba2YCu3O7−x superconductor

Abstract The electrical resistivity and low field ac and dc magnetic suspectibility of an inhomogeneous or composite Ba 2 YCu 3 O 7−x superconductor have been measured. The observation of a rather large resistive transition (T≅3.6 K) allows to get a clear separation among diamagnetism setting of isolated Superconducting grains and a phase transition involving grain boundaries Josephson juntions and leading to electrical connectivity and thus to diamagnetic shielding.

Critical currents and relaxation effects in Nd2−xCexCuO4−y single crystals

Abstract Inductive critical current and field cooled (FC) magnetic relaxation measurements were carried out on a Nd 2 − x Ce x CuO 4 − y ( x = 0.15) single crystal for H t c. A zero field J c t value of 1.5 × 10 4 A cm −2 was obtained at 4.2 K, together with logarithmic magnetic field dependence. The relaxation data collected after a FC process revealed a logarithmic time dependence. At 4.2 K two clearly different time dependences were observed over distinct time scales. The corresponding thermal activation energies, U o t , for flux creep were found to be ≈ 3 and 1 meV, which are much smaller than typical values found for other high temperature superconductors. A.c. susceptibility data show a two step diamagnetic response below T c and complete magnetic shielding at 4.2 K and 1.1 Oe. The d.c. susceptibility ( H ≅ 4 Oe) reveals unusual behaviour near T c and paramagnetic rare earth polarization at 4.2 K.

Y-Sm twinned and untwinned high temperature superconductors: a comparative study

Abstract Electrical transport, low field d.c and a.c. magnetic susceptibility and inductive critical current measurements on polycrystalline Ba 2 Y 0.5 Sm 0.5 Cu 3 O 7 – x samples, which have different oxygen content and particle sizes, are reported. The sample having smaller particle size appears untwinned in TEM. From our data it is concluded that twin boundaries do not play a relevant role in the onset of the superconducting state. Even if the twin boundaries act as weak links, as suggested by absorption effects on χ ac , the possibility that the planes confine the screening currents seems very unlikely.

Critical state models for inter and intragranular flux pinning in HTS ceramics: universal scaling laws

Abstract Critical state models have been used in the qualitative and quantitative analysis of the ac susceptibility of HTS ceramics. A systematic procedure to derive the characteristic parameters that within a given model define each sample has been established. Good theory-experiment agreements are found.

Influence of Sb and Pb substitution on the physical properties of the BiSrCaCuO compounds

Abstract Under our synthesis conditions, the partial substitution of Bi by Sb and Pb on Bi Sr Ca Cu O stabilizes the 107K superconducting phase. Only small changes in H c1 and in the critical currents are produced.