Investigação dos mecanismos de transporte de massa em eletrólitos quase-bidimensionais

• Autor(a) principal: Cunha, Thiago Henrique Rodrigues da
• Data de Publicação: 2010
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/4270
• topic: Teoria do transporte; Campos magnéticos; Ferro - Eletrodeposição; Massa - Transferência; Theory of transport; Magnetic fields; Iron - Electroplating; Mass - Transfer; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA::FISICA DA MATERIA CONDENSADA

In this study, we analyzed the effects of a uniform magnetic field on the electrodeposition of iron quasi-two-dimensional electrolytic cells. We used circular cells - with an outer ring anode and a central point cathode, and rectangular cells - with parallel line electrodes. When a voltage difference is applied between the electrodes of the electrolytic cell, we observed the formation of a ramified aggregate as result of the reduction of the metallic ions. It is believed that the process of mass transport (of ions) giving rise to the deposit is governed mainly by diffusion and migration. However, it has been shown by experiments that the convection in an ultra thin layer of electrolyte, could be driven by two different sources: Coulomb forces due to electrical charges located at the ends of branches, and by buoyancy forces due to concentration gradients that lead to density gradients. The application of a magnetic field induces forces that can changes these transport process. We found that when a magnetic field is applied parallel to the cell, the deposit show a branched structure different from that observed in the absence of it. In the case of low current density and 2D circular cells, the deposit loses its radial symmetry and grows in a centered rectangular symmetry at the cathode. For higher current density, a new radial symmetry break is observed; the deposit grows in the lozenge shape centered on the cathode. Viewed from the magnetic field direction, it was observed a predominant growth in the lozenge right side, resulting in a small asymmetry in the pattern; fact not observed in the rectangular pattern. In the rectangular cells, we found that when a magnetic field is applied to the growth direction and perpendicular to the electric current, the deposit show morphology different from that structure with slender filaments arranged parallel to each other observed in the absence of it. The field produces an interlacement of the branches of the aggregate that leads to a structure compound by a set of entangled filaments, with gaps as a regular network. Based on morphological transitions and the continuous monitoring of electrical current, we interpret the formation and selection of the patterns as result of the competition between the electric and magnetic forces. Beyond the directly interaction with the ions in motion through Lorentz force, the magnetic field also induces the magnetization of the deposit, which give rise magnetic dipolar forces in the system. The interplay of electric, dipolar and Lorentz forces with the movement of the free ions in solution and with the movement of liquid as a whole, limits the freedom of orientation that a new particle has to join the aggregate. Therefore, the morphology of the deposit is conditioned by the map of these forces in its neighborhood.