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Search Results: 1 - 10 of 257743 matches for " Néstor David Espinosa Torres "
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Ab Initio Molecular Orbital Calculation for Optical and Electronic Properties Evaluation of Small and Medium Size Silicon Nano-Clusters Found in Silicon Rich Oxide Films  [PDF]
Néstor David Espinosa Torres, José Francisco Javier Flores Gracia, José Alberto Luna López, Juan Carlos Ramírez García, Alfredo Morales Sánchez, José Luis Sosa Sánchez, David Hernández de la Luz, Francisco Morales Morales
Journal of Modern Physics (JMP) , 2013, DOI: 10.4236/jmp.2013.411A2001

In systems in atomic and nano scales such as clusters or agglomerates constituted of particles from a few to less than one hundred of atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nano-structures display optical and electronic properties significantly different of those found in corresponding bulk materials. Silicon agglomerates found in Silicon Rich Oxide (SRO) films have optical properties, which have reported as depended directly on nano-crystal size. Furthermore, the room temperature photoluminescence (PL) of Silicon Rich Oxides (SRO) has repeatedly generated a huge interest due to their possible applications in optoelectronic devices. However, a plausible emission mechanism has not yet widespread acceptance of the scientific community. In this research, we employed the Density Functional Theory with a functional B3LYP and a basis set 6 - 31G* to calculate the optical and electronic properties of small (six to ten silicon atoms) and medium size clusters of silicon (constituted of eleven to fourteen silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism experimentally found in thin SRO films.

Forecast of the Luminescent Phenomena of Silicon Rich Oxide Films off Stoichiometry by Means of the Global Reaction Model  [PDF]
Néstor David Espinosa-Torres, José álvaro David Hernández de la Luz, José Francisco Javier Flores-Gracia, José Alberto Luna-López, Javier Martínez-Juárez, Gregorio Flores-Carrasco
Journal of Modern Physics (JMP) , 2015, DOI: 10.4236/jmp.2015.611170
Abstract: The Global Reaction Model describes a set of chemical reactions that can potentially occur during the process of obtaining silicon rich oxide (SRO) films, regardless of the technique used to grow such films which are an outside stoichiometry material. Particularly, chemical reactions that occur during the process of growing of SRO films by LPCVD technique are highlighted in this model. We suggest and evaluate either some types of molecules or resulting nanostructures and we predict theoretically, by applying the density functional theory, the contribution that they may have to the phenomenon of luminescence which is measured in SRO films. Also, we have calculated the opto-electronic properties of SRO films. The suggested model provides enough information required to identify the molecular structures resulting from the presence of defects in SRO films and also those corresponding to charged structures. It is also possible to detect the molecular structures which are modified due to the effect of heat treatment, and identify the presence of different oxidation states inclusive the formation of siloxanes.
Rendimiento de semilla de cruzas simples fértiles y androestériles progenitoras de híbridos de maíz
Tadeo-Robledo, Margarita;Espinosa Calderón, Alejandro;Beck, David;Torres, José Luis;
Agricultura técnica en México , 2007,
Abstract: to reduce production costs maize seed multiplication, the use of male sterility is a viable option. researchers at the national research institute for forestry, agriculture and livestock (inifap) and the national autonomous university of mexico (unam), introduced a male sterile gene since 1992 have been pursuing the introduction of this that into maize lines and in the search for fertility restoring lines. the objective of this study was to evaluate seed production and quality of male sterile single crosses and to compare them with their fertile version. the experiment was established at el batan, state of mexico, mexico and tula, hidalgo, in a complete randomized block design with three replicates. during the flowering period the sterility trait was visually verified. no difference in seed production was observed between male sterile and normal versions of the crosses. male sterility increased the percentage of large seeds and in the majority of the crosses the 100 seed weight. the male sterility trait was confirmed on all simple crosses.
Estudio teórico de la combustión de pellets de biomasa procedente de la ca?a de azúcar
Verdecia Torres,David; carrás,Idalberto; Gaskins Espinosa,Benjamín Gabriel;
Revista Ciencias T??cnicas Agropecuarias , 2012,
Abstract: in the follow work they are an a examinations of the kinetic quimestry of the combustions process, we obtain the combustions time in functions of the kinetic combustions model and we made a experiment design for determinations of the theory's mathematics models in that process.
Modelling change in color and firmness of baby banana (Musa acuminata AA) in modified atmosphere packaging
Castellanos Espinosa Diego,Algecira Enciso Néstor
Agronomía Colombiana , 2012,
Abstract: To determine the change in the ripening stage and quality through associated variables such as firmness and peel color is a useful tool for predicting the behavior and involvement of the product stored at different changing conditions. The change in O2 and CO2 concentration, pulp firmness and peel color were measured in a test of modified atmosphere packaging for baby banana to develop a mathematical model to represent the change in firmness and color as a function of temperature, mixture of gas and time. The fruits were packaged at three temperatures (11, 13 and 17°C) and a range of combinations of steady state concentrations of modified atmospheres (5.3 to 15.7 kPa O2 and 0 to 11.0 kPa for CO2), local atmospheric pressure of 74.9 kPa (0.74 atm) and 80 % relative humidity constant in polyethylene bags (HDPE) micro-perforated for a period of 30 days. The model considers the product respiration rate by an Michaelis-Menten equation of noncompetitive inhibition, and takes into account the transfer of gases through packaging film and through micro-perforations of this; the change of firmness is considered through a first-order model, and depending on the concentration of both O2 and CO2 by Michaelis-Menten equations; color change (in Hunter Lab coordinates) was considered separately for each color coordinate, representing the coordinate L* using an increase-logistic model, a* by a zero-order model and b* by first-order model, all temperaturedependent by Arrhenius functions. The predictive capability of the developed model is appropriate, explaining 97.0% of the modified atmosphere effect on the loss of firmness, 90.6% of the change in the coordinate L*, 88.9% of the change in a* and 96.7 % of the change in b*.
Model-Based Analysis of a Phenol Bio-Oxidation Process by Adhered and Suspended Candida tropicalis  [PDF]
Hugo Velasco-Bedrán, José A. Hormiga, Guido Santos, Néstor V. Torres
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.48A004

Phenol is an important commodity for the chemical industry, used for many processes and deemed to be a major pollutant due its xenobiotic nature and high toxicity. For the purpose of phenol bioremediation a biotechnological set up consisting of a continuous packed column bioreactor with Candida tropicalis adhered onto activated carbon beads has been previously described. In this work, we show how the integration of available experimental data of such a biotechnological set up into a mathematical model, can lead both to a better comprehension of the underlying physiological mechanisms operating in the cell culture, and to the identification of the system parameters optimum performance. The model so constructed describes the dynamics of phenol uptake and growth rates by the adhered and suspended biomass; the lethality rates; the adhered biomass removal into suspension or adherence onto carbon beads rates and the phenol and biomass (adhered and suspended) concentrations. It also serves to identify different physiological states for the adhered and the suspended biomass; its predictions being verified by comparing with experimental observations. Based on the model description, different optimization strategies are proposed, some of which have been experimentally tested, encompassing changes in bioreactor operation conditions, process development and strain development.

Development and Application of a Modified Genetic Algorithm for Estimating Parameters in GMA Models  [PDF]
José A. Hormiga, Carlos González-Alcón, Néstor V. Torres
Applied Mathematics (AM) , 2014, DOI: 10.4236/am.2014.516236

In this work we introduce a modified version of the simple genetic algorithm (MGA) and will show the results of its application to two GMA power law models (a general theoretical branched pathway system and a mathematical model of the amplification and responsiveness of the JAK2/STAT5 pathway representing an actual, experimentally studied system). The two case studies serve to illustrate the utility and potentialities of the MGA method for concerning parameter estimation in complex models of biological significance. The analysis of the results obtained from the application of the MGA algorithm allows an evaluation of the potentialities and shortcomings of the proposed algorithm when compared with other parameter estimation algorithm such as the simple genetic algorithm (SGA) and the simulated annealing (SA). MGA shows better performance in both studied cases than SGA and SA, either in the presence or absence of noise. It is suggested that these advantages are due to the fact that the objective function definition in the MGA could include the experimental error as a weight factor, thus minimizing the distance between the data and the predicted value. Actually, MGA is slightly slower that the SGA and the SA, but this limitation is compensated by its greater efficiency in finding objective values closer to the global optimum. Finally, MGA can lead to an early local optimum, but this shortcoming may be prevented by providing a great population diversity through the insertion of different selection processes.

New Targets for Drug Discovery against Malaria
Guido Santos, Néstor V. Torres
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0059968
Abstract: A mathematical model which predicts the intraerythrocytic stages of Plasmodium falciparum infection was developed using data from malaria-infected mice. Variables selected accounted for levels of healthy red blood cells, merozoite (Plasmodium asexual phase) infected red blood cells, gametocyte (Plasmodium sexual phase) infected red blood cells and a phenomenological variable which accounts for the mean activity of the immune system of the host. The model built was able to reproduce the behavior of three different scenarios of malaria. It predicts the later dynamics of malaria-infected humans well after the first peak of parasitemia, the qualitative response of malaria-infected monkeys to vaccination and the changes observed in malaria-infected mice when they are treated with antimalarial drugs. The mathematical model was used to identify new targets to be focused on drug design. Optimization methodologies were applied to identify five targets for minimizing the parasite load; four of the targets thus identified have never before been taken into account in drug design. The potential targets include: 1) increasing the death rate of the gametocytes, 2) decreasing the invasion rate of the red blood cells by the merozoites, 3) increasing the transformation of merozoites into gametocytes, 4) decreasing the activation of the immune system by the gametocytes, and finally 5) a combination of the previous target with decreasing the recycling rate of the red blood cells. The first target is already used in current therapies, whereas the remainders are proposals for potential new targets. Furthermore, the combined target (the simultaneous decrease of the activation of IS by gRBC and the decrease of the influence of IS on the recycling of hRBC) is interesting, since this combination does not affect the parasite directly. Thus, it is not expected to generate selective pressure on the parasites, which means that it would not produce resistance in Plasmodium.
Efectos del bloqueo económico, financiero y comercial de Estados Unidos en el Sistema Nacional de Salud Impact of the US economic, financial and commercial blockage on the national health care system
Néstor Marimón Torres,Esther Torres Martínez
Revista Cubana de Salud Pública , 2013,
Abstract: Introducción: Cuba ha estado sometida a un feroz bloqueo económico, comercial y financiero de Estados Unidos de América desde el 1959, con serias afectaciones para el pueblo cubano y el sistema de salud, lo que se vio recrudecido a partir del Período Especial a principios de los 90. Objetivos: proporcionar elementos que permitan realizar un análisis del impacto del bloqueo en el Sistema Nacional de Salud cubano y la experiencia de Cuba ante crisis anteriores. Métodos: investigación cualitativa descriptiva acerca de los da os causados por el bloqueo al sistema de salud. Resultados: se demuestran las afectaciones sobre la salud con cifras y ejemplos concretos, las medidas que se adoptaron para mantener los principales servicios a la población y las transformaciones necesarias en las cuales trabaja hoy el Ministerio de Salud Pública para conservar sus logros. Conclusiones: la salud cubana es una de las esferas más afectadas por el bloqueo de los Estados Unidos, pero a pesar de ello, ha mantenido los servicios a la población y buscado soluciones y alternativas para resolver cada dificultad. Las experiencias adquiridas sirven de ejemplo para estar mejor preparados y continuar enfrentando los efectos del bloqueo y la crisis económica que se presenta en el escenario mundial. Introduction: since 1959, Cuba has suffered a ruthless economic, commercial and financial blockade imposed by the USA, which have heavily affected the Cuban population and the health care system. This situation worsened in the so called "special period of the 90's"after the tightening of the blockade. Objectives: to provide the necessary elements which allow analyzing the impact of the US blockade on the national health system as well as the Cuba's experience in previous crises and the learned lessons. Methods: qualitative and descriptive research study of the damages inflicted by the blockade on the health system. Results: the impact on the health care system was proven, based on figures and specific examples, the measures adopted to keep the main services for the population and the required changes in which the Ministry of Public health works in order to preserve its achievements Conclusions: the Cuban health care is one of the most affected spheres by the US blockade; however, it has kept the services in operation for the people and has looked for new alternatives to solve every difficulty. The learned lessons help us to be better prepared and to continue facing the blockade impact and the world economic crisis.
Espinosa,Luisa Fernanda; Ramírez,Gustavo; Campos,Néstor Hernando;
Boletín de Investigaciones Marinas y Costeras - INVEMAR , 1995,
Abstract: concentrations of six organochlorine pesticide residues (lindane, heptachlor, aldrin, pp'dde, pp'ddd y pp'ddt) wereanalized in surface mangrove sediments at four sites at ciénaga grande de santa marta and in one site in bahía de chengue, colombian caribbean. analyses were carried out by gas-liquid chromatography. samples were taken between march and december 1993, corresponding to four climatic seasons (dry, minor rainy, minor dry and major rainy). significant differences in lindane, heptachlor and pp'dde concentrations among seasons were determined by multifactorial analysis of variance; the other three compounds (aldrin, pp'ddd an pp'ddt) did not show significant differences for any of the likely sources of variation. highest concentrations of lindane were in dry season and those of heptaclore and pp'dde were in rainy season. a canonical correlation was run between the groups formed with organochlorine concentratios and water salinity plus relative organic matter content. the correlation coefficients indicate that only pp'ddt presented a inverse correlation with the percentage of organic matter.
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