Difüzyon yoluyla katkılanmış süper iletken MgB2

This thesis reports the change of the physical, structural, electrical, mechanical, pinning mechanismand superconducting properties of Cu-diffused MgB2 superconducting samples prepared at differentannealing temperature (650 -850 °C) with the aid of the X-ray diffraction (XRD), scanning electronmicroscopy (SEM), Vickers microhardness and dc resistivity measurements. As for the XRD results,all the samples prepared contain the MgB2 phase only and exhibit the polycrystalline superconductingphase with more intensity of diffraction lines, resulting in the increase of the lattice parameter a and c.Moreover, SEM investigations are performed for the microstructural characterization of the samples.The results obtained show that the sample annealed at 850 °C has the best surface morphologyand grain connectivity,largest grain size and lowest porosity. At the same time, Vickers microhardness,elastic modulus, yield strength, fracture toughness and brittleness index values ofsamples are extracted from the microhardness measurements. These mechanicalproperties of the Cu-diffused samples are found to depend strongly on thediffusion-annealing temperature. The onset (Tconset) and offset (Tcoffset) criticaltemperatures, activation energies (U0), irreversibility fields (?0Hirr), upper criticalfields (?0Hc2), residual resistivity ratios (RR), cross-sectional area fractions (AF),penetration depths (?), coherence lengths (?) and electronic mean free path ( ? )values of the samples are determined from the magnetoresistivity curves.Additionally, the diffusion coefficient is observed to increase from 6.81x10-8 to4.69 x10-7 cm2s-1 as the diffusion-annealing temperature increases. Temperaturedependence of the Cu diffusion coefficient is described with the aid of theArrhenius relation D=3.75x10-3 exp (-1.15±0.10eV/kBT) and the correspondingactivation energy of copper in MgB2 system is found to be about 1.15 eV. Thepossible reasons for the observed improvement in physical, structural, electrical,mechanical, pinning mechanism and superconducting properties of the samplesdue to Cu diffusion are also discussed.Keywords: Cu-diffused MgB2; Diffusion Coefficient; Scanning electronmicroscopy; X-ray diffraction; Vickers Hardness.