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Search Results: 1 - 10 of 44260 matches for " Antonio Sergio Bezerra Sombra "
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Preparation of Bi4Ti3O12 (BIT) Ceramics via a High-Energy Ball Milling Process Doped with Multi-Walled Carbon Nanotubes (MWNTs)  [PDF]
Alexandre Gon?alves Pinheiro, Gilberto Dantas Saraiva, Josué Mendes Filho, Antonio Sergio Bezerra Sombra
Materials Sciences and Applications (MSA) , 2013, DOI: 10.4236/msa.2013.49067
Abstract:

We prepared the Nano-sized bismuth titanate Bi4Ti3O12 (BIT) powders, through a high-energy ball milling process from their oxides Bi2O3 and TiO2. This BIT phase can be formed after a milling process for 40 min. With an increasing milling time, this particle size of mixture is gradually reduced, thus, we have mostly an amorphous phase. The BIT ceramics were duly obtained by sintering the synthesized powders at temperatures ranging from 850°C to 1000°C. The BIT ceramics sintered at 1020°C for 1 h, exhibiting a density with 7.52 g/cm3 of a crystaline phase and a dielectric of K = 288.11 (100 Hz), as well as a dielectric loss of 0.05 (100 kHz). The High-energy ball milling process is a promising way to prepare BIT ceramics. After the preparation of the BIT, we doped it with the Multi-Walled Carbon Nanotubes which are properly obtained by a chemical vapour deposition (CVD), using nickel as a catalyst, as well as using acetilene at 720°C, and then proceeded with the dielectric and optical measurements.

Periodic Modulation of Nonlinearity in a Fiber Bragg Grating: A Numerical Investigation  [PDF]
Antonio Francisco Gomes Furtado Filho, José Rubens Rodrigues de Sousa, Apiano Ferreira de Morais Neto, Jose Wally Mendon?a Menezes, Antonio Sergio Bezerra Sombra
Journal of Electromagnetic Analysis and Applications (JEMAA) , 2012, DOI: 10.4236/jemaa.2012.42007
Abstract: We present numerical studies on the switching characteristics of a fiber Bragg grating (FBG) with modulation in the third order nonlinear index of refraction along it’s length. The FBG is operating in a continuous wave regime (CW). This study was done taking into account the possible asymmetry brought by the non harmonic modulation of the nonlinearity, leading to different reflection and transmission characteristics, that depend on the direction of propagation along the modulated nonlinear FBG. This phenomenon may be useful for applications like an optical isolator. It was found that for a set of values of the modulation parameter, the FBG can exhibit multistable states. The numerical studies were obtained starting from the coupled-mode equations solved from the coupled-mode theory and simulated using the fourth-order Runge-Kutta method.
The Thermal Stability of (CaTiO3)1-x (Cr3/4Fe5/4O3)x Ceramic Composites in the Microwave Region  [PDF]
Francisco Nivaldo Aguiar Freire, Manoel Roberval Pimentel Santos, Hélio Henrique Barbosa Rocha, Ana Fabiola Leite Almeida, Selma Elaine Mazzetto, Antonio Jefferson Mangueira Sales, Antonio Sergio Bezerra Sombra
Materials Sciences and Applications (MSA) , 2016, DOI: 10.4236/msa.2016.74020
Abstract: The thermal stability, structural and dielectric properties of the (CaTiO3)1-x(Cr3/4Fe5/4O3)x ceramic composites with x = 0, 0.1, 0.5, 0.9 and 1 have been examined in microwave region. The samples were produced via the solid-state reaction. The orthorhombic structural phase of CaTiO3 and trigonal structural phase of Cr3/4Fe5/4O3 were confirmed by the X-Ray Powder Diffraction (XRPD). The XRPD patterns for composites reveal the quantities of each original phase present. The infrared spectra of the samples reinforce this structural verification indicating no or minimal occurrence of unwanted reactions. The first sample (x = 0) and last sample (x = 1) in this series exhibit the maximum and the minimum of the relative dielectric permittivity and values range from 140.1 to 8.3 respectively. The measured temperature coefficient of the matrix CaTiO3 was +921 ppm·-1 and for the matrix Cr3/4Fe5/4O3 was -56 ppm·-1. With the study of series of composites, it was possible to make a mathematical prediction for a composition reach the temperature coefficient near zero. The proposed ceramic has potential use as thermostable material in the microwave region and can be applied to resonators, low-noise amplifiers, filters, and so on.


Optical Bistability in an Acousto-Optic Tunable Filter (AOTF) Operating with Short Optical Pulses  [PDF]
Karlo David Alves Sabóia, Alex Sander Barros Queiroz, Francisco Tiago Lima, Cícero Saraiva Sobrinho, Jose Wally Mendon?a Menezes, Antonio Sergio Bezerra Sombra
Journal of Electromagnetic Analysis and Applications (JEMAA) , 2012, DOI: 10.4236/jemaa.2012.43015
Abstract: In this paper we report for the first time the presence of bistability in an acoustic-optic tunable filter (AOTF) operating with ultrashort (2 ps) optical light pulses. The results for the study of bistability has shown the dependence of the hysteresis curve with the product of the coupling constant (κ) by the length of the device (ξL) and the conversion power-coupling constant factor (G). The range of bistability varies significantly with both G and with κξL parameters. The variation of κξL directly increases the size of the range of bistability hysteresis while the increase in G causes the bistability to occur at low powers. The phenomenon of optical bistability (OB) is the object of increasing interest due to its possibilities for important device applications. A bistable device is a device with a capability to generate two different outputs for a given input and the physical requirements for this are an intensity-dependence refractive index and an optical feedback mechanism.
Analysis of the Performance of Optical Code-Division Multiple-Access System (OCDMA), Operating with Gold Codes under Nonlinear Effects  [PDF]
Glendo de Freitas Guimar?es, Agliberto Melo Bastos, Alisson da Concei??o Ferreira, Alex Sander Barros Queiroz, José Wally Mendon?a Menezes, José Rubens Rodrigues de Sousa, Antonio Sergio Bezerra Sombra
Journal of Electromagnetic Analysis and Applications (JEMAA) , 2013, DOI: 10.4236/jemaa.2013.52008
Abstract:

A numerical simulation study of the performance of encoding and decoding of short optical pulse in OCDMA (Optical Code Division Multiple Access) systems under non-linear optical effects is presented. The performance of encoding and decoding short pulses is obtained through use of device FBG (fiber Bragg grating) where the codes are inserted through discrete jumps in the optical phase (±π). The multiuser interference is also the object of the present study, where a figure of merit (interference figure) is used to quantify the interference in multiuser auto and cross correlation. We evaluate the dependence of the multiuser interference for the coupling constant of FBG. Finally, the interference inserted in the autocorrelation due to nonlinear effects generated in the nonlinear switching of the coded pulse in a dual core nonlinear directional coupler is examined, where temporal broadening of pulses decoded is observed.

Prevalência de excesso de peso e hipertens?o arterial, em popula??o urbana de baixa renda
Feij?o, Adelina Maria Melo;Gadelha, Francisco Vieira;Bezerra, Antonio Alberto;Oliveira, Antonio Maurício de;Silva, Maria do Socorro Sombra;Lima, José Wellington de Oliveira;
Arquivos Brasileiros de Cardiologia , 2005, DOI: 10.1590/S0066-782X2005000100007
Abstract: objective: to study the relation between body mass and blood pressure in a low-income urban population. methods: from july to december 1998, a cross-sectional study was carried out in a representative sample of a low-income urban community with individuals of both sexes, aged > 30 years and living in 67 (30%) blocks selected out of 224. blood pressure, weight, and height were measured, and completion of a questionnaire provided information on sex, age, familial income, educational level, and occupation. body mass index was obtained by dividing weight (kilogram) by height squared (square meter), and the following values were considered: bmi < 25, normal; 25 > bmi < 30, overweight; bmi > 30, obesity. in addition, excessive weight was defined as bmi > 25, and arterial hypertension was defined as systolic blood pressure > 140 mm hg and diastolic blood pressure > 90 mm hg. results: in 1078 dwellings, 1137 eligible individuals resided, and complete information was obtained from 1032 (91%) individuals. the prevalences of arterial hypertension and excessive weight were 22.58% and 51.26%, respectively. before adjustment, the or for arterial hypertension for overweight individuals was 1.85 (95% ci: 1.52-2.25) and, for obese individuals, 3.7 (95% ci: 3.04-4.50); after adjustment, the ors were 2.04 (95% ci: 1.65-2.54) and 4.08 (95% ci: 3.30-5.08), respectively. conclusion: a strong association between body mass and blood pressure exists, independently of sex, age, familial income, educational level, and occupation.
Temperature Dependence of the Magnetic and Electric Properties of Ca2Fe2O5  [PDF]
Cléber Candido da Silva, Antonio S. B. Sombra
Materials Sciences and Applications (MSA) , 2011, DOI: 10.4236/msa.2011.29183
Abstract: Ca2Fe2O5 powder sample, were prepared to investigate the origin of the weak ferromagnetic component reported in literature for calcium ferrite single crystals. In this work, the calcination method was used to produce nanocrystalline powders of Ca2Fe2O5. XRD measurement has shown the presence of Fe3O4 magnetite and CaO as impurity phases. The ferrimagnetic phase deeply influences the magnetic behavior with features very similar to those reported in literature for Ca2Fe2O5, both powders and single crystals. Our results support the hypothesis that the weak ferromagnetic component observed in Ca2Fe2O5 can be also due to the presence of magnetite impurity traces in the samples. The powders were submitted to calcination processes at 500℃ for 2 hours and 950℃ for 16 hours. The sintered sample was submitted at 1050℃ for 6 hours and characterized by X-Ray Powder diffraction (XRD), dielectric measurements, Magnetization and Scanning Electron Microscopy (SEM) analysis.
Obsessed with grasses?: The case of mara Dolichotis patagonum(Caviidae: Rodentia)
SOMBRA,MOIRA S.; MANGIONE,ANTONIO M.;
Revista chilena de historia natural , 2005, DOI: 10.4067/S0716-078X2005000300004
Abstract: the diet of mara (dolichotis patagonum, zimmeraman 1780) within the limits of sierra de las quijadas national park (san luis, argentina) is described. other studies regarding mara?s diet showed that mara, feeds on different species of grasses and shrubs. however, there is controversy regarding the proportions of these items in the diet or whether this proportion varies seasonally. considering mara?s body size, anatomical features and physiology, we suggest that mara feeds on grasses in a greater proportion than shrubs and forbs. moreover, giving that in this region precipitation varies considerably between seasons; it is also expected to find a correlation between the diet of mara and seasons. fecal pellets of mara were collected during four periods including two dry, and two wet seasons. relative frequency of each item in scats was determined by microhistological analysis. results support the hypothesis that mara shows preference for grasses, despite they are less available than shrubs and forbs and that this preference remains constant through seasons regardless of the precipitation regime. the characteristics of mara?s diet and the ecological implications of these findings are discussed
Obsessed with grasses?: The case of mara Dolichotis patagonum(Caviidae: Rodentia) Obsesionada con gramíneas?: El caso de mara Dolichotis patagonum (Caviidae: Rodentia)
MOIRA S. SOMBRA,ANTONIO M. MANGIONE
Revista chilena de historia natural , 2005,
Abstract: The diet of mara (Dolichotis patagonum, Zimmeraman 1780) within the limits of Sierra de las Quijadas National Park (San Luis, Argentina) is described. Other studies regarding mara’s diet showed that mara, feeds on different species of grasses and shrubs. However, there is controversy regarding the proportions of these items in the diet or whether this proportion varies seasonally. Considering mara’s body size, anatomical features and physiology, we suggest that mara feeds on grasses in a greater proportion than shrubs and forbs. Moreover, giving that in this region precipitation varies considerably between seasons; it is also expected to find a correlation between the diet of mara and seasons. Fecal pellets of mara were collected during four periods including two dry, and two wet seasons. Relative frequency of each item in scats was determined by microhistological analysis. Results support the hypothesis that mara shows preference for grasses, despite they are less available than shrubs and forbs and that this preference remains constant through seasons regardless of the precipitation regime. The characteristics of mara’s diet and the ecological implications of these findings are discussed En este trabajo se describe la dieta de mara (Dolichotis patagonum Zimmeraman 1780) en el Parque Nacional Sierra de las Quijadas (San Luis, Argentina). Los estudios realizados sobre su dieta indican que se alimenta de varias especies de gramíneas y arbustos. Sin embargo, existe cierta controversia acerca de la proporción en que se presentan estos ítemes, y si las mismas varían estacionalmente. En base al tama o corporal de mara, y a sus características anatómicas y fisiológicas se propone que, mara consume una mayor proporción de gramíneas que de otras hierbas no graminiformes y dicotiledóneas en general. Además esperamos una variación estacional de la dieta, correlacionada a una posible variación estacional en los recursos alimentarios asociada a las precipitaciones. Se colectaron muestras de heces correspondientes a cuatro estaciones en el parque, dos estaciones secas y dos lluviosas. Mediante análisis microhistológico se determinó la proporción de cada ítem medido como frecuencia de aparición en las heces. Los resultados apoyan la hipótesis de un consumo preferencial de gramíneas por parte de mara. Las gramíneas aparecen en mayor proporción en la dieta a pesar de estar en menor disponibilidad en el hábitat y sin que el régimen de precipitación tenga algún efecto Se discuten las características de la dieta de mara y las implicancias ecológicas relacionadas con esta
Intrinsic palindromic numbers
Antonio J. Di Scala,Martin Sombra
Mathematics , 2001,
Abstract: We introduce a notion of palindromicity of a natural number which is independent of the base. We study the existence and density of palindromic and multiple palindromic numbers, and we raise several related questions.
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