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Öğe Effect of annealing ambient conditions on crack formation mechanisms of bulk Bi-2212 ceramic systems(Taylor & Francis Ltd, 2021) Erdem, U.; Akkurt, B.; Ulgen, A. T.; Zalaoglu, Y.; Turgay, T.; Yildirim, G.This study paves way to examine the influence of different annealing conditions (temperature range of 830-850 degrees C and duration intervals 24-48 h) on the fundamental mechanical performance and characteristic quantities of polycrystalline Bi2.1Sr2.0Ca1.1Cu2.0Oy (Bi-2212) superconducting ceramics by means of Vickers microindentation hardness tests at the various indentation test loads (0.245 N <= F <= 2.940 N) and some available theoretical approaches. The annealing ambient plays an important role on the operable slip systems and crystal quality. The bulk Bi-2212 superconducting compound prepared at 840 degrees C and 24 h is found to be the least sensitive to the applied test load due to less structural problems, voids, cracks and stress raisers in the crystal system. Conversely, the excess annealing ambient complicates remarkably the control of crack growth size and velocity. Thus, relatively lower load can lead to the formation of crack and acceleration of crack rate up to the critical size and terminal velocity. The samples exhibit the typical indentation size effect (ISE) behavior as a result of predominant character of elastic recovery mechanism. As for the theoretical examination in the saturation limit regions, the indentation-induced cracking (IIC) model wins the comparison as it provides the most accurate results to the experimental findings.Öğe Effect of vanadium addition on fundamental electrical quantities of Bi-2223 crystal structure and semi-empirical model on structural disorders-defects(SPRINGER, 2020) Ulgen, A. T.; Erdem, U.; Zalaoglu, Y.; Turgay, T.; Yildirim, G.The primary contribution of the present study is to determine the effect of vanadium addition on the fundamental aspects of characteristic crystalline and electrical quantities for the Bi1.8Sr2.0Ca2.2Cu3.0VxOy (0.00 <= x <= 0.30) crystal system using the powder X-ray diffraction (XRD), temperature-dependent electrical resistivities and semi-empirical approaches founded on the structural disorders-defects. The de electrical resistivity results show that every electrical quantity is found to degrade regularly with the increment in the addition level as a consequence of the induced permanent structural disorders-defects, intergranular grain boundary coupling interaction problems and non-superconducting barrier regions in the bulk Bi-2223 superconducting system. The vanadium addition brings also about the characteristic transition from over-doped state to under-doped state due to the suppression in the overlapping of Cu-3d and O-2p wave functions. The XRD results indicate that the vanadium addition leads to shift the characteristic peaks towards the larger/lower angles in terms of the peak positions in the reference data, enlarge the diffraction peak widths (line broadening of X-ray diffraction), appear or disappear new peaks, increase/decrease the average grain size, lattice cell parameters and superconducting phase fractions founded on the diffraction intensities. Based on the evidences, the presence of vanadium particles in the bulk Bi-2223 superconducting phase damages crucially the fundamental characteristic features. Moreover, it is found that characteristic two-stage (bulk genuine, T-c(mid) and coherence, T-co) transition temperatures decrease systematically with the addition level. On this basis, the presence of vanadium impurity in the system leads to degrade the stabilization of superconductivity in the small homogeneous clusters in the paths and especially effective electron-phonon coupling (bipolaron in the polarizable lattices) probabilities due to the reduction of hole trap energy per Cu ions in the valence band of system. Additionally, the results display that the vanadium particles affect negatively on both the dirty limit characteristic feature and gap coefficient of Bi-2223 ceramic compound as a result of the decrement in the minimum required energy for breaking up the cooper-pairs in the system. At the same time, the electrical resistivity curves enable us to develop a sensitive semi-empirical approach to find the possible highest onset critical transition temperature for the ideal crystallinity. The model founded on the crystallinity quality displays that the possible highest onset transition temperature is about 116.037 K +/- 1.25587 K with R-adj(2) = 0.948.Öğe Evaluation of crystallographic and electrical-superconducting features of Bi-2223 advanced ceramics with vanadium addition(Springer, 2021) Akkurt, B.; Erdem, U.; Zalaoglu, Y.; Ulgen, A. T.; Turgay, T.; Yildirim, G.In the current study, the effect of vanadium particles on the electrical, superconducting, crystallographic, key structural and morphological features of Bi1.8Sr2.0Ca2.2Cu3.0VxOy superconducting materials is examined with the aid of powder X-ray diffraction (XRD), scanning electron microscope (SEM), electron-dispersive X-ray (EDX) and dc electrical resistivity over the temperature (rho-T). The vanadium-added Bi1.8Sr2.0Ca2.2Cu3.0VxOy (Bi-2223) superconducting materials are prepared within the molar ratios between x = 0.00 and 0.30 using the conventional solid-state reaction technique. The temperature-dependent electrical resistivity measurements show that the existence of vanadium atom in the superconducting system damages seriously the Bi-2223 (high-T-c) phase content in the crystal structure as a result of the formations/disappearances of new impurity phases. On this basis, the amplitude psi(0) of wave function founded on the super-electrons is considerably reduced with the vanadium addition. The critical onset and offset transition temperature values truncate from the values of 110.92 K and 97.45 K to 103.17 K and 18.38 K in case of the maximum vanadium addition level of x = 0.30. Similarly, the XRD results present that the average crystallite size and c-axis length parameters are noted to decrease considerably whereas a-axis length, strain and relativistic dislocation density ratios are calculated to enlarge harshly depending on the addition content level. It is also obtained that the vanadium inclusions lead to increase seriously the permanent crystal structure problems, disorders, misorientations, lattice strains, crack-producing omnipresent flaws and grain boundary coupling problems in the active Cu-O-2 consecutively stacked layers in the superconducting core, being assured by SEM analyses. Besides, the SEM results show that the enhancement of vanadium addition level in the crystal structure damages remarkably the flaky layers of platelet-like shape for the grains. In fact, the excess vanadium addition seriously damages the general characteristic view (flaky layer structure) of Bi-2223 compound. Based on the EDX findings, the main reason for the degradation of fundamental characteristic properties of Bi-2223 system may stem from the possible replacement of aliovalent vanadium impurities for the copper-sites in the crystal structure. Namely, the vanadium addition in the crystal structure is ploughed to improve the fundamental crystallographic and electrical-superconducting features of bulk Bi-2223 superconducting materials.Öğe Excitation Functions of Some Neutron Production Targets on (d,2n) Reactions(Springer, 2010) Kaplan, A.; Buyukuslu, H.; Aydin, A.; Tel, E.; Yildirim, G.; Bolukdemir, M. H.Deuterons are weakly bound nuclei which easily undergo break up reactions. This is well known at low energies, around 10 MeV, where deuteron beams have been used to produce neutrons very efficiently. These neutrons have a widely application field such as fission energy production and hybrid reactor systems. In this study, neutron emission cross sections produced by (d,2n) reactions for spallation neutron targets such as Cr-52, Fe-56, Ni-60,Ni-62,Ni-64, Cu-63,Cu-65, W-182,W-183,W-184,W-186, Th-232, U-235 and U-238 have been investigated. Hybrid model and geometry dependent hybrid model were used to calculate the pre-equilibrium neutron-emission cross sections. For the reaction equilibrium process, Weisskopf-Ewing model calculations were selected. The obtained results have been discussed and compared with the available experimental data and found agreement with each other.Öğe Experimental and theoretical approaches on mechanical evaluation of Y123 system by Lu addition(Springer, 2013) Turkoz, M. B.; Nezir, S.; Ozturk, O.; Asikuzun, E.; Yildirim, G.; Terzioglu, C.; Varilci, A.This work is the continuation of a systematic study on the characterization of the Lu-added Y123 bulk superconducting materials prepared by the nitrate compounds and derivatives at 970 degrees C for 20 h. In this part, the effect of Lu inclusions on the physical and mechanical properties of the Y123 superconductors is examined with the aid of microhardness measurements performed at various applied loads in the range of 0.245-2.940 N. The microhardness measurement results allow us to determine the important mechanical characteristics such as Vickers microhardness, elastic (Young's) modulus, yield strength and fracture toughness values being responsible for the potential industrial applications. It is found that all the properties given above are strongly dependent upon the Lu concentration in the Y123 matrix. Especially, Vickers microhardness (H-nu) values of the samples studied in this work are found to suppressed considerably with the enhancement of the Lu addition in the system due to the degradation in the connectivity between superconducting grains. Moreover, the H-nu values of the pure Y123 sample are observed to increase with increasing the applied load whereas those of the Lu-doped superconducting materials are obtained to decrease with the load. In other words, the pure sample exhibits the reverse indentation size effect (RISE) behavior while the others obey the indentation size effect (ISE) feature, confirming the degradation in the mechanical properties with the Lu inclusions in the Y123 matrix. In addition, the microhardness measurement results are estimated using the 5 different models such as Meyer's law, proportional sample resistance model, elastic/plastic deformation model, Hays-Kendall (HK) approach and indentation-induced cracking (IIC) model. According to the results obtained from the simulations, of the mechanical analysis models, the Hays-Kendall (HK) approach is determined as the most successful model for the description of the mechanical properties of the Lu-doped superconducting materials (exhibiting the ISE behavior) where both the both the reversible (elastic) and irreversible (plastic) deformations are produced. On the other hand, the IIC model is found to be superior to other approaches for the pure sample (presenting the RISE feature) where the irreversible deformation becomes more and more dominant compared to the reversible deformation.Öğe Improvement in fundamental electronic properties of Bi-2212 electroceramics with trivalent Bi/Tm substitution: a combined experimental and empirical model approach(Springer, 2021) Zalaoglu, Y.; Erdem, U.; Bolat, F. C.; Akkurt, B.; Turgay, T.; Yildirim, G.This study delves into the variation in the fundamental aspects of electrical quantities with the partial substitution of Tm impurities at Bi-site in the Bi2.1-xTmxSr2.0Ca1.1Cu2.0Oy (0.00 <= x <= 0.30) ceramic system with the derivatives of electrical resistivity examinations and theoretical approaches. It is found that all the electrical characteristic properties tend to improve with the trivalent Bi/Tm substitution level up to x = 0.07 beyond which they degrade considerably due to the increment of non-superconducting barrier regions, permanent disorders, inhomogeneity, porosity, grain misorientation distribution, internal and surface omnipresent defects, microscopic cracks, and coupling interaction problems throughout the grain boundaries in the Bi-2212 crystal system. Thus, the optimum dopant level of x = 0.07 results in the transition from the over-doped state to optimally doped state in the Bi-2212 crystal system as a consequence of augmented hybridization mechanism. Further, the characteristic two-stage transition temperatures, gap coefficient, Josephson coupled, and thermal energies for the isolated grains and inter-grains are explored. The findings show that the optimum Bi/Tm substitution leads not only to stabilize the superconductivity in the homogeneous superconducting clusters as a result of the increment in the formation of active Cooper pairs but also to decrease significantly the location of resistivity in long-range coherent state due to the increment of hole trap energy. Additionally, a strong link is established between the structural disorders-defects and onset/offset (T-c(onset)/T-c(offset)) transition temperatures using the electrical resistivity features for the first time. The empirical model based on the impurity scattering and lattice strain in the crystal lattices displays that it is possible to achieve the possible highest T-c(onset) and T-c(offset) values of about 86.558 K and 86.445 K, respectively. To sum up, the paper with strong methodology between electrical quantities and structural disorders-defects depending on Tm impurity may be a pioneering research to explain why the characteristic features improve with the optimum substitution and especially open up a novel and feasible area for the advanced engineering, heavy industrial technology, and large-scale applications of ceramic materials.Öğe Improvement of the Nature of Indentation Size Effect of Bi-2212 Superconducting Matrix by Doped Nd Inclusion and Theoretical Modeling of New Matrix(Springer, 2014) Ozturk, O.; Asikuzun, E.; Kaya, S.; Yildirim, G.; Turkoz, M. B.; Kilic, A.Neodmium (Nd) inclusions at different stoichiometric ratios (x=0.0, 0.001 %, 0.005 %, 0.01 %, 0.05 %, 0.1 %) are doped in the Bi-2212 superconducting samples and the samples obtained are subjected to the sintering process at 840 C-a similar to constant temperature for 72 hours. The effect of Nd doping on the structural and mechanical properties of prepared samples is investigated by the standard characterization measurements. XRD and SEM measurements are performed to obtain information about surface morphology, phase ratios, lattice parameters and particle size. Moreover, Vickers microhardness (H (V) ) measurements are exerted to investigate the mechanical properties of the all samples in detail. It is found that all the properties given above retrogress with the increase of the Nd concentration in the Bi-2212 superconducting core. However, the ISE nature of the materials improves systematically. Additionally, the experimental results of microhardness measurements are analyzed using Meyer's law, PSR, MPSR, EPD models and HK approach. The results show that Hays-Kendall approach is determined as the most successful model.Öğe Neutron Emission Spectra of 104,105,106,108,110Pd Isotopes for (p,xn) Reactions at 21.6 MeV Proton Incident Energy(Springer, 2010) Buyukuslu, H.; Kaplan, A.; Tel, E.; Aydin, A.; Yildirim, G.Palladium, which is a rare and lustrous silvery-white color from precious metals, plays important role in fusion-fission reactions and different fields of nuclear technology. In addition, it is used for not only cold fusion experiments but also separation of hydrogen isotopes researches for fusion reactors. In this study, neutron-emission spectra produced by (p,xn) reactions for structural fusion material 104,105,106,108,110Pd isotopes have been investigated by a proton beam at 21.6 MeV. Moreover, multiple pre-equilibrium mean free paths constant from internal transition, and the pre-equilibrium and equilibrium level density parameters have been analyzed for some (p,xn) neutron-emission spectra calculated. New evaluated hybrid model and geometry dependent hybrid model, full exciton model and cascade exciton model were used to calculate the pre-equilibrium neutron-emission spectra. For the reaction equilibrium component, Weisskopf-Ewing model calculations were preferred. The obtained results have been discussed and compared with the available experimental data and found agreement with each other.Öğe A novel research on the subject of the load-independent microhardness performances of Sr/Ti partial displacement in Bi-2212 ceramics(SPRINGER, 2020) Zalaoglu, Y.; Turgay, T.; Ulgen, A. T.; Erdem, U.; Turkoz, M. B.; Yildirim, G.This work is interested in the critical changes in the load-independent microhardness performance parameters with the partial substitution of Sr2+ inclusions for the Ti4+ impurities in the Bi-2212 inorganic solids with the help of the theoretical approximations as regards Meyer's law (ML), proportional sample resistance (PSR), modified proportional sample resistance (MPSR), elastic/plastic deformation (EPD), Hays-Kendall (HK) and indentation-induced cracking (IIC) models found on the experimental microhardness tests applied to a variety of test loads between 0.245 and 2.940 N for the first time. Moreover, Ti-substituted Bi-2212 bulk ceramics (Bi2.1Sr2.0-xTixCa1.1Cu2.0Oy) are prepared within mole-to-mole ratios of x = 0.000, 0.010, 0.030, 0.050, 0.070, 0.100 by the standard solid-state reaction method in the atmospheric pressure conditions. It is provided that Ti partial substitution in the superconducting system descends unsmilingly the mechanical durability, stability, strength, toughness, critical stress, stiffness and flexural strengths of Bi-2212 superconducting solids studied owing to the increment of crystal structural problems. Moreover, it is obtained that the degradation in the crystal structural leads to diminish the typical ISE characteristic of Bi-2212 superconducting ceramic compounds. At the same time, the results show that all the models (especially IIC approach) can serve as the suitable descriptors for the determination of the variation in the load-independent mechanical performances of the Bi-2212 superconducting materials.Öğe Role of active slip systems induced with holmium impurity in Bi-2212 ceramics on mechanical design performance and morphological properties(Elsevier Sci Ltd, 2022) Ulgen, A. T.; Cetin, Samet; Zalaoglu, Y.; Turkoz, M. B.; Erdem, U.; Yildirim, G.Effect of Ho/Bi partial replacement in Bi2.1-xHoxSr2.0Ca1.1Cu2.0Oy (Bi-2212) superconductors on the fundamental structural, morphological and mechanical performance properties are investigated by Scanning Electron Mi-croscopy (SEM) and Vickers hardness (Hv) measurement techniques. Crystallinity quality and surface morphology including the microcrystal coalescence orientations, grain alignment distributions, microscopic structural problems, microvoids, internal defects, uniform surface view, porosity and particle growth distribution are visually examined with the aid of SEM. Basic mechanical performance and characteristic features of Bi/Ho substituted Bi-2212 superconducting ceramics (0.00 <= x <= 0.10) are also determined with Vickers tests con-ducted at various loads intervals 0.245-2.940 N. Experimental findings show that the characteristic features enhance seriously in case of x = 0.01 due to refinement of crystallinity quality and slip systems. Thus, the op-timum Ho concentration presents the highest mechanical fracture strength to the load applied as a result of better uniform surface appearance and grain orientations, well-connected flaky layers, larger particle size distribution and denser structure, confirmed by the SEM investigations. Namely, much more load is required to accelerate the dislocation movement and crack propagation to the terminal velocity for critical size growth. The fracture predominantly takes place in the transcrystalline regions and hence the propagations are easily controlled with the optimum Ho dopant ions. On the other hand, the increase in the Ho ions in Bi-2212 structure induces the crack-initiating defects for new stress concentration sites. In conclusion, the permanent and non-recoverable deformations appear at even lower indentation test loads. All samples present indentation size effect feature depending on the dominant character of elastic recovery mechanism. Further, original hardness parameters are semi-empirically analyzed in the plateau limit regions using mechanical modelling approaches for the first time. Based on the analyses, Hays-Kendall model exhibits the closest results to the experimental findings.Öğe Self-Sensing of Flexural Damage in Large-Scale Steel-Reinforced Mortar Beams(Amer Concrete Inst, 2019) Sarwary, M. H.; Yildirim, G.; Al-Dahawi, A.; Anil, O.; Khiavi, K. A.; Toklu, K.; Sahmaran, M.The majority of self-sensing studies conducted in current literature uses small-scale specimens without steel reinforcement which might be misleading for damage monitoring in actual structures. To account for this, herein, the main emphasis was placed on self-sensing sensing evaluation of large-scale (100 x 150 x 1000 mm(3) [3.94 x 5.91 x 39.4 in.(3)] width x height x length) reinforced mortar beam elements tested under four-point bending. For the purposes of self-sensing, either chopped carbon fibers (CFs) or multi-walled carbon nanotubes (CNTs) were used in beams to achieve desirable electrical properties. Assessment of self-sensing was made by tracking the fractional changes in electrical resistivity (FCER) with respect to midspan beam displacement under flexural loading. Results related to electrical properties were recorded from brass electrodes embedded in specimens in fresh state using a resistivity meter using alternating current. Self-sensing results of large-scale beams were also backed by mechanical/structural characterization. Experimental findings suggest that use of CF and CNT in beam elements is significantly effective in modifying the overall failure types for the given reinforcement configuration. Proposed measurement setup is successful in capturing the flexural self-sensing data regardless of the type of carbon-based material. At low levels of damage, for both CF- and CNT-bearing beams, self-sensing and damage occurrence measured by the level of beam deformation are well-fit to each other, although this is clearer for specimens with CF However; at high levels of damage, most probably due to rupturing of individual fibers, clearer abrupt changes in FCER results of CF-bearing beams are monitored, although this is not the case for beams with CNT. Taking into account the production cost, performance, and easier mixability of different carbon-based materials within the relatively dense cementitious systems of beam elements, use of CF seems to be more advantageous than the use of CNT for an efficient self-sensing assessment of real-time structural elements.
