الفهرس 
CHAPTER 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
A series of superconducting composite YBa2Cu3O7- δ+ x Ni metal (x = 1, 2.5, 5, 7.5, 10 and 15 wt.%) was prepared by solid state reaction route. X-rays diffraction (XRD) and scanning electron microscope (SEM) characterization were terminated for structural and microstructural analysis.
The results showed the presence of orthorhombic peaks of YBa2Cu3O7- δ (or YBCO) in the whole composite samples how Ni addition affects the distortions in the orthorhombic unit cell of YBCO. Scanning electron microscope (SEM) images revealed that the grain size of YBCO reduces gradually as Ni content is increased. At the same time segregation of Ni as NiO in patches forms throughout the composite.
Density and molar volume were determined using the immersion method. A new technique has been used to measure the ultrasonic wave velocities (longitudinal and transvers) at 4 MHz ultrasonic frequency at room temperature (300 K). The mechanical parameters of the sintered superconductor samples including longitudinal modulus, shear modulus, Young’s modulus, Poisson’s ratio and microhardness have been investigated from the view point of the elastic anomalies and the porosity. Besides, the Debye temperature was also determined as a function of the additions of Ni contents. The elastic moduli, microhardness and Debye have been improved up to 2.5 wt.% Ni, then a decrease in all data has been observed with further addition of Ni. Quantitative analysis has been implemented o obtain more details about the structure of the composite samples, based on orthorhombic phases, the oriented grain size, the porosity and the agglomeration between grains. Measurements of temperature dependent resistivity, ρ(T), were made employing the standard four-probe technique. The magnetization (M) vs. applied magnetic field (H), or M-H curve, data of the composites was taken at liquid N2 temperature (~ 79 K) via a vibrating sample magnetometer. At low Ni wt.%, it was found that the superconducting state properties improved and then dropped with higher Ni wt.%. Segregated patches of NiO on different length scales were observed. The change in the structural and superconducting state parameters of YBCO and the variation of the magnetic critical current density with improved flux pinning in some composites with Ni additives were ascribed to the Jahn-Teller (JT) distortions and pinning of vortices due to antiferromagnetic NiO patches within the samples.
The attenuation coefficient (α) of the ultrasonic longitudinal waves at frequency 4 MHz was measured for the samples in the temperature range 60-300 K. The results showed the clear effect of Ni2+-presence in CuO2 plane on anti-ferromagnetic (AF) correlations. Increasing Ni2+ ions in CuO2 plane decreases the height of an attenuation peak (P1) in the vicinity of the superconducting transition temperature TC. Decreases of both P1 and TC with Ni wt.% were found to have similar trends suggesting that the relaxation species of P1 may be correlated with superconductivity.
Another series of superconducting composite YBa2Cu3O7- δ+ x Ag metal (x = 1, 2.5, 5, 10 and 15 wt. %) was also prepared by solid state reaction route. Most of the above characterizations were also made for the YBCO-Ag samples. The differences in the electrical and ultrasonic properties between YBCO-Ag and -Ni composites were investigated and discussed.