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Fokin V.M.,Si Vavilovs State Optical Institute | Cabral A.A.,Federal Institute of Maranhao IFMA | Reis R.M.C.V.,Federal University of São Carlos | Nascimento M.L.F.,Federal University of São Carlos | Zanotto E.D.,Federal University of São Carlos
Journal of Non-Crystalline Solids | Year: 2010

Microscopy methods are usually employed to estimate the number density of super critical nuclei and the resulting crystal nucleation rates, I(T). These traditional techniques rely on a double-stage treatment, i.e. the development of the nuclei at a temperature higher than the previous nucleation temperature up to a size large enough to be visible with optical or electron microscopy. These methods can give reliable results for I(T), but are rather laborious and time-consuming. On the other hand, non-isothermal (DTA/DSC) methods are, in principle, much faster. In this paper, we experimentally test two non-isothermal methods by comparison with a traditional optical microscopy method. We found that, if they are properly employed, non-isothermal methods can give useful kinetic information, which includes the crystal number density and nucleation rates, but to get accurate quantitative data they need some preliminary information about nucleation and growth rates in the studied glass and, in addition, are as laborious as the traditional microscopy methods! © 2009 Elsevier B.V. All rights reserved.

Rivas Mercury J.M.,Federal University of Maranhão | Costa Pereira D.J.,Odylo Costa Filho Creativity Center | Silva Vasconcelos N.S.L.,São Paulo State University | Cabral Jr. A.A.,Federal Institute of Maranhao IFMA | Angelica R.S.,Federal University of Pará
Revista Escola de Minas | Year: 2013

This work involved the first-ever characterization of antique Portuguese ceramic wall tiles in the Historic Center of São Luis do Maranhão, Brazil. The tiles were characterized by optical microscopy, X-ray diffraction (XRD) and chemical analysis to identify the possible raw materials used in the fabrication process, as well as the firing temperature of these materials. The results indicate that the microstructure of these materials consists of pores of varying sizes with calcite incrustations and quartz grain sizes smaller than 500 μm distributed in a pinkish yellow matrix, which were identified by XRD as calcite, gehlenite, wollastonite, quartz, and amorphous mineral phases. Based on this information, it can be inferred that the original raw materials probably consisted of a mixture of kaolinitic clays (Al2O3·2SiO2·2H2O) rich in calcium carbonates and quartz, or mixtures of kaolinitic clays, quartz and calcite, which did not reach the pre-firing temperature of 950°C.

Lisboa A.J.T.,Postgraduate Program in Materials Engineering PPGEM IFMA | Vasconcelos N.S.L.S.,Federal Institute of Maranhao IFMA | Rangel J.H.G.,Federal Institute of Maranhao IFMA | Oliveira M.M.O.,Federal Institute of Maranhao IFMA | And 3 more authors.
Ceramics International | Year: 2014

The purpose of this study was to increase the thermal resistance of a cellulose substrate, using films (a mixture of corundum-phase aluminum oxide, α-Al2O3, and a PVA solution). The corundum phase, which was chosen due to its high thermal resistance, was obtained using an adaptation of Pechini's polymeric precursor method. A polyvinyl alcohol (PVA) solution was used as a medium for mixing the oxide for subsequent deposition on a paper substrate. The materials were characterized by X-ray diffraction (XRD) and field emission gun scanning electron microscopy (FEG-SEM) coupled with energy dispersive X-ray spectrometry (EDS). The thermal behavior of the material was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC). The EDS results indicate that all the samples with deposited α-phase aluminum oxide contain the chemical elements (Al and O) originating from the α-phase, while the surface analysis indicated that the paper substrates were completely coated with the films. The TG and DSC analyses of the thermal resistance of the film-coated substrate indicate that it was impossible to increase the thermal resistance of the paper substrate because the events that occurred (decomposition of cellulose and pyrolysis of lignin, hemicellulose and PVA) took place in the same temperature range, as has been reported in the literature for samples of the paper substrate without deposited film. © 2014 Elsevier Ltd and Techna Group S.r.l.

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