The superconducting and normal state transport properties of ceramic samples with composition Bi 2 Sr 2− x K x CaCu 2 O y ( x  = 0, 0.1, 0.2, 0.3, 0.4) have been investigated by measuring thermoelectric power. The samples were prepared by the solid state route, and single phase Bi-2212 was found for all compositions. The T c (0) and thermoelectric power data were collected from 50 to 300 K. The thermoelectric power changes slightly with increasing doping level and increases linearly with decreasing temperature reaching a maximum value at T c (0). The T c (0), hole concentration and thermoelectric data were correlated. The results obtained are analysed in terms of phenomenological theoretical models that makes it possible to determine the band spectrum parameters in the normal state and to trace their changes with varying composition. Following models applied such as (a) Hubbard model, (b) two-band model, (c) Fermi-liquid behaviour. Applicability of these models to the present HTSC system is discussed and the parameters related to the models have been evaluated.

Thermoelectric power in Bi2Sr2−xKxCaCu2Oy

Improvement in high-Tc phase formation in (thallium–antimony) doped bismuth-based superconducting materials

Samples of stoichiometric composition Bi1.8Pb0.4Sr2−xKxCa2−yNdyCu3Oz with x = 0, 0.01, 0.02, and y = 0, 0.01, 0.02 were prepared by the solid state reaction method. Post-annealing experiments were carried out at 500 °C for 30 h in order to improve the oxygen homogeneity. They were characterized by the x-ray diffraction (XRD) technique, field emission scanning electronic microscopy (FE-SEM), resistivity, magnetization and Raman measurements. The superconducting transition temperature Tczero of the samples varies from 99 to 110 K for different doping. The superconductivity and the critical current density (Jc) of the sample single doped with K are improved compared with the undoped one. We attribute the change of Tc to the variation of carrier concentration. Additionally, the observed change of the normal state resistivity is related not only to carrier concentration but also to the nature of grain boundary links. These analyses are confirmed by Raman and SEM measurements.

Effect of K and Nd substitutions on superconductivity of Bi2223 superconductors

Bi2Sr2CaCu2O8+δ (Bi-2212) nanowires were fabricated employing electrospinning. A subsequent thermal treatment based on thermal gravity analysis is required to obtain the final crystal structure and superconductivity; however, if this preparation temperature is too high, the nanowire structure may completely collapse during the treatment, which is even more pronounced when aligning the nanowires. Therefore, Li acetate was added in various amounts to the starting composition, which enabled to produce the desired Bi-2212 phase already at about 750 °C instead of 840 °C. An X-ray and a TEM analysis proved that the resulting nanowires were single-phase Bi-2212. After the electro-spinning process, the resulting samples form fabric-like networks, and in the heat treatment, the nanowire structure was found to survive. The nanowires exhibited a polycrystalline structure with grain sizes of 20–50 nm, an average wire diameter of ∼100 nm, and a length of up to several micrometers. The resulting nanowire network samples were characterized by magnetic measurements and electric transport measurements in fields up to 9 and 4 T, respecitively.

Characterization of Electrospun Bi 2 Sr 2 CaCu 2 O 8+δ Nanowires With Reduced Preparation Temperature

A systematic study of the weak link behaviour for (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ ( x  = 0, 0.25) superconductors samples has been carried out using electrical resistivity and AC-susceptibility techniques. The K-doped (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ samples were synthesized by solid-state reaction method. In magnetic susceptibility measurements, the real ( χ ′) and imaginary ( χ ″) parts of the magnetic susceptibility of (Cu 0.5 Tl 0.5− x K x )Ba 2 Ca 2 Cu 3 O 10− δ ( x  = 0, 0.25) samples were measured as a function of temperature under various DC-magnetic fields up to 172 Oe. It is observed from these studies that the magnitude of the diamagnetism is substantially enhanced by K-doping. The possible reasons for the enhanced magnitude of diamagnetism have been investigated. It is observed from in-field magnetic measurements that the inter-grain coupling is improved with the K-doping. It is concluded from these studies that potassium atoms appearing at the crystal boundaries enhance inter-grain coupling and pinning mechanism in K-doped (Cu 0.5 Tl 0.25 K 0.25 )Ba 2 Ca 2 Cu 3 O 10− δ superconductors.

Effect of K-doping to the weak link behaviour of (Cu0.5Tl0.5−xKx)Ba2Ca2Cu3O10−δ superconductors

Samples with the nominal composition Bi 2 Sr 2 Ca 1−x Li x Cu 2 O y (x = 0.2, 0.4, and 0.6) and Bi 2 Sr 2 CaCu 2−x Li x O z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of Li at both Ca-and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the Li-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T c and the Li-doped at the Cu site gives T c (0) above the liquid nitrogen temperature. It was observed that the Li doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method

Effect of lithium doping in the Bi-2212 superconducting system

Samples with the stoichiometric composition with x = 0.1 and y = 0.1, 0.2, 0.3 and 0.4 were prepared by the solid state reaction method. The role of Na and K doped at the Ca and Sr sites simultaneously in the Bi-2212 composition was studied. From the x-ray diffraction data it was found that the structure is single-phase Bi-2212. From the resistivity data the varies from 63 to 67 K for different dopant levels. The diamagnetic nature of the samples was confirmed by ac susceptibility measurements. The oxygen content was determined by the two-step iodometric titration technique. The present results on the effect of double doping are compared with those of single dopants published earlier by our group. The microstructures of the samples were examined by scanning electron microscopy.

Superconductivity in the Bi - Sr/K - Ca/Na - Cu - O system

Samples with the stoichiometric composition Bi 2−x -Pb x Sr 2 Ca 2 Cu 3−y B y O z (x = 0.3, y = 0.2, 0.25, 0.3, 0.4) ceramics were prepared by a solid state reaction method. The samples were annealed at 850°C for 100 hours (treatment A), and the other at 850°C for 200 hours (treatment B). From the X-ray diffraction data of a ceramic sample it is revealed that all the samples were mixed phases of 2212 and 2223. The variation of the lattice parameters with the dopant level are represented. From the D.C. four-probe electrical resistivity data it was found that for the samples subjected to treatment B the T c (0) values were higher than those with treatment A. The A.C. susceptibility data were collected by change in the inductance method. The effect of boron doping on the phase formation and T c (0) is presented and the volume fraction of the phases estimated from the X-ray data. The presence of boron in the samples was confirmed by the inductive coupled plasma method. The microstructure of the samples was studied by scanning electron microscopy

B. Gopalakrishna

Papers

Effect of lithium doping in the Bi-2212 superconducting system

Superconductivity in the Bi - Sr/K - Ca/Na - Cu - O system

Effect of Boron Doping in Bi‐based 2223 Superconductors