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    Effects of different dwell-times under low pelletization pressure on the physical properties of the Bi-2212 ceramics
    (Springer, 2022) Kara, Emine; Ozkurt, Pinar; Ozkurt, Berdan
    We reported the evaluation of superconducting properties of Bi-2212 ceramics prepared under different dwell times between 12 and 72 h under 114 MPa constant pelleting pressure using precursors produced through the conventional solid-state reaction. From the XRD patterns, it is clear that all samples contain a high Bi-2212 phase ratio. Moreover, they include only Bi2CaO4 as secondary phase(.) This means that even though the applied pelleting pressure was low, it allowed the formation of the Bi-2212 phase due to its high-thermodynamic stability. SEM results showed that all samples have similar grain morphology with random grain orientation and formation of homogeneous phases, except that they have different grain sizes. T-c values obtained from the M-T curve (magnetization vs. temperature) for all samples are in the range of 77-82 K, indicating that the Bi-2212 phases are formed in the ideal ratio. The magnetic properties of the prepared samples were investigated using M-H (Magnetic hysteresis) and magnetic J(c) curves (critical transport current density calculated in the framework of the Bean model), at 15 K. It has been found that the dwell time of 12 h at constant pelleting pressure of 114 MPa provided the best superconducting properties and the largest J(c) values (1.605 x 10(4) A/cm(2) at T = 15 K and H = 1.5 T) for the Bi-2212 phase in this work. In addition, to better examine the flux pinning properties of samples, the pinning force density (Fp) was calculated from the J(c) and B values. The maximum pinning force (3.929 Oe A/cm(2) at T = 15 K and H = 1.95) has been obtained in the sample including dwell time of 12 h under 114 MPa.
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    Öğe
    Enhanced Superconducting Properties in Bi2Sr2Ca1Cu1.75Na0.25Oy Ceramics Prepared by Hot-Pressing Under Different Pressures and Temperatures
    (Springer, 2022) Oner, Bulut; Ozkurt, Pinar; Madre, M. A.; Ozkurt, Berdan; Sotelo, A.
    Bi2Sr2Ca1Cu1.75Na0.25Oy ceramics have been prepared by the classical solid state reaction method followed by hot-pressing process under several applied pressures and temperatures: 22 and 33 MPa at 750 degrees C, and 22 MPa at 800 degrees C. X-ray diffraction data indicate that the major phase in all samples is the Bi2Sr2Ca1Cu1.75Na0.25Oy phase, accompanied by small amounts of Bi2CaO4 and Bi3Sr4CaCu3O4 secondary phases. Microstructural results clearly showed that the best grain orientation has been obtained under 33 MPa at 750 degrees C. Critical temperatures, T-C (onset) and T-C (offset), of these samples were significantly increased to 96.6 and 85.6 K, respectively, pointing out to a good intergranular coupling. M-H hysteresis curves for all samples display a very large area due to the microstructural characteristics obtained in these samples. The highest critical current density, calculated from M-H measurements according to Bean's model, has been obtained in samples prepared under 33 MPa at 750 degrees C (201.5 x 10(4) A/cm(2) at 1.9 T and 15 K), which are much larger than those reported for samples with the same composition prepared through the classical solid state method.
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    Öğe
    Enhanced thermoelectric properties in Bi2Sr2-xBaxCo2Oy via doping and texturing for integration in more efficient thermoelectric generators
    (Elsevier, 2025) Ozkurt, Pinar; Madre, M. A.; Ozkurt, Berdan; Torres, M. A.; Sotelo, A.
    Bi2Sr2-xBaxCo2Oy (0 <= x <= 0.15) thermoelectric samples have been sintered, and textured through the laser floating zone process using a Nd:YAG laser. Powder XRD studies showed that the thermoelectric phase is the major one in all cases, with higher amount of secondary phases in the textured ones due to their incongruent melting. Microstructural characterization revealed a drastic microstructural modification in the textured samples, when compared to the sintered ones, producing much larger and well oriented grains along the growth direction. These characteristics led to lower electrical resistivity in textured samples, reaching the minimum at 650 degrees C (14.8 mS2 cm) in 0.125Ba-doped samples, which is lower than those typically reported in this system. On the other hand, no significant variation in Seebeck coefficient has been found between the samples. This behaviour is associated to the isovalent doping which does not modify the charge carrier concentration in the material, and the highest values at 650 degrees C (166 mu V/K) are in the order of the reported in the literature. As a consequence, power factor values are mainly driven by the electrical resistivity values, leading to the highest values at 650 degrees C in 0.125Ba-doped textured samples (0.19 mW/K2m) due to their lowest resistivity. These values are higher than the reported for textured materials and in the order of the best reported for this compound in bulk form. All these properties, together with the possibility of the direct integration of these compounds in thermoelectric modules, make them very attractive for practical applications ensuring access to affordable, reliable, and sustainable energy for all.
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    Öğe
    Wavelength-induced modifications of thermoelectric properties of laser-textured Bi2Sr2-xBaxCo2O8 ceramics for sustainable energy generation
    (Elsevier, 2025) Ozkurt, Pinar; Madre, M. A.; Ozkurt, Berdan; Torres, M. A.; Sotelo, A.; Rasekh, Sh.
    Ba-doped Bi2Sr2-xBaxCo2O8 samples have been directionally grown using the Laser Floating Zone technique using two different lasers, leading to significant differences between the samples. Powder XRD patterns revealed that the major phase in all samples is the thermoelectric one, without notable differences with the type of laser. On the other hand, microstructure studies showed significant differences between Nd:YAG and CO2-grown samples. Use of CO2 laser promoted a reduction of secondary phases content and a higher grain alignment. Furthermore, Ba-doping further decreases the secondary phases content and increases grain alignment. The electrical resistivity was affected by the different microstructures, being lower for the CO2-grown samples, reaching the lowest values at 650 degrees C in 0.125Ba-doped samples, 12.8 m Omega cm, which are among the best reported in the literature. However, S is maintained practically unchanged independently of the laser and composition. As a consequence, PF values mainly depend on the electrical resistivity and, consequently, the highest values at 650 degrees C, 0.22 mW/K2m, have been achieved in 0.125Ba samples textured with CO2 laser. These values are around the best ones presented in the literature for this material. Additionally these textured materials possess the advantage of avoiding the typical machining processes which are necessary in bulk materials for their integration into thermoelectric modules. These characteristics may enhance the interest on these materials in order to be used in practical devices to help their use to produce affordable, reliable and sustainable energy for all mankind.