Browsing by Author "Turkoz M.B."

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Effect of Co/Cu partial replacement on fundamental features of Y-123 ceramics
(2020-05-01) Ozturk O.; Nefrow A.R.A.; Bulut F.; Ada H.; Turkoz M.B.; Yildirim G.
This study is liable for the effect of sample production processes including the standard solid-state reaction (SSR) and classical sol–gel (SG) preparation methods on the fundamental characteristic features, namely electrical, superconducting, crystal structure quality, crystallinity, morphological, strength quality of grain boundary couplings, and interaction between the grains of YBa2Cu3−xCoxO7−δ (Y-123) advanced ceramic compounds within the weight ratio intervals x = 0–20%. The main heat treatments are exerted at two main steps: (I) annealing at 950 °C for 24 h in air medium conditions and (II) annealing at 500 °C during 5 h under the oxygen annealing ambient. The standard measurement methods such as powder X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, temperature-dependent electrical resistance, and Vickers hardness measurements are performed for the characterization of materials. It is found that the samples prepared at SSR route present much more superior characteristic features as compared to those fabricated at SG technique, being one of the most striking points deduced this work. In more detail, every material prepared crystallizes in the orthorhombic symmetry and exhibits the superconducting nature but considerable decrement in the critical transition temperatures. The onset and offset transition temperatures are noted to decrease regularly from 92.96 K (92.28 K) to 90.20 K (83.59 K); and 90.05 K (90.03 K) to 45.97 K (30.49 K) for the materials prepared by the SSR (SG) route. Similarly, the variation in the lattice cell and average grain size parameters confirm that the Co/Cu substitution damages Y-123 superconducting phase. Additionally, the Co/Cu partial replacement mechanism leads to increase significantly the Vickers hardness results. To sum up, the Co/Cu partial substitution (produced by either SSR or SG method) is plowed to improve the fundamental characteristic features for new, novel, and feasible market application areas of Y-123 cuprate ceramics in the universe economy.
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.