Propriedades inter e intragranulares de amostras mesoscópicas de YBa2Cu3O7-δ ; preparadas por síntese química

• Autor(a) principal: Deimling, Cesar Vanderlei
• Data de Publicação: 2010
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da UFSCAR
• Texto Completo: https://repositorio.ufscar.br/handle/ufscar/4926
• topic: Supercondutividade; Corrente crítica; Granularidade; Síntese química; CIENCIAS EXATAS E DA TERRA::FISICA

This work consists of a systematic study of the evolution of the morphological characteristics and the superconducting properties of YBCO samples prepared using the Modified Polymeric Precursors Method (MPPM). Aiming at producing YBCO powder exhibiting the smallest possible intergranular contribution, we have studied, through scanning micrographs, x-ray difractograms and magnetic measurements; the effects of milling, ultrasonic treatments and the pelletizing pressure; the influence of the chelating agents citric acid, tartaric acid ethylenediaminetetraacetic acid; as well as the calcinating temperature, on the morphology and the superconducting properties of this class of samples. From this studies we could conclude that the powder presents mesoscopic granulometry, being formed by aggregates, very difficult to dismantle without insertion a large amount of defects. AC-susceptibility measurements as a function of temperature were used to monitor the evolution of the line, Hc wl(T), which in a field versus temperature magnetic phase diagram, H x T, delineates the region above which the intergranular critical current density of the sample vanishes. Studying the effects of calcinating temperatures below 900°C, we have verified the possibility of monitoring the intergranular characteristics of powder YBCO samples. Above this temperature, the aggregates coalesce in such a way that the intergranular magnetic response is totally suppressed. In order to produce samples exhibiting a narrow intergranular critical current distribution, we have adopted the preparation route involving the chelating agent citric acid since, among all studied samples, those prepared using this route gave the best intergranular contribution to the magnetic response. Due to the mesoscopic granulometry and the high coalescence presented by those powders, sintering under temperatures in the interval from 920°C to 980°C resulted in pellets with the largest degree of inhomogeneity associated with the magnetic response, so that the line Hc wl(T) was impossible to determine. Preparing pellets pressed under 1500 kgf/cm2, 3000 kgf/cm2, 6000 kgf/cm2 and sintered in temperatures ranging from 870°C to 900°C, we could observe several kinds of singular behavior, such as a double superconducting transition, the Paramagnetic Meissner effect (PME) and the occurrence of two minima in magnetization versus magnetic field curves, M x H. Such effects are related with the intergranular properties, being more evident in superconducting granular samples presenting a narrow intergranular critical current distribution. Due to the narrow intergranular critical current distribution, which is a consequence of similar properties of the weak links (WLs), it became possible to build a phase diagram containing the Hc wl(T) line for such samples, including also Hc1(T), which separates the Meissner from the Mixed states. Both lines are extrinsic properties of the samples, since they depend on specific procedures adopted during sample preparation. Associating Hc wl(T) with the upper limit of superconductivity for the intergranular matrix, and Hc1(T) with the frontier of the Meissner state, we have verified two major classes of WLs were present. We were then able to model those boundary lines as if they were type II superconductors. The Ginzburg-Landau parameters obtained from this modeling process are: _0 wl = 19,7 for the WL class having Tc wl = 85,5 K and _1 wl = 238,2 for the class presenting Tc wl = 51,4 K. Simulations of ac-susceptibility measurements as a function of the excitation field, _ac x T, were in good agreement with the experimental results for the powder samples, revealing that such specimens have broad intergranular critical current distributions. For pellets sintered in the 870°C - 900°C temperature interval, simulations of _ac x T are inefficient, since they do not take into account the positive component of the magnetic moment associated to flux trapped at intergranular regions of samples exhibiting PME.