According to an experimental data set on the superconducting transition temperature (Tc) of 21 BiPbSrCaCuOF superconductors under different process parameters including the amount of bismuth (n(Bi)), amount of oxygen (n(O)), sintering time (t) and sintering temperature (T), support vector regression (SVR) combined with particle swarm optimization (PSO) for its parameter optimization, was proposed to establish a model for prediction of the Tc of BiPbSrCaCuOF superconductors. The performance of SVR model was compared with that of back-propagation neural network (BPNN) and multivariable linear regression (MLR) model. The results show that the mean absolute error (MAE) and mean absolute percentage error (MAPE) of test samples achieved by SVR are smaller than those achieved by MLR or BPNN. This study suggests that SVR as a novel approach has a theoretical significance and potential practical value in development of high-Tc superconductor via guiding experiment.

Predicting the Superconducting Transition Temperature Tc of BiPbSrCaCuOF Superconductors by Using Support Vector Regression

Bulk pure-phase samples of HgBa 2 Ca 2 Cu 3 O y with high quality were prepared using a solid-state reaction with BaF 2 as a part of the starting material. X-ray diffraction data show that the crystal structure is nearly the same as for starting compounds without fluorine with a = 3.85 A and c = 15.85 A . The effect of fluorine in the starting material on the magnetic properties was studied by DC susceptibility and hysteresis-loop measurements, and our results revealed a remarkable enhancement of hysteretic magnetization in samples prepared using BaO/BaF 2 . Fluorine in Hg-1223 also shows an effect similar to that of oxygenation, which increases T c onset of the as-synthesized samples to 133 K. The asymmetric magnetization-field data indicate the presence of a surface barrier, and the field dependence and time dependence of the magnetization data display an enhancement of hysteresis upon replacement of 10% of BaO by BaF 2 as a result of increased concentration of pinning centers without changing the nature of the individual pinning.

Influence of fluorine in magnetic properties of Hg-1223 high-temperature superconductors

F-doped HBCCO samples with different amounts of fluorine at different temperatures (from 650°C to 860°C) have been synthesized using a precursor made by nitrates of Ba, Ca and Cu. The samples were examined by powder X-ray diffraction, AC susceptibility and resistance measurements. The influence of fluorine on the formation process of superconducting phases and the superconducting properties, and the role of fluorine in the Hg-system superconductors have been investigated. The location of fluorine in the crystals of the superconducting phase is discussed. The results show that F-doping promotes the formation of superconducting phases, increases the Tc of the as-sintered samples and causes a contraction of the unit cell.

Effect of F-doping in the HgBaCaCuO system

Superconducting ceramics of Bi1.6Pb0.4Sr2Ca2Cu3OyFx (x = 0–0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high Tc phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. From resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtained critical current density at 77 K under zero magnetic field also increases with fluorine doping.

Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics

A glass-ceramic Bi1.7V0.3Sr2Ca2Cu3Ox superconductor was prepared by the melt-quenching method. The compound was characterized by scanning electron microscopy, x-ray diffraction, differential thermal analysis, current-voltage characteristics, transport resistance measurements, and Hall effect measurements. Two main phases (BSCCO 2212 and 2223) were observed in the x-ray data and the values of the lattice parameters quite agree with the known values for 2212 and 2223 phases. The glass transition temperature was found to be 426 °C while the activation energy for crystallization of glass has been found to be Ea = 370.5 kJ / mol. This result indicates that the substitution of vanadium increased the activation energy for the BSCCO system. An offset Tc of 80 K was measured and the onset Tc was 100 K. The Hall resistivity ρH was found to be almost field-independent at the normal state. A negative Hall coefficient was observed and no sign reversal of ρH or RH could be noticed. The mobility and carrier density at different temperatures in the range 140-300 K under different applied magnetic fields up to 1.4 T were also measured and the results are discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructural, thermal, and electrical properties of Bi1.7V0.3Sr2Ca2Cu3Ox glass-ceramic superconductor

The effect of F doping in the (Bi, Pb)SrCCuO system has been investigated by resistance and AC susceptibility measureme The influence of fluorine on the forming process of superconducting phases and the superconducting properties are shown, and the role of fluorine in the Bi system superconductors is discussed. The effect of the heat treatment of the F doped samples has also been studied. The results indicate that F doping promotes the forming of 2223 phase and increases the bulk Tc.

X.-G. Wang

Papers

Effect of F doping in the (Bi, Pb)-Sr-Ca-Cu-O system