Master Degree / Yüksek Lisans Tezleri

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Access type: Open accessSubject: search.filters.subject.Superconductivity

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  • Master Thesis
    Synthesis and Characterization of Mgb2 Superconducting Wires
    (Izmir Institute of Technology, 2008) Horzum Polat, Nesrin; Özgen, İsmet Tamerkan; Özgen, Tamerkan; 04.01. Department of Chemistry; 04. Faculty of Science; 01. Izmir Institute of Technology
    In this study, the superconducting properties of laboratory synthesized MgB2 was investigated. In the first part, MgB2 synthesis using commercial magnesium and boron (95-97% purity), and its microstructural and electrical characterization was investigated.Effects of sheath material and annealing temperatures were also examined. The microstructural studies showed that when Cu tubes were used as sheath material, MgCu2 forms instead of MgB2 even at 700oC, while on Fe clad cores, the major phase was MgB2 with minor MgO constituent. The transition temperatures of Fe clad wires were measured between 39K and 40K, whereas no transition temperature was observed for Cu clad wires. The Ic value of the Fe clad MgB2 wire was about 25 A at 4K, while the copper clad wire could not carry current and formed resistance. In Fe clad wires, better results were obtained at annealing temperature of 800°C for 30 minutes. In the second part, MgB2 synthesis using commercial magnesium and boron (90% purity) was tried. 0-5-10-15 wt% of Mg doping and, additionally annealing temperatures were examined. Powder-In-Tube method was used for wire production. 10 wt% Mg addition was seen to be beneficial as compared to the stoichiometric MgB2. 750°C was found to be the most suitable temperature for the formation of MgB2 phase. The Ic value of the wire was measured as 13 A at 4K and it showed a broader transition with non-zero resistivity, transition temperature of 24K.In the third part, 200 m long four filament MgB2/Cu wire was successfully produced in laboratory conditions.
  • Master Thesis
    High Temperature Superconducting Yba2cu3o7-Δ Thin Flims and Bolometers
    (Izmir Institute of Technology, 2006) Öktem, Bülent; Abukay, Doğan; Abukay, Doğan; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    In this work, we report the results on the response characteristics of high temperature superconducting bolometers under the infrared and optical radiation. The bolometers were made of YBa2Cu3O7-. (YBCO) thin films deposited on <100>-oriented and 10x10x1-0.5 mm3 sized single crystal MgO substrates by using DC inverted cylindrical magnetron sputtering (ICMS) technique with 200 nm thickness. The thin films were characterized in detail by employing structural (XRD, SEM, EDX, and AFM), magnetic (.-T) and electrical (R-T) measurements. We have investigated effects of temperature and modulation frequency on the magnitude and phase of the infrared response of the both large and small area bolometers. Dependence of the phase and magnitude of the response of MgO substrate YBCO edge transition bolometers to a near infrared radiation on the superconducting transition width has also been investigated in this work. We have observed that the superconductivity transition width has major effects on the response of the bolometers such as; on a dip of the phase of the response versus modulation frequency curve around 1 Hz, the rate of decrease of the magnitude of the response, and dependence of the phase of the response on temperature at mid-range modulation frequencies. Wavelength dependence of the photoresponse of YBCO edge transition bolometers has been studied. It was observed that the optical response of the devices was almost independent of the wavelength of incident light from 0.4 .m to 0.73 .m. Here we present the analysis and the possible mechanisms that can affect the response of the bolometers in the superconducting transition region.
  • Master Thesis
    Spin Polarized Tunneling Spectroscopy of Intercalated Bi2sr2cacu2o8+
    (Izmir Institute of Technology, 2006) Özdemir, Mehtap; Özyüzer, Lütfi; Özdemir, Mehtap; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There has been continuing interest in decades in heterostructure tunnel junctions that combine ferromagnetic material (F), insulator (I) and superconductor (S). This junction has been known to a good probe to analyze the physical properties of ferromagnetic and superconducting materials. In order to understand the influence of the spin injection on the c-axis tunneling characteristics of a Bi-2212 single crystal as a function of temperature and magnetic field, two sets of samples have been prepared; in one set, the surface of HgBr2 intercalated Bi-2212 crystals have been deposited with merely Au while the other set has ferromagnetic multilayer (Au/Co/Au) on top of the crystal. The micron sized mesa arrays have been patterned using photolithography and ion beam etching techniques. The surface topography and height of the mesa were investigated using atomic force microscopy. Tunneling characteristics were examined by means of the novel technique, point contact tunneling (PCT), and experiment were performed wide range of temperatures from 4.2 K to 300 K and that of magnetic field from 0 G to 2600 G. The results of spin polarized tunneling measurements compared with spin degenerate tunneling. Magnetic field dependence hysteretic I-V curves with multiple branches were examined to show the suppression of the superconductivity with spin polarized current. The distinct temperature dependence of depression in tunneling conductance near Fermi level was discussed for spin degenerate and polarized currents above Tc. The zero bias conductance and resistivity versus temperature plots were investigated to show the existence of the pseudogap in tunneling spectroscopy of investigated samples.
  • Master Thesis
    Microstructural, Electrical and Mechanical Properties of Mgb2/Mg Metal Matrix Composites
    (Izmir Institute of Technology, 2004) Eğilmez, Mehmet; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The recent discovery of superconducting MgB2 (2001) has risen a great interest because of its relatively high Tc (39 K). Furthermore, simple crystal structure, large coherence lengths, high critical current densities (Jc) and critical fields (Bc2), promise that MgB2 will be a good material for large-scale applications based on production of superconducting wires. However, MgB2 is not a stand-alone material to produce wires because of its brittle nature. The potential of MgB2/Mg composites for producing superconducting wires, is investigated. MgB2/Mg composites were prepared using metal matrix composite fabrication technique, which is an alternative to conventional wire fabrication methods such as powder in tube. MgB2 and Mg powders were mixed at different weight fractions and uniaxially pressed in a cylindrical dye under the pressure of 0.5 GPa and 1GPa for two hours at different temperatures in the air. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Electron Dispersive X-Ray Spectroscopy (EDX) techniques were used for phase identification and microstructural studies. D.C resistivity measurements were carried on the pellet segments of the MgB2/Mg composites in the temperature range 20-300 K. Magnetization measurements were done on the composites using a vibrating sample magnetometer to characterize the magnetic properties of the samples. The effects of pressing temperature on the mechanical properties of MgB2/Mg composites were also investigated. For this purpose, compressive mechanical testing were performed to measure elastic modulus and strength at failure values of the composites. It was found that the relative weight fractions of Mg and the pressing temperature have some considerable effect on the electrical, magnetic, microstructural and the mechanical properties of the composites.