Showing papers by "Simón Bolívar University published in 2002"

Homogeneous nucleation and fractionated crystallization in block copolymers

Abstract: The confinement of crystallizable blocks within AB or ABC microphase-separated block copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight, and chemical structure. In this work we have examined the crystallization behavior of a series of AB and ABC block copolymers incorporating one or two of the following crystallizable blocks: polyethylene, poly(e-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or any other isolated morphology, is much higher than the number of heterogeneities available in each crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from homogeneous nuclei was obtained. The results show that, regardless of the specific morphologica...

Expected‐posterior prior distributions for model selection

Abstract: SUMMARY We consider the problem of comparing parametric models using a Bayesian approach. A new method of developing prior distributions for the model parameters is presented, called the expected-posterior prior approach. The idea is to define the priors for all models from a common underlying predictive distribution, in such a way that the resulting priors are amenable to modern Markov chain Monte Carlo computational techniques. The approach has subjective Bayesian and default Bayesian implementations, and overcomes the most significant impediment to Bayesian model selection, that of ensuring that prior distributions for the various models are appropriately compatible.

Nutritional improvement of beans (Phaseolus vulgaris) by natural fermentation

Abstract: The effect of natural fermentation of flour and whole bean seeds (Phaseolus vulgaris) on the content of nutrients (protein, fat, starch, dietary fiber, vitamins B1 and B2) and antinutritional factors (α-galactosides, trypsin inhibitor activity and inositol phosphates) has been studied. After fermentation, total protein content decreased (2–15%) but the in vitro protein digestibility increased (4–8%) and the fat content did not change. In fermented bean flours, fructose and sucrose decreased (67–83% and 99–100% respectively), glucose increased (175–750%) and galactose was present at the concentration 1:4 g/ml (flour:water), but when whole beans were fermented none of these soluble sugars were detected. Total starch, available starch, insoluble fiber content decreased after fermentation (5–13%, 10–24%, and 5–26%, respectively), which was less pronounced in whole fermented beans. The soluble dietary fiber decreased (61–71%). The vitamin B1 content decreased (17–38%) and the vitamin B2 content increased (16–35%) after fermentation. Fe, P, Mg, Ca, and K decreased and Zn content did not change after natural fermentation. In either fermented flour or whole beans, the alpha-galactosides and IP6 decreased (99–100%, 7–39% respectively); however in fermented flour beans IP5 content did not modify or decrease and IP4 content could also increase. When whole bean grains were fermented no IP5 and IP4 was detected. TIA levels and tannins content decreased (58–71% and 61–70%) after fermentation. Natural fermentation of beans has been a very effective process for increasing functionality of Phaseolus vulgaris and the whole bean fermentation is very promising due to the lower cost.

Responses to drought of five Brachiaria species. I. Biomass production, leaf growth, root distribution, water use and forage quality

Abstract: The introduction of African grasses in Neotropical savannas has been a key factor to improve pasture productivity. We compared the response of five Brachiaria species to controlled drought (DT) in terms of biomass yield and allocation, pattern of root distribution, plant water use, leaf growth, nutrient concentration and dry matter digestibility. The perennial C4 forage grasses studied were B. brizantha (CIAT 6780), B. decumbens (CIAT 606), B. dictyoneura (CIAT 6133), B. humidicola (CIAT 679) and B. mutica. Two DT periods, which mimic short dry spells frequent in the rainy season, were imposed by suspending irrigation until wilting symptoms appeared. They appeared after 14 days in B. brizantha, B. decumbens and B. mutica, and after 28 days in B. humidicola and B. dictyoneura. The impossed drought stress was mild and only the largest grass, B. brizantha, showed reduced (23%) plant yield. The other grasses were able to adjust growth and biomass allocation in response to DT leaving total plant yield relatively unaffected. Brachiaria mutica, had a homogeneous root distribution throughout the soil profile. In the other species more than 80% of root biomass was allocated within the first 30 cm of the soil profile. Brachiaria brizantha and B. decumbens had the lowest proportion of roots below 50 cm. Drought caused a general reduction in root biomass. The shoot:root ratio in B. mutica and B. humidicola increased in response to DT at the expense of a reduction in root yield down to 50 cm depth. Although the total water volume utilized under DT was similar among grasses, the rate of water use was highest (0.25 l day−1) in B. brizantha, B. decumbens and B. mutica and lowest (0.13 l day−1) in B. humidicola and B. dictyoneura. In all species leaf expansion was reduced by DT but it was rapidly reassumed after rewatering. Drought increased specific leaf mass (SLM) only in B. brizantha compensating for leaf area reduction, but leaf area ratio (LAR) was unaffected in all species. In almost all grasses DT increased leaf N and K concentration and in vitro dry matter digestibility. The results indicate that B. brizantha, B. decumbens and to a lesser extent, B. mutica are better adapted to short dry periods, whereas B. humidicola and B. dictyoneura are better adapted to longer dry periods.

Power system analysis using space-vector transformation

Abstract: In this paper, the space-vector transformation used in machine vector control is applied to power system analysis. The proposed method is used to model electric machines, power electronic converters, transformers, and transmission lines and to analyze power sources and loads with different connections (delta and wye). This method can also be applied to analyze steady-state (or transient phenomena) and unbalanced sources, including harmonics. Models obtained with this method are as simple as those of the per-phase approach. With the space-vector transformation, instantaneous active and reactive power concepts can be generalized, and new power system control strategies can be developed when power electronic converters are used. Steady-state, transient behavior, and harmonic analyses examples and applications are presented to illustrate the performance and advantages of the proposed method. This method can be extended to unbalanced systems (e.g., unsymmetric faults) using instantaneous symmetrical components in polyphase balanced circuits.

Dense packing of binary and polydisperse hard spheres

Abstract: The packing of binary and polydisperse unimodal and bimodal ensembles of hard spheres in the limit of high pressure is studied using a sequential addition algorithm. Upon fixing the number of particles, and their size distribution, the average (maximum) packing fraction is determined for systems of up to 20 000 particles. The structures obtained correspond to amorphous states close to the dense random close packing density. Binary distributions obtained are denser than the equivalent monodisperse distribution and agree with the theoretical prediction for an infinite size ratio limit. Unimodal normal and lognormal polydisperse distributions obtained compare favourably with available simulation and experimental data. Results for bimodal lognormal distributions are presented. In all cases it is seen how an increase in polydispersity increases the packing fraction of the system. The results can be employed to gain insight into optimal formulations for dense emulsions.

Self-affine tiling via substitution dynamical systems and Rauzy fractals

Abstract: In this paper we show that a class of sets known as the Rauzy fractals, which are constructed via substitution dynamical systems, give rise to self-affine multi-tiles and self-affine tilings. This provides an efficient and unconventional way for constructing aperiodic self-affine tilings. Our result also leads to a proof that a Rauzy fractal R associated with a primitive and unimodular Pisot substitution has nonempty interior.

Thermodynamic Studies of Anion Adsorption at Stepped Platinum(hkl) Electrode Surfaces in Sulfuric Acid Solutions

Abstract: The thermodynamics of the so-called perfectly polarized electrode was employed to analyze the total charge densities for stepped Pt(hkl) electrodes in solutions of H2SO4 with an excess of an inert electrolyte (0.1 M HClO4). Three Pt single-crystal electrodesPt(10,10,9) = 20(111)×(111), Pt(7,7,6) = 14(111)×(111), and Pt(5,5,4) = 10(111)×(111)vicinal to the Pt(111) surface were employed in these studies. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The Gibbs excess, Gibbs energy of adsorption, electrosorption valency, and Esin−Markov coefficients for (bi)sulfate adsorption at these surfaces have been determined. The thermodynamic data display a dependence on the crystallographic structure of the Pt electrode surface. Both the Gibbs excess and the Gibbs energy of adsorption decrease with increasing step density or decreasing terrace length. This result indicates that adsorption of (bi)sulfate is stronger at larger (111) terraces.

Sensorless speed measurement of AC machines using analytic wavelet transform

Abstract: A new method for the sensorless speed estimation of AC machines, using the analytic wavelet transform of the stator current signal, is proposed for a direct torque control drive. A comparison with results obtained using the short-time Fourier transform is included. The proposed method can be implemented in real time on a digital signal processor. The time-frequency resolution obtained and the computation time required by the proposed algorithm are improved in comparison to existing techniques, and the method can be applied over the entire speed range.

Efficient evaluation of queries in a mediator for WebSources

Abstract: We consider an architecture of mediators and wrappers for Internet accessible WebSources of limited query capability. Each call to a source is a WebSource Implementation (WSI) and it is associated with both a capability and (a possibly dynamic) cost. The multiplicity of WSIs with varying costs and capabilities increases the complexity of a traditional optimizer that must assign WSIs for each remote relation in the query while generating an (optimal) plan. We present a two-phase Web Query Optimizer (WQO). In a pre-optimization phase, the WQO selects one or more WSIs for a pre-plan; a pre-plan represents a space of query evaluation plans (plans) based on this choice of WSIs. The WQO uses cost-based heuristics to evaluate the choice of WSI assignment in the pre-plan and to choose a good pre-plan. The WQO uses the pre-plan to drive the extended relational optimizer to obtain the best plan for a pre-plan. A prototype of the WQO has been developed. We compare the effectiveness of the WQO, i.e., its ability to efficiently search a large space of plans and obtain a low cost plan, in comparison to a traditional optimizer. We also validate the cost-based heuristics by experimental evaluation of queries in the noisy Internet environment.

Linamarase Expression in Cassava Cultivars with Roots of Low- and High-Cyanide Content

Abstract: This paper reports the expression and localization of linamarase in roots of two cassava (Manihot esculenta Crantz) cultivars of low and high cyanide. Two different patterns of linamarase activity were observed. In the low-cyanide type, young leaves displayed very high enzyme activity during the early plant growing stage (3 months), whereas in root peel, the activity increased progressively to reach a peak in 11-month-old plants. Conversely, in the high-cyanide cultivar (HCV), root peel linamarase activity decreased during the growth cycle, whereas in expanded leaves linamarase activity peaked in 11-month-old plants. The accumulation of linamarin showed a similar pattern in both cultivars, although a higher concentration was always found in the HCV. Linamarase was found mainly in laticifer cells of petioles and roots of both cultivars with no significant differences between them. At the subcellular level, there were sharp differences because linamarase was found mainly in the cell walls of the HCV, whereas in the low-cyanide cultivar, the enzyme was present in vacuoles and cell wall of laticifer cells. Reverse transcriptase-PCR on cassava tissues showed no expression of linamarase in cassava roots, thus, the transport of linamarase from shoots to roots through laticifers is proposed.

New method for determination of harmonic distortion in SOI FD transistors

Abstract: We present a new method for calculating the total harmonic distortion (THID) and the third harmonic distortion (HD3) of the output current-voltage characteristics of a semiconductor device. The method is based on the calculation of two functions which we call D and D3 and are based on a specific integration of the DC current-voltage characteristic of the device. In this paper we demonstrate that function D can be correlated with the THD and function D3 with the HD3, so that they can be determined in a much simpler way, with no need to use derivatives, Fourier coefficients or fast Fourier transforms. The new method is applied to calculate the harmonic distortion of a silicon-on-insulator (Sol) fully depleted (FD) MOS transistor in the triode regime to be used as an active resistor at the input of an operational amplifier in a MOSFET-C filter configuration. It is also demonstrated that the transistor I-DS-V-DS characteristics used in these calculations can be obtained from either measurements, analytical models or numerical simulations. (C) 2002 Elsevier Science Ltd. All rights reserved.

Symmetric Functions in Noncommuting Variables

Abstract: Consider the algebra Q > of formal power series in countably many noncommuting variables over the rationals. The subalgebra Pi(x_1,x_2,...) of symmetric functions in noncommuting variables consists of all elements invariant under permutation of the variables and of bounded degree. We develop a theory of such functions analogous to the ordinary theory of symmetric functions. In particular, we define analogs of the monomial, power sum, elementary, complete homogeneous, and Schur symmetric functions as will as investigating their properties.

Measuring household food security in poor Venezuelan households.

Abstract: Objective: To validate abbreviated methods that estimate food security level among poor communities in Caracas, Venezuela. Design: Two independent cross-sectional studies were undertaken to internally and externally validate simple quantitative/qualitative methods. The quantitative measure was constructed from data on household food availability, gathered using the list-recall method. It is a count of the foods that explain 85% or more of household energy availability. The qualitative measure is a score of female-perceived food insecurity level estimated with a modified ‘hunger index’, reflecting food resource constraints and hunger experiences within the home. Socio-economic and food behaviour data that may predict household food security (HFS) levels were gathered. The second study was repeated a year later to measure the impact of an increase in the minimum wage on HFS levels. Setting: Two poor urban communities in Caracas, Venezuela. Subjects: All households in both communities that complied with selection criteria (poor and very poor families that share food resources) and were willing to participate. The sample comprised 238 and 155 female household food managers in the two communities. Results: In 1995, data from females in 238 urban poor households provided evidence for the overall validity of the method. Its application in 1997 to 155 households in the other community gave support to the external validity of the method. Measures were repeated in 1998 on 133 subjects of the above sample, when the minimum wage was increased by 23%. Evidence is presented showing the sensitivity of the method to changes in the determinants of HFS. Data analysed during these three periods suggest that the method can be simplified further by using the food diversity score instead of the quantitative measure since these variables correlate highly with one another ( r ≥ 0:854). Conclusions: This simple method is a valid and precise measure of food security among poor urban households in Caracas. The qualitative/quantitative measures complement each other as they capture different dimensions of HFS.

Etnobotánica médica de una comunidad campesina del estado Trujillo, Venezuela: un estudio preliminar usando técnicas cuantitativas.

Abstract: In order to document the relative importance of medicinally used taxa, the use of medicinal plants in a rural community in Trujillo State, Venezuela, was quantitatively evaluated. The diagnostic of the most common health problems was made with the help of information provided by the state’s Direction of Epidemiology. Ethnopharmacological information was gathered through interviews at the homes of the participants, using the TRAMIL structured interview. Ethnopharmacological data was analyzed using the quantitative indexes of use value for each species (UVIs) and significative level of use (TRAMIL). 32 species of medicinal plants from 25 botanical families were recorded as well as their traditional uses and modes of administration. Among them 10 species are wild and 22 species are cultivated plants. PALABRAS CLAVE

Setting the mutation rate: scope and limitations of the 1/ L heuristic

Abstract: An important decision to make when designing a GA is how to set the evolutionary parameters Among these parameters, the mutation rate has been acknowledged as the most sensitive one All approaches so far for a near-optimal setting of the mutation rate have intrinsic limitations A promising guideline is, however, the heuristic suggesting pm = 1/L where L is the string length This paper is a first attempt to explore the scope and limitations of this heuristic on GAs with bit-string representation Specifically, we select two real-world domains as test problems and explore (i) whether optimal mutation rates change with time; and (ii) the interactions between the mutation rate and other evolutionary parameters Results suggest that a constant mutation rate of 1/L is useful for a GA with a controlled 'moderate' selection pressure It should be, however, revised for a weak or extremely strong selection pressure, and for a small population size

Canonical quantization of constrained theories on discrete spacetime lattices

Abstract: We discuss the canonical quantization of systems formulated on discrete spacetimes. We start by analysing the quantization of simple mechanical systems with discrete time. The quantization becomes challenging when the systems have anholonomic constraints. We propose a new canonical formulation and quantization for such systems in terms of discrete canonical transformations. This allows us to construct, for the first time, a canonical formulation for general constrained mechanical systems with discrete time. We extend the analysis to gauge field theories on the lattice. We consider a complete canonical formulation, starting from a discrete action, for lattice Yang–Mills theory discretized in space and Maxwell theory discretized in space and time. After completing the treatment, the results can be shown to coincide with the results of the traditional transfer matrix method. We then apply the method to BF theory, yielding the first lattice treatment for such a theory ever. The framework presented deals directly with the Lorentzian signature without requiring a Euclidean rotation. The whole discussion is framed in such a way so as to provide a formalism that would allow a consistent, well-defined, canonical formulation and quantization of discrete general relativity, which we will discuss in a forthcoming paper.

A linear programming methodology for the optimization of electric power-generation schemes

Abstract: A mathematical model, based upon the application of a linear-integer programming algorithm, is presented for the optimum selection of independent electric power generation schemes in industrial power systems, taking into account reliability considerations. The problem is formulated as a mathematical programming problem, considering investment costs, fuel costs, operation and maintenance costs, power balance, maximum and minimum limits on the generated power of the units, as well as reliability considerations such as the unavailability of the generation scheme. These considerations include assumptions taken and simplifications performed in order to obtain an accurate enough linear model. The problem is solved using a conventional branch and bound algorithm for linear-integer programming, yielding to the optimum number of units, as well as the correspondent size and type. Results are presented for the application of the proposed methodology to a real case of an industrial power system. The technique is proved as a valuable tool for the planning engineer.