Browsing by Author "Zalaoglu Y."

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A study on magnetoresistivity, activation energy, irreversibility and upper critical field of slightly Mn added Bi-2223 superconductor ceramics
(2012-05-01) Dogruer M.; Zalaoglu Y.; Varilci A.; Terzioglu C.; Yildirim G.; Ozturk O.
This study discusses the effect of Mn addition on the superconducting and physical properties in Bi 1.8Pb 0.4Sr 2Mn xCa 2.2Cu 3.0O y bulk superconductors with x = 0, 0.03,0.06,0.15, 0.3, and 0.6 by means of themagne-toresistivity measurements. The magnetoresistivity of the samples prepared using the standard solid-state reaction method was measured for different values of the applied magnetic field strengths. The superconducting and physical properties of the samples such as the zero resistivity transition temperatures (T c), irreversibility fields (μ 0Hi rr), and upper critical fields (μ 0H c2) were deduced from the magnetoresistivity curves. Moreover, thermally activated flux creep model was studied for activation energy (U0) values of the samples. According to the results of the measurements, not only were the T c and U0 values of the samples found to decrease significantly but the μ 0Hi rr and μ 0H c2 values were also observed to reduce with the increase in the Mn addition, indicating that the doping degrades the physical and superconducting properties of the samples. © 2012 Springer Science+Business Media, LLC.
Comparative study on mechanical properties of undoped and Ce-doped Bi-2212 superconductors
(2013-07-01) Zalaoglu Y.; Bekiroglu E.; Dogruer M.; Yildirim G.; Ozturk O.; Terzioglu C.
This study discusses the mechanical performances of Bi1.8Sr 2.0CexCa1.1Cu2.1Oy ceramics with x = 0, 0.001, 0.003, 0.005, 0.01, 0.03, 0.05 and 0.1 by way of Vickers microhardness (Hv) measurements performed at different applied loads in the range of 0.245-2.940 N. For the potential industrial applications, the important mechanical characteristics such as Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are extracted from the microhardness measurements. All the results obtained indicate that the Vickers hardness, Young's (elastic) modulus, yield strength, fracture toughness and brittleness index values suppress with the increment of the Ce concentration in the system as a consequence of the degradation in the connectivity between superconducting grains. The decrement in the Hv values with the applied load is attributed Indentation Size Effect behaviour of the samples studied. Moreover, the experimental results of Vickers microhardness measurements are estimated using the 5 different models such as Kick's law, proportional sample resistance model, modified proportional sample resistance model, elastic/plastic deformation model and Hays-Kendall approach. According to the results obtained from the simulations, Kick's law is not useful model to obtain information about the origin of the indentation size effect feature of the Ce-doped bulk Bi-2212 superconductors. On the other hand, the Hays-Kendall approach is determined as the most suitable model for the description of the mechanical properties of the superconducting samples. In addition, the bulk porosity analysis for the samples reveals that the porosity increases monotonously with the Ce inclusion in the Bi-2212, leading to the degradation of the grain connectivity. © 2013 Springer Science+Business Media New York.
Effect of Ce addition on the magnetoresistivity, irreversibility field, upper critical field and activation energies of Bi-2212 superconducting ceramics
(2012-05-01) Yildirim G.; Dogruer M.; Ozturk O.; Varilci A.; Terzioglu C.; Zalaoglu Y.
This study aims to analyze the effect of Ce addition on the microstructural, superconducting and physical properties of Bi 1.8Sr 2.0CexCa 1.1Cu 2.1O y ceramics with x = 0, 0.001, 0.003, 0.005, 0.01, 0.03, 0.05 and 0.1 via X-Ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-Ray (EDX) and magnetoresistivity measurements. The ceramics produced in this work are prepared using the standard solid-state reaction method. The zero resistivity transition temperatures (T c), activation energies (U0), irreversibility fields (μ 0Hi rr) and upper critical fields (μ 0H c2) are determined from the resistivity versus temperature (R-T) curves under dc magnetic fields up to 7 T The results show that T c and U0 values of the samples are found to decrease dramatically with the increase in the Ce-content and applied magnetic field. Moreover, XRD results indicate that all the samples contain the Bi-2212 phase only and exhibit the polycrystalline superconducting phase with less intensity of diffraction lines with the increase of the Ce addition. As for the results of SEM images, the texturing, crystallinity, grain size distribution, layered grain growth and grain connectivity are observed to degrade with the increase of the Ce doping. Besides, the irreversibility fields and upper critical fields are found to degrade as Ce doping increases. Penetration depths (λ) and coherence lengths (ξ) are also discussed. © 2012 Springer Science+Business Media, LLC.
Effect of homovalent Bi/Ga substitution on propagations of flaws, dislocations and crack in Bi-2212 superconducting ceramics: Evaluation of new operable slip systems with substitution
(2019-12-01) Turkoz M.B.; Zalaoglu Y.; Turgay T.; Ozturk O.; Yildirim G.
This study defines a strong methodology between the mechanical performance behaviors and formation of possible operable slip systems in the crystal structure of Bi-2212 superconducting phase with trivalent Bi/Ga substitution with the aid of Vickers hardness tests exerted at various indentation load intervals 0.245 N–2.940 N. It is found that the mechanical performance behaviors improve regularly with the increment in the trivalent Bi/Ga partial substitution level up to the value of x = 0.05 due to the formation of new operable slip systems. Namely, the optimum gallium (Ga) impurities serve as the strain fields and associated forces for the interaction of dislocations within the different orientations with each other to impose the surface residual compressive stresses orienting favorably the superconducting grains. Thus, the propagation of dislocations, flaws and cracks divert in the crystal structure. On this basis, the presence of optimum Ga impurity in the Bi-2212 crystal system strengthens the mechanical strength, critical stress, resistance to the plastic deformation, stiffness and durability nature. Moreover, the experimental results advance in-depth understanding of fundamental links between the porosity and Young's moduli of elasticity founded on the impurity level and applied test loads. It is observed that in case of the optimum level of x = 0.05 the propagation of flaws, dislocations and cracks proceed along the transgranular regions instead of the intergranular regions as a consequence of improvement in the durable tetragonal phase. On the other hand, the excess Ga content level in the polycrystalline Bi-2212 system results in the augmentation in the stress raisers, crack surface energy and crack-initiating flaws, activating the stress-induced phase transformation.
Evaluation of key mechanical design properties and mechanical characteristic features of advanced Bi-2212 ceramic materials with homovalent Bi/Ga partial replacement: Combination of experimental and theoretical approaches
(2019-12-01) Turkoz M.; Zalaoglu Y.; Turgay T.; Ozturk O.; Akkurt B.; Yildirim G.
This study models the variations in the key mechanical design properties and mechanical characteristic features of Ga substituted Bi-site Bi-2212 ceramics prepared within the different molar ratios of x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.100 and 0.300 with the assistant of available theoretical approaches; namely, Meyer's law, proportional sample resistance, elastic/plastic deformation, modified proportional sample resistance model, Hays Kendall and indentation-induced cracking methods for the first time. The mechanical modeling parameters are gathered from the microhardness (Vickers) experimental tests performed at various applied loads interval 0.245 N–2.940 N. The results provide that the key mechanical design features improve systematically with the augmentation of trivalent Bi/Ga partial replacement level up to x = 0.05 due to the rapid decrement in the main structural problems; namely, the grain orientations, lattice strains, distortions, dislocations, grain boundary interaction/coupling problems, crack-initiating and crack-producing omnipresent flaws in the advanced Bi-2212 ceramic system. Accordingly, the optimum Ga inclusions strengthens the mechanical durability towards the applied stress due to the increased stabilization in the durable tetragonal phase. After the critical substitution amount of x = 0.05, the mechanism turns reversely, and the general mechanical characteristic features including the stiffness, mechanical durability and strength degrade remarkably. Additionally, the mechanical modeling results demonstrate that the Bi/Ga impurity leads to vary positively the quality of standard indentation size effect (ISE) feature until x = 0.05, beyond which the excess Ga additives damage seriously ISE feature of Bi-2212 inorganic compounds. Besides, the indentation-induced cracking (IIC) model is noticed as the best method to describe the true microhardness parameters of Bi/Ga substituted Bi-2212 compounds for the mechanical characterization.
Evaluation of microstructural and mechanical properties of ag-diffused bulk MgB2 superconductors
(2014-01-01) Yilmazlar M.; Terzioglu C.; Dogruer M.; Karaboga F.; Soylu N.; Zalaoglu Y.; Yildirim G.; Ozturk O.
Electrical, microstructural, and mechanical properties of undiffused and Ag-diffused bulk MgB2 superconductors are systematically studied using dc resistivity, scanning electron microscopy (SEM), and Vickers microhardness (H V ) measurements. The resistivity (at room temperature), critical (onset and offset) temperature, variation of transition temperature, hole-carrier concentration, surface morphology, Vickers microhardness, elastic modulus, and yield strength values of the samples are obtained and compared with each other. One can see that all superconducting parameters given above depend on the Ag diffusion on MgB2 system. The obtained results illustrate that the room temperature resistivity reduces with the increment of diffusion annealing temperature because of the hole filling when the onset (Tconset) and offset (Tcoffset) critical temperatures determined from the resistivity curves are obtained to enhance from 38.4 to 39.7 K and from 36.9 to 38.8 K, respectively. Further, SEM studies carried out for the microstructural characterization demonstrate that the surface morphology and grain connectivity also improve with the increase of the diffusion annealing temperature. In fact, the best surface morphology is observed for the Ag-diffused bulk MgB2 superconductor exposed to 850 C annealing temperature. Besides, it is obtained that the load-dependent microhardness values reduce nonlinearly as the applied load increases until 2 N, beyond which the curves shift to the saturation region, presenting that all the samples exhibit the indentation size effect (ISE) behavior. Further, the elastic modulus and yield strength values observed decrease with the enhancement of the applied load. © 2013 Springer Science+Business Media New York.
Influence of diffusion-annealing temperature on physical and mechanical properties of Cu-diffused bulk MgB2 superconductor
(2013-02-01) Dogruer M.; Zalaoglu Y.; Gorur O.; Ozturk O.; Yildirim G.; Varilci A.; Yucel E.; Terzioglu C.
This study reports not only the effect of Cu diffusion on physical and mechanical properties of bulk MgB2 superconductors with the aid of Vickers microhardness (Hv) measurements but also the diffusion coefficient and the activation energy of copper (Cu) in the MgB2 system using the resistivity measurements for the first time. Cu diffusion is examined over the different annealing temperature such as 650, 700, 750, 800 and 850 C via the successive removal of thin layers and resistivity measurement of the sample. Further, Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are evaluated from microhardness measurements. It is found that all the results obtained depend strongly on the diffusion annealing temperature and applied load. The microhardness values increase with ascending the annealing temperature up to 850 C owing to the increment in the strength of the bonds between grains but decreasing with the enhancement in the applied load due to Indentation Size Effect behaviour of the bulk samples. Moreover, the diffusion coefficient is observed to enhance from 2.84 × 10-8 to 3.22 × 10 -7 cm2 s-1 with the increase of the diffusion-annealing temperature, confirming that the Cu diffusion is more dominant at higher temperatures compared to lower ones. Besides, temperature dependence of the Cu diffusion coefficient is described by the Arrhenius relation D = 2.66 × 10-3 exp(-1.09 ± 0.05 eV/k BT) and the related activation energy of the Cu ions in the MgB 2 system is obtained to be about 1.09 eV. Based on the relatively low value of activation energy, the migration of the Cu ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, resulting in the improvement of the physical and mechanical properties of the bulk MgB2 samples. © 2012 Springer Science+Business Media, LLC.
Quantum chemical calculations and interpretation of electronic transitions and spectroscopic characteristics belonging to 1-(3-Mesityl-3-methylcyclobutyl)-2-(naphthalene-1-yloxy)ethanone
(2015-02-25) Koca M.; Arici C.; Muglu H.; Vurdu C.; Kandemirli F.; Zalaoglu Y.; Yildirim G.
This comprehensive study reports the synthesis of the title compound, 1-(3-Mesityl-3-methylcyclobutyl)-2-(naphthalene-1-yloxy)ethanone (C26H28O2), and identification of the molecule by means of the standard experimental methods such as single-crystal X-ray diffraction, ultra violet-visible (UV-vis) spectra, Fourier transform infrared (FTIR) spectra, 13C and 1H NMR chemical shifts and quantum chemical calculations using density functional theory (B3LYP) method for the first time. The experimental results observed display that the synthesis of the C26H28O2compound is perfectly conducted without any impurities. Additionally, the little deviations are noticed on the bond lengths and bond angles, confirming that the strong intra-molecular charge transfers appear in the due to the presence of the electron engagements and conjugative effects (bond weakening). Besides, the intermolecular CH⋯O distance presents the interaction between the methylcyclobutyl CH group and oxygen atom in the ethanone group. At the same time, the absorption wavelength (λmax) appears at 292 nm and interval 297-269 nm in the solvent of chloroform and THF as a consequence of the presence of effective π-π∗conjugated segments in the molecule studied. Besides, optical band gap energy of 3.22/3.25 eV (chloroform/THF), verifies the existence of the strong electronic donating groups in the structure. As for the quantum chemical computations, the determination of the optimized molecular structures, vibrational frequencies including infrared intensities, vibrational wavenumbers, thermodynamic properties, atomic charges, electronic transitions, dipole moment (charge distribution), optical band gap energy, 1H and 13C NMR chemical shifts are conducted using density functional theory/Becke-3-Lee-Yang-Parr (DFT/B3LYP) method with the standard 6-311++G(2d,2p) level of theory. The results obtained show that the strong intra-molecular charge transfer (ICT) appears between the donor and acceptor in the title compound due to the existence of the strong electronic donating groups and effective π-π∗conjugated segments with high electronic donor ability for the electrophilic attack (intermolecular interactions). Additionally, the presence of the non-uniform charge distributions (polar behavior) on the various atoms makes the title compound be useful to bond metallically.
Role of diffusion-annealing time on the superconducting, microstructural and mechanical properties of Cu-diffused bulk MgB2 superconductor
(2013-01-01) Dogruer M.; Gorur O.; Zalaoglu Y.; Ozturk O.; Yildirim G.; Varilci A.; Terzioglu C.
In this study, the effect of various annealing time (0.5, 1, 1.5 and 2 h) on microstructural, mechanical and superconducting properties of the Cu-diffused bulk MgB2 superconducting samples is investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers microhardness (H v ) and dc resistivity measurements for the first time. The critical transition temperature, grain size, phase purity, lattice parameter, surface morphology, crystallinity and room temperature resistivity values of the bulk samples prepared are compared with each other. Electrical-resistivity measurements show that the sample (annealed at 850 C for 1 h), exhibiting the highest room temperature resistivity, obtains the maximum zero resistivity transition temperature (T c ). From the XRD results, all the samples contain MgB2 as the main phase with a very small amount of Mg 2Cu phase. Moreover, SEM investigations conducted for the microstructural characterization illustrate that not only does the grain size of the samples studied enhance gradually, but the surface morphology and grain connectivity also improve with the increase in the diffusion-annealing time up to 1 h beyond which all the properties obtained start to degrade. Indeed, the worst surface morphology is observed for the Cu-diffused bulk MgB2 superconductor exposed to 2 h annealing duration. At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values are calculated separately for the pure and Cu-diffused samples. It is found that the microhardness values depend strongly on the diffusion-annealing time. Furthermore, the diffusion coefficient of the Cu ion in the bulk MgB2 superconductor is obtained to change from 1.63 × 10-7 to 2.58 × 10-7 cm2 s-1. The maximum diffusion coefficient is observed for the sample prepared at 850 C for 1 h whereas the minimum one is noted for the sample annealed at 850 C for 2 h, confirming that the annealing-time of 1 h is the best ambient to improve the mechanical, microstructural and superconducting properties of the samples produced. © 2012 Springer Science+Business Media, LLC.