The effect of Ag 2 CO 3 addition (0–25 wt%) on the Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O δ (Bi-2223) precursor powder has been investigated in terms of X-ray diffraction, scanning electron microscopy, dc electrical resistivity, critical temperature, ac magnetic susceptibility and critical current density. The samples were prepared by the conventional solid-state reaction method. The room temperature X-ray diffraction patterns of all the samples indicated the presence of large amount of Bi-(2223) phase along with minor amount of Bi-(2212) phase. The lattice structure of the material belongs to the orthorhombic unit cell. The volume fraction was estimated from the intensities of Bi-(2223) and Bi-(2212) phases. The sample with 5 wt% of added Ag shows the higher volume fraction of Bi-(2223) phase formed (86%). The dc electrical resistivity of all the samples decreases as the wt% of Ag increases. Both the onset critical T c (onset) and zero electrical resistivity T c ( R  = 0) temperatures were determined from the dc electrical resistivity and ac magnetic susceptibility measurements, which remains within the temperature range 110–113 ± 1 K and 101–106 ± 1 K respectively. Resistivity data agrees with ac magnetic susceptibility measurements. The critical current density for the 25 wt% Ag doped samples increased by three-times compared to the un-doped sample.

Effect of Ag2CO3 addition on the morphology and physical properties of Bi-based (2223) high-Tc superconductors

Two series of the superconducting materials with nominal composition Bi 1.6− y V y Pb 0.4 Sr 2 Ca 2 Cu 3 O δ and Bi 1.6− y V y Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O δ (where y  = 0.0, 0.1, 0.2, 0.3) were prepared by the conventional solid-state reaction method. The effect of vanadium and barium substitution at Bi and Sr sites respectively, have been investigated by the X-ray diffraction, scanning electron microscopy, dc electrical resistivity, ac susceptibility and critical current density. Room temperature X-ray diffraction studies show that there exists two phases i.e. Bi-(2223) and Bi-(2212) in the analyzed samples. The evaluated crystalline lattice structure of the prepared sample mainly belongs to the superconductive orthorhombic Bi-(2223) and Bi-(2212) phases. The dc electrical resistivity data agree with the ac susceptibility measurements. The onset temperature T c (onset) and zero resistivity critical temperature T c ( R  = 0) of all the samples remains within the temperature range ≈112–116 ± 1 K and 104–108 ± 1 K respectively. The measurements showed that the highest T c ( R  = 0) value of the superconducting sample is 108 ± 1 K, with composition Bi 1.6 Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O δ . By increasing the vanadium concentration, an increase in the critical current density in both the series has been observed.

Effect of vanadium and barium on the Bi-based (2223) superconductors

The electrical resistance, Meissner signal, magnetoresistance and Hall coefficient of Bi1.75Pb0.25Sr2Ca2Cu3Ox and Bi1.5Pb0.5Sr2Ca2Cu3Ox (nominal compositions) have been measured. Resistance of the Bi1.75Pb0.25 sample becomes zero for T≤106 K while the Bi1.5Pb0.5 sample shows a 65% drop in the resistance around 106 K and TR = 0c ≈ 73 K. Powder X-ray diffraction analysis reveals that the major phase in the Bi1.75Pb0.25 sample is the high-Tc phase and in the Bi1.5 Pb0.5 sample the low-Tc phase. The Hall coefficients (RH) of both samples are positive with R−1H> linear in temperature. The temperature dependence of RH is stronger in the Bi1.75Pb0.25 sample than in the Bi1.5Pb0.5 sample. The carrier concentration determined R−1H for the Bi1.75Pb0.25 and the Bi1.5Pb0.5 samples at 300 K are 1.59 × 1021 cm−3 and 3.66 × 1021 cm−3, respectively. The excess conductivity of both samples is analyzed using the Aslamazov-Larkin expression. The critical exponent λ obtained for the Bi1.75Pb0.25 sample is 0.78 and that for the Bi1.5Pb0.5 sample, in the temperature region where the high-Tc phase contributes only, is 0.77.

Electrical resistivity, magnetoresistance, magnetisation, hall coefficient and excess conductivity in Pb-doped Bi-Sr-Ca-Cu oxides

Vanadium substituted 2212 and 2223 superconducting ceramics

In this work, effects of compaction pressure on the structural and superconducting properties of BSCCO ceramic superconductors were investigated. The study was carried out on two systems which were, System I: Bi1.7Pb0.3Sr2Ca2Cu3Oy and System II: Bi1.6Pb0.3Ag0.1Sr2Ca2Cu3Oy, respectively. Ceramic powders were prepared by conventional solid-state reaction method and sintered at 850°C after compaction at five different pressures in the 150–750 MPa range. Critical temperatures of samples were determined by resistivity-temperature determinations made by four-point probe method in liquid nitrogen conditions. XRD analysis was conducted by powder X-ray diffraction method. Morphology of the grains present in the samples were determined by using scanning electron microscope (SEM) photographs at 2 K× and 2.5 K× magnifications for System I and System II, respectively. Sintered densities of the superconducting ceramics were measured by Archimedes water displacement method and unit cell parameters were additionally obtained from XRD data. Tc values for System I was determined to be in the 109–115 K with sample D having the highest Tc of 115 K while Tc varied in the 104–109 K range and sample B had the highest Tc value of 109 K for System II. The transition width, which is a sign of the purity of the samples, was determined to be narrow for both systems. The data obtained from X-ray diffraction measurements have shown that 2223 high-Tc phase was dominant in both systems. The determination of the optimum pellet compaction pressure for BSCCO ceramic superconductors was the main purpose of this work. The results of this work indicated that compaction at around 450 MPa improves the superconducting and structural properties of the BSCCO ceramic superconductors.

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Xu Mingkang

Papers

Preparation and superconductivity of the high-Tc phase of Pb-doped Bi-Sr-Ca-Cu-O superconductor