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    Effect of postannealing processes on the properties of hot-pressed Bi2Sr2Ca1Cu1.75Na0.25Oy (Bi-2212) ceramics
    (Springer, 2023) Ozkurt, Berdan; Madre, M. A.; Sotelo, A.
    In the present work, the effect of different annealing-time on the properties of Bi2Sr2Ca1Cu1.75Na0.25Oy (Bi-2212) superconducting compound is presented. First, the samples were produced by hot-pressing method under 33 MPa pressure at 750 degrees C in accordance with previous works. After annealing, the samples were kept at 840 degrees C for 15, 30 and 60 h, respectively, for postannealing process. XRD results demonstrate the formation of Bi-2212 phase in large amount in all samples. SEM images show the formation of different grain morphologies depending on the postannealing time. The sample postannealed for 60 h at 840 degrees C has the highest superconducting transition temperature (T-c, onset), 91 K, obtained from M-T measurements. The magnetic properties of samples have been investigated using M-H hysteresis loops at 15 K in the range - 1.6 x 10(6) to + 1.6 10(6) A/m. The widest M-H curves were obtained in the postannealed sample for 60 h at 840 degrees C, probably as a result of both the improvement of crystallinity, cleaner grain boundaries and better grain orientation. In addition, the widest hysteresis curve observed in that sample in the entire applied magnetic field range is an indication that it has strong pinning behavior. The critical current density (J(c)) was investigated via Bean's model from these data. Finally, the sample postannealed at 840 degrees C for 60 h reached a J(c) value of 2.903 x 10(6) A/cm(2) at H = 1.9 T and T = 15 K, which is, approximately, 44% higher than the determined in annealed samples (2.015 x 10(6) A/cm(2)).
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    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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    Enhanced thermoelectric properties in Bi2Sr2-XBaxCo2Oy ceramics by Ba doping
    (Elsevier, 2022) Ozkurt, Berdan; Madre, M. A.; Sotelo, A.; Torres, M. A.
    Bi2Sr2-xBaxCo2Oy polycrystalline samples (0.00 <= x <= 0.15) were prepared through the classical solid-state reaction method. It has been found that Ba-doped Bi2Sr2Co2Oy was obtained as major phase in all samples, based on XRD results. On the other hand, SEM studies have shown the presence of small amounts of secondary phases. The electrical resistivity values of all Ba doped samples decreased when compared to the undoped one, being minimum for the 0.075Ba doped ones. Seebeck coefficient increase with the increasing temperature in all samples, reaching the highest values in the 0.025 Ba doped sample. Finally, power factor has been calculated using the Seebeck coefficient and electrical resistivity values to determine the thermoelectric performances of samples. The maximum PF value of 0.14 mW/K2 m at 650 degrees C was obtained for 0.075Ba doped sample, which is around 25% higher than the undoped sample at the same temperature. All these results clearly show that the thermoelectric properties of the Bi2Sr2Co2Oy system can be improved by Ba doping at optimal values.
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    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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    Improving thermoelectric properties of Ca3Co4O9+ through both Na doping and K addition at optimal values
    (Springer, 2019) Ozkurt, Berdan; Aytekin, M. Ersin; Madre, M. A.; Sotelo, A.; Torres, M. A.
    Ca2.93Na0.07Co4Oy/x wt% K2CO3 (x=0.00, 0.01, and 0.03) polycrystalline ceramics were prepared by conventional solid-state method. XRD results have shown that all samples predominantly include Ca3Co4O9 phase together with small amounts of secondary phases. SEM images show that all samples have randomly oriented plate-like grains in different sizes. The electrical resistivity measurement showed that electrical properties of Ca3Co4O9 ceramics can be improved significantly by both, Na doping and K addition at their optimal values. The effect of dopants on thermoelectric properties of Ca3Co4O9 was examined by both, Seebeck coefficient and power factor, being higher in K-added than in the pure samples, indicating that thermoelectric properties of samples are positively affected when alkaline elements enter into their structure.
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    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.