Yazar "Rasekh, Sh." seçeneğine göre listele

Listeleniyor 1 - 3 / 3
  • Küçük Resim Yok
    Öğe
    Decrease of Ca3Co4O9+? thermal conductivity by Yb-doping
    (Elsevier Sci Ltd, 2015) KIirat, G.; Aksan, M. A.; Rasekh, Sh.; Madre, M. A.; Diez, J. C.; Sotelo, A.
    In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the Ca3Co4O9+delta system has been investigated in the low temperature region (between 10 and 300 K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85 K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E-0, value also decreases with Yb-substitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, kappa, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300 K. Highest figure of merit, ZT, value is found to be 5.4 10(-3) at 300 K for the unsubstituted sample and the ZT value decreases by the substitution. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
  • Küçük Resim Yok
    Öğe
    Effect of Secondary Annealing Process on Critical Current Density in Highly Textured Bi-2212 Superconducting System
    (Springer, 2015) Aksan, M. A.; Madre, M. A.; Rasekh, Sh.; Constantinescu, G.; Torres, M. A.; Diez, J. C.; Sotelo, A.
    Bi-2212 samples prepared by a solid-state reaction technique have been grown from the melt using the laser floating zone method. After annealing the as-grown bars, the samples showed a good grain alignment and a high transport critical current density. Secondary annealing processes were performed on the annealed samples with the aim of producing Bi-2212 phase controlled decomposition. Hence, the Bi-2201 phase and the secondary phases, which act as effective pinning centers, were obtained with the secondary annealing process. After these thermal treatments, the transport critical current densities of samples significantly increased, when compared to the annealed ones. The maximum critical current density was achieved when the samples were subjected to secondary annealing at 680A degrees C for 168 h with an improvement of similar to 80%, compared to the annealed ones. Moreover, it was found that magnetization of the secondarily annealed samples was also increased. The magnetic critical current densities in these secondary annealed samples were about 3 times higher than the values obtained for the annealed ones. These results clearly indicate that the secondary annealing processes lead to the formation of effective pinning centers in the bulk material.
  • Küçük Resim Yok
    Öğe
    Growth Speed and Substitution Effects on Alignment and Thermal Transport Properties of Bi-2212 Textured Superconductors
    (Springer, 2015) Ozabaci, M.; Rasekh, Sh.; Kizilaslan, O.; Madre, M. A.; Sotelo, A.; Yakinci, M. E.
    Fe-substituted superconducting thin BiSrCaCuO rods with nominal compositions of Bi2Sr2Ca1Cu2-x Fe (x) O8+delta (x = 0, 0.01, 0.03, 0.05, and 0.1) were fabricated using the laser floating zone technique at two different growth speeds, 15 mm h(-1) and 30 mm h(-1). The influences of growth speed and Fe substitution on the grain alignment in the rods were evaluated by means of x-ray pole figure studies. The obtained results showed that both applied growth speed and Fe substitution play a crucial role on grain alignment, which is strongly connected with the current-carrying capacity of the rods. It was found that the rods grown at 15 mm h(-1) (G15) have stronger orientation than the rods grown at 30 mm h(-1) (G30). However, in contrast to the G15 rods, an increased substitution rate improved the orientation of the G30 rods. Another important observation is that the increase on the substitution caused a decrease on the grain size of all the rods. The decrease of critical temperature values of the rods upon substitution was ascribed to both grain size effect and formation of a nonsuperconducting Fe-rich phase detected in scanning electron microscope/energy-dispersive x-ray analyses. The thermal conductivity values of the G15 and G30 rods were found to be in the range of 0.9-1.9 and 1.1-1.18 W m(-1) K-1 at 150 K, respectively. The higher values of figure of merit (ZT), at all temperature ranges, were obtained from the highest substituted rods (x = 0.1) for both of the applied growth speeds. In addition, it was observed that the ZT of G30 rods are up to three times higher than that of G15 ones.