Citation
Dihom, Mustafa Mousa Ali
(2018)
Microstructure and superconducting properties of YBa(Ca/K)CuO (Y123 and Y358) systems synthesized using thermal treatment method.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
There is a lack of reports on superconducting behavior of Y3Ba5Cu8O18δ (Y358) superconductor which belong to YBaCuO family. In addition, the role of alkali metals, Ca and K, substitution on the Ba site on the microstructural property of YBaCuO superconductors have not been well understood. In this work, bulk Y358 and YBa2Cu3O7δ (Y132) superconductors were synthesized by a new technique based on thermal treatment method using PVP as capping agent. The samples were sintered in flowing O2 at 980°C for 24 hour. In addition, the effect of alkali metals (M = Ca and K) substitutions in Ba site of Y123 and Y358 on the microstructure and superconducting properties were systematically investigated using Xrays diffraction (XRD), field emission scanning electron microscope (FESEM), energydispersive Xray spectroscopy (EDX), resistivity behaviour (ρ–T), temperature dependence of resistance measurement, and alternating current susceptibility (AC) and electron spin resonance (ESR) techniques.
From XRD results, the Y123 and Y358 showed orthorhombic crystal structure besides small amount of secondary phased such as Y211. In the case of Y123, the orthorhombicity and crystallite size changed differently with Ca and K substitutions. For Ca substituted Y358, the orthorhombicity and crystallites size increases up to x = 0.01 and 0.02 respectively and then decreases. The intensity of XRD peaks changed unsystematically with K substitution in Y358, however it shows improvement at x = 0.03 and 0.14.
From Field Emission Scanning Electron Microscope (FESEM) micrographs, the grain size of pure Y358 is larger and more compact compared to Y123. The grain size was found to be larger when Ba is substituted with either Ca or K than the pure samples in both Y123 and Y358. For both Y123 and Y358 samples, the grains become much finer, almost with different shape and wellconnected as K contents increases.
Both Y123 and Y358 samples exhibited good metallic behaviour in the normal state and one step transition. The Y123 and Y358 showed critical temperature Tc(R=zero) at 87 and 92 K and onset of superconducting transition Tc (onset) at 93 K and 98 K, respectively. The Tc(R=zero) for Ca substituted Y123 and Y358 was decreased. The changing of lattice parameters in Y123 and Y358 structure due to Ca substitution may disturb the oxygen content and hence affect Tc. The Tc(R=zero) was increased as K substitution in Y123 increased. In general Y123 and Y358 samples with initial Ca and K substitution show sharper superconducting transition (Tc) than pure, which could be due to good microstructural morphology and better crystallinity. The AC susceptibility measurement show that for the Ca substituted Y123 and Y358 samples a decrease in diamagnetism onset temperature Tconset, was observed from real part (χ') which exhibited twostep transitions related to the superconducting intra and intergrain coupling. The Tconset decreased in the case of Ca substituted samples and increased in the case of K substituted samples. This decrease is mainly due to the decrease and increase of hole concentration respectively. The intergranular critical current density, Jcm, of pure Y123 and Y358, 34.4 A cm2 and 34.7 A cm2 respectively, increased to 35 Acm2 at (x=0.08) and 36.3 Acm2 at (x=0.2) for Ca substituted Y123 and Y358, respectively and K to 35 Acm2 at (x=0.1) and 38.8 Acm2 at (x=0.12) for Ca substituted Y123 and Y358, respectively, which could be due the improvement of the grain boundary and the hence the grains’ coupling. On the other hand, Josephson current, Io, and Josephson energy, Ej decreased and increased with the Ca and K concentration respectively due to degrading and coupling between the grain connectivity. From electron spin resonance (ESR), the Ca and K substituted in Y123 and Y358 showed ESR spectra consisted of two peaks. The gfactors increased with increment of Ca and K content in both Y123 and Y358 samples, which could be due to changes in the oxygen ordering in Y123 and Y358.
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