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São Luís, Brazil

Furtado P.S.,IFMA DAQ PPGEM | Oliveira M.M.,IFMA DAQ PPGEM | Vasconcelos J.S.,IFMA DAQ PPGEM | Rangel J.H.G.,IFMA DAQ PPGEM | And 2 more authors.
Ceramica | Year: 2012

An investigation was made of the microstructural and electrical properties of SnO2-based varistors microwave sintered at 1200 °C, applying a heating rate of 120 C/min and treatment times of 10, 20, 30, 40, 50 and 60 min. The system used in this study was (98.95-X)%SnO2.1.0%CoO.0.05%Cr2O3.X%Ta 2O 5, where X corresponds to 0.05 and 0.065 mol%. Sintering was carried out in a domestic microwave oven (2.45 GHz) fitted for lab use. Silicon carbide was placed in a refractory vessel to form a heating chamber surrounding the sample holder. The pellets were examined by scanning electron microscopy, X-ray diffractometry, direct current measurements and impedance spectroscopy. The parameters of density, medium grain size, coefficient of nonlinearity, breakdown electrical field, leakage current, and height and width of the potential barrier were analyzed. Source

Oliveira M.M.,IFMA DAQ PPGEM | Pessoa P.A.P.,IFMA Campus Caxias | Brito R.L.,IFMA Campus Caxias | Rangel J.H.G.,IFMA DAQ PPGEM | And 2 more authors.
Revista Materia | Year: 2016

Varistors are elements that are part of electric power transmission and distribution systems or of special electrical installations. Varistors are manufactured in a wide variety of types, depending on their application, such as low-voltage devices with a layer of a few grains of thickness and low breakdown voltage, to varistors with a breakdown voltage of several kilovolts, such as those used in the lightning arresters of electric power distribution networks. Varistors, whose physical operation has been extensively studied, share common elements such as metal-semiconductor Schottky barrier junctions. Several studies have focused on the development of varistor ceramics for high and low voltage applications, including ZnO, SnO2, TiO2, and more recently, WO3-based ceramics. Unlike the first three compositions, WO3-based ceramics present an intrinsic varistor behavior because they contain monoclinic and triclinic phases. The addition of electron donor and acceptor dopants and heat treatments in different atmospheres also alter the non-linear properties of these systems, since they affect the formation of the Schottky barrier. This paper offers a review of the literature on the new varistor ceramic composition based on tungsten oxide (WO3). © 2016, Universidade Federal do Rio de Janeiro. All rights reserved. Source

Brito R.L.,IFMA DAQ PPGEM | Pessoa P.A.P.,IFMA DAQ PPGEM | Vasconcelos J.S.,IFMA DAQ PPGEM | Dos Santos L.P.S.,IFMA DAQ PPGEM | And 5 more authors.
Ceramica | Year: 2012

This paper reports on a study of the electrical properties of SnO 2-based varistor blocks. The nonlinearity of these ceramics was evaluated based on their voltage-current characteristics, while the height, width and charge carriers were evaluated by impedance spectroscopy and activation energies. The a values were relatively low, with the SCNCrPr system showing the highest value: 6.3. Although the average grain size of the SCNCrLa system was small, i.e., 1.66 mm, its Er value was very low, which may be attributed to the non-formation of effective barriers in the grain boundary region, thus generating a high leakage current in the order of 230 μA. Source

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