M. Zouaoui

Effect of nano-size ZrO2 addition on the flux pinning properties of (Bi, Pb)-2223 superconductor

We investigated the microstructure and flux pinning capability of the bulk (Bi,Pb)2Sr2Ca2Cu3Oy system (denoted as (Bi, Pb)-2223) with 0.0–0.3 wt% nano-size ZrO2 particles (10–20 nm) addition synthesized by solid state reaction. Phase analysis by x-ray diffraction (XRD), granular structure examination by scanning electron microscopy (SEM), microstructure investigation by transmission electron microscopy (TEM) equipped with energy dispersive x-ray spectrometer (EDS), critical current density dependence on the applied magnetic field Jc(H) and electrical resistivity ρ(T,H) were carried out. The results show that activation energies of vortex flux motion U(H), critical current density Jc(H) behavior in applied magnetic field and volume pinning force density Fp are enhanced by the addition of 0.1 wt% ZrO2. The pinning force analysis and microstructure observations indicated that the improvement of flux pinning is suggested to originate from the introduction of extrinsic pinning centers due to the formation of normal nano-sized Zr-rich phase.

Electron transport properties of silver added polycrystalline MgB2

Abstract The temperature dependence of resistivity in the normal state of polycrystalline MgB 2 samples including various amounts of Ag powder has been studied systematically. The samples show different temperature dependences of normal state resistivity and residual resistance ratios (RRRs) although their superconducting temperatures are nearly the same. Ag powder admixture was found to increase the RRR, suggesting a decrease of the defect scattering density. We have shown that the normal state of samples exhibits a power law dependence of the temperature dependent resistivity ( T n with n >1) and changes to become linear as we increase the silver content. The normal state behaviour of polycrystalline MgB 2 samples is thus thought to be drastically affected by carrier scatterings at grain boundaries defects, which most probably arise from Mg deficiency.

Enhancement of superconductivity properties in nano ZrO2 particles added Bi1.8Pb0.4Sr2Ca2Cu3Ox ceramics

Polycrystalline samples of (Bi,Pb)-2223 superconductor have been prepared by standard solid state reaction method with the addition of a nanometer ZrO2 particles. Electrical transport properties have been studied by current density measurements in liquid nitrogen, and the pinning energy parameters have determined by Z H Wang and X W Cao approach. Using the right amount of ZrO2 addition (0.1wt.%), the flux pinning ability of (Bi,Pb)-2223 has been enhanced.

Electron localization induced by grain boundary disorder in the normal state of high-Tc superconductors

Abstract We have studied the effects of superconducting grain boundary disorder on the normal state transport properties of cuprate films. Dip-coated granular YBa 2 Cu 3 O 7− y (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the crossover from strong to weak inter-grain disorder. Grain boundary passivation was achieved by metallic inclusions of different forms. We have shown that the normal state of samples exhibit a semiconducting behavior and changes to ‘metallic’ with sharper transitions to the superconducting state as we reduce grain-interfaces disorder, i.e. increase metallic inclusion content. On the basis of electron localization mechanisms, the normal state conductivity is thus shown to undergo a dimensional crossover from 3D to 2D in the frame of the variable-range hopping (VRH) regime. The transition threshold was found to depend on the form of metallic inclusions.

Fluctuation conductivity in nano-sized A12O3 added (Bi,Pb)-2223 superconductors under applied magnetic field

We report electrical conductivity fluctuation analyses on (Bi, Pb)2Sr2Ca2Cu3Ox (denoted as (Bi.Pb)-2223) granular samples added with nano-sized A12O3 particles. A12O3 is added to the precursor powders during the final sintering cycle of a multi-step preparation process. Phase analysis by X-ray diffraction (XRD) and granular structure examination by transmission electron microscopy (TEM) were carried out. TEM and energy dispersive X-ray spectroscopy (EDS) analysis have shown nanometric Al-rich phase intergrowth within the (Bi.Pb)-2223 superconducting matrix. The temperature dependence of electrical resistivity as function of magnetic field has been measured on free and 0.2 wt. % Al2O3-added samples. The effect of microscopic inhomogeneities in the paraconductivity region has been reported. Data about the dimensionality of the thermodynamic fluctuation is obtained by analyzing the excess of conductivity on the basis of the Aslamazov-Larkin theory. Samples show predominately two dimensional (2D) behaviors with cross-over to three dimensional (3D) fluctuations in the mean field region. Magneto-conductivity fluctuation results have shown that the Al-rich phase inclusion in superconducting matrix affects the mean field region of superconducting order parameter of fluctuation.

Pinning mechanism in (Bi,Pb)-2223 polycrystalline samples prepared with Al2O3 nano-particles

Nano-size Al2O3 (40 nm) addition effect on the polycrystalline (Bi,Pb)-2223 system properties was studied by XRD, SEM, TEM/EDX and electrical measurements. Samples with 0.0 and 0.2.wt% Al2O3 are synthesized in air by solid state reaction. Nanometer Al2O3 particles were added during the final sintering cycle of a multi-step preparation process. Analysis of magnetoresistivity has shown that the dissipation phenomenon follows the thermally activated flux creep model. Both onset temperature of dissipation, effective activation energy U, critical current density Jc behaviour in applied magnetic field and volume pinning force density Fp are enhanced by addition of 0.2wt.% Al2O3. Enhancement of the vortices flux pinning in doped samples is mainly originated from the surface normal-like pinning centers.

Magnetoresistance studies of PbO addition in YBa2Cu3O7-δ superconductor

The effect of PbO addition on the superconducting properties of YBa2Cu3O7-? (Y-123) polycrystalline phase has been investigated through samples with x wt.% PbO (x = 0-4.5) addition prepared using standard solid state reaction method. Electrical transport measurements indicate that samples with x = 1-3 have their current density, at self magnetic field, J(0), increased about 50%. Their magnetic field dependent Jc(H) has been measured in liquid nitrogen, and the activation energies have been determined using the TAFF model which shows an enhanced flux pinning proprieties. TEM and EDX analyses have shown that Pb reacts with Y-123 phase to form nanometric Pb-rich phase intergrown in Y-123 superconductors matrix. These inhomogeneities may be the origin of strong pinning.

Kinetic formation of (Bi, Pb)-2223 compound during the last stage of a multi-step thermal preparation process

A kinetic study of the (Bi,Pb)-2223 phase formation has been performed through a two-cycles annealing process at a temperature of 835°C by X-ray diffraction and SEM observations. Samples were Pb-doped (part 0.4 of Bi substituted by Pb) during the first thermal cycle. A second substantial addition of up to 10% excess of PbO in weight was added at the beginning of the second thermal cycle. PbO addition was shown to be more effective during the second cycle. Using the Avrami equation, a kinetic study of the (Bi,Pb)-2223 phase formation mechanism was conducted. The reaction order, n has been shown to depend on PbO excess and thus the (Bi,Pb)-2223 formation mechanism. With lower excess of PbO (0% and 4%), n is about 0.5 and the related formation mechanism corresponds to the thickening of plates after their edges have been impinged. With higher excess of PbO (10%), n is about 2,2 and the related mechanism corresponds to the initial growth of particles nucleated at a constant rate.

Magneto-resistance analysis of nanometer A12O3 added Bi-2223 polycrystalline superconductors

We report magneto-resistivity analyses on (Bi,Pb)2Sr2Ca2Cu3Ox (denoted as (Bi.Pb)-2223) granular samples added with nano-sized A12O3 particles. A12O3 is added to the precursor powders during the final sintering cycle of a multi-step preparation process. Resistivity measurements of 0.0 and 0.2 wt. % A12O3 added (Bi.Pb)-2223 samples in applied magnetic field ranging from 0 to 250 mT have been compared and analyzed by the thermally assisted flux creep model. The effective activation energie in the A12O3 added sample is higher than in free one. The vortex phase diagrams in the H-T space show that characteristic temperatures Tg and Tp shift to higher values with 0.2 wt.% A12O3 addition.

Effect of Ag admixture on the electron localization mechanism in the normal state of dip-coated YBa2Cu3O7−δ thick films

Abstract The effects of superconducting grain boundary disorder on the normal-state transport properties of cuprate films was investigated. Dip-coated granular YBa2Cu3O7−δ (YBaCuO) thick films on polycrystalline MgO substrates were synthesized and networked grains were systematically made less disordered in order to probe the cross-over from strong to weak inter-grain (between grains) disorder. Grain boundary passivation was achieved by AgNO3 metallic inclusions. The normal state of samples was shown to exhibit a semi-conducting behaviour and then changes to metallic with sharper transitions to the superconducting state as the grain-interfaces disorder is decreased, i.e. increase metallic inclusion content. The normal-state conductivity is thus shown to undergo a dimensional cross-over from 3D to 2D in the frame of the variable range-hopping regime.