S. Çelebi

Comparative AC susceptibility analysis on Bi-(Pb)-Sr-Ca-Cu-O high-Tc superconductors

Abstract We discuss the comparative AC susceptibility analysis on rectangular bar shaped high-temperature Bi–(Pb)–Sr–Ca–Cu–O superconductors prepared by the liquid ammonium nitrate method and the solid state reaction method. Five samples used in this study were obtained from different starting compositions. The analysis for comparison is based on (i) how much rapidly the relative peak temperature ( T c − T p ) in χ ″( T ) increases with increasing fields; (ii) the suppression degree of the diamagnetic behavior with respect to fields; (iii) the sharpness of the transition of χ ′( T ) for intergranular component for the same field amplitude; (iv) the steepness of the data of χ ″( T =55 K) vs. the field amplitude. We also qualitatively discuss experimental results in the framework of the critical state model, estimate the effective volume fraction of the grains and determine the temperature dependence of the critical current density by means of the best fit of the calculated data for the experimental matrix susceptibility.

Frequency dependence of the intergranular AC loss peak in a high-Tc Bi–(Pb)–Sr–Ca–Cu–O bulk superconductor

Abstract We have performed AC susceptibility measurements as a function of temperature; frequency and AC field amplitude on a rectangular bar shaped high-temperature superconductor with nominal composition of Bi 1.8.4 Pb 0.34 Sr 1.91 Ca 2.03 Cu 3.06 O 10 prepared by the liquid ammonium nitrate method. We have observed that as the frequency (AC field amplitude) increases, the intra- and intergranular AC loss peaks move higher (lower) temperature, which indicates that AC losses are due to both hysteretic bulk pinning losses and viscous (flux flow) losses. M – H curves exhibit a hysteresis which is similar to that of hysteretic superconductors. Activation energy at the grain boundary is extracted from the loss peak at some selected AC fields. Experimental results are qualitatively discussed in the framework of the critical state model (CSM).

Coexistence of critical and normal state magnetostrictions in type II superconductors : A model exploration

Exploiting the simple formula proposed by Ikuta et al. [Phy. Rev. Lett. 70, 2166 (1993)] to describe the magnetostriction of high TC superconductors and introducing a B2 dependent magnetic flux density component, as proposed by Koziol and Dunlap [J. Appl. Phys. 79, 4662 (1996)], we reproduce families of magnetostriction curves measured by Nabialek et al. [Physica B 319, 286 (2002)] and de Visser et al. [Phys. Rev. B 45, 2962 (1992)], using a simple expression for the dependence of the critical current density JC on the magnetic field in each analysis.

Flux line cutting and cross-flow in a high-Tc superconducting YBCO tube

We present strong evidence observed in a YBCO superconducting tube that helical flux lines can concurrently enter and leave across the inner surface of the wall of a hollow cylinder of sintered high-Tc superconductors at 77 K, and hence cut and traverse each other. The evidence for flux line cutting and cross-flow is examined in the perspective of basic concepts. The traffic of the flux lines is also described mathematically. The penetration fields across the wall and into the grains of the sintered ceramic tube are seen to correlate with salient features (peaks and valleys) of the flux line cutting and cross-flow phenomena. This enables us to claim that we are witnessing flux line cutting and cross-flow in the weak links as well as in the interior of the grains in the YBCO tube. The Clem/Perez-Gonzalez phenomenological theory is exploited in a simplified framework to describe the crucial features of the data semi-quantitatively. This analysis confirms the above conclusions and provides estimates of jc||m and jc||g, the critical current densities for intergranular and intragranular flux line cutting and their dependence on the magnetic flux density.

Contribution of the Meissner current to the magnetostriction in a high Tc superconductor

We show that the magnetostriction hysteresis curves, measured at different temperatures by Chabanenko et al (1998 Supercond. Sci. Technol. 11 1181) in a La1.85Sr0.15CuO4 single crystal, can be well reproduced by exploiting the critical state framework proposed by Ikuta et al (1993 Phys. Rev. Lett. 70 2116) to describe this phenomenon in high Tc superconductors, provided that the role of the Meissner current circulating at the surface of the crystals is taken into account.

Fabrication and characterization of Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 superconductors prepared by a wet technique

Abstract We have carried out measurements of resistivity on Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 granular samples prepared by the liquid ammonium nitrate method. The standard four-probe technique was used to measure the resistivity and the critical current of the specimen. We estimated the irreversibility line from the broadening of the resistive–superconducting transition in external DC magnetic fields up to 447 mT for the sample fabricated. A non-linear Arrhenius plot of resistive tail has been found. The power law relation RBirr=B0(1−T/Tc)3/2 gives a good fit to our data obtained from zero-resistivity temperature in selected magnetic fields. We have also characterized the prepared samples in terms of XRD, SEM and critical currents.

AC losses and irreversibility line of Bi(Pb)SrCa CuO high-tc superconductors

Abstract We performed measurements of χ′( T , H ac , f ) and χ′( T , H ac , f ) on a nominal composition of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu

Effect of ZnO Additions on the Magnetic Behaviour in YBCO Superconductors

We have investigated the effect of ZnO additions on YBCO prepared by conventional solid state reaction method by ac susceptibility measurements. We measured the ac susceptibility of sintered YBCO pellets as a function of temperature and ac magnetic field amplitude. The superconducting transition temperatures and critical current density depend on the Zn content of the samples. The intergrain critical current density was increased significantly by 1 wt% of ZnO compared to that of undoped samples. This conclusion was drawn by various aspects. The values of the extrapolated j cm (0) are 160, 308 and 1000 A/cm 2 for samples S1, S2, and S3 respectively. We also qualitatively discuss experimental results in the framework of the Bean critical state model, estimate the effective volume fraction of the grains and determine the temperature dependence of the critical current density by means of the best fit of the calculated data for the experimental matrix susceptibility.

AC susceptibility study in Bi1.6Pb0.4Sr2(Ca1−xNdx)2Cu3Oδ ceramic superconductors

Abstract We have studied the frequency dependence of the AC susceptibility in polycrystalline samples of Bi1.6Pb0.4Sr2(Ca1−xNdx)2Cu3Oδ. AC susceptibility measurements in the range 20–1000 Hz show that as the frequency increases, the intra and intergranular AC loss peaks move to higher temperature, and also the height and the width of the peaks increase. When we plot the peak height as a function of the AC field amplitudes, we observe a valley-like behavior. We qualitatively discuss experimental results in the framework of the critical state model, estimate the effective volume fraction of the grains and determine the temperature dependence of the intergranular critical current density by means of the best fit of the calculated data for the experimental matrix susceptibility. Since the granularity effect was very pronounced at low field range, we separated the contribution of the grains and matrix from the total measured AC susceptibility.

Production of BiPbSrCaCuO Thin Films on MgO and Ag/MgO Substrates by Electron Beam Deposition Techniques

Superconducting BiPbSrCaCuO thin films were prepared on MgO(001) and Ag/MgO substrates using an electron beam (e-beam) evaporation technique. The effects of annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties, respectively, were investigated by X-ray diffraction, atomic force microscopy, and by measurements of the critical temperature and the critical current density. It was shown that an annealing of both types of films at 845 or 860 °C resulted in the formation of mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. The slight increase of the critical temperature from 103 K to 105 K, the enhancement of the critical current density from 2 x 10 3 to 6 x 10 4 A/cm 2 , and the improved surface smoothness are due to a possible silver doping from the substrate.