Resumo O objetivo deste trabalho foi investigar as propriedades magnéticas e a dependência da condutividade elétrica com a temperatura associada ao modelo de barreiras de potencial entre os cristalitos. A cerâmica de espinélio de gálio e magnésio possui baixa coercividade magnética e alta resistividade elétrica. As amostras de MgGa2-xFexO4 (x= 0,01, 0,05, 0,15, 0,25, 0,35) foram preparadas pelo método de estado sólido e sinterizada a 800 °C durante 8 h. A análise de difração de raio X confirmou a formação de única fase com estrutura de espinélio cúbico compacto. As medições magnéticas mostraram que a magnetização de saturação e remanescência de todas as amostras aumentaram com a concentração de ferro. O campo coercitivo diminuiu até a concentração x= 0,15 e acima de x= 0,25 observou-se um aumento no campo coercivo. Através da caracterização eléctrica verificou-se que as amostras apresentaram comportamento isolante para x= 0,01 e acima de x= 0,15 observou-se um comportamento semicondutor compatível com o modelo de barreira de potencial entre cristalitos, ou seja, preenche a condição de L/2 > LD (tamanho de cristalito L em comparação com o comprimento de Debye) e a condução é limitada por barreiras de potencial entre os cristalitos.

Abstract The aim of this work was to investigate the magnetic properties and the electrical conductivity temperature dependence associated to the potential barrier between the crystallites model. Gallium and magnesium containing spinel ceramic has low magnetic coercivity and high electrical resistivity. MgGa2-xFexO4 samples (x= 0.01, 0.05, 0.15, 0.25, 0.35) were prepared by solid-state method and sintered at 800 °C for 8 h. X-ray diffraction analysis confirmed the formation of a single phase with compact cubic spinel structure. The magnetic measurements show that the saturation magnetization and remanence of all samples increased with increasing iron concentration. The coercive field decreased up to the concentration x= 0.15, and above x= 0.25 it was observed an increase in the coercive field. Through electrical characterization it was found that the samples presented highly insulating behavior for x= 0.01, and further increase in x above 0.15 gives a semiconductor behavior compatible with the potential barrier between the crystallites model, i.e. fulfills the condition L/2 > LD (crystallite size L in comparison with the Debye length LD), and the conduction is limited by potential barriers between the crystallites.

Resumo Os objetivos deste trabalho foram a obtenção de uma ferrita de chumbo e cobre com a estrutura do espinélio, a caracterização da sua microestrutura e a determinação das suas propriedades magnéticas. A ferrita de cobre e chumbo é um material com baixa coercividade magnética, alta resistividade elétrica e alta permeabilidade magnética. Amostras com estequiometria PbxCu1-xFe2O4, com (x= 0, 0,1 e 0,2), foram produzidas pelo processo de metalurgia do pó e sinterizadas a 800 °C por 6 h em atmosfera ambiente. As caracterizações estrutural, microestrutural e magnética foram realizadas utilizando as técnicas de difração de raios X, microscopia eletrônica de varredura, magnetômetro de amostra vibrante e análise termogravimétrica magnética. Os resultados mostram que as amostras são formadas exclusivamente pela estrutura do espinélio tetragonal. As curvas de histerese mostram características de materiais magnéticos moles e sua magnetização de saturação aumentou com a concentração de chumbo. O valor da temperatura de Curie diminuiu com o aumento da concentração de chumbo.

Abstract The objectives of this work were to obtain a lead and copper ferrite with spinel structure, characterizing the microstructure and the determination of their magnetic properties. The copper and lead ferrite is a material with low magnetic coercivity, high electrical resistivity and high magnetic permeability. Samples with stoichiometry PbxCu1-xFe2O4 with (x = 0, 0.1, and 0.2) were prepared by powder metallurgy process and sintered at 800 ºC for 6 h in air. The structural, microstructural and magnetic characterizations were performed using the techniques of X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer, and magnetic thermal gravimetric analysis. The results showed that samples are formed exclusively by the tetragonal spinel structure. Hysteresis loops show the characteristics of a soft magnetic material and the saturation magnetization increased with the lead concentration. The value of the Curie temperature decreased with increasing lead concentration.

Atualmente há uma crescente demanda no cenário mundial com relação ao tratamento de resíduos industriais, dentre estes o fenol e pilhas inutilizadas. Óxidos e cerâmicas a base de ferro, tais como ferritas são utilizadas como catalisadores no processo Fenton heterogêneo. Os processos de Fenton e foto-Fenton apresentam alta eficiência no tratamento de resíduos tóxicos utilizando catalisadores a base de ferro. Neste contexto foram desenvolvidas ferritas a partir de resíduos de pilhas inutilizadas. O uso destas ferritas como catalisadores é bastante interessante, uma vez que, são materiais ferromagnéticos podendo ser recuperados no final do processo utilizando campo magnético, evitando a liberação de Fe2+ e Fe3+ no efluente. Os resultados mostraram que a ferrita proveniente de pilhas inutilizadas e calcinada à temperatura de 500 °C apresentou estrutura correspondente a fase espinélio e magnéticas (M S= 37,04 emu.g-1) e como catalisador de fotodegradação atingiu-se 90% de remoção de fenol em pH 2,5 durante 2 h de irradiação.

Currently there is a growing global demand with respect to industrial waste treatment, among them phenol and spent batteries. Iron oxides and ceramics containing iron, such as ferrites, have been used with success as catalysts for heterogeneous Fenton process. The Fenton and photo-Fenton processes have high efficiency in treatment of toxic residue using iron as catalysts. In this respect, ferrites were developed from spent batteries. The use of these ferrites as catalysts is very interesting because these oxides are ferromagnetic and can be recovered at the end of the process using a magnetic field, avoiding the discharge of Fe2+ and Fe3+ in the environment. The results showed that the ferrite from spent batteries calcined at 500 °C presents structural of spinel phase and magnetic properties (Ms= 37.04 emu.g-1) and applied in photo-Fenton process, it was reached 90% of phenol removal at pH 2.5 with 2 h of irradiation.

The rough grind machine operation in metallic material produce a sub product with the followings components such as: metallic residue, small parts come from the resin grinds, diatomaceous earth material (filter material) and soluble oil in water for tools cooling. According [HU1UH] from ABNT, this sub product is toxic, not inert material, aggress the environment with pollution in the currency water and contaminate the ground when incorrectly go to the municipal solid waste. The final destination of this sub product, is the incineration, in the cements company, being the incorporated the sub product to the cement, during your production. With the qualitative result using the scanning electron microscopy (SEM), coupled to the dispersive energy spectroscopy (EDS), two experiments were made with objective to separate the magnetic material of the non-magnetic material, using a permanent magnet. We conclude that the magnetic material can be used by powder metallurgy process in production of pieces.

Geomagnetic storms are intervals of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial injection of energy into the magnetosphere-ionosphere system, to an intensified ring current, strong enough to exceed some key threshold of the quantifying storm time Dst index. We have studied all the 9 great magnetic storms (peak Dst < -200 nT) observed during the rise and maximum of solar cycle 23 (from 1997 to early 2001), in order to identify their solar and interplanetary causes. Apart of one storm occurred during the period without observations from the Solar and Heliospheric Observatory (SOHO), all of them were related to coronal mass ejections observed by the Large Angle and Spectroscopic Coronagraph (LASCO). The sources of interplanetary southward magnetic field, Bs, responsible for the occurrence of the storms were related to the intensified shock/sheath field, interplanetary magnetic cloud's field, or the combination of sheath-cloud or sheath-ejecta field. It called our attention the fact that one of the events was related to a slow CME, with CME expansion speed not greater than 550 km/s. The purpose of this paper is to address the main sources of large geomagnetic disturbances using the current satellite capability available. As a general conclusion, we found that shock/sheath compressed fields are the most important interplanetary causes of great magnetic storms during this period.