Programa de Pós-graduação em Física

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Navegando Programa de Pós-graduação em Física por Por Orientador "CARVALHO, Jesiel Freitas"

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    Estudos da sinterização de Bi12TiO20 (BTO) visando obter cerâmicas transparentes
    (Universidade Federal de Goiás, 2011-02-16) AMARAL, Thiago Martins; CARVALHO, Jesiel Freitas; http://lattes.cnpq.br/5841291496427516
    This work presents the systematic results of studies concerning the sintering of the Bi12TiO20 (BTO) aiming the production of transparent ceramic samples. All that because the BTO is a material that has great scientific and technologic visibility due to its optical, electro and electro-optical properties, but still little explored as ceramic. The sintering starting material was obtained by solid state reaction between Bi2O3 e TiO2; the synthesized BTO was grinded in ball milling until sub micrometric sizes. The conditions for preparing green samples were also investigated, allowing samples with green relative density between 58% and 62%, with uniaxial and isostatic pressing techniques. The study of the sintering process involved different sintering programs, under different atmospheres conditions and additive utilization. Optimized conditions were established and the limiting factors discussed. The quality of the obtained ceramics was assessed by their final relative density, phase homogeneity, microstructure and optical properties (transmission, optical activity and electro-optical performance). Samples with relative density above 99,9% and 50% of the monocrystal´s optical transmission in the 633 nm wavelength and equal optical activity were obtained. Nevertheless, the ceramics still show scattering centers and none liquid electro-optical effect was observed.
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    Crescimento de cristais de Bi2TeO5
    (Universidade Federal de Goiás, 2010-07-30) FABRIS, Zanine Vargas; CARVALHO, Jesiel Freitas; http://lattes.cnpq.br/5841291496427516
    Bi2TeO5 (bismuth tellurite) crystals are photorefractive materials appropriate for permanent holographic recording. They are capable to retain a hologram for a long time, several years in the dark, without further fix processes. Just some groups around the world have grown bismuth tellurite single crystals even with their interesting properties. It may be caused by the relatively difficulty for grow this crystal, mainly due to the high vapor pressure of tellurium oxide and the easy crystal cleavage. The present work was dedicated to study the crystal growth process of Bi2TeO5 by Czochralski method in a low thermal gradient system. Tellurium oxide excess was used in the starting composition to compensate its lost during crystal growth, and also this starting material was previously synthesized by solid state reaction before fusion. We have performed tests with 1Bi2O3 : 1,03TeO2 and 1Bi2O3 : 1,10TeO2 liquid phase compositions. Also, we tried to use a double crucible configuration, with pure tellurium oxide between the walls. Pulling and rotate rates are critical parameters. Pulling and rotate rates of 0,1 to 0,4 mm/h and 10 to 15 rpm, respectively, were appropriated parameters for our growth system. Good size and crystalline quality crystals have been grown. We have obtained samples of sufficient quality to perform preliminary optical measurements and structural characterization.
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    Estudo de fases cristalinas no sistema Bi2O3-Tb4O7
    (Universidade Federal de Goiás, 2009-08-25) MATOS, Lucyene Nascimento; CARVALHO, Jesiel Freitas; http://lattes.cnpq.br/5841291496427516
    A systematic study concerning the crystalline phases in the Bi2O3-rich region of the system Bi2O3-Tb4O7 was made. X-ray powder diffraction method, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were used in the study of the obtained crystalline phases. Sixteen samples with composition in the range of 4.21 to 33.33 mol% of Tb4O7 were synthesized at 800oC by solid state reaction synthesis. Three distinct crystalline phases were initially identified: a tetragonal phase in the composition range of 4.21 to 5.69 mol% of Tb4O7, a cubic phase for compositions between 6.12 and 29.03 mol% of Tb4O7, and a monoclinic one with 33.33 mol% of Tb4O7. The thermal stability of these compositions was rigorously investigated and the results shown that they are metastable phases in almost all studied range. Under thermal annealing at 500oC, tetragonal phase undergoes decomposition into two phases, monoclinic and rhombohedral ones. The cubic phase, under the same thermal annealing, decomposes partially into a rhombohedral one, except for the interval between 25.0 and 29.1 mol% of Tb4O7. In this case, there was not decomposition under thermal annealing at 500oC during 373 hours, suggesting the cubic phase is stable in this composition range. The monoclinic phase was also tested concerning its thermal stability, but no phase transition was verified. The existence of a possible composition range in the system Bi2O3-Tb4O7 with stable cubic -Bi2O3 type structure is a new and promise result, because of the high ionic conductivity presented by this phase