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- MakaleYalnız künye2016Effect of laser irradiation on activation energy, irreversibility field and upper critical magnetic field of bulk MgB2 superconductor
Erdem, O.; Yanmaz, E.
In this study, the role of laser irradiation on the upper critical field (Hc2), the irreversibility field (Hirr) and the activation energy (U0) of bulk MgB2 superconductor was first time investigated with the aid of magnetoresistivity measurements conducted at different applied magnetic fields in a range of 0–15 kOe as a function of temperature from 25 to 40 K. For this aim, a disk shaped MgB2 superconductor was produced and cut into two pieces. One of the pieces was irradiated under high vacuum conditions by using an Nd:YVO4 laser. The obtained results showed an increase in the critical temperature (Tc) by about 1 K after irradiation. Also, the values of Hc2(0) were respectively found to be around 143 and 151 kOe while the values of Hirr(0) were about 122 and 130 kOe for the reference and the irradiated samples. Using the thermally activated flux flow model the dependence of U0 on magnetic field was determined. The maximum U0 values of 0.43 and 0.83 eV in zero magnetic field were respectively obtained for the reference and the irradiated samples. Also, the magnetic field dependence of the critical current density Jc(B) values at different temperatures were slightly increased after the irradiation. These results point out that the superconducting properties of bulk MgB2 can be improved via laser irradiation.
AtıfScopus11WoS10
İncele - MakaleYalnız künye2017Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures
Erdem, O.; Yanmaz, E.
The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 °C, after hot pressing at 200 °C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 °C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 °C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor.
AtıfScopus4WoS4İncele - MakaleYalnız künye2014Attractive method for thickness-independent permittivity measurements of solid dielectric materials
Hasar, Ugur Cem; Barroso, Joaquim José; Bute, Musa; Kaya, Yunus; Kocadagistan, Mustafa Engin; Ertugrul, Mehmet
We propose a non-resonant-type method for reference-plane invariant and thickness-independent permittivity measurement of solid dielectric materials when there is no information from permittivity. To achieve this, we derive a branch-index-independent function and verify that it determines the sample thickness and permittivity without resorting to finding the correct branch index value for three different measurement scenarios: (a) measurements at closely separated frequencies of non-dispersive samples, (b) measurements at largely separated frequencies of non-dispersive samples, and (c) measurements at largely separated frequencies of dispersive samples. Via a differential uncertainty analysis, the effect of inaccurate sample thickness on permittivity determination is also presented for demonstration of the improvement achieved by our method. For validation, we measured the permittivity of an approximately 20 mm PTFE sample using the presented method and compared its result with those from other similar methods. From the comparison, we note that the accuracy of tested methods from literature drastically decreases when accurate information of sample thickness is not known (in addition to a further decrease in accuracy when correct transformation factors from reference-planes are not known a priori), while the accuracy of our method is not influenced.
AtıfScopus25WoS22İncele - MakaleYalnız künye2018Self-Calibrating Noniterative Complex Permittivity Extraction of Thin Dielectric Samples
Hasar, Ugur Cem; Kaya, Yunus
A microwave method relying on uncalibrated scattering (S-) parameters is proposed to measure the complex permittivity (ϵr) of thin dielectric samples. It has the following two main advantages. First, it takes into account effect of the sample holder, used for holding the sample, especially important for thin sample electromagnetic property characterization. Second, it does not require any specific information about the location of the sample (and its holder) inside its measurement cell for ϵr extraction. For validation of our method, we applied a commercial three-dimensional electromagnetic simulation program-CST Microwave Studio-and the Lorentz dispersion model. Uncalibrated (as well as calibrated) S-parameter measurements were conducted to measure ϵr of a 0.7 mm thick polyethylene sample (the sample holder was a 5.18 mm thick PVC sample) by our method and other similar methods in the literature. From the comparison, we observed that while the accuracy of tested methods significantly changed with inaccurate knowledge of the sample position inside its cell, the accuracy of our method did not much alter.
AtıfScopus18WoS16İncele - MakaleYayıncıda açık erişim2019Uniform boundedness of kantorovich operators in variable exponent Lebesgue spaces
Ayazoglu, Rabil; Akbulut, Sezgin; Akkoyunlu, Ebubekir
In this paper, the Kantorovich operators Kn, n ϵ N are shown to be uniformly bounded in variable exponent Lebesgue spaces on the closed interval [0, 1]. Also an upper estimate is obtained for the difference Kn (f) − f for functions f of regularity of order 1 and 2 measured in variable exponent Lebesgue spaces, which is of interest on its own and can be applied to other problems related to the Kantorovich operators.
AtıfScopus3WoS3İncele