Influência dos arranjos randômicos de grãos nas propriedades de transporte de eletrocerâmicas do tipo SmBaCuO

• Autor(a) principal: Capucho, Ivan Meloti
• Data de Publicação: 2015
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Espírito Santo (riUfes)
• Texto Completo: http://repositorio.ufes.br/handle/10/1646
• topic: Supercondutividade; Oxigênio; Resistência elétrica; Susceptibilidade magnética; Eletrocerâmicas; Engenharia Mecânica; Supercondutores; 621

Samples of polycrystalline superconducting Sm-123 type were prepared by solid-vapor reaction and treated with six different gas flows oxygen (O2). Samples were called: Sample A with flow rate of 0.2 cm3/s, Sample B with a flow of 0.5 cm3/s, Sample C with a flow of 0.7 cm3/s, Sample D with flow of 0.9 cm3/s, Sample E with flow of 1.2 cm3/s and Sample F with flow of 1.4 cm3/s. These samples were properly characterized by X-ray diffraction, density, SEM, electrical resistivity critical AC and AC magnetic susceptibility. The results of X-ray diffraction (XRD) showed the predominance of Sm-123 phase with tetragonal and orthorhombic crystal structure as well as the appearance of other spurious phases. Moreover, with the increased flow of O2, there was an increase in the fraction of phase Sm-123 with tetragonal structure and in particular the phase BaCuO2+. From the XRD results were obtained network parameters both of the phase Sm-123 with orthorhombic crystalline structure and the tetragonal structure where the sample B showed slightly smaller unit cell volume slightly when compared to the other samples. The results of determination of density samples showed a dependence on the flow of O2. It was observed that increasing the flow of O2, the porosity decreases, that is, the hardness increased. Through SEM images obtained was the average size of the junction decreased with the increase of the doping of O2 . The sample A, which had the lowest flow the O2 in synthesis, had an average size Junction 17(2) m. An investigation into the electrical resistivity was performed only after careful analysis of the excitation current considering the linear range limit, so that the measurements were performed by applying current = 20 mA. Moreover, calibration of the measurement system was performed with a sample already characterized by magnetic susceptibility. This sample was named Sample Reference (SR). And according to our results, sample A showed the lowest electrical resistivity at room temperature and the higher Tc (95 K). It was also observed that the sample A had smaller width transition ( T) than the sample B. This indicated that such a sample must possess better distribution of oxygen across the grain. However, Samples C, D, E and F showed superconducting transition above 77 K. In relation to the critical current density measurements (Jc), the sample A showed higher Jc than the sample B and the Sample Reference. However all the experimental data have collapsed into a single curve represented by S-N-S model equation. This suggests that the transport mechanism is the same for both samples, independently of the amount of oxygen applied. As the Sample Reference has other synthesis parameters, it can be suggested that the transport mechanism also independent of heat treatment of these ceramics. The interpretation of magnetic susceptibility measurements () indicate that the magnetic intensities are different for each sample. This variation is associated to the magnetic moment of the particle, this is, the magnetic signal depends on the intragranular and intergranular shielding currents. Thus, the greater the grain size (larger effective junction area) increased the intensity of magnetic shielding. This explains the dependence of magnetic shielding with the passage of electrical current through the junction and circuitry area. The sample A showed higher magnetic shielding and higher critical temperature. Corroborating the X-ray analysis, as this sample has a higher fraction of Sm-123 composition with orthorhombic crystal structure.