Francisco M. S. Garrido

Pilhas a combustível de óxido sólido: materiais, componentes e configurações

In this paper the current status of fuel cells is described with particular emphasis on high (T > 800 oC) and intermediate (T < 800 oC) temperature solid oxide fuel cells. Also the importance of the fuel cell technology is shown. Reviewed are the fundamental features, the basic principles, types of fuel cell, fabrication methods, cell configurations and the development of components (cathodes, anodes, electrolytes, interconnect) and materials.

Chemometric analysis of nonlinear optical chromophores structure and thermal stability

AbstractIn this work we explore experimental data on thermal stability of some nonlinear optical chromophores with second ordermolecular hyperpolarizability coefficients (b 0 ) ranging from 17 to 119 £10 230 esu. A detailed analysis of the thermal stabilityinvolves multiple factors, so we decided to work with multivariate analysis using chemometric methodology. The chemicalvariables chosen to relate thermal stability (T d ) were: heat of formation (DH), selected bond lengths (CDB, CNB), conjugatedchain length (CC), dipole moment (m), charge over selected sites (QD and QW), frontier orbital energy (E HOMO and E LUMO ). Ourchemometric analysis highlights the relation between structural and electronic chemical variables and thermal stability of thisset of nonlinear optical chromophores. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear optics; Organic chromophores; Thermal stability; Multivariate analysis 1. IntroductionNonlinear Optics has been recognized for severaldecades as a promising field with important applica-tions in the domain of opto-eletronics and photonics[1]. Chromophore-doped polymers are being vigor-ously studied for use in nonlinear optical applications[2–4].Organic chromophores to be used in this applica-tions must not only have high optical nonlinearities(related with hyperpolarizability) but also requiregood thermal and chemical stability and low opticalloss (high transparency) [1–7]. As a consequence thedesign of new chromophore is challenging, sincethese properties are to a larger degree mutually exclu-sive.Recently the tradeoff that has received the mostattention has been the nonlinearity-thermal stabilitytradeoff [1,6–9]; aromatic groups incorporation intoa chromophore’s delocalized bridge generallyincrease the thermal stability but decrease the mole-cular hyperpolarizability. Moylan and co-workerscircumvented this dilemma by replacing aliphaticdialkylamino donor groups with diarylamino groups,which results in a substantial increase in thermal stabi-lity of a wide range of chromophores without compro-mising the nonlinearity of the chromophore [7,8].The Moylan approach to improve the chromophorethermal stability is empirical, more recently Twiegand co-workers used thermogravimetric analysis

Verdete de cedro do abaeté como fonte de potássio: caracterização, tratamento térmico e reação com CaO

The potassium is one of the most important macronutrients to the soil. However, in the Brazilian soils this nutrient becomes scarce due to the high acidity of tropical soils. Thus, the demand for alternative sources of potassium grows considerably. In this context, we performed a chemical and mineralogical characterization of a verdete from Cedro do Abaete, Minas Gerais, as well as an evaluation of the verdete thermal treatment, with or without addition of calcium oxide (CaO), in order to solubilize the potassium present in the rock. The characterization of this rock was developed by the techniques of X-ray diffraction (XRD), X-ray fluorescence (XRF) and infrared spectroscopy (IR). The evaluation of the XRF results showed that the rock has 6,95 mass% of K2O, related to the presence of glauconite. This was confirmed by the XRD due to the presence of characteristics peaks at 1.0, 0.45, 0.24 e 0.15 nm and the occurrence of IR bands at 3520, 3440, 1020 and 630 cm-1. There is also a high content of SiO2 (64,65%), indicative of the presence of quartz, which is characterized by an intense peak at 0.335 nm. During the thermal treatment, the verdete was heated for 2 hours at a temperature of 1200 oC. In order to increase the solubility of potassium present in the rock, reactions were carried out with CaO in proportions of 10, 20 and 30% by weight. The structural changes in the rock were followed by XRD and IR. The changes in the position of the characteristic bands of Si-O, in the IR spectra at around 1100 cm-1, and the disappearance of X-ray diffraction peaks, related to glauconite, confirmed the reaction between the rock and the CaO. The reaction with 20% of CaO allowed an extraction of 7.7 % of the potassium present in the sample.

Alvejamento químico de caulins Brasileiros: efeito do potencial eletroquímico da polpa e do ajuste do pH

Samples of Kaolin from different regions in Brazil were characterized by XRD, SEM and chemical analysis. A chemical bleaching study with pH adjustment was accomplished with the fractions below 37 μm, after classification by screening. The main objective was to evaluate the conditions of chemical bleaching that most increase the brightness of these kaolins samples. Increases between 2.63 and 2.98% in the brightness (ISO) were observed after the chemical bleaching. We could say that the reduction of Fe3+ to Fe2+ during the chemical bleaching promoted an increase in the brightness, based on the Pourbaix Diagrams.

Study on the Characterization of Marginal Bauxite from Pará/Brazil

Bauxite from Para is divided into five different layers. However, only one is processed. The crystallized-amorphous (CAB) phase is considered a marginal bauxite because it presents a high quantity of SiO2reactive and its use depends on special technologies. CAB was characterized and the results were compared with the bauxite used nowadays in the alumina plant. Characterization was performed by XRD, IR, XRF, chemical analysis, TGA and SEM. XRD determined the mineral content: such bauxite is gibbsitic and has been associated with kaolinite and hematite. IR data supported the XRD results. XRF was used to determined the sample’s chemical composition. The chemical content of Al2O3available and SiO2reactive was determined by potentiometric titration and FAAS. The results found for the Bayer process sample were 41.7% and 7.1%, respectively. TGA observed the bauxite decomposition and SEM supplied chemical and thermal analysis. Thus, based on stoichiometric relations of the bauxite components decomposition, it was possible to confirm the presence of the following phases: gibbsite and kaolinite.

Síntese de óxidos mistos SiO2 /Mn xOy para aplicação na reação de redução de O2

The mixed oxides SiO2/MxOy obtained by the sol-gel process, designated as composites, usually combine both mechanical and chemical properties of silica gel with the chemical properties of metal oxide (MxOy), showing properties such as: high degree of dispersion and homogeneity of metal oxide particles in the silica matrix, as well as high mechanical and chemical resistance. The goal of this work was to prepare and characterize materials in the system SiO2/MnxOy obtained by the sol-gel process, using different concentrations of Mn (II) precursors (acetate or nitrate). The technique of X-ray fluorescence showed that the concentration of metal oxide in the materials is dependent on the precursor used and its concentration, ranging from 9.4% up to 20.9% by mass. The highest concentration of manganese oxide was obtained when (CH3CO2)2Mn.4H2O was used as precursor. The infrared spectra indicate that the network of SiO2 is not disturbed by the increase in the quantity of manganese oxide, suggesting that the MnxOy is dispersed on the surface of the silica matrix. The X-ray diffraction showed the low crystallinity of these materials. However, there is a change in the XRD on 2θ = 35.70 when the material is heated to 400 0C, probably due to the formation of Mn3O4 crystallites. The proposed synthesis method for materials SiO2/MnxOy is reproducible. Application of these materials as electrodes for the reduction of O2 was evaluated and it was found that the electrocatalytic activity of these materials can be increased by heating them at 400 0C.

Synthesis and Characterization of Sodalite and Cancrinite from Kaolin

Cancrinite and sodalite are characterized by high capacity of adsorption due to its porous structure. The aim of this work was to use kaolin from Borborema-Serido/Brazil as a source of aluminium and silicon to hydrothermal synthesis of sodalite and cancrinite and to evaluate the best conditions to the formation of these phases. Therefore, kaolin was heated at 700 and 900 °C for 2 h in order to transform kaolinite in metakaolin. Thereafter, experiments with the calcined kaolin’s were carried out between 150 and 230 °C for 30–270 min on a stainless steel pressure vessel using NaOH, Na2CO3 and H2O. The samples were characterized by X-ray diffraction, nuclear magnetic resonance, scanning electronic microscopy, thermogravimetry, and Zeta potential. The most pure sodalite phase was obtained at 230 °C, with the kaolin/NaOH ratio of 1:1, low concentration of Na2CO3 and smaller time of synthesis (30 min). On the other hand, for the synthesis of cancrinite phase at 230 °C, it was necessary to use kaolin heated at 900 °C, a longer time of synthesis (270 min) and a higher concentration of Na2CO3.

Caracterização e classificação quanto ao risco ambiental do estéril da mina de cromita do município de Andorinha, Bahia

The mineral waste studied was host rock from a chromite mine located in Andorinha/Bahia, extracted and stockpiled in yards without specific application. Host rock was submitted to chemical analysis, XRD, SEM-EDS, IR and TGA and classified according to ABNT standards for solid waste classification. Analyses confirmed that this host rock, classified as ultrabasic, consists mainly of dolomite, calcite and diopside. Hazard assessment results showed this host rock should be classified as class II B - inert waste, important for its potential application in agriculture as a soil acidity correction agent.

Mechanochemical Activation of Bauxite

The crystallized layer that compounds the bauxite’s geological profile from NE Para was ore dressed through the processes of crushing, screening, washing and grinding, in order to reach the same size distribution of that in the alumina production industry. After the preparation process, the sample (90%, < 0.21 mm) containing 5.3% reactive silica and 47.2% available alumina was submitted to mechanochemical activation with different kinds of reagents (CaO, Ca(OH)2, CaCO3, CaO+HCl), in different concentrations, which resulted in the formation of a calcium aluminum hydrosilicate (hydrogarnet). This phase was formed in the first 30 min of grinding with the use of CaO and Ca(OH)2. However, chemical characterization showed that the principal phase obtained, katoite hydrogarnet, had high aluminum content in its chemical composition thus proving that its formation is not good for the Bayer process.

Interação de átomos leves com clusters de metais de transição

Density Functional Theory (DFT) calculations on the interactions of small atoms (H, C, O, and S) on first-row transition metal clusters were performed. The results show that the adsorption site may vary between the metal surface and the edge of the cluster. The adsorption energies, adatom-nearest neighbor and adatom-metal plane distances were also determined. Finally, the authors present a discussion about the performance of these metals as anodes on solid oxide fuel cells. The results obtained agree with empirical data, indicating that the theoretical model used is adequate