F. Ben Azzouz

Synthesis, microstructural evolution and the role of substantial addition of PbO during the final processing of (Bi,Pb)-2223 superconductors

Abstract The evolution of the phase formation and microstructure in (Bi,Pb)–Sr–Ca–Cu–O (BPSCCO) superconducting materials have been studied through a two-cycle annealing process by X-ray diffraction and SEM observations. Samples were Pb-doped (part 0.4 of Bi substituted by Pb) during the first thermal cycle. Yet, the synthesizing efficiency of the (Bi,Pb)-2223 phase was found to be greatly re-enhanced during the second thermal cycle by a second and substantial addition of up to 20% excess of PbO in weight. No significant increase of secondary phases was observed. Pb addition was shown to be more effective at the end of a multi-step thermal cycle. Therefore the lack of Pb that arise in BPSCCO system after some heat treatment duration is shown to be one of the most important rate-limiting (Bi,Pb)-2223 transformation. The second sintering duration at 835°C was optimized with respect to the weight percentage of PbO in excess. The present work attempts also to point out the complex mechanisms of the (Bi,Pb)-2223 formation reactions. Some intermediate phases are identified and the liquid phase is suggested to derive from a partial decomposition of the (Bi,Pb)-2212 phase. This decomposition is highly sensitive to Pb addition.

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.

Fluctuation induced magneto-conductivity of Y3Ba5Cu8O18±x and YBa2Cu3O7−d

A comparative study of the fluctuation magneto-conductivity in YBa2Cu3O7−d (noted Y-123) and Y3Ba5Cu8O18±x (noted Y-358) polycrystalline samples was carried out. Samples were synthesized in oxygen atmosphere using a standard solid state reaction technique. Phases and microstructure have been systematically investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magneto-conductivity was measured in a magnetic field ranging from 0 T to 7 T applied perpendicular to the current path direction. The magneto-conductivity data were analyzed in terms of the temperature derivative of the resistivity and the logarithmic temperature derivative of the conductivity χσ = −d(ln Δσ) dT, where Δσ is the fluctuation conductivity. Analyses of the magneto-conductivity data reveal that Y-123 possesses a better quality of intrinsic Josephson junction compared to Y-358 one while in the former one the depression in superconducting temperature is more pronounced. The applied magnetic field narrowed substantially the three dimensions (3D) Gaussian regime and for Y-358 sample this regime is vanished for high magnetic field ( >1 T). Results are discussed in relation with the difference in the structure and microstructure Y-358 and Y-123. The upper critical and the irreversibility magnetic fields were estimated and were found to be higher in the Y-358 sample.

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.

Effect of ZnO and Zn0,95Mn0,05O nano-particle inclusions on YBCO polycrystalline pinning properties

The effect of nano-size ZnO or Zn0.95Mn0.05O (ZnMnO) addition on the microstructure and pinning properties of polycrystalline YBa2Cu3Oy (YBCO) was analyzed. Samples with nano-particles ZnO and Zn0.95Mn005O addition were synthesized by solid state reaction. Microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), critical current density dependence on applied magnetic field Jc (B) and electrical resistivity as a function of temperature ρ(T) were carried out to evaluate the relative performance of samples. Using the right amount of ZnO or Zn0.95Mn0.05O addition (0.2 wt.%), the flux pinning ability of YBCO has been enhanced. The best flux pinning was observed for Zn0.95Mn0.05O added sample. The flux pinning enhancement suggests magnetic interaction of pinning centers via vortices.

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.

Excess conductivity analysis in YBa2Cu3O7−d added with SiO2 nanoparticles and nanowires: Comparative study

The effect of nanosized silicon oxide nanoparticles (denoted NP-SiO2) and nanowires (denoted NW-SiO2) additions during the final processing stage on electrical fluctuation conductivity of polycrystalline YBa2Cu3Oy (Y-123 for brevity) in the mean field region has been reported. Series of samples were synthesized in air using a standard solid-state reaction technique by adding nanosized entities up to 0.5 wt.%. Phases, microstructure and superconductivity properties have been systematically investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM) and electrical measurements. TEM investigations show the presence of inhomogeneities embedded in the superconducting matrix along with the presence of columnar defects in the case of SiO2 nanoparticles added samples, however nanowires tend to agglomerate by entangling with each other in the intergrain regions. The fluctuation conductivity was analyzed as a function of reduced temperature using the Aslamazov–Larkin model. Using the Lawrence–Doniach equations, the Ginzburg–Landau (GL) number (NG) and equations, the coherence length, the effective layer thickness, the lower critical field Bc1(0), the upper critical field Bc2(0) and the critical current density Jc(0) were estimated. It was found that the addition of an optimum concentration of SiO2 nanomaterials, that depends on the shape, effectively controlled the microstructure, the grains coupling and hence improved the physical properties of Y-123 compound.