Abstract A preliminary study of the magnetic relaxation in (Bi, Pb)-2223 superconducting ceramics doped with ®-Al2O3 nanoparticles is presented, taking as starting point the measurements of the M(t) dependence, both in samples in the form of powder as in pellet. The relaxation of the three types of vortices that can exist inside these materials is analyzed, emphasizing on the intragranular region where the planar defects exist and, consequently, Abrikosov-Josephson vortices emerge. Finally, interesting results of the comparison between the behavior of the samples in the form of powder and in the form of pellet are shown from the estimation of the pinning energy of the vortices.

Abstract We present a preliminary study on the effects of nano-size α-Al2 O3 (40 nm) in the transport properties of Bi1.65 Pb0.35 Sr2 Ca2 Cu3 Oy ((Bi,Pb)-2223) ceramic samples. The (Bi,Pb)-2223 samples were synthesized by the solid state reaction method and α-Al2O3 nanoparticles (NPs), with concentrations ranging from 0.1 to 0.7 wt. percent (% wt.), were added before the last heat treatment. All samples were characterized by means of X-rays diffraction patterns, and temperature and magnetic field dependence of the electrical resistivity, ρ(T, Ba). From the experimental ρ(T,Ba) data we extracted the critical temperature dependence of the % wt. of α-Al2O3 NPs, and by using the Arrhenius plot the effective pinning energy has been also determined. The results indicate that mostly of the transport properties are optimized for the sample doped with 0.3% wt. of α-Al2O3 NPs.

Abstract The aim of this review is to give a summary of some hidden features related to penetration, trapping, and relaxation of the magnetic flux in (Bi,Pb)-2223 granular superconductors. The discussion is based on experimental and theoretical results obtained in samples prepared under different conditions and by different research groups. Fundamental concepts such as granularity and grain boundaries (GB), superconducting levels, types of vortices, and microstructural characteristics of (Bi,Pb)-Sr-Ca-Cu-O superconductors are invoked for supporting the discussion. As conclusions we focus on the main insufficiencies in the treatment and understanding of the phenomena mentioned above. We also propose some experiments for improving the understanding of the general physical features of granular superconductors.

We performed measurements of electrical resistivity as a function of temperature, ρ(Τ), in polycrystalline samples of YBa2Cu3O7-δ (Y-123) subjected to different uniaxial compacting pressures. We observed by using X-ray diffractometry that samples have a very similar composition. Most of the identified peaks are related to the superconducting Y-123 phase. Also, from the X-ray diffraction patterns performed, in powder and pellet samples, we estimated the Lotgering factor along the (00l) direction, F(00l). The results indicate that F(00l) increases from 0.13 to 0.16. From electrical resistivity measurements as a function of temperature, we were able to separate contributions arising from both the grain misalignment and microstructural defects. We found appreciable degradation in the normal-state transport properties of samples with an increase in uniaxial compacting pressure. It seems that this type of behavior is associated with an increase in the influence of microstructural defects at the intergranular level. The experimental results are analyzed in the framework of a current conduction model of granular samples.

Se presentan mediciones de resistividad eléctrica como función de la temperatura en muestras policristalinas de YBa2Cu3O7-δ (Y-123) sometidas a diferentes presiones de compactación antes de la sinterización final. Mediante el análisis de difracción de rayos-X se observó que las muestras poseen una composición muy parecida. En todos los casos la fase mayoritaria es de Y-123. Además, de los patrones de rayos X medidos en polvos y en pastillas, pudimos estimar el factor de Lotgering en la dirección (00l), F(00l). Los resultados obtenidos indican que F(00l) aumenta ligeramente desde 0.13 hasta 0.16. De las mediciones de resistividad eléctrica como función de la temperatura separamos las contribuciones asociadas con la desorientación de los granos y los defectos microestructurales. Se encontró un deterioro de las propiedades eléctricas de las muestras en estado normal con el aumento de la presión de compactación. Los resultados sugieren que el comportamiento obtenido esta asociado a un incremento de defectos microestructurales a nivel intergranular. Los resultados experimentales son analizados en el marco de un modelo de conducción eléctrica en estado normal propuesto para materiales granulares.

We have studied the effects of the uniaxial compacting pressure on the physical properties of polycrystalline Bi1.65Pb0.35Sr2Ca2Cu3O10+delta (Bi-2223) superconductors. Powders of this material were pressed at different uniaxial compacting pressures ranging from ~ 90 to ~ 600 MPa and heat-treated at the same temperature. A characterization of samples by using Scanning Electron Microscopy and X-ray diffractometry indicated an appreciable improvement of the degree of texture with increasing pressure. The temperature dependence of the electrical resistivity rho(T) exhibits a T-linear behavior at temperatures higher than T<FONT FACE=Symbol>*</FONT> ~ 235 K. The deviation of rho(T) from the linear behavior below T<FONT FACE=Symbol>*</FONT> indicates the opening of the pseudogap, a feature confirmed by magnetic susceptibility measurements performed in powder samples. From linear fittings of the normal-state electrical resistivity we were able to separate contributions to rho(T) arising from both the grain misalignment and microstructural defects. The results suggest that the grain orientation and the connectivity between them are improved with increasing compacting pressure. Also, based on the linearity of the electrical resistivity data both the transport electron-phonon coupling constant, lambdatr, and the mean free path, l, were estimated. We have found that in the sample with the highest degree of texturelambdatr ~ 0.53, a value comparable with the one obtained in Bi-2223 single crystals. However, the result for l ~ 12.7 Å at 300 K, in the same ceramic sample, is close to 3 times lower than the single crystal value. The influence of the intergranular electrical resistivity in determining band-theory parameters was analyzed within the framework of a current conduction model for granular superconducting materials.