Polikristal MgB2 süperiletken örneklerde demir (Fe) difüzyonu

The main objective of this thesis is to investigate iron (Fe) diffusion into the MgB2 superconductor bulk samples. Diffusion of Fe has been performed at different sintering temperatures for different sintering time durations. For Fe diffusion, a 50 micrometer thick Fe layer was coated on MgB2 by metal evaporation in vacuum. Experimental studies of this thesis are divided in to two parts. In the first part, the Fe diffusion in superconducting MgB2 bulk samples has been studied over the temperature range of 650-900C for 1 hour. Fe coating was made on two sets of pelletized bulk samples; either after initial sintering at 800C/1h (Set 1S) or without initial sintering (Set 1N). Effects of Fe diffusion on the crystal structure and superconducting properties of MgB2 have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR) and resistivity measurements. Fe diffusion coefficients were determined for both sets from values of lattice parameter c and room temperature resistivity. The temperature dependence of the Fe diffusion coefficient in this temperature range is described by the Arrhenius relation. It has been found that the Fe diffusion coefficient increases with increasing sintering temperature, as expected. The plausible explanations for the observed improvement in microstructure and superconducting properties of the samples due to Fe diffusion are discussed. In the second part, coating process was made on two sets of samples; either after initial sintering at 800C/1h (Set 2S) or without initial sintering (Set 2N). The Fe diffusion in both sets of superconducting MgB2 samples has been studied over the time range of 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours and 8 hours at 900C. Similar to the first part, effects of Fe diffusion on the crystal structure and superconducting properties of MgB2 have been investigated by XRD, SEM and resistivity measurements. Fe diffusion coefficients for sintering process were determined from lattice parameter c and room temperature resistivity values. It has been found that the Fe diffusion coefficient decreases with increasing sintering time, as expected. Finally, since diffusion coefficient increases with increasing annealing temperature and decreasing annealing time, heat treatment processes for superconductor should be re-considered in the light of these findings. High quality superconductors in the forms of bulk, wire and tape may be produced by sintering at lower temperatures for longer durations if other parameters are optimized.