A. Varilci

Investigation of some physical properties of silver diffusion-doped YBa2Cu3O7?x superconductors

Silver diffusion in superconducting YBa2Cu3O7?x (YBCO) has been studied over the temperature range 500?800??C by x-ray diffraction measurements. The diffusion coefficient of Ag in YBCO was determined from the observed increase (0.12%) in the lattice parameter c as compared with the undoped sample. It was shown that the diffusion coefficient of Ag in the temperature range 500?800??C changed from 1 ? 10?10 to 5 ? 10?9?cm2?s?1 with an activation energy of about 1.06?eV. For the Ag-doped sample, the critical transition temperature and the critical current density were increased from 90 to 92.5?K and from 60 to 98?A?cm?2 respectively in comparison with those of undoped YBCO. The possible reasons for the observed increases in Tc and Jc due to Ag diffusion were discussed.

Effect of diffusion-annealing time (0.5 h ≤ t ≤ 2 h) on the mechanical and superconducting properties of Cu-diffused bulk MgB2 superconductors by use of experimental and different theoretical models

This study reports the effect of different annealing time (0.5 h ≤ t ≤ 2 h) on the electrical, physical, microstructural, mechanical and superconducting properties of Cu-diffused bulk magnesium diboride (MgB2) system by means of dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and microhardness measurements (Hv). The room temperature resistivity (at 300 K), critical transition (Tcoffset and Tconset) temperature, variation of transition temperature, grain size, phase purity, lattice parameter, texturing, surface morphology, crystallinity and Vickers microhardness values of the samples are evaluated and compared with each other. The resistivity results obtained reveal that the (Tcoffset and Tconset) values of the samples produced ascend with the enhancement in the annealing time up to 1 h beyond which these values start to reduce systematically and in fact the smallest Tconset of 38.1 K and Tcoffset of 36.2 K are observed for the sample annealed for 2 h. Similarly, the SEM micrographs display that the surface morphology, crystallinity and grain connectivity improve until a certain diffusion-annealing time (1 h), and after this point, all the properties obtained start to degrade with the increase of the annealing time. Furthermore, the peak intensities, grain sizes and lattice parameters deduced from the XRD measurements illustrate that a systematic elongation in the a and c axis lengths is detected with the annealing time until 1 h beyond which a regular contraction in the lattice parameters is observed for the samples. Likewise, the peak intensities belonging to MgB2 phase enhance with the increment of the annealing time up to 1 h after which they reduce slowly; however, a new peak belonging to Mg2Cu phase appears in the sample annealed for the duration of 2 h, confirming both the reduction of the grain size and degradation of the critical temperature. Additionally, we have focused on the microhardness measurements for the examination of the mechanical properties of the samples studied. Experimental results of microhardness measurements are estimated using the various models such as Meyer’s law, proportional sample resistance model, modified proportional sample resistance model and Hays-Kendall (HK) approach. Based on the simulation results obtained, the Hays-Kendall (HK) approach is determined as the most suitable model describing the mechanical properties of samples prepared.

Influence of magnetic fields on hysteretic ac losses in bulk MgB2 superconductor investigated by using Hall probe ac susceptibility method

We report the results of an investigation of the influence of magnetic fields on hysteretic ac losses in bulk MgB2 superconductor by using Hall probe ac susceptibility. The external magnetic field in this study had an ac part with frequency 10 Hz and magnitude in the range 240–1200 A m−1 and no dc part. We have measured the imaginary part of the magnetic susceptibility and analysed data by using Beans critical state model of cylindrical geometry for four different temperature values 39.55, 39.51, 39.47 and 39.41 K. The result of this analysis indicates that Beans model is adequate to explain the experimental findings. Calculated hysteretic ac loss versus applied magnetic field curve is given by using the critical state model. We have also measured the magnetization versus applied magnetic field and determined the first critical magnetic field of a MgB2 sample as 500 A m−1 at 35 K.

Influence of annealing temperature on the microstructure and mechanical properties of MgB2

We have investigated the effect of annealing temperature on the microstructure, magnetic and mechanical properties of MgB2 superconducting samples employing X-ray diffraction (XRD), scanning electron microscopy (SEM), ac susceptibility and Vickers microhardness measurements. XRD patterns and SEM micrographs are used to obtain information about lattice parameters and grain size, respectively. These measurements indicate that MgB2 grain size, lattice parameters, and critical temperature are increased, and grain connectivity is improved, with increasing the sintering temperature up to 850°C. It is also observed that the Vickers microhardness of the samples is dependent of the sintering temperature and applied load. In addition, we calculate the load dependent mechanical properties of MgB2 samples such as the Youngs modulus, yield strength, and fracture toughness. The possible reasons for the observed improvements in microstructure, superconducting and mechanical properties due to annealing temperature are discussed.

Transport and ac susceptibility measurements on silver wrapped pellets of Bi1.7Pb0.35Sr1.9Ca2.1Cu3Oy superconductors

The effect of the cooling rates on oxygen content of silver sheathed Bi-2223 bulk superconducting samples was investigated performing dc electrical resistivity, transport critical current density, ac susceptibility and x-ray diffraction (XRD) measurements. The bulk samples with Ag sheathing were annealed under identical conditions and cooled with rates of 25° C / h, 50° C / h, 75° C / h, and 100° C / h. We estimated the transition temperatures from both dc resistivity and ac susceptibility as a function of temperature measurements. It is observed that Tc and Jc depend on cooling rates of the samples. Tc and Jc decrease with increasing cooling rates. The imaginary part of ac susceptibility measurements is used to calculate inter-granular critical current density using the Bean Model. Jc(Tp) is also seen to decreases with increasing cooling rates. The peak temperature, Tp, in the imaginary part of the ac susceptibility is shifted to a lower temperature with increasing cooling rates as well as increasing ac magnetic fields. The force pinning density decreased with increasing the cooling rates. XRD patterns are given to determine lattice parameter c and obtain information about Bi-2223 and Bi-2212 phases.