28 resultados para Henríquez Ureña


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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

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Transparent monoliths and films of urea cross-linked tripodal siloxane-based hybrids (named tri-ureasils) were prepared by the sol-gel process, under controlled atmosphere (inside a glove box) and ambient conditions and their structure and optical features were compared. X-ray diffraction data point out that all the materials are essentially amorphous and Si-29 NMR reveal an increase in the condensation degree (0.97) for the hybrids prepared under controlled atmosphere relatively to that found for those prepared under ambient conditions (0.84-0.91). The tri-ureasils are white light emitters under UV/Visible excitation (from 250 to 453 nm) being observed for the composites prepared inside the glove box a significant enhancement (60-80 %) of the absorption coefficient and higher emission quantum yield values (similar to 0.27 and similar to 0.20 for monoliths and films, respectively) relatively to those synthesized under ambient condition.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

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Gaseous losses are the main factors affecting the efficiency of nitrogenous fertilizers in pastures. To evaluate NH3-N volatilization losses in Tanzania grass fertilized with urea in autumn, spring and summer, a completely randomized design with repeated measurements over time and fifteen replicates was used. Plots were represented by urea levels (50; 100 and 150 kg ha-1 N) and subplots by time after fertilization (1; 2; 3; 6; 9; 12 and 15 days). The interaction between fertilization leveland time after urea application was significant for the accumulated NH3-N volatilization. Urea application leads to higher percentage N losses in the first three days after application. The average cumulative NH3- N loss for the three occasions (different seasons of the year) was 28%, 20% and 16% of N applied for fertilizer doses of 50; 100 and 150 kg ha-1 of N, respectively. The season of the year influenced NH3-N loss pattern and volume, with the lowest values recorded in spring, followed by summer and autumn. The cumulative NH3-N volatilization loss varies from 78 to 90% up to the third day after application of the total NNH3 loss.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

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The bamboo waste can be an alternative material to sustain the crescent demand for particleboards, also bringing ecological benefits as reduction of the pressure for raw materials and landfill space demands. In this context, this research aimed to manufacture and determine some physical and mechanical properties of particleboards with bamboo waste particles (Dendrocalamus giganteus), obtained from different sources, bonded with four different percentages of urea–formaldehyde (UF) based resin (6%, 8%, 10% and 12% related to dry material of particles). Response variables investigated were: density; moisture content; thickness swelling in 2 and 24 hours; water absorption in 2 and 24 hours; internal adhesion (STpe); strength in tension parallel to faces (STpa); modulus of elasticity (MOE) and modulus of rupture (MOR). Results permitted to conclude that particleboards as mentioned showed good performance only in the physical properties requirements imposed by Brazilian Standard NBR 14810, but this was not observed to mechanical properties considered. New researches are needed in order to optimize the producing process parameters.