Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Subject: search.filters.subject.High temperature superconductors

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Now showing 1 - 7 of 7
  • Master Thesis
    Fabrication of Lu Doped Ybco Thin Films by Pulsed Laser Deposition Technique and Their Characterization
    (Izmir Institute of Technology, 2010) Sezer, Bahtiyar Şirvan; Abukay, Doğan; Abukay, Doğan; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    Nearly twenty years ago YBCO was the first superconductor discovered with a transition temperature above the boiling point of liquid nitrogen, the .fascinating. limit for high temperature superconductivity. From the day forward, the interest in this ceramic compound has not diminished. YBCO is one of the most promising materials for the application of high temperature superconductors (HTS) because it is able to carry a technically useful current density in applied fields at 77 K. A lot of experiments guided to investigate the basic properties of the HTS and to further the theoretica understanding of them also used YBCO, because this progress has been achieved in the preparation of bulk samples, and especially thin films deposited by a various methods. The aim of the experimental investigation presented in this thesis was to produce high quality epitaxial Lutetium doped YBCO thin films on MgO substrates prepared by pulsed laser deposition. For this purpose, bulk Lu2O3 powder was mixed into YBCO by using solid-state reaction method and pressed to make a stoichiometric target for PLD process. KrF excimer laser was worked at 14 Kv with repetition rates ranging from 3 to 5 Hz to deposited Y0.9Lu0.1Ba2Cu3O7-. thin films at a substrate temperature of 800 oC. The surface of the films were characterized by employing XRD, SEM, EDX and AFM techniques.
  • Master Thesis
    Reactive Ion Beam Etching and Characterization of High-Tc Superconductor Bi2212
    (Izmir Institute of Technology, 2009) Köseoğlu, Hasan; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The lack of coherent, continuous and tunable compact solid-state sources of electromagnetic radiations at terahertz (THz) frequency range (0.3-30 THz) can be solved by high temperature superconductors (HTS) which are better candidates for generation of THz radiation. THz sources have potential application areas in materials characterization, biology, communication, medicine and security. The HTSs have large energy gap intervals which are available for radiations at THz frequency range, so recently, they were used as better converters from DC-voltage to high frequency radiation in junction technologies. Bi2Sr2CaCu2O8+. (Bi2212) HTS single crystals include natural superconductor-insulator-superconductor multi-junctions called intrinsic Josephson junctions. For generation of coherent continuous and powerful THz radiations, we have fabricated large and tall mesas on Ca rich Bi2212, although mesas with small planar and lateral dimensions were preferred in recent studies because of the heating effect. We have used underdoped Bi2212 to control the heating problem. By using the vacuum thermal evaporation, optical photolithography, magnetron sputtering and reactive ion beam etching techniques, the mesas with size of 40-100x300 um2 have been fabricated by using single layer mask (PR) and two different multilayer masks, which are Ta/PR and PR./Ta/PR. Their heights and lateral dimensions were analyzed by Atomic Force Microscopy, profilometer and SEM. We have investigated temperature dependence of c-axis resistivity and current-voltage (I-V) tunneling characteristics of under-doped Bi2212 which exhibit exponentially increases in resistivity from 300 K to Tc. The influences of heating effect were analyzed at temperature evolution of I-V measurements. Finally, bolometric detections of emission from long edges of mesas were done.
  • Master Thesis
    Spin Polarized Tunneling in Large Area Mesas of Superconducting Bi2sr2cacu2o8+d for Terahertz Emission
    (Izmir Institute of Technology, 2010) Türkoğlu, Fulya; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There is an increasing interest in science and technology of electromagnetic waves in terahertz frequency range (0.1-10 THz) because of their emerging application areas including physics, biology, chemistry, astronomy, medicine etc. The observation on generation of THz radiation emitted from lateral dimension of high temperature superconductor (HTS) Bi2Sr2CaCu2O8+.. (Bi2212) and responses to THz waves increase the importance of these HTSs. Single crystal of HTS Bi2212 forms natural superconductor-insulator-superconductor (SIS) layered junctions, which are called intrinsic Josephson junctions (IJJ). The stacks of IJJs in Bi2212 can be used such a voltage-frequency converter and their large energy gap allows the emissions at THz frequency range. Recently, it has been demonstrated that rectangular IJJ mesa structures of Bi2212 can be used as a source of continuous, coherent and polarized THz radiation. It was shown that all THz emitting mesas are below a certain underdoped level, which has relatively small critical current in contrast to optimally doped and overdoped Bi2212. In this work, rectangular Au/Co/Au/Bi2212 mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals using standard photolithograph and Ar ion beam etching techniques. In order to characterize the mesas, c-axis resistance versus temperature (R-T) and current-voltage (I-V) characteristics were investigated. During I-V characterization, Si composite bolometer was used to detect the emission. We obtained small critical current from as-grown mesas due to injection of spin polarized current. We observed THz emission peak for one of the mesas which has low quasiparticle conductivity and low dissipation due to its small critical current density. It means that the adjustment of doping level can be eliminated for THz emission by the injection of spin polarized current through the c-axes of the asgrown mesas.
  • Master Thesis
    High Temperature Superconducting Yba2cu3o7-X Transitation Edge Bolometers (tebs) for Infrared Region
    (Izmir Institute of Technology, 2008) Günel, Hacı Yusuf; Abukay, Doğan; Abukay, Doğan; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The thesis focuses on electrical and bolometric characterization of high temperature superconducting YBCO transition edge bolometers. In this study, superconducting YBCO thin films grown on MgO, LaAlO3 and SrTiO3 substrates were first characterized structurally via x-ray diffraction and scanning electron microscopy.They then were patterned into bolometers and their electrical and bolometric characterizations were carried out. The results of these characterizations were fitted to the RC model. In addition, figures of merit for these devices, such as noise equivalent power (NEP) and detectivity, were measured and calculated.During this work, the patterning process was carried out by standard photolithographic processes and chemical etching. We have also investigated the effect of dimensions of our devices on the performance of bolometers. The electrical and bolometric characterizations comprised of resistance versus temperature (R-T), amplitude of response versus temperature, amplitude of response versus chopping frequency (A-F) and noise voltage measurements. Finally, the performance of our devices were compared to literature and found to be similar to or better than those of similar devices in literature.
  • 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.
  • Master Thesis
    Fabrication of Mesa Structures on Superconducting Bi2sr2cacu2o8+& Single Crystals
    (Izmir Institute of Technology, 2005) Kurter, Cihan; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There have been tremendous efforts to understand the relatively much more sophisticated mechanism of superconductivity in high temperature superconductors (HTSC). In order to investigate the inherent features and tunneling characteristics just only peculiar to HTSC, micron-sized mesa structures were fabricated on the surfaces of both pristine optimally doped and HgBr2 intercalated Bi2Sr2CaCu2O8+& (Bi-2212) single crystals using photolithography and Argon ion beam etching techniques. The surface topography and heights of the mesas were examined with atomic force microscopy.Hysteretic I-V curves with multiple branches and temperature dependence of tunneling characteristics were investigated by means of a novel technique, point contact tunneling (PCT) and experiments were carried out in a large range of temperatures from 4.2 K to 300 K. The results of SIN single junctions and SIS break junctions obtained by tunneling measurements using PCT technique on bulk crystals were compared withtunneling measurements using PCT technique on bulk crystals were compared with intrinsic Josephson junction quasiparticle spectra generally showing sharp peaks at the gap voltages and no dip/hump structures; which are reconciled with overheating in the mesa. The IJJ measurements performed with HgBr2 intercalated Bi-2212 samples showed far more enhanced characteristics indicating less heating. The zero bias conductance versus temperature plots were examined to scrutinize the existence of pseudogap in electronic excitation spectra of investigated samples. Besides, the normalized gap voltages were plotted against normalized temperature to show the deviation from BCS fit, which displays the novelty of HTSC.
  • Master Thesis
    Electrical and Structural Characterization of Metallic Superconductor Mgb2
    (Izmir Institute of Technology, 2003) Günel, Aylin; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The brittleness of recently discovered superconductor MgB2 requires novel technique to prepare them as a superconducting wire. In this study, the metal matrix composites consisting of Mg and MgB2 were fabricated by uniaxial hot pressing with a pressure application of 0.5 GPa. While some of the composite pellets was exposed to a heat treatment at 400 0C, the other samples prepared by a heat treatment at 500 0C. One of the samples in each group was a pure pellet of commercially available powders and the others include excess Mg with 5wt %, 10wt %, 15wt % and 20wt % of the MgB2 powders in the pure pellets. X-ray results illustrate that the initial excess Mg consitutient is still present in the final metal matrix composite in unreacted form, which is what we wanted. Microstructural studies performed by SEM and EDX indicate the possible presence of secondary phases like MgB4, MgB6 and MgO in very small amounts. While the MgB4 and MgB6 probably originated from the initial commercial MgB2 powder, the concentration of MgO might be increased with the heat treatment. Microstructural studies also point out that all the composites exhibited a porous structure with no significant amount of secondary phases. The temperature dependence of resistivity was measured for each sample with an application of 1 mA, 5 mA, 10 mA, 50 mA and 100 mA by four-point measurement technique. The midpoint critical temperatures at 10 mA were found in a range between 22.67 and 32.50 different sample preparation conditions. Present data imply a possible improvement in superconducting transition temperature of sample annealed at 500 °C with increasing Mg concentration up to some value between 15% and 20%, and an increase in critical current relative to samples annealed at 400 °C. For the samples prepared with lower heat treatment, there is an improvement in superconducting critical temperature up to an excess Mg concentration around 10% weight of the constituted MgB2 powder.