Beginning point of metal to insulator transition for Bi-2223 superconducting matrix doped with Eu nanoparticles

Abstract

Abstract This comprehensive study examines the change of the microstructural, electrical and superconducting properties of the Eu doped Bi1.8Pb0.4EuxSr2Ca2.2Cu3.0Oy ceramic cuprates (with x ⩽ 0.7) produced by the conventional solid state reaction method at the constant annealing temperature of 840 °C for 24 h with the aid of the standard characterization measurements such as bulk density, dc resistivity (ρ-T), transport critical current density (Jc), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) examinations. For the full characterization of the pure and Eu doped Bi-2223 samples, the degree of granularity (from the bulk density and porosity measurements); the room temperature resistivity, onset–offset critical transition temperature, variation of transition temperature, hole carrier concentration, spin-gap opening temperature and thermodynamic fluctuations (from the dc resistivity experiments); the texturing, crystal structure, crystallite size, phase purity and cell parameters (from the XRD investigations); the variation of the flux pinning centers and the boundary weak-links between the superconducting grains (from the critical current density values); the crystallinity, specimen surface morphology, grain connectivity between the superconducting grains and grain size distribution (from the SEM examinations), the elemental compositions and distributions (from EDX measurements) of the samples are determined and discussed clearly. The results obtained confirm that all the properties degrade with the enhancement of the Eu concentration in the Bi-2223 superconducting matrix up to x = 0.5 beyond which they are destroyed surprisingly due to not only the distortion between the Bi-2223 slabs but also the increase in the porosities and grain boundary weak-links. For example; the onset (offset) critical temperature decreases from 117.6 K (109.9 K) to 68.1 K (14.8 K) with the impurity content. The critical temperatures ( T c onset and T c offset ) are not measurable for the sample doped with the Eu content level of x = 0.7 as a consequence of the metal to insulator transition (MIT). On the other hand, the critical current density (Jct) is measured in the range from 3201 A/cm2 (for the pure sample) to 29 A/cm2 (for the sample doped with x = 0.5). For the sample doped with x = 0.7, each value is not measurable due to the nonsuperconducting behavior. Besides, the SEM pictures display that the surface morphology and grain connectivity degrade considerably with the Eu concentration. Moreover, the XRD measurements show that the Eu inclusions enter into the crystal structure by reducing the formation velocity of the Bi-2223 phase. Similar to the XRD evidences, the EDX measurement results demonstrate that all the elements used in the samples successfully introduce into the Bi-2223 structure, and the observed peaks of Ca, Cu and especially Pb reduce gradually with the Eu individuals in the Bi-2223 matrix, meaning that the Eu nanoparticles may substitute for the elements given above. This is enough to explain why the superconducting properties retrograde rapidly with the Eu impurities.