Navegando por Autor "Varela, José A."

Agora exibindo 1 - 20 de 23

An investigation of metal oxides which are photoluminiscent at room temperature
(Elsevier, 2004-01) Leite, Edson Roberto; Pontes, Fenelon M.; Lee, Eduardo J. H.; Aguiar, Rosiana; Silva, Elson Longo da; Pontes, D. S. L.; Nunes, M. S. J.; Pizani, Paulo Sérgio; Lanciotti Júnior, Franchesco; Boschi, Tania M.; Varela, José A.; Paskocimas, Carlos Alberto; Pinheiro, Carlos Davidson; Taft, Carlton Anthony
Amorphous thin films, based on different network formers, were processed by a soft chemical process called the polymeric precursor method. The resultant amorphous metal oxides, displayed intense photoluminescence (PL) at room temperature. Heat treatment increases the PL intensity of these materials. Theoretical ab initio calculations are correlated with the observed experimental trends
BaMoO4:Tb3+ phosphor properties: synthesis, characterization and photophysical studies
(Elsevier, 2011) Marques, Ana Paula de Azevedo; Motta, Fabiana Villela da; Cruz, Mariana A.; Varela, José A.; Longo, Elson; Rosa, Ieda Lúcia Viana
BaMoO4:Tb3+ powders were synthesized by the complex polymerization (CP) method and characterized by powder X-ray diffraction patterns (XRD), Fourier transform infra-Red (FTIR) and Raman spectroscopy as well as photoluminescence (PL) measurements. The results confirmed the presence of a crystalline scheelite-type phase in BaMoO4:Tb3+crystalline powers. Excitation spectra, λem. = 543 nm revealed the predominance of a broad band of charge transfer that was ascribed to the absorption of [MoO4]2- ionic tetragonal clusters. Emission spectra of BaMoO4:Tb3+ samples (λex. = 294 nm) have a group of sharp emission bands (456–487 nm; 543–546 nm; 581–589 nm and 617–620 nm) assigned to the 5D4 → 7 F6, 5D4 → 7 F5, 5D4 → 7 F4 and 5D4 → 7 F3 transitions of Tb3+ ions, respectively. The samples show the exponential decay curves of 5D4 → 7 F5 transition, λexc. = 294 nm and λem. = 543 nm, where the greater decay time was evaluated as 1.07 ms for the BaMoO4:Tb3+ annealed at 900 °C
Effect of calcium on the structural properties of Ba(1−x)Ca x TiO3 particles synthesized by complex polymerization method
(Springer, 2014-01-14) Araújo, Vinícius Dantas; Motta, Fabiana Villela da; Marques, Ana Paula de Azevedo; Paskocimas, Carlos Alberto; Delmonte, Maurício Roberto Bomio; Silva, Elson Longo da; Varela, José A.
Ferroelectric materials, such as barium titanate (BaTiO3), have been extensively studied for application in electronic and optical devices. The substitution of Ba by Ca is an effective method to improve the piezoelectricity temperature stability, as it can greatly lower the tetragonal–orthorhombic phase transition temperature, whereas the change of the Curie point is negligible. Ba(1−x)Ca x TiO3 (x = 0, 0.05, 0.10, 0.15, and 0.20) powders were prepared by complex polymerization method. The effect of calcium on the tetragonality of the BaTiO3 system was monitored using basic characterization techniques: X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. The results indicate that increased calcium contents raise the Curie temperature (T c) and that the addition of calcium in the BT matrix reduces tetragonality
Effect of different starting materials on the synthesis of Ba0.8Ca0.2TiO3
(SpringerOpen, 2015-01-31) Medeiros, P. N.; Araújo, Vinícius Dantas de; Marques, Ana Paula de Azevedo; Tranquilin, Ricardo Luis; Paskocimas, Carlos Alberto; Delmonte, Maurício Roberto Bomio; Varela, José A.; Silva, Elson Longo da; Motta, Fabiana Villela da
Literature has reported the synthesis of barium calcium titanates by various synthesis methods such as solid state reaction, co-precipitation and polymer precursors. These compounds are usually obtained using calcium carbonate (CaCO3), barium carbonate (BaCO3) and titanium oxide as starting materials. This study investigated the effect of different starting reagents on the synthesis of Ba0.8Ca0.2TiO3 (BCT) by complex polymerization method (CPM). Two sets of starting precursors were used: titanium citrate, CaCO3 and BaCO3, and titanium citrate and Ba1−x Ca x CO3 solid solution precursor. Samples were crystallized at a temperature range from 400 °C to 700 °C for different time. The obtained powders were characterized by X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA), and Raman and infrared spectroscopy. The infrared spectroscopy indicated that the chelation processes of Ba, Ca, Ti and CA ions are very similar. The results showed that the use of CaCO3 and BaCO3 or Ba1−x Ca x CO3 solid solution as precursors does not affect the final properties of BCT powders obtained by CPM
Experimental and theoretical correlation of very intense visible green photoluminescence in BaZrO3 powders
(American Institute of Physics, 2008) Cavalcante, L. S.; Zampieri, M.; Espinosa, J. W. M.; Pizani, Paulo Sérgio; Paskocimas, Carlos Alberto; Sambrano, Julio Ricardo; Silva, Elson Longo da; Varela, José A.; Simões, M. L.; Longo, V. M.
Very intense visible green photoluminescence PL was observed at room temperature in structurally ordered-disordered BaZrO3 powders. Ab initio calculations, ultraviolet-visible absorption spectroscopy, electron paramagnetic resonance, and PL were performed. Theoretical and experimental results showed that local defects in the cubic structure caused by ZrO5 ·VO z complex clusters, where VO z =VO x , VO • , and VO ••, play an important role in the formation of hole-electron pairs, giving rise to a charge gradient in the structure which is responsible for PL emission
Foundry sand recycling in the troughs of blast furnaces: a technical note
(Elsevier, 2005-01-10) Andrade, Ricardo Magnani; Cava, Sérgio; Silva, Sidiney Nascimento; Soledade, Luiz Edmundo Bastos; Rossi, Cláudia C.; Leite, Edson Roberto; Paskocimas, Carlos Alberto; Varela, José A.; Silva, Elson Longo da
The foundry sand agglomerated with alkaline phenolic resin, used for the manufacture of molds, was found to be a residue which is able to be recycled, minimizing the costs of disposal and the environmental impact. This paper analyzes the thermomechanical regeneration and leaching processes and also assesses the influence of additives on the improvement of the mechanical properties of the sands. Besides, the industrial experiments carried out at CSN aiming at the foundry sand recycling in the covering of the blast furnace troughs are presentedThe foundry sand agglomerated with alkaline phenolic resin, used for the manufacture of molds, was found to be a residue which is able to be recycled, minimizing the costs of disposal and the environmental impact. This paper analyzes the thermomechanical regeneration and leaching processes and also assesses the influence of additives on the improvement of the mechanical properties of the sands. Besides, the industrial experiments carried out at CSN aiming at the foundry sand recycling in the covering of the blast furnace troughs are presented
Freezing distortions and photoluminescence property in PbMoO4 Micro- Octahedrons: an experimental and theoretical study
(Bentham Science Publishers, 2014-02) Moraes, Eduardo de; Delmonte, Maurício Roberto Bomio; Longo, Valéria M.; Silva, Elson Longo da; Varela, José A.
In this paper, we report a detailed structural and electronic characterization of PbMoO4 crystals by using a conventional hydrothermal (CH) method. The samples were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), field-emission gun scanning electron microscopy (FEG-SEM) and photoluminescence (PL) measurements. In addition, first-principles quantum mechanical calculations based on the density functional theory were employed in order to understand the band structure and density of states for the PbMoO4. Analysis of both theoretical and experimental results allows to rationalize the role of order-disorder effects in the observed green PL emissions in these ordered powders
Highly intense violet-blue light emission at room temperature in structurally disordered SrZrO3 powders
(American Institute of Physics, 2007-02-27) Longo, V. M.; Cavalcante, L. S.; Figueiredo, A. T. de; Santos, L. P. S.; Silva, Elson Longo da; Varela, José A.; Sambrano, Julio Ricardo; Paskocimas, Carlos Alberto; Vicente, F. S. de; Hernandes, C.
Violet-blue photoluminescence was produced at room temperature in a structurally disordered SrZrO3 perovskite structure with a 350.7 nm excitation line. The intensity of this emission was higher than that of any other perovskites previously studied. The authors discuss the role of structural order-disorder that favors the self-trapping of electrons and charge transference, as well as a model to elucidate the mechanism that triggers photoluminescence. In this model the wide band model, the most important events occur before excitation
In2O3 microcrystals obtained from rapid calcination in domestic microwave oven
(Elsevier, 2010) Motta, Fabiana Villela da; Lima, Renata C.; Marques, Ana Paula de Azevedo; Leite, Edson Roberto; Varela, José A.; Silva, Elson Longo da
The simple way to prepare In2O3 microcrystals is reported in this paper. The precursor, In(OH)3 microstructures, were obtained using the Microwave-Assisted Hydrothermal (MAH) Method. By annealing as-prepared In(OH)3 precursor at 500 °C for 5 min in a domestic microwave oven (MO), In2O3 microcrystals were prepared, inheriting the morphology of their precursor while still slightly distorted and collapsed due to the In(OH)3 dehydration process which was studied by thermal analysis. The In(OH)3 and In2O3 were characterized using powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and Raman spectroscopy. These techniques confirm the chemical dehydration of In(OH)3 and the formation of In2O3 powders. The domestic MO promotes a rapid structural organization as compared with a CF (conventional furnace). The MAH method and the subsequent annealing in a domestic MO were shown to be a low cost route for the production of In2O3, with the advantages of lower temperature and smaller time
Indium hydroxide nanocubes and microcubes obtained by microwave-assisted hydrothermal method
(Elsevier, 2010) Motta, Fabiana Villela da; Lima, R. C.; Marques, Ana Paula de Azevedo; Li, Maximo Siu; Leite, Edson Roberto; Varela, José A.; Silva, Elson Longo da
Single crystalline indium hydroxide (In(OH)3) architectures were prepared by a rapid and efficient microwave-assisted hydrothermal (MAH) method using indium chloride as the precursor. Nanostructures were obtained at a low temperature (140 °C with a time of 1 min). In(OH)3 samples were prepared at the same temperature for 8 with a time of 64 min. Using this method, In(OH)3 samples were obtained at milder conditions of temperature and time compared to the conventional hydrothermal method. Field emission scanning electron microscopy images confirm that these samples are composed of 3D nanocubic, microcubic and irregular structures of about 70 nm to 5 μm in size. Fourier transform Raman spectroscopy and photoluminescence (PL) measurements were used to characterize the products
O papel dos modificadores de rede na produção da fotoluminescência no CaWO4
(ABCERAM, 2004-01) Ciaco, F. R. C.; Pontes, F. M.; Pinheiro, C. D.; Leite, Edson Roberto; Lázaro, S. R. de; Varela, José A.; Pizani, Paulo Sérgio; Paskocimas, Carlos Alberto; Souza, Antônio Gouveia de; Silva, Elson Longo da
Discutiu-se a fotoluminescência para o tungstato de cálcio amorfo, observada à temperatura ambiente. Verificou-se que há concordância entre os resultados experimentais e teóricos. Neste trabalho foram simuladas as estruturas cristalinas e amorfas do tungstato de cálcio (CaWO4), comparando-se as respectivas estruturas eletrônicas. Os resultados dos cálculos teóricos indicam a formação de novos níveis de energia na banda de valência e de condução do amorfo. Estes níveis eletrônicos extras são os responsáveis pela formação da cauda na curva do espectro de absorção. Correspondentemente, medidas experimentais de absorção óptica mostraram a presença da cauda, com relação ao espectro do cristalino. Desta forma, pode-se interpretar a formação da cauda, como sendo associada aos defeitos promovidos na estrutura desordenada do material amorfo
Photoluminescence of BaZrO3 explained by a order/disorded transformation
(Springer, 2015-02-13) Aguiar, E. C.; Simões, A. Z.; Paskocimas, Carlos Alberto; Cilense, M.; Silva, Elson Longo da; Varela, José A.
A hydrothermal (HTMW) microwave method was used to synthesize ordered–disordered BaZrO3 (BZ) nanoparticles at temperature of 140 °C with times ranging from 15 min to 2 h. X-ray diffraction results verified the formation of BZ crystallites at a soaking time of 2 h while infrared data showed no traces of carbonate. Field emission scanning microcopy revealed a homogeneous size distribution of nanometric BZ powders. HTMW produced nanoparticles of pure BZ phase, with a size ranging from 40 to 80 nm. These results are in agreement with Raman scattering values which show that the HTMW synthesis route is rapid and cost effective. This method could be used as an alternative to obtain BZ nanoparticles as compared to other chemical methods. Intense photoluminescence in disordered BZ powders was observed at room temperature. The key of mystery of the intense PL emission is related to order–disorder structural in BZ lattice
Preparation and photoluminescence characteristics of In(OH)3:xTb3+ obtained by Microwave-Assisted Hydrothermal method
(Elsevier, 2013-03-15) Motta, Fabiana Villela da; Marques, Ana Paula de Azevedo; Li, Maximo Siu; Abreu, Maria Fernanda C.; Paskocimas, Carlos Alberto; Delmonte, Maurício Roberto Bomio; Souza, Ricardo Pires de; Varela, José A.; Silva, Elson Longo da
Crystalline terbium-doped indium hydroxide structures were prepared by a rapid and efficient Microwave-Assisted Hydrothermal (MAH) method. Nanostructures were obtained at a low temperature. FE-SEM images confirm that these samples are composed of 3D nanostructures. XRD, optical diffuse reflectance and photoluminescence (PL) measurements were used to characterize the products. Emission spectra of terbium-doped indium hydroxide (In(OH)3:xTb3+) samples under excitation (350.7 nm) presented broad band emission referent to the indium hydroxide matrix and 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3 terbium transitions at 495, 550, 590 and 627 nm, respectively. Relative intensities of the Tb3+ emissions increased as the concentration of this ion increased from 0, 1, 2, 4 and 8 mol%, of Tb3+, but the luminescence is drastically quenched for the In(OH)3 matrix
Room temperature photoluminescence of BCT prepared by Complex Polymerization Method
(Elsevier, 2010) Motta, Fabiana Villela da; Marques, Ana Paula de Azevedo; Espinosa, José Waldo Martínez; Pizani, Paulo Sérgio; Silva, Elson Longo da; Varela, José A.
It was used the Complex Polymerization Method to synthesize barium calcium titanate powders (BCT). Crystalline Ba0.8Ca0.2TiO3 perovskite-type phase could be identified by X-ray diffraction and confirmed by Raman spectroscopy in the powders heat treated at 600 °C. Inherent defects, linked to structural disorder, facilitate the photoluminescence emission. The photoluminescent emission peak maximum was around of 533 nm (2.33 eV) for the Ba0.8Ca0.2TiO3. The photoluminescence process and the band emission energy photon showed dependence of both the structural order–disorder and the thermal treatment history. The results revealed that Ba0.8Ca0.2TiO3 (BCT20) is a highly promising candidate material for optical applications.
Síntese e caracterização do sistema ZrO2-SiO2 com adição de cobalto para uso como pigmentos cerâmicos
(ABCERAM, 2005-07) Cava, Sérgio; Tebcherani, Sérgio Mazurek; Pianaro, Sidnei Antônio; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Varela, José A.
Pós de ZrSiO4 dopado com cobalto foram sintetizados por meio do método dos precursores poliméricos para obtenção de pigmentos cerâmicos. O efeito de diferentes concentrações de Co2+ na transição de fase t-ZrO2 para ZrSiO4 foi estudado por meio de difração de raios X e análise térmica diferencial. Resultados de espectroscopia na região do UV-visível permitiram comparar o deslocamento de bandas em função da transição de fase. Além disso, foi realizado um estudo de estabilidade térmica e intensidade da alteração de cor do azul ao vermelho dos pós sintetizados em função da presença de cobalto e temperatura de calcinação, sendo estes parâmetros comparados com a transição de fase. Os resultados obtidos permitem identificar a transição de fase t-ZrO2 para ZrSiO4
Structural analysis of Ti and Pb citrate using NMR and FT-Raman signals and quantum mechanics simulations
(Springer, 2006-01-02) Zampieri, M.; Lazaro, S. R.; Paskocimas, Carlos Alberto; Ferreira, A. G.; Silva, Elson Longo da; Varela, José A.
Titanium citrate and lead and titanium citrate were prepared by polymeric precursor method in aqueous solution. This citrate was analyzed by 1H, 13C NMR and gHMBC–NMR (Hetero-nuclear multiple-bond correlation) to investigate the formation of the citrate complexs and influence of the Pb2+ ion in this complex. These complexs were characterized by interaction between Ti4+ ion and citric acid carboxyls. Quantum mechanic simulations in level ab initio were used to study the electronic structure and natural charges (NBO) to both the complexs. Such techniques indicated the formation of an octahedral complex with an arrangement similar to Ti atom in the crystalline structure of the PbTiO3. A study using the technique FT-Raman made possible the confirmation of the interaction among the Ti4+ and Pb2+ ions with the citric acid carboxyls
Structural and spectroscopic analysis of y-Al2O3 to α-Al2O3-CoAl2O4 phase transition
(Elsevier, 2005-07-25) Cava, Sérgio; Tebcherania, Sérgio Mazurek; Pianaroa, Sidnei Antônio; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Varela, José A.
Co-doped alumina powders were synthesized by means of the polymeric precursor method to obtain ceramic pigments. The effect of different contents of Co2+ on phase transition to -Al2O3 and appearing of CoAl2O4 spinel were studied by means of X-ray diffraction. A partial phase diagram of the system CoAl2O3 was proposed from these data by means of determination of the percentages of these phases according to the calcining temperature. Critical particle size to phase transition was determined by means of calculations of crystallite size and determination of superficial area through the BET method. UV–vis spectroscopy of the samples allow to compare the band shift with the phase transition. Besides, a study of thermal stability and intensity of the blue coloration of the synthesized powders with the presence of cobalt in relation to the calcining temperature was accomplished and compared to the phase transition. The results show that the higher blue color intensity was obtained for the powders with Co-doped -Al2O3 closest of phase transition to -Al2O3 + CoAl2O4
Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method
(Springer, 2007-03-21) Cava, Sérgio; Benincá, Renata; Tebcherani, Sérgio Mazurek; Souza, Iedo A.; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Varela, José A.
Al2O3 and Al2−xCrxO3 (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL∗a∗b∗ color data). The obtained results allow to identify the γ -Al2O3 to α-Al2O3 phase transition. The single-phase α-Al2O3 powder was obtained after heat treatment at 1050 ◦C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al2−xCrxO3 are related to the γ to α-Al2O3 phase transition and the temperature and time range for such transition depends on the chromium content
Structural characterization of phase transition of Al2O3 nanopowders obtained by polymeric precursor method
(Elsevier, 2007-06-15) Cava, Sérgio; Tebcherani, Sérgio Mazurek; Souza, Iedo A.; Pianaro, Sidnei Antônio; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Varela, José A.
Nanocrystalline Al2O3powders have been synthesized by the polymeric precursor method. A study of the evolution of crystalline phases of obtained powders was accomplished through X-ray diffraction, micro-Raman spectroscopy and refinement of the structures through the Rietveld method. The results obtained allow the identification of three steps on the -Al2O3 to -Al2O3 phase transition. The single-phase -Al2O3 powder was obtained after heat-treatment at 1050 ◦ C for 2 h. A study of the morphology of the particles was accomplished through measures of crystallite size, specific surface area and transmission electronic microscopy. The particle size is closely related to -Al2O3 to -Al2O3 phase transition
Study on the orientation degree of Pb1 − xLaxTiO3 thin films by the rocking curve technique and its morphological aspects
(Elsevier, 2007-04-02) Rangel, J. Horta G.; Bernardi, Maria Inês Basso; Paskocimas, Carlos Alberto; Silva, Elson Longo da; Varela, José A.
Thin films of perovskite-type materials such as PbTiO3, BaTiO3, (Pb,La)TiO3, (Pb, La)(Zr,Ti)O3, KNbO3, and Pb(Mg,Nb)O3 have been attracting great interest for applications like non-volatile memories, ultrasonic sensors and optical devices. Thin film should be epitaxially grown or at least highly textured since the properties of this anisotropic material depend on the crystallographic orientation. For optical devices, in particular, an epitaxial thin film without defects are essential to reduce optical propagation losses. Pb1 − xLaxTiO3 (PLT) where x=0, 13 and 27% thin films were prepared by a chemical method (polymeric precursors method), and deposited by the spin coating technique onto substrates of SrTiO3 (STO) and LaAlO3 (LAO). The films were then heat treated at 500 °C in a controlled atmosphere of O2. The orientation degree of the thin films was obtained from rocking curve technique, by means of X-ray diffraction analysis. A microstructural study revealed that the films were crack-free, homogeneous and have low roughness

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Navegando por Autor "Varela, José A."