Advances in processing and fabrication of high critical current density ( J c ) long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Gd, Sm, Nb, and Ag concentration. The critical current density J c measured at 77 K from I–V data shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube were found to be in the range 7×10 8  A/m 2 . X-ray diffraction results showed that the volume fraction of the high- T c (2223) phase decreases and that of low- T c (2212) phase increases with increase of these rare-earth ion contents. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition.

Thermal and phase characterization of Bi-2223 superconductors

The role of Gd addition in the Bi1.6Pb0.4Sr2Ca3GdxCu4Oy (x = 0.0, 0.1, 0.2, 0.3 and 0.4) system was examined by X-ray powder diffraction, scanning electron microscopy, critical current density (Jc) and magneto-transport measurements. The samples were fabricated by the conventional solid state reaction method. The analysis showed that the critical temperature (Tc) and hole number (p) of the materials decreased while the room temperature resistances increased with the addition of Gd. The resistance–temperature dependence for the low-resistance regions of the transition can be determined by the thermally activated flux flow model. The upper critical field [Hc2(T)] values calculated by using this model decreased with the addition of Gd. Furthermore, it was observed that the activation energy (U0) decreased both with the increase in the amount of Gd and applied magnetic field. The degradation of the superconducting properties of the samples may be related to the alteration of the structural, electronic and magnetic properties of the materials depending on the presence of the Gd ions. As a result, the decrease in the number of holes and the magnetic properties of Gd ions are thought to be the fundamental reasons for the deterioration of the superconducting properties of the materials. It has been observed that Gd additions from the microstructure studies of the materials affected the surface morphology negatively.

Study of microstructure and magneto-transport properties in Bi1.6Pb0.4Sr2Ca3GdxCu4Oδ superconducting systems

The superconducting properties of Bi1.7Pb0.3Sr2Ca2−xRx(R = Eu, Yb and Ag)Cu3OY have been investigated by X-ray diffraction, electrical resistance and the peritectic transition of these superconductors was studied kinetically under different atmospheres and temperature gradients. X-ray diffraction results show that the volume fraction of the high-Tc (2 2 2 3) phase decreases and that of the low-Tc (2 2 1 2) phase increases as Eu, Yb and Ag concentrations increase. The resistivity measurements reveal that the Tc onset decreases down to 100 K for Eu, 85 K for Yb and 106 K for Ag concentrations. These results are explained on the basis of possible variations of hole concentration with trivalent rare earth ion substitutions. Activation energies and frequency factors for crystallisation were determined by non-isothermal differential thermal analysis (DTA), employing different models. It was found that both peritectic transition and reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2 2 1 2 phase was greatly enhanced under oxygen atmosphere.

Effect of rare-earth (Eu, Yb and Ag) substitutions on superconducting properties of the Bi1.7Pb0.3 Sr2Ca2-xRx(R = Eu, Yb, and Ag)Cu3OY system

The polycrystalline Bi1.8Pb0.4Sr2.0Ca1.1Cu2.1 MxO
 y
 , with M = Zr (x = 0.0, 0.02, 0.04), were synthesized by solid-state reaction method and studied by X-ray diffraction analysis (XRD), scanning electron microscopy equipped with energy dispersive of X-ray analysis (SEM/EDX) and resistivity versus temperature measurements. The influence of the Zr addition on the Tc and microstructure properties of the superconducting compounds has been studied. SEM observations show whiskers grains randomly distributed and microstructural change due to the addition of Zr. The ZrO2 was incorporated into the crystalline structure of BSCCO system in all samples. The crystallographic structure remains in a tetragonal form where a= b≠c. Generally, all samples exhibit semiconductor behaviour above $T_{\mathrm {c}}^{\text {onset}}$
 . The onset critical temperature $T_{\mathrm {c}}^{\text {onset}}$
 increases up to 86 with x = 0.02. There is an enhancement in the critical temperature for doped samples as compared with pure Bi1.8Pb0.4Sr2.0Ca1.1Cu2.1O
 y
 .Changes in superconducting properties of ZrO2 nanoparticle added Bi-2212 system were discussed.

Effect of Zr Addition on Bi 1.8 Pb 0.4 Sr 2.0 Ca 1.1 Cu 2.1 O y Superconductor

The effects of the BCO3 addition on the formation and properties of the Bi-based superconductors were studied. Phase identification, and structural and microstructural analyses were carried out using the powder X-ray diffraction method (XRD), scanning electron microscopy (SEM/EDX), AC magnetic susceptibility and resistivity measurements. The experiment results showed an increase of the superconducting phase content (BCO3) with x=0.01 and all samples doped with B contain the 2212 phase. In the BCO3 added samples (with x=0.01), the c lattice parameter decreased slightly from the nonadded sample showing the possibility of B incorporating into the crystal structure of the Bi-2212 phase. The variation in cell parameters is due to the difference in the ionic radii of B (0.2 A) and Ca (1.06 A). The SEM observation showed whisker grains, which are randomly distributed. The microstructural change is due to the addition of B. From the resistivity measurement, the $T_{\mathrm{c}}^{\mathrm{onset}}$
 was found to be increased by 4 K for the doped sample compared to the undoped sample. An improvement of the normal state conductivity is obtained with a small amount of the addition (0.01). In the normal state, all the samples exhibit a metallic behavior.

Effects of BCO3 Addition on the Formation and Properties of the Bi-Based Superconductors

Long lengths of Bi-2223 conductors doped with Ag, Nb and B were fabricated. The recrystallization of the glasses during various heat treatments was studied by differential thermal analysis (DTA), differential scanning calorimetry, X-ray diffraction and resistivity measurements. Activation energies and frequency factors for crystallization were determined by non-isothermal DTA, employing different models. It was found that both the peritectic transition and the reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2223 was greatly enhanced under an oxygen atmosphere. Jc measurements at 77 K showed an increase on silver addition. Furthermore it was found that the silver addition does not destroy the superconductivity. © 1998 Chapman & Hall

Maham Khan

Papers

Fabrication and transport properties of long-length doped Bi1.7Pb0.3Sr2Ca2-xMxCu3Oy conductors (M=B, Nb or Ag)